, AI: , , .

-AI Context. .

frameworks :

gstack framework slash commands -skills AI coding agents. -AI , gstack : product thinking, engineering review, design review, code review, QA, security, -shipping. AI coding -delivery pipeline.

Lattice framework composable AI skills engineering discipline . -Atoms, Molecules, Guardrails, Refiners , workflows , . , Lattice .lattice/ - -design, , , code reviews, .

, .

:
gstack -AI .
Lattice -AI , -guardrails.
, - AI -AI Engineering Workflow.

AI : , , review, , , -diff 1,200 .

-AI . . :

.
domain logic -infrastructure.
APIs
idempotency.
test .
-failure modes .
abstraction .

. compile -production incident - 02:17.

: -AI , , . .

: , , , , review, , .

-AI.

gstack -Lattice .

gstack?

gstack framework AI coding agents, : -AI , . -Personas .

-gstack :

/office-hours
/plan-ceo-review
/plan-eng-review
/review
/qa
/ship

. . - .

• /office-hours

• /plan-ceo-review -scope.

• /plan-eng-review , data flow, , -test plan.

• /review .

• /qa .

• /ship merge release.

, gstack . -AI .

:

Prompt  Code  Review  Pain

:

Think  Plan  Build  Review  Test  Ship  Learn

. -AI , , artifacts, , .

Lattice?

Lattice , .

gstack -AI ?"
Lattice -AI ?

Lattice framework composable AI skills. :

1. Atoms - guardrails . - clean code, architecture, secure-coding, test-quality, design-first -context-anchoring.

2. Molecules - workflows Atoms . , requirement-forge, design-blueprint, code-forge, bug-fix, refactor-safely

3. Refiners - . architecture standards, review standards, requirement standards.

-Lattice . , .lattice/. :

.lattice/
 config.yaml
 standards/
    architecture.md
    clean-code.md
    secure-coding.md
    test-quality.md
    review-standards.md
 requirements/
 context/
 reviews/
 learnings/

: . -artifact.
-AI . , , , .

Lattice Martin Fowler / Thoughtworks AI-assisted development. patterns Knowledge Priming, Design-First Collaboration, Context Anchoring, Encoding Team Standards -Feedback Flywheel.

: AI coding assistant , , onboarding, . AI, - . .

-PR .

?

gstack -Lattice, :

1. - -AI .

2. -

3. - .

AI . , , .
. . -state . . design . .

gstack -Lattice .
gstack .
Lattice engineering memory.

: refund workflow .
, :

Build a refund workflow for paid invoices.
Include validation, audit log, event publishing, and admin status view.

. , :

• invoice?

• refund ?

• ?

• external payment provider timeout?

• -queue -transaction?

• ?

• -audit log?

• -UI projection source of truth?

• -rollback behavior

• events ?

AI . . , , , .

gstack :

/office-hours
    
/plan-ceo-review
    
/plan-eng-review
    
build
    
/review
    
/qa
    
/ship

, Lattice standards -context:

/requirement-forge
    
/design-blueprint
    
/code-forge
    
/review
    
learnings saved into .lattice/

:
gstack = ?
Lattice = ?

, Lattice Atoms : architecture, clean-code, secure-coding, test-quality, context-anchoring.
-review, gstack review role bugs, scope, -production risk, -Lattice -standards -learnings .

workflow -AI . :

Unclear request
    
Structured requirement
    
Approved design
    
Constrained implementation
    
Delta review
    
Behavior validation
    
Shipping summary
    
Reusable learning

review . . .
review adapter?, :

• -design ?

• -scope ?

• -failure modes ?

• ?

AI leverage .

-workflow gstack -Lattice.
-TypeScript billing. -refund . HTTP API, application, domain logic, postgreSQL, queue publishing -admin UI refund state.

-AI refund.
artifact .

1: Lattice context

-Lattice , -skill : /lattice-init. Lattice , , . . :

.lattice/
 config.yaml
 standards/
    knowledge-base.md
    architecture.md
    clean-code.md
    secure-coding.md
    test-quality.md
    requirement-standards.md
    review-standards.md
 requirements/
 context/
 reviews/
 learnings/
     operational-learnings.md

.lattice/config.yaml :

version: 1

paths:
  knowledge_base: .lattice/standards/knowledge-base.md
  architecture: .lattice/standards/architecture.md
  clean_code: .lattice/standards/clean-code.md
  secure_coding: .lattice/standards/secure-coding.md
  test_quality: .lattice/standards/test-quality.md
  requirement_standards: .lattice/standards/requirement-standards.md
  review_standards: .lattice/standards/review-standards.md
  operational_learnings: .lattice/learnings/operational-learnings.md

-refiners. -architecture :

---
mode: overlay
---

# Architecture Standards

## Layering

Allowed dependency direction:

- `interfaces/http`  `application`
- `interfaces/workers`  `application`
- `application`  `domain`
- `application`  ports
- `infrastructure`  ports
- `infrastructure`  external SDKs

Forbidden:

- `domain` importing `infrastructure`
- HTTP controllers querying PostgreSQL directly
- queue consumers duplicating business rules
- application services depending directly on vendor SDKs

## Messaging

Every queue publisher or consumer must define:

- event contract
- idempotency strategy
- retry behavior
- dead-letter behavior
- observability signals
- poison-message ownership

## External Calls

Every external API call must include:

- timeout
- bounded retry policy
- error classification
- correlation id
- clear fallback or fail-fast behavior

## Money Movement

Any operation that moves, refunds, captures, reserves, or releases money must define:

- authorization boundary
- audit trail
- idempotency key
- consistency model
- reconciliation strategy

. "style guide". boundary contract.

AI , . -PostgreSQL HTTP controller, . queue event idempotency, . -refund audit trail, .

2: gstack

: :

/office-hours

We need to add a refund workflow for paid invoices.

Goal:
Allow admins to request a full or partial refund for an already paid invoice.

Initial requirements:
- Validate invoice state.
- Prevent duplicate refunds.
- Write an audit trail.
- Publish RefundRequested event.
- Expose refund status to the admin UI.

Do not propose implementation yet.
Clarify product scope, missing assumptions, and risk areas.

. friction. -AI .

:

- Are partial refunds allowed more than once?
- Who can approve a refund?
- Is refund execution synchronous or asynchronous?
- What happens if the payment provider accepts the refund but event publishing fails?
- Does the admin UI need real-time status or eventual consistency?
- What is the expected audit format?
- Are refunds reversible?

ambiguity .

3: Lattice

-Lattice :

/requirement-forge

Create a structured requirement for the refund workflow.

Input:
Admins can request a full or partial refund for a paid invoice.
The system must validate invoice state, prevent duplicate refunds,
write an audit trail, publish RefundRequested, and expose refund status.

Use project requirement standards.
Persist the result under .lattice/requirements/refund-workflow.md.

Explicitly include:
- business rules
- authorization rules
- state transitions
- API expectations
- event expectations
- failure modes
- open questions

:

# Refund Workflow Requirement

## Capability

Admins can request a full or partial refund for a paid invoice.

## Business Rules

- Refund can only be requested for invoices in `PAID` state.
- Total refunded amount must not exceed paid amount.
- Duplicate refund requests must be rejected or deduplicated.
- Every refund request must be auditable.
- Refund execution is asynchronous.

## State Transitions

PAID
   REFUND_REQUESTED
   REFUND_PROCESSING
   REFUNDED

PAID
   REFUND_REQUESTED
   REFUND_FAILED

## Events

- `RefundRequested`
- `RefundCompleted`
- `RefundFailed`

## Failure Modes

- Duplicate request.
- Payment provider timeout.
- Event publish failure.
- Audit write failure.
- Admin UI reads stale status.

## Open Questions

- Are multiple partial refunds allowed?
- Is approval required above a threshold?
- What is the SLA for status visibility?

artifact. . . .

4: Engineering review gstack

design , review .

/plan-eng-review

Review .lattice/requirements/refund-workflow.md.

Focus on:
- data model implications
- transaction boundaries
- event publishing guarantees
- idempotency
- race conditions
- rollback behavior
- observability
- test matrix

Do not write implementation code.
Produce engineering risks and required design decisions.

:

# Engineering Review Notes

## Critical Decisions

1. Refund request creation and audit write must be atomic.
2. Event publishing should use outbox pattern or equivalent reliability mechanism.
3. Refund request must use idempotency key scoped by invoice and request source.
4. Admin status should read from refund state stored internally, not directly from provider.
5. Provider timeout must not imply failure; status reconciliation is required.

## Race Conditions

- Two admins request refund for same invoice simultaneously.
- Provider callback arrives before local state transition completes.
- Retry publishes duplicate RefundRequested event.

## Required Tests

- Full refund success.
- Partial refund success.
- Duplicate refund request.
- Refund exceeds paid amount.
- Provider timeout.
- Event publish retry.
- Audit trail write failure.

: , feature .
-ROI process.

5: Design Blueprint Lattice.

-design:

/design-blueprint

Create a design blueprint for .lattice/requirements/refund-workflow.md.

Use:
- .lattice/standards/architecture.md
- .lattice/standards/secure-coding.md
- .lattice/learnings/operational-learnings.md
- engineering review notes from /plan-eng-review

Do not write implementation code.

The blueprint must include:
- capabilities
- components
- interactions
- contracts
- data model changes
- state machine
- failure handling
- test strategy

:

# Refund Workflow Design Blueprint

## Components

### HTTP Interface

`POST /admin/invoices/:invoiceId/refunds`

Responsibilities:
- Authenticate admin.
- Validate request shape.
- Delegate to application use-case.
- Return refund request id and initial status.

Must not:
- Query PostgreSQL directly.
- Call payment provider directly.
- Publish events directly.

### Application Use-Case

`RequestRefundUseCase`

Responsibilities:
- Load invoice.
- Validate business rules.
- Create refund request.
- Write audit record.
- Persist outbox event.
- Return refund status.

### Domain

`InvoiceRefundPolicy`

Responsibilities:
- Validate refund eligibility.
- Validate partial refund limits.
- Prevent invalid state transitions.

### Infrastructure

- `RefundRepository`
- `AuditLogRepository`
- `OutboxRepository`
- `PaymentProviderRefundClient`

## Consistency Model

Refund request creation, audit write, and outbox event creation occur in one database transaction.

Actual provider call is asynchronous and triggered by outbox processing.

## Idempotency

Idempotency key is required per request.
Scope:
`adminId + invoiceId + idempotencyKey`

## Failure Handling

- Provider timeout leaves refund in `REFUND_PROCESSING`.
- Reconciliation job resolves unknown provider states.
- Outbox retry is safe because event handling is idempotent.
- Duplicate admin requests return existing refund request when idempotency key matches.

design-first . . .

6: Code Forge Lattice

:

/code-forge

Implement the approved refund workflow design blueprint.

Use:
- .lattice/context/refund-workflow-design-blueprint.md
- .lattice/standards/architecture.md
- .lattice/standards/clean-code.md
- .lattice/standards/test-quality.md
- .lattice/standards/secure-coding.md

Rules:
- Implement only the approved design.
- Do not modify unrelated modules.
- Do not introduce new architectural patterns.
- Generate tests with the implementation.
- Stop if existing code contradicts the approved design.

-"build refund workflow.
-AI:

• product discovery .

• refactor .

• public contracts

• design .

: , .

7: Review

, review .
gstack:

/review

Review the implementation delta.

Focus on:
- production bugs
- hidden assumptions
- scope creep
- missing tests
- incorrect error handling
- race conditions
- security issues

Classify findings by severity.

Lattice:

/review

Review the implementation against:
- .lattice/standards/architecture.md
- .lattice/standards/secure-coding.md
- .lattice/standards/test-quality.md
- .lattice/context/refund-workflow-design-blueprint.md

Persist reusable findings into:
.lattice/reviews/refund-workflow-review.md
.lattice/learnings/operational-learnings.md

.
gstack shipping.
Lattice .

:

# Review Findings

## High Severity

### Missing idempotency enforcement in worker

The HTTP layer validates idempotency key, but the worker does not enforce
deduplication when consuming RefundRequested.

Impact:
Duplicate event delivery may trigger duplicate provider calls.

Required fix:
Persist provider operation key and enforce uniqueness before calling provider.

## Medium Severity

### Audit trail does not include actor role

Refund audit records include admin id but not admin role.

Required fix:
Include role and permission context in audit metadata.

## Learning

For money movement workflows, idempotency must be enforced both at request
creation and at asynchronous execution boundary.

-Learning . - .lattice/learnings/operational-learnings.md, .
AI AI workflow .

gstack -Lattice . .
-AI .

AI coding agents: . . , . , business logic -retry.

, .
gstack -Lattice, , -artifacts.

,

. , -AI . -AI , , artifact , review.
-AI design .
? .

-ADRs.
.
-review.
.
, , .

Lattice .
gstack .
: , .

Durable artifacts

AI context erosion. . , logs, .
, .

artifacts

requirement.md
design-blueprint.md
architecture.md
review-log.md
operational-learnings.md

. -AI .
: " architecture standards .
,

Design first ;

AI . .
syntax error. design .

Design-First . .
Lattice design-blueprint.
gstack planning -engineering review.
:

, ?

, -diff .
.

Role separation

gstack mode .
scope .
UI.
review release summary.

, . -AI product , -test strategy, -refactor . , .

gstack :

Product thinking ?
Engineering planning ?
Implementation ?
Review ?
QA ?
Ship ?
Reflect ?

. .

,

Lattice .
: legacy policy. , .

.lattice/standards/architecture.md , -AI . , .
.lattice :

• review .

• refactor

• .

: -AI , .

-Lattice , .
, , review .

, idempotency -worker, -operational learning:

## Money Movement Learning

For workflows involving money movement, idempotency must be enforced at every
asynchronous boundary, not only at the initial HTTP request boundary.

Check:
- HTTP request deduplication
- queue consumer deduplication
- provider operation key
- reconciliation behavior

flow , -AI . -context.

AI workflow - , : , .

Failure modes

-failure mode over trust.
/review review . . , , , , data migrations - .
.

-failure mode stale context.
, -AI .

ceremony overload.
full workflow. type , , engineering review -QA. .

local optimization.
AI . review , diff .

ambiguity laundering.
, AI . framework , ambiguity .
AI:

AI - .

AI - guardrail.

gstack -Lattice AI .

.
workflow -AI : , , code review, QA, shipping,

gstack :

Think Plan Build Review Test Ship Learn

Lattice :

Atoms Molecules Refiners .lattice/ living context

: AI-assisted development chat-based coding process-aware engineering.

, AI . . , , standards, ambiguity, .

:

AI .
AI , architecture, learnings, .

Extract -Tableau Cloud Airflow ( -MWAA), -Personal Access Tokens (PAT) (Linear Session). (Error 401002). -Airflow Pools -API, -JWT Connected Apps, -Providers -MWAA

: -BI Orchestration

, AWS Managed Workflows for Apache Airflow (MWAA) 2.10.3, : BI 4 .

: DAG , Flow -dbt cloud -Datasource -Tableau. -Flow , . -API Tableau.

( -PAT )

Tableau REST API. -PAT, -Site logic :

1. sign_in() -1

2. ( ) sign_in() PAT -2

3. Tableau Cloud -1

4. (Unauthorized Access) 401002

, PAT.

:

, -MFA Wall. 2022, Tableau MFA, .

JWT Connected App, Stateless Authentication . , -MWAA : -Tableau Provider 4.5.0, -JWT -5.2.0. requirements.txt -Constraints AWS.

:

: Airflow Pools

-DAG, Airflow .

Airflow Pool tableau_refresh_pool Slot . -dbt -Workers, . Worker -Tableau , -Race Condition.

: DAG Factory

:

# Deploy TableauOperator with Pool
for luid in conf['tableau_datasource_luids']:
    TableauOperator(
        task_id=f"refresh_{luid[:8]}",
        site_id="my_site",
        resource="datasources",
        method="refresh",
        find=luid,
        pool="tableau_refresh_pool" #   401002
    ) >> dbt_task

1. Auth is Architecture: , -Scaling .

2. : MWAA, constraints.txt provider .

3. Pools : -DAG API .

Apache Spark . execution engine : pipeline , -Spark cluster. , -RDD immutable, partitioned -DataFrame -Spark SQL, -optimization. , groupBy, join, count() syntax : CPU, , , shuffle.

: Spark ?

Backend , Spark .

: "pandas .
: "SQL engine .

, . Spark, , :

multi-language engine for executing data engineering, data science, and machine learning on single-node machines or clusters

, ; -API .

-Spark . . Driver, Executors, Resource Allocation, Parallelism, Locality, Failure Recovery, state nodes. -Backend latency, fan-out, hot-keys -network overhead - -Spark data flow request flow.

Spark :

RDD:

Spark RDD - Resilient Distributed Dataset. :

collection of elements partitioned across the nodes of the cluster that can be operated on in parallel

-API :

immutable, partitioned collection of elements

:

• Dataset -

• Distributed - -partitions nodes .

• Resilient - , Spark .

-"Resilient" . -Spark intermediate result fault tolerant. , - dataset - partitions . -mental model Spark -runtime .

Partitioning: parallelism

RDD . -partitions, partition . -Spark executors. -parallelism , , partitions , , .

. partition , cluster . ? hot shard, .

Transformations -Actions: lazy

RDD operations:

• Transformations: flatMap, filter, map

• Actions: save, collect, count

: transformations -Spark lazy. - filter Spark . . actions, execution . reactive programming reactor publisher .

, Spark. . .

Driver, Executors, -Cluster Manager

Spark processes . -Driver -SparkContext. Spark -Cluster Manager ( Kubernetes) Executors worker nodes. -Executors processes -tasks .

: cluster shared cache" "database". -executors , -state . , external storage, -executors.

Jobs, Stages, Tasks:

-action , Job. -job -Stages, stage -Tasks. -tasks -executors. , tasks -partitions .

. , Spark execution graph, pipeline, redistribution . latency : Shuffle.

Shuffle: distributed data

, RDD DataFrame. Shuffle.

Shuffle . groupBy, join, distinct, orderBy. shuffle -operation , data serialization, disk I/O, -network I/O.

. groupBy . key , . , , intermediate data, .

: . .

DataFrame -Spark SQL?

RDD, DataFrame/DataSet/SQL.

, Spark SQL structured data, -RDD API, . , . Spark SQL

columnar storage, cost-based optimizer, and code -generation

-Quick Start ( ): 2.0 -main programming interface -RDD Dataset, -RDD . -RDD API , -workloads structured

:
RDD . DataFrame , .

, typed protocol -runtime .

: RDD

rdd = spark.sparkContext.textFile("s3a://logs/app/")  
errors = rdd.filter(lambda line: "ERROR" in line)  
count = errors.count()

?

• textFile RDD

• filter , transformation pipeline

• count() , job

, . . ,

: groupBy

from pyspark.sql import functions as F  

users = spark.read.parquet("s3a://warehouse/users")  

result = (  
    users  
    .filter(F.col("is_active") == True)  
    .groupBy("country")  
    .count()  
)

: , , .

, . filter . groupBy("country") . shuffle.

Spark API .

Join: ,

JOIN Spark SQL execution. -performance tuning, Spark SQL join , broadcast, -join hints BROADCAST, MERGE, SHUFFLE_HASH -SHUFFLE_REPLICATE_NL. , Spark 3.5 -Adaptive Query Execution sort-merge join -broadcast join, coalescing post-shuffle partitions, -skew join optimization.

-JOIN shuffle . , Spark -executors local join . execution, query.

Spark Infrastructure:

-Spark . , Spark , : cluster manager, storage, submission, monitoring, logging orchestration . cluster mode overview applications -independent sets of processes cluster, -cluster manager resources -Spark applications.

Spark infrastructure, -Spark binaries. workloads : , runtime, resource isolation, observability, retries, pipeline scheduler .

Backend engineers

-Spark . . partitioning -distribution -design, .

-collect() . -driver, distributed execution.

-API . , .

, , RDD . , -production workloads DataFrames -SQL, partitions, lazy evaluation, actions -shuffle, dataset .

Spark : RDD immutable -partitions . DataFrame -Spark SQL, optimization. -API , executors, tasks, network, memory -shuffle.

-Backend, . . boundaries, hot spots, fan-out, . Spark .

:
groupBy .
groupBy .

-Data Lakehouse Data Warehouse -Data Lake, . Table Format ( Apache Iceberg Delta Lake), -Lakehouse Optimistic Concurrency Control, - Object Storage. : ACID, -O(1), -Decoupling Compute -Storage - (open source).

:

-Lakehouse , :

1. 1: -Data Warehouse ()
   Teradata, Redshift. Coupled Storage & Compute Shared-Nothing MPP.

   1. : -OLAP, (Schema-on-Write) ACID .

   2. : . - (Unstructured), , .

2. 2: -Data Lake (The Swamp)
   -Big Data , -HDFS -S3/GCS. Schema-on-Write.

   1. : , (JSON, CSV, Parquet)

   2. : Data Governance. ACID. Spark , -Bucket . (UPDATE/DELETE), partition .

-Data Lakehouse (3) -ACID -Warehouse, -Lake. ? (Metadata) .

? -Lakehouse

-Lakehouse :

( Trino) -S3 ( O(n) ) -Table Format.

: -UPDATE

, upsert ( ) ETL . -Lakehouse, , merge-on-read copy-on-write

PySpark Apache Iceberg

from pyspark.sql import SparkSession

# Config session with Iceberg Catalog configuration
spark = SparkSession.builder \
    .config("spark.sql.extensions", "org.apache.iceberg.spark.extensions.IcebergSparkSessionExtensions") \
    .config("spark.sql.catalog.my_catalog", "org.apache.iceberg.spark.SparkCatalog") \
    .config("spark.sql.catalog.my_catalog.type", "hadoop") \
    .config("spark.sql.catalog.my_catalog.warehouse", "s3a://my-lakehouse-bucket/") \
    .getOrCreate()

# The MERGE INTO was available only on Data Warehouse
spark.sql("""
MERGE INTO my_catalog.db.users t
USING (SELECT * FROM staging_users) s
ON t.user_id = s.user_id
WHEN MATCHED THEN 
  UPDATE SET t.email = s.email, t.last_login = s.last_login
WHEN NOT MATCHED THEN 
  INSERT *
""")

, Iceberg . Snapshot , Parquet (), - .

: Lakehouse

- -Query Planning -O(1)

Hive, -Directory Structure. -Iceberg, - :

Catalog -> Metadata file (current snapshot) -> Manifest List -> Manifest Files -> Data Files (Parquet)

, -Manifests Min/Max Filtering -S3. .

Optimistic Concurrency Control (OCC)

-Lakehouse (Writer A Writer B)?
-OCC: . -Commit, A Swap -Metadata Pointer. B , A -Metadata , B . - Commit . Throughput .

, (Named-Based) (Position-Based). -Lakehouse, (ID). ? ? ? -ID -. Backfill .

Decoupling Compute & Storage Vendor Lock-in

Snowflake , . -Lakehouse, -Bucket , Parquet . Spark (-ETL ), Trino ( BI ), -Amazon Athena - .

-Data Lakehouse , -Big Data. (State), (ACID) - -Compute () - , , Data Science, Data Engineer, -Data Analysts .

(Medalion Architecture) Framework Data Lake, : Bronze ( ), Silver ( ), Gold ( ). -ETL , ML -AI.

: -ETL

, -ETL (Extract, Transform, Load) . , . , Big Data , -Silos . (Data Swamps) . -Data Estate .

:

-Bronze:

-Landing . (Raw data), .

• : Single Source of Truth. (Full Fidelity), .

• : -Ingestion Schema Drift ( ), -Bronze -Binary, VARIANT String

• : , -Bronze -Time Travel ( Delta Lake) Reprocessing .

-Silver:

-Integrity. . , -Type Casting.

• -Data Science: . (Non-aggregated), .

• : Z-Ordering Liquid Clustering (Low Latency).

-Gold:

(Consumer-Facing). ( Star Schema Kimball) Power BI, AI.

• Semantic Backbone: -Gold . -LLM ( ?) , .

: Silver vs. Gold

:

:

- . -AI , .

Backend-for-Frontend ( BFF) JSON . Public Client ( ) -Confidential Client . -BFF (Goodbye, LocalStorage), Over-fetching, Next.js BFF .

: One Size Fits All

( ): -Go Node.js. API . Web, IOS, .

? . ? . Endpoint 5MB, .

, : -Access Token -Web? localStorage, . -BFF

:

, -BFF (Facade) . API , -Web.

-Deep Tech BFF: .

-OAuth 2.0, SPA ( ) Public Clients. ( ). . - XSS (Cross-Site Scripting).

-BFF -Confidential Client. .

1. () -BFF HttpOnly Cookie ( Javascript).

2. -BFF -Tokens (API Keys, OAuth Tokens) -Session Store .

3. -BFF -Microservices (Downstream Services) .

? . .

: -BFF

-BFF . Next.js, Remix, SvelteKit Integrated BFF.

-Next.js (App Router). -Route Handler :

// app/api/user-dashboard/route.ts
import { cookies } from 'next/headers';
import { NextResponse } from 'next/server';

// This acts as our BFF layer
export async function GET() {
  // 1. Security: Get the session ID from a secure, HttpOnly cookie
  const cookieStore = cookies();
  const sessionId = cookieStore.get('secure_session_id')?.value;

  if (!sessionId) {
    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
  }

  // 2. Downstream Call: Fetch heavy data from the core Microservice
  // The actual API Key is stored safely on the server environment
  const upstreamResponse = await fetch('https://api.internal.corp/heavy-user-data', {
    headers: {
      'Authorization': `Bearer ${process.env.CORE_SERVICE_API_KEY}`,
      'X-Session-ID': sessionId
    }
  });

  const heavyData = await upstreamResponse.json();

  // 3. Transformation: The "Frontend" part of BFF
  // Instead of sending 50 fields, we send exactly what the UI needs
  const optimizedPayload = {
    displayName: `${heavyData.firstName} ${heavyData.lastName}`,
    avatar: heavyData.profileImages['thumbnail_200px'], // Logic tailored for UI
    pendingTasks: heavyData.tasks.filter((t: any) => t.status === 'URGENT').length
  };

  return NextResponse.json(optimizedPayload);
}

?

1. Security Boundary: -API key (process.env.CORE_SERVICE_API_KEY) .

2. Payload Optimization: , JSON .

: (Trade-offs)

, .

1. -Fan-Out Problem: -BFF -5 , . , . , .

   1. : -Circuit Breakers ( Netflix Hystrix ) Caching -BFF.

2. : BFF -BFF -Web, .

   1. : (Shared Libraries) GraphQL ( WunderGraph) BFF- , Build.

3. ? ( ), BFF- . -Federated Supergraphs ( Apollo Federation), , .

-BFF . -Backend . Web Mobile, - -BFF , .

. -Database -UI? -Local Storage? .

? . : , -Git Worktree , -Cursor Rule (.mdc) , AI.

:

, , AI. , :

!
how to worktree

. , . -IDE --

1: (The Setup)

-Workflow, . . Workspace :

-root :

$ ll
drwxr-xr-x  common-libraries  #  
drwxr-xr-x  rnd-documentation #   -Software Designs
drwxr-xr-x  first-repo         #   - Production Branch
drwxr-xr-x  second-repo          #   - Production Branch

? -first-repo VS COde -Source of Truth. . ( -AI) , , -History. . second-repo

, git checkout . .

2: (Git Worktree)

git clone , -git worktree. , Branch .

( ), :

git worktree add -b bugfix/PROJ-123-fix-bridge ../worktrees/proj-123-bridge master

:

git worktree add -b <new branch name> <path to the worktree new dir> <base branch to checkout>

git checkout? . , , , . first-repo , . -Build Build , .

-worktree :

git worktree rm ../worktrees/proj-123-bridge

:

git worktree rm <worktree directory>

, ? : -Monorepos . , git worktree . , , Code Reveiew ? git stash, , , git stash pop - Worktree .

: , -Cursor . agent :

• - local -

• - worktree - worktree agents -ui ( )

• - cloud - , .

3: (Cursor Rules & Personas)

. Cursor , -Context -Persona .

.cursor/rules - repos - -.mdc. . :

(The Researcher)

. . :

# Bug Researcher Character
...
## Core Principles
1. Systematic Investigation: Follow precise hierarchy of investigation.
2. Evidence-Based: Base conclusions on code evidence, not assumptions.

## Workflow
1. **Identify**: Extract error message, timestamp, patterns.
2. **Hypothesize**: Generate 3 plausible root causes based ONLY on the error.
3. **Investigate**: Request specific code snippets (Do NOT ask for the whole codebase).
4. **Explain**: Provide a short explanation of WHY the bug is happening.
5. **Wait for Approval**: DO NOT provide fix details until user explicitly approves.

( )

(The Coder)

, . -Clean Code -Builds:

# Coding Character
...
## Core Principles
- **Test-Driven Development**: Always write tests before implementation.
- **No Comments**: Code must be self-documenting.
- **Build Must Pass**: Never commit if build fails locally.

## Workflow
1. **Analyze Requirements**: Read the Jira ticket or Prompt.
2. **Audit Existing Tests**: Check coverage.
3. **Generate Test Suite**: Happy path, Edge cases.
4. **Implement**: Minimal implementation to pass tests.
5. **Refactor**: Apply strict coding standards (Immutable objects, Constructor injection).

4:

, . : -Bridge .

Cursor ( -ui), Bug Researcher :

@.cursor/rules/characters/bug-researcher-character.mdc I am researching an upgrade to master - but bridge does not finish the upgrade process. In the logs I see..." [Logs Attached]

Failed to handle MapRuleTasks , . -Researcher -Stack Trace Recovery. -finalize() .
:

Root Cause: Recovery scenario bug. When bridge restarts during upgrade... if upgrade instruction documents are missing... it calls updateAllStatuses which throws NotFoundException.

. -AI . . , -Researcher:

"Write a summarize prompt here in chat for another agent to fix this issue."

Prompt -Agent . , , .

. -worktree cd ../worktrees/fix-bridge), -Agent Cursor :

cursor-agent --promp "<prompt from previous step>"

-Agent ( -IDE ) , , , , .

? IDE .

:

AI.

1. : , Worktrees .

2. : , .

3. : -AI -AI .

( , Worktrees), ? .

AI Cursor . Monorepo , ? -Workflow Git Worktrees, VS Code Workspace -Cursor Rules () , ( ).

:

-Workflow -Tufin, . , . , Repositories -UI, Repos (Common Libraries).

? , (Dependencies)

1: ()

, -Cursor -IntelliJ : . -Gradle -VS Code Engine (: , -LSP IntelliJ).

, . , . , , () .

: AI - , .

2: (Workspaces )

VS Code Multi-root Workspaces. , -ide: X, Y -Z .

. -Cursor . . -: -.

( 30 ). , git clone . Repo , .

3: -Game Changer (Git Worktree & Personas)

-Workflow .

Git Worktree

-Repo ( Clone), -git worktree. -Git Object Database, Branch . , ( - , )

- Cursor Rules & Personas

. : , ( High Level Design), Code Review, . .mdc .cursor/rules

:

• : , Clean Code, TDD Tufin.

• software design: software design agent .

• : , ,

• -Reviewer: code review

: . , .

-workflow :

, : . .

:

1. : . .

2. : . software design. , pr -agent .

3. :

   1. git worktree .

   2. : , -cursor agent .

   3. -agent -design , -.

agent ?

-ui , , -agents Worktrees .

AI . , Context -Agent.

:

• /

• -AI.

:

• . -Worktrees .

? .

(Specialization) (Generalization) . ? AI --- . , . -Expert Generalist , , , .

:

, : -PostgreSQL Internals, , ?

:

Specialists are seen as people with a deep skill in a specific subject, while genralists have broad but shallow skills

. -T-Shaped Person - ( ) ( ).

Expert Generalist. T, (Trade-offs) .

. .

: -AI -Specialist

(LLMs) AI ( Cursor) .

:

• Paxos -Rust? -AI

• Terraform -AWS? -AI

-T, -AI . -AI -Specialist .

? -AI -T. . , .

-Expert Generalist . .

:

, ( ).

: (5 ), -Product Market Fit . (Background Jobs)

-AI (): -AI : Design a robust, scalable background job system for our e-commerce platform

-AI, , . Microservices Kafka RabbitMQ, Consumers -Go, Kubernetes State -Dead Letter Queues. : , (Operational Complexity)

-Expert Generalist ( ): -AI. , . ( ): " 5 . DevOps . . -Overhead ?" : Redis ( BullMQ) -PostgreSQL , -Polling.

: 3 -Expert Generalist

, :

(Contextual Humility):

-AI . -Generalist .

Effective generalists react to that by first understanding why this odd behavior is the way it is Humility encourages the Expert Generalist to not leap into challenging things until they are sure they understand the full context

? ,

- (Value-Directed Curiosity)

- ,

Customer-focus is the necessary lens to focus curiosity. Expert generalists prioritize their attention that the things that will help them help their users to excel

-Generalist . .

(The Art of Inquiry)

-AI.

Rather than just typing in some code from Stack Overflow There is an art to asking questions that elicit deeper answers without leading the witness

-AI ( X), ( -Trade-offs ?).

:

Expert Generalist , . () AI, ( ).

, , .

, , '.

, Atomic Kotlin

.
.
- package
:

Kotlin

package com.yoururl.libraryname

// Componentes to reuse...
fun f() = "result"

components , components .
component f().

, .
yoururl.com

' .
, com.yoururl.libraryname com/yoururl/libraryname.
, ' .

, -import

Kotlin

import com.yoururl.libraryname.*

fun main() {
  val x x = f()
}

libraryname -components libraryname.
component , -component.

atomic test

, atomictest.
, .

atomictest, -eq ( ) -neq ( ).

Kotlin

import atomictest.*

fun main() {
  val pi = 3.14
  val pie = "A round dessert"
  pi eq 3.14
  pie eq "A round dessert"
  pi neq pie
}

/* Output:
3.14
A round dessert
3.14
*/

-eq neq infix notation.
infix : pi.eq(3.14) infix notation, : pi eq 3.14.
eq neq assersions .
, .

atomictest.trace

Kotlin

import atomictest.*

fun main() {
  trace("Hello,")
  trace(47)
  trace("World!")
  trace eq """
    Hello,
    47
    World!
  """
}

println() .

.
Object Oriented -Functional Programming.

vals -vars ( properties).
.

var val , .

-container collection.
.
List Doubles.
listOf() List

Kotlin

import atomictest.eq

fun main() {
  val lst = listOf(19.2, 88.3, 22.1)
  lst[1] eq 88.3 // Indexing
  lst.reversed() eq listOf(22.1, 88.3, 19.2)
  lst.sorted() eq listOf(19.2, 22.1, 88.3)
  lst.sum() eq 129.6
}

-import -List.
[] , -0.

-List out of the box, (sorted()), (reversed()), (sum()).

-sorted() -reversed() , .
- - , .

, -class, .

Kotlin

class NoBody

class SomeBody {
  val name = "Shaked Eyal"
}

class EveryBody {
  val all = listOf(SomeBody(), SomeBody(), SomeBody())
}

fun main() {
  val noBody = NoBody()
  val someBody = SomeBody()
  val everyBody = EveryBody()
}

', ,
-new.
.
SomeBody String EveryBody List SomeBody

Kotlin

package summary_2

import atomictest.eq

class Temperature {
    var current = 0.0
    var scale = "f"

    fun setFahrenheit(now: Double) {
        current = now
        scale = "f"
    }

    fun setCelsius(now: Double) {
        current = now
        scale = "c"
    }

    fun getFahrenheit(): Double =
        if (scale == "f")
            current
        else
            current * 9.0 / 5.0 + 32.0

    fun getCelsius(): Double =
        if (scale == "c")
            current
        else
            (current - 32.0) * 5.0 / 9.0
}

fun main() {
    val temp = Temperature()
    temp.setFahrenheit(98.6)
    temp.getFahrenheit() eq 98.6
    temp.getCelsius() eq 37.0
    temp.setCelsius(100.0)
    temp.getFahrenheit() eq 212.0
}

.
.

temp val, .
-val -temp, .

Kotlin

package summary_2

import atomictest.eq

class Cell {
    var entry = ' '

    fun setValue(e: Char): String {
        return if (entry == ' ' && (e == 'X' || e == 'O')) {
            entry = e
            "Successful move"
        } else {
            "Invalid move"
        }
    }
}

class Grid {
    val cells = listOf(
        listOf(Cell(), Cell(), Cell()),
        listOf(Cell(), Cell(), Cell()),
        listOf(Cell(), Cell(), Cell())
    )

    fun play(e: Char, x: Int, y: Int): String =
        if (x !in 0..2 || y !in 0..2)
            "Invalid move"
        else
            cells[x][y].setValue(e)
}

fun main() {
    val grid = Grid()
    grid.play('X', 1, 1) eq "Successful move"
    grid.play('X', 1, 1) eq "Invalid move"
    grid.play('O', 1, 3) eq "Invalid move"
}

Cell entry -var setValue().
' Char String, entry Char.
setValue() -Cell String
play() , -setValue() Cell .

- Constructors

.
' .

. , .

Badger , .
id -years .

, . - Snake -Moose

-val -var .

' -String , -toString()

Kotlin

package summary_2

import atomictest.eq

class Badger(id: String, years: Int) {
    val name = id
    val age = years

    override fun toString(): String {
        return "Badger: $name, age: $age"
    }
}

class Snake(var type: String, var length: Double) {
    override fun toString(): String {
        return "Snake: $type, length: $length"
    }
}

class Moose(val age: Int, val height:Double) {
    override fun toString(): String {
        return "Moose, age: $age, height: $height"
    }
}

fun main() {
    Badger("Bob", 11) eq "Badger: Bob, age: 11"
    Snake("Garden", 2.4) eq "Snake: Garden, length: 2.4"
    Moose(16, 7.2) eq "Moose, age: 16, height: 7.2"
}

-CPP JAVA.
-public, private, protected, internal.

public private , .

public

, .
public .
, -public .

private

, , -private ( )

-counter private, " , . " -counter.

Kotlin

package summary_2

import atomictest.trace

private var count = 0

private class Box(val dimension: Int) {
    fun volume() =
        dimension * dimension * dimension

    override fun toString(): String {
        return "Box volume: ${volume()}"
    }
}

private fun countBox(box: Box) {
    trace("$box")
    count++
}

fun countBoxes() {
    countBox(Box(4))
    countBox(Box(5))
}

fun main() {
    countBoxes()
    trace("$count boxes")
    trace eq """
        Box volume: 64
        Box volume: 125
    """.trimIndent()
}

-fuel -warning -private JetPack. JetPack .

burn() -private, JetPack

fly() -check() public

Kotlin

package summary_2

import atomictest.eq

class JetPack(private var fuel: Double) {
    private var warning = false

    private fun burn() =
        if (fuel - 1 <= 0) {
            fuel = 0.0
            warning = true
        } else {
            fuel -= 1
        }

    public fun fly() = burn()

    fun check() =
        if (warning) {
            "Warning"
        } else {
            "OK"
        }
}

fun main() {
    val jetPack = JetPack(3.0)
    while (jetPack.check() != "Warning") {
        jetPack.check() eq "OK"
        jetPack.fly()
    }
    jetPack.check() eq "Warning"
}

internal

.
.
-internal .
-[Gradle](https://gradle.org/) -[Maven](https://maven.apache.org/) , .

Exceptions

toDouble(), String -Double.
?

Kotlin

fun main() {
  // val i = "$1.9".toDouble()
}

2, .
, ' , .
-stack trace .
-atomictest

Kotlin

import atomictest.*

fun main() {
  capture {
    "$1.9".toDouble()
  } eq "NumberFormatException: For input string: \"$1.9\""
}

generate

Flux generate -Flux.

, - . -sync SynchronousSink, **next()** -callback. -error(Throwable) -complete(), .

-state , . BiFunction<S, SynchronousSink<T>, S> <S> state -<T> . Supplier<S> -state, state.

, -int -state :

, generate -state 0. , -state . 3 -state
3, , . state -1 .
. -state .
8, -flux.doOnNext, , -blockLast(). , , complete error.
, -sink , , sink.next(), sink.complete().

:

3 X 0 = 0 3 X 1 = 3 3 X 2 = 6 3 X 3 = 9 3 X 4 = 12 3 X 5 = 15 3 X 6 = 18 3 X 7 = 21 3 X 8 = 24 3 X 9 = 27 3 X 10 = 30

mutable -state .
AtomicLong -state , .
:

, AtomicLong -state .
, Flux , .

generate, : `generate(Supplier, BiFunction, Consumer)` **Consumer<S>** -state ,
-state .

, -state .
state: 11 2 -state 10 i, -state 11.

create

create Flux , threads .
**FluxSink** next, error, complete , -generate state.
, threads .
create API -listeners .

create , , . block -create -deadlock . -subscribeOn, , - sink.next(t). .
- subscirbeOn(Scheduler, false)

-API listener. -chunks :

• chunk

• (terminal event)

:

create -Flux<T>

-EventListener<String> -API -Flux<T>
chunk -Flux sink::next. processComplete() -sink.complete()
-myEventProcessor .

, -**create** API backpressure, -backpressure -overflowStrategy:

• IGNORE -

• ERROR - -downstream

• DROP - -downstream

• LATEST - -downstream

• BUFFER - , -downstream .

its all binary

-thread

generate() create() , thread push().
-create backpressure, -push thread . -thread -next, error, complete

:

Flux push

push/pull

reactor, **create**, push/pull. ( push), pull, .
-consumer , -producer -consumer , .
, push(), create() onRequest consumer , -sink

-onRequest , , .

create, push

publisher onDispose, onCancel
onDispose Flux , , .
onCancel -Flux onDispose

handle

handle . -instance method. ( map, filter, doOnNext '). Flux Mono.

generate . , .
map filter ( ).
Flux<R> handle(BiConsumer<T, SynchronusSink<R>>);

. -reactive streamse specification null . -map , null?
:

-handle -null

alpahbet , null.

M I T

, threads thread .
.

, ,

IAM USER?

AWS.
AWS , IAM user .

Identity and Access Management (IAM) AWS.
.

- AWS

-AWS.
aws , - .

Bob AWS management Console ( AWS )

:

• AWS

• Root User ( )

• IAM IAM

• , Users

• Create User

• : Bob, AWS Management Console Next

 :     AWS  canse insensitive.      Bob  BOB.

?

2011, 1.0 2016. JetBrains, .
-C++ C, Java. .

' , .
' . , .

Readability

. , ( ), .

:

• JVM - -bytecode ( .class) JVM

• Android - - ART .dex

• JavaScript -

• Native Binaries - CPU

Hello, World!

, .
', -// /* */ .
main()

println() String ( String). -String .
print()

-; .

,

, , -val.

val identifier: Type = value

, , .

val identifier = value

val daysInFebruary = 28
val daysInMarch: Int =31

, -var.
,

var identifire1 = value
var indentifier2: Type = value

-val var

var hoursSpent = 20
hourSpent = 25

-type .

hoursSpent = 30.5

-hoursSpent Int Float

fun , . .
.
.

, .
expression body

fun functionName(arg1: Type1, arg2: Type2, ..): ReturnType = result

cube() , -return.
bang() String

:

• ! -

• && -

• || -

isOpen -&&. hour >= opens false false.

isClosed -|| true hour < opens

if

if-else , , .

, -if .

.
activity, hour -if .
-if opens, closes .
true 14 .

-String.format() ', .
String , $.
' String Templates

3 answer . , 42
5 , if condition ( -$ Bash
, 6, $1, 1 .

, -String Templates .
trimIndent()
-escaping -" -""".
-String -escaping.

(Int, Long) (Double).
, -Int, L , Long.
, Double

Int - 231- 231-1. Overflow, Int

Int -Int, Int , .
1/2 0. Double , -Int -Double .
1.0/2 0.5

d1 3.4, . 2/5, Int 0. -Double 3.0 + 0. d2 Double Int , .

while

, while true
, .

-iterable,

-String .
, c val . c += 1

in -range
-range ..

Expression & Statements

statement expression
,

• -statement (state)

• -expression .

expression

val hours = 10
val minutesPerHour = 60
val minutes = hours * minutesPerHour

= expression
println() , expression, .
- Unit

- Subscribe

-publisher -Reactor
Flux -Mono Java 8 lambdas.
.
subscribe() :

subscribe(); subscribe(Consumer<? super T> consumer); subscribe(Consumer<? super T> consumer, Consumer<? super Throwable> errorConsumer); subscribe(Consumer<? super T> consumer, Consumer<? super Throwable> errorConsumer, Runnable completeConsumer); subscribe(Consumer<? super T> consumer,
Consumer<? super Throwable> errorConsumer,
Runnable completeConsumer,
Consumer<? super Subscription> subscriptionConsumer);

, -subscription subscribe()

" -subscription .
-subscription , .
Reactor -Disposable interface.
.

Flux/Mono

Flux - 1,2,3. -Flux .

, .
.
-subscribe .

:

1 2 3

, :

, :

2 Flux , 4 - 1,2,3,4
map .

.
7 -Flux :

**1 2 3 Error: java.lang.RuntimeException: Got to 4**

,

-Flux
, -Flux .

**1 2 3 4 Done**

1
2
3
4
Done

(terminals) .
, publisher error signal completion signal.
-completion signal .

subscribe Disposable

, -subscribe Disposable.
, -Disposable -subscription dispose().

Flux -Mono cancellation signal .
, , .

Disposable Disposables. , Disposables.swap() Disposable Disposable .
, , -UI .
**dispose()** ', .

Disposables.composite(). Disposable .
, , . dispose() , Disposable .

Subscribe

subscribe, .
Subscriber .
Reactor BaseSubscriber Subscriber .

 !!
 -BaseSubscriber        -,   -BaseSubscriber     -publisher      -publisher .

. SampleSubscriber:

SampleSubscriber BaseSubscriber, Subscribers .
hooks -Subscriber.
, -BaseSubscriber subscribe , .
, .

, subscriber -publisher, -subscriber , -publisher .

-Reactor , hookOnSubscribe(**Subscription subscription)** hookOnNext(T value) . hookOnSubscribe "Subscribed" . , hookOnNext , , .

-SampleSubscriber **Flux**:

Subscribed 1 2 3 4

-BaseSubscriber requestUnbounded() , -subscribe() ( -request(Long.MAX_VALUE)), **cancel()**.

hookOnComplete, hookOnError, hookOnCancel hookFinally ( , SignalType).

hookOnError, hookOnCancel hookOnComplete. -**SampleSubscriber**

Backpressure

backpressure?

Back pressure (or backpressure) is a resistance or force opposing the desired flow of fluid through pipes, leading to friction loss and pressure drop.
The term back pressure is a misnomer, as pressure is a scalar quantity, so it has a magnitude but no direction.

Wikipedia

, publisher , subscriber .
backpressure -publisher -subscriber .

backpressure -Reactor, -subscriber request -publisher ( , ).
" "" " ". **Long.MAX_VALUE**, . -publisher " ", -backpressure.

-request -subscriber , , subscription -backpressure.
:

• subscribe() .

• block(), blockFirst(), blockLast() ( )

• toIterable(), toStream()

, hookOnSubscribe BaseSubscriber

.
doOnRequest -Flux .
-BaseSubscriber .
subscription -publisher, .

:

**request of 1 Cancelling after having received 1**

subscriber , -publisher , -Flux "". BaseSubscriber .

Reactor
Flux Mono subscription .
, subscription .
subscriber , -backpressure.

, .
, , .

,
-AI, , .
, .
AI Tufin, .

, .

Andrew Ng

machine learning AI, ? ?
, , .

Arthur Samuel

Field of study that gives computers the ability to learn without being explicitly programmed

Arthur Samuel

, .

Arthur . , , , .

. 10 .

:

• - supervised learning

• - unsupervised learning

, .

- Supervised Learning

X Y.
() " " () .
X -Y .

:

• - (0 1).

• - (audio) .

• ( ) ().

. .
.

, , .

Applied Materials .

. X -Y ( , '), .

- Regression

supervised learning .

.

-X , -Y .
750?
, ,

, :

, -scope .
, (-Y ) .

- Classification

supervised learning .
.

.

.
:

0 -1 .
:

-X , -Y . .
.
(class) .

( ), .

, , 0 1. .
, .
:

, .
, .

, .
:

.
X O

Supervised Learning

" "

supervised learning:

• • ()

, Node.js. : (boileplate) , , Express -TypeScript. , Best Practices .

- ?

, API. , : , -, ?

"The Pragmatic Programmer" . , -ecosystem , , - .

: . -backend : Node.js TypeScript -Express.

. , , . Best Practices , , . .

- API .

- (Dependencies)

, . Node.js

Express

. Express micro-framework HTTP, (routing), -Middleware. , .

TypeScript -TS-Node

JavaScript , TypeScript , . (types) JavaScript, (compilation) (runtime). ts-node TypeScript , -JavaScript /

-Middleware

• helmet: helmet . (app.use(helmet())), HTTP headers XSS, Clickjacking . .

• cors (Cross-Origin Resource Sharing) : -web , -frontend ( React ) -backend ( -API -AWS) . , . cors -Middleware -API .

• compression: : ( Gzip) (response body) . ? payload , , , .

• morgan: . morgan logger HTTP. - (GET/POST), (URL), -status code . -debugging .

-Pipeline : Middleware

Middleware Express. . HTTP . , ( -Middlewares). : , , , .

-Middlewares app.use() , . :

1. app.use(helmet()): . .

2. app.use(cors()): , . , .

3. app.use(compression()):

4. app.use(morgan('combined'): , . combined , Apache.

5. app.use(express.json()): . (body) JSON, , JavaScript req.body. , POST PUT .

-Middleware , - -Route Handler.

(Routes) - -API

, (Endpoints) -API . -Express, app.post(), app.get() .

Route :

TypeScript

app.get('/path', (req, res) => {
  // Business logic here
});

app.get('/path', (req, res) => {
  // Business logic here
});

:

• (Path): -URL -Endpoint ( 'api/users/).

• -Handler: (callback) (req) (res).

  • req (Request): - headers, -URL, , .

  • res (Response): . -status code ( res.status(200)), ( res.json({ message: 'Success' }).

endpoint /health

TypeScript

app.get5('/health', (_req, res) => {
  res.status(200).json({
    status: 'ok',
    uptime: process.uptime(),
  });
});

app.get5('/health', (_req, res) => {
  res.status(200).json({
    status: 'ok',
    uptime: process.uptime(),
  });
});

-endpoint nice to have, best practice . Kubernetes -Health Checks ( liveness and readiness probes) . -endpoint 200, .

.

-404 (Not Found):

-api/non-existent-route? , timeout. , -routes , -routes :

TypeScript

app.use('*', (req, res) => {
  res.status(404).json({
    error: 'Not Found',
    message: `The requested URL ${req.originalUrl} was not found on this server.`,
  });
});

app.use('*', (req, res) => {
  res.status(404).json({
    error: 'Not Found',
    message: `The requested URL ${req.originalUrl} was not found on this server.`,
  });
});

(*) -wildcard . , route .

-Global Error Handler:

-Route Handlers ? , ? , Node.js .

-Middleware . (err, req, res, next):

TypeScript

app.use((err: Error, _req, res, _next) => {
  console.error('Error:', err.stack) //Log the full error for debugging

  // Send a generic message in production to avoid leaking detail
  const message = process.env.[NODE_ENV] === 'development'
    ? err.message
    : 'An unexpected error occurred.';

  res.status(500).json({ error: 'Internal Server Error', message });
});

app.use((err: Error, _req, res, _next) => {
  console.error('Error:', err.stack) //Log the full error for debugging

  // Send a generic message in production to avoid leaking detail
  const message = process.env.[NODE_ENV] === 'development'
    ? err.message
    : 'An unexpected error occurred.';

  res.status(500).json({ error: 'Internal Server Error', message });
});

, ( ), 500 . : , -debugging. prod, .

?

, - .

TypeScript

if (process.env[NODE_ENV] !== 'test') {
  app.listen(PORT, () => {
    console.log(`Server is running on http://localhost:${PORT}`);
  }
}
<div></div>
export default app;

if (process.env[NODE_ENV] !== 'test') {
  app.listen(PORT, () => {
    console.log(`Server is running on http://localhost:${PORT}`);
  }
}

export default app;

HTTP . ? (integration tests), -app ( supertest), . export default app .

:

-"Hello World. :

• Middleware.

• .

• .

• .

:

. , . -unit testing , node.js .
.

, Factory Aggregates . Repository Aggregates persistence ( database), -Aggregate . , .

-DDD, Aggregates . (Entities, Value Objects) (Invariants) .

:

1. Aggregate , ?

2. -Aggregates , ?

Patterns -DDD: -Factory -Repository. , , .

-Factory:

Factory -DDD : Aggregates . ? -new Object().

Aggregate Order (). :

• (ID).

• ( ).

• ( Pending").

• .

(Invariants) . -constructor Order . -Factory , , Order .

Kotlin

import java.util.UUID
<div></div>
// Value Objects for Type Safety
data class OrderId(val value: String)
data class CustomerId(val value: String)
<div></div>
// Other domain classes...
data class Product(val id: String, val price: Double, private val inStock: Boolean) {
    fun isInStock() = inStock
}
data class Customer(val id: CustomerId, private val active: Boolean) {
    fun isActive() = active
}
data class LineItem(val productId: String, val price: Double)
<div></div>
enum class OrderStatus {
    PENDING, SHIPPED, CANCELED
}
<div></div>

class Order private constructor(
    val id: OrderId,
    val customerId: CustomerId,
    initialItems: List<LineItem>
) {
    private val _lineItems: MutableList<LineItem> = initialItems.toMutableList()
    val lineItems: List<LineItem> get() = _lineItems.toList()
<div></div>
    var status: OrderStatus = OrderStatus.PENDING
        private set
<div></div>
    companion object {
        fun createNew(customer: Customer, initialItem: Product): Order {

            // Enforcing invariants (business rules)
            require(customer.isActive()) { "Cannot create order for an inactive customer." }
            require(initialItem.isInStock()) { "Cannot create order with an out-of-stock item." }
<div></div>
            val newOrderId = OrderId(UUID.randomUUID().toString())
            val firstLineItem = LineItem(initialItem.id, initialItem.price)

            return Order(newOrderId, customer.id, listOf(firstLineItem))
        }
    }
<div></div>
    fun ship() {
        if (this.status == OrderStatus.PENDING) {
            this.status = OrderStatus.SHIPPED
        }
    }
}

import java.util.UUID

// Value Objects for Type Safety
data class OrderId(val value: String)
data class CustomerId(val value: String)

// Other domain classes...
data class Product(val id: String, val price: Double, private val inStock: Boolean) {
    fun isInStock() = inStock
}
data class Customer(val id: CustomerId, private val active: Boolean) {
    fun isActive() = active
}
data class LineItem(val productId: String, val price: Double)

enum class OrderStatus {
    PENDING, SHIPPED, CANCELED
}

class Order private constructor(
    val id: OrderId,
    val customerId: CustomerId,
    initialItems: List<LineItem>
) {
    private val _lineItems: MutableList<LineItem> = initialItems.toMutableList()
    val lineItems: List<LineItem> get() = _lineItems.toList()

    var status: OrderStatus = OrderStatus.PENDING
        private set

    companion object {
        fun createNew(customer: Customer, initialItem: Product): Order {

            // Enforcing invariants (business rules)
            require(customer.isActive()) { "Cannot create order for an inactive customer." }
            require(initialItem.isInStock()) { "Cannot create order with an out-of-stock item." }

            val newOrderId = OrderId(UUID.randomUUID().toString())
            val firstLineItem = LineItem(initialItem.id, initialItem.price)

            return Order(newOrderId, customer.id, listOf(firstLineItem))
        }
    }

    fun ship() {
        if (this.status == OrderStatus.PENDING) {
            this.status = OrderStatus.SHIPPED
        }
    }
}

, -Factory createNew Order . Order .

-Repository:

Order , . , -. -Repository .

Repository (-Aggregates) -persistence ( , , ). . -MongoDB, PostgreSQL CSV. -Repository -Order .

-Repository :

• getById(id): Aggregate

• save(aggregate): Aggregate ( ).

• findBySomeCriteria(...): Aggregates .

Kotlin

interface OrderRepository {
  fun findById(orderId: OrderId): Order?
  fun save(order: Order)
  fun findPendingOrdersForCustomer(customerId: CustomerId): List<Order>
}

interface OrderRepository {
  fun findById(orderId: OrderId): Order?
  fun save(order: Order)
  fun findPendingOrdersForCustomer(customerId: CustomerId): List<Order>
}

,

?

Factory -Repository DDD .

• -Factory , .

• -Repository -Aggregates .

; , , .

• : (Fat Interfaces) . , .

• : (ISP) : . , .

• : , , (low coupling)

: IWhateverService 15 , . ? NoImplementedException?
, (Fat Interface), , (coupling) . - Interface Segregation Principle (ISP), -SOLID .

: (Fat Interaces)

, -clients . . , Exception .

, coupling . , , . .

: The Interface Segregation Principle (ISP)

-ISP :

.

, (cohesive) . .

:

.
microservices. , .

ISP: Repository

, IUserRepository,

Kotlin

// Fat interface that contains read, write, and cache management actions
interface UserRepository {
  fun getUserById(id: String): User
  fun findUsersByCountry(countryCode: String): List<User>
  fun saveUser(user: User)
  fun deleteUser(id: String)
  fun invalidateCacheForUser(id: String)
}

// Fat interface that contains read, write, and cache management actions
interface UserRepository {
  fun getUserById(id: String): User
  fun findUsersByCountry(countryCode: String): List<User>
  fun saveUser(user: User)
  fun deleteUser(id: String)
  fun invalidateCacheForUser(id: String)
}

. :

1. (Reporting Service): . .

2. (User Management Service): - , , cache.

:

Kotlin

class ReportingUserRepository(private val dbConnection: DbConnection) : UserRepository {
<div></div>
    override fun getUserById(id: String): User {
        // logic to read from DB
        println("Fetching user $id for report")
        return User(id, "Test User")
    }
<div></div>
    override fun findUsersByCountry(countryCode: String): List<User> {
        // login to read from DB
        println("Fetching users from $countryCode for report")
        return listOf()
    }
<div></div>
    // The real problem
    override fun saveUser(user: User) {
        throw UnsupportedOperationException("Reporting repository is read-only!")
    }
<div></div>
    override fun deleteUser(id: String) {
        throw UnsupportedOperationException("Reporting repository is read-only!")
    }
<div></div>
    override fun invalidateCacheForUser(id: String) {
       // Leave empty? That is confusing 
    }
}

class ReportingUserRepository(private val dbConnection: DbConnection) : UserRepository {

    override fun getUserById(id: String): User {
        // logic to read from DB
        println("Fetching user $id for report")
        return User(id, "Test User")
    }

    override fun findUsersByCountry(countryCode: String): List<User> {
        // login to read from DB
        println("Fetching users from $countryCode for report")
        return listOf()
    }

    // The real problem
    override fun saveUser(user: User) {
        throw UnsupportedOperationException("Reporting repository is read-only!")
    }

    override fun deleteUser(id: String) {
        throw UnsupportedOperationException("Reporting repository is read-only!")
    }

    override fun invalidateCacheForUser(id: String) {
       // Leave empty? That is confusing 
    }
}

:

• read-only ( ) .

• , .

• ? , .

ISP: ( )

, , CORS (Command Query Responsibility Segregation)

Kotlin

// Specific interface for reading actions (Query)
interface UserReader {
  fun getUserById(id: String): User
  fun findUserByCountry(countryCode: String): List<User>
}
<div></div>
// Specific interface for write actions (Command)
interface UserWriter {
  fun saveUser(user: User)
  fun deleteUser(id: String)
}
<div></div>
// Specific interface for manage cache
interface CacheManager {
    fun invalidateCahceForUser(id: String)
}

// Specific interface for reading actions (Query)
interface UserReader {
  fun getUserById(id: String): User
  fun findUserByCountry(countryCode: String): List<User>
}

// Specific interface for write actions (Command)
interface UserWriter {
  fun saveUser(user: User)
  fun deleteUser(id: String)
}

// Specific interface for manage cache
interface CacheManager {
    fun invalidateCahceForUser(id: String)
}

, :

Kotlin

// Implement the read-only repo
class ReportingUserRepository(private val dbConnection: DbConnection) : UserReader {
    override fun getUserById(id: String): User {
        println("Fetching user $id for report")
        return User(id, "Test User")
    }
<div></div>
    override fun findUsersByCountry(countryCode: String): List<User> {
        println("Fetching users from $countryCode for report")
        return listOf()
    }
}
<div></div>

//Implement the full repo for the read-write and cache
class FullUserRepository(private val dbConnection: DbConnection) : UserReader, UserWriter, CacheManager {

    override fun getUserById(id: String): User { /* ... */ return User(id, "") }
    override fun findUsersByCountry(countryCode: String): List<User> { /* ... */ return listOf() }

    override fun saveUser(user: User) { /* ... */ }
    override fun deleteUser(id: String) { /* ... */ }
<div></div>
    override fun invalidateCacheForUser(id: String) { /* ... */ }
}

// Implement the read-only repo
class ReportingUserRepository(private val dbConnection: DbConnection) : UserReader {
    override fun getUserById(id: String): User {
        println("Fetching user $id for report")
        return User(id, "Test User")
    }

    override fun findUsersByCountry(countryCode: String): List<User> {
        println("Fetching users from $countryCode for report")
        return listOf()
    }
}

//Implement the full repo for the read-write and cache
class FullUserRepository(private val dbConnection: DbConnection) : UserReader, UserWriter, CacheManager {

    override fun getUserById(id: String): User { /* ... */ return User(id, "") }
    override fun findUsersByCountry(countryCode: String): List<User> { /* ... */ return listOf() }

    override fun saveUser(user: User) { /* ... */ }
    override fun deleteUser(id: String) { /* ... */ }

    override fun invalidateCacheForUser(id: String) { /* ... */ }
}

. UserReader, . , .

: ?

-ISP :

• Decoupling: -ISP -coupling . , ( client ) .

• : , . , .

• Clarity & Reusability: . , .

-Interface Segregation Principle , . ( ).

, : ?. , .

;

-Visitor . ; -Visitors . Acyclic Visitor . , .

Visitor?

. (Assets) . Apartment, Car, Salary.

, .

• : calculateArnonna(), calculateIncomeTax() . .

• -Visitor: ( ) (). Visitor , , IncomeTaxVisitor. -Visitor .

: , Visitor . .

- Visitor

? , .

: CryptocurrencyWallet ( ).

-Visitors , Visitor:

Kotlin

// Before
interface Visitor {
  fun visit(apt: Apartment)
  fun visit(salary: Salary)
  fun visit(car: Car)
}
<div></div>
// After adding new asset
interface Visitor {
    fun visit(apt: Apartment)
    fun visit(salary: Salary)
    fun visit(cw: CryptocurrencyWallet) //this line breaks all implements
}

// Before
interface Visitor {
  fun visit(apt: Apartment)
  fun visit(salary: Salary)
  fun visit(car: Car)
}

// After adding new asset
interface Visitor {
    fun visit(apt: Apartment)
    fun visit(salary: Salary)
    fun visit(cw: CryptocurrencyWallet) //this line breaks all implements
}

visit(CryptocurrencyWallet), . -Visitors - ArnonaVisitor, IncomeTaxVisitor - ! , .

(Cyclic Dependency). Element .

- Acyclic Visitor

(Uncle Bob). -Interface Segregation Principle .

Visitor :

• Visitor (Marker Interface).

• Asset ( Apartment) Visitor ( ApartmentVisitor)

• -Concrete Visitors -Visitors .

, -Visitor, -Visitor . .

?

Double Dispatch . -accept Visitor , ( dynamic_cast) -Visitor .

• -> -Visitor , -visit .

• -> -Visitor . .

.

-

, . Acyclic Visitor

:

Kotlin

// Empty and basic Visitor. Use for marker
interface Visitor
<div></div>
// Basic interface for all assets
interface Asset {
  fun accept(visitor: Visitor)
}

// Empty and basic Visitor. Use for marker
interface Visitor

// Basic interface for all assets
interface Asset {
  fun accept(visitor: Visitor)
}

: (Elements) -Visitors

Kotlin

class Apartment : Asset {

  // Specific visitor interface for the Apartment asset
  interface ApartmentVisitor : Visitor {
    fun visit(apt: Apartment)
  }

  override fun accept(visitor: Visitor) {
    // The magic: 
    if (visitor is ApartmentVisitor) {
      visitor.visit(this)
    }
  }
}
<div></div>
class StockPortfolio : Asset {
<div></div>
  interface StockPortfolioVisitor : Visitor {
    fun visit(sp: StockPortfolio)
  }

  override fun accept(visitor: Visitor) {
    if (visitor is StockPortfolioVisitor) {
      visitor.visit(this)
    }
  }
}
<div></div>
// Implemnt other assets

class Apartment : Asset {

  // Specific visitor interface for the Apartment asset
  interface ApartmentVisitor : Visitor {
    fun visit(apt: Apartment)
  }

  override fun accept(visitor: Visitor) {
    // The magic: 
    if (visitor is ApartmentVisitor) {
      visitor.visit(this)
    }
  }
}

class StockPortfolio : Asset {

  interface StockPortfolioVisitor : Visitor {
    fun visit(sp: StockPortfolio)
  }

  override fun accept(visitor: Visitor) {
    if (visitor is StockPortfolioVisitor) {
      visitor.visit(this)
    }
  }
}

// Implemnt other assets

: (Concrete Visitors)

Kotlin

// The IRS is interesetd in the Salary and StockProtfolio
class IncomeTaxVisitor : Salary.SalaryVisitor, StockPortfolio.StockPortfolioVisitor {

  override fun visit (sl: Salary) {
    println("IRS: calculate tax for salary")
  }

  override fun visit(sp: StockPortfolio) {
    println("IRS: calculate tax for stock portfolio")
  }
}

// The IRS is interesetd in the Salary and StockProtfolio
class IncomeTaxVisitor : Salary.SalaryVisitor, StockPortfolio.StockPortfolioVisitor {

  override fun visit (sl: Salary) {
    println("IRS: calculate tax for salary")
  }

  override fun visit(sp: StockPortfolio) {
    println("IRS: calculate tax for stock portfolio")
  }
}

: -

Kotlin``` class CryptocurrencyWallet : Asset { interface CryptoVisitor : Visitor { fun visit(cw: CryptoCurrencyWallet) }

override fun accept(visitor: Visitor) { if (visitor is CryptoVisitor) { visitor.visit(this) } } }

class CryptocurrencyWallet : Asset { interface CryptoVisitor : Visitor { fun visit(cw: CryptoCurrencyWallet) }

override fun accept(visitor: Visitor) { if (visitor is CryptoVisitor) { visitor.visit(this) } } }

   - `IncomeTaxVisitor`    -asset 

Kotlin

class IncomeTaxVisitor : Salary.SalaryVisitor, StockPortfolio.StockPortfolioVisitor, CryptocurrencyWallet.CryptoVisitor {

override fun visit (sl: Salary) { println("IRS: calculate tax for salary") }

override fun visit(sp: StockPortfolio) { println("IRS: calculate tax for stock portfolio") }

// new override fun visit(vw: CryptocurrencyWallet) { println("IRS: calculate tax for crypto profits") } }

class IncomeTaxVisitor : Salary.SalaryVisitor, StockPortfolio.StockPortfolioVisitor, CryptocurrencyWallet.CryptoVisitor {

override fun visit (sl: Salary) { println("IRS: calculate tax for salary") }

override fun visit(sp: StockPortfolio) { println("IRS: calculate tax for stock portfolio") }

// new override fun visit(vw: CryptocurrencyWallet) { println("IRS: calculate tax for crypto profits") } }

####  :   

Kotlin

fun main() { val assets: List = listof(Apartment(), StockPortfolio(), Salary(), CryptocurrencyWallet())

val arnona = ArnonaVisitor() for (asset in assets) { asset.accept(arnona) // print only for Apatment }

val incomeTax = IncomeTaxVisitor() for (asset in assets) { asset.accept(incomeTax) // print for all assets beside Apartment } }

fun main() { val assets: List = listof(Apartment(), StockPortfolio(), Salary(), CryptocurrencyWallet())

val arnona = ArnonaVisitor() for (asset in assets) { asset.accept(arnona) // print only for Apatment }

val incomeTax = IncomeTaxVisitor() for (asset in assets) { asset.accept(incomeTax) // print for all assets beside Apartment } } ```

- , -

, -

• : Element .

• : -Elements -Visitors.

• SOLID: -Open/Close Principle -Interface Segregation Principle.

• Compile-Time Saftey: , . , .

• -Boilerplate: .

• ( ): -dynamic_cast .

-

Visitor -Acyclic Visitor .

• , - -Visitor , .

• , - Acyclic Visitor , .

-Visitor, : , ?.
.