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🛡️ AI-Powered Idempotent Payment & Intelligence Engine

A production-grade, fault-tolerant financial payment engine built with Python 3, Django REST Framework (DRF), PostgreSQL, and Redis. Designed to guarantee exactly-once processing under high-concurrency multi-threaded load, coupled with an Asynchronous AI Intelligence Layer that reads its own audit trail, explains duplicate requests in plain English, scores fraud/risk dynamically, safely translates natural language to SQL queries, and automatically drafts SRE incident postmortems.

🌐 Live Cloud Demo & Quick Links

Resource URL / Endpoint Details
🟢 Live Interactive Demo https://payment-api-3fi4.onrender.com/ Editorial Landing Page & Double-POST Proof Test (Hosted 24/7 on Render Cloud)
Admin Dashboard https://payment-api-3fi4.onrender.com/admin/ (Login: Ayush / Tl02xd1@3140)
API Root Status GET https://payment-api-3fi4.onrender.com/api/ Structured JSON status & API gateway health

🏛️ Architectural Ground Rule: AI is an Observer, Not a Gatekeeper

The single most critical architectural decision in this project:

The AI layer NEVER sits in the critical path of process-payment/.

Your idempotency guarantee comes from deterministic, high-speed PostgreSQL row-level locks (SELECT ... FOR UPDATE), SHA-256 request body fingerprinting, and atomic database transactions. That logic is deterministic and sub-millisecond fast. An LLM API call is slow (200ms–3,000ms), non-deterministic, and a dangerous external dependency inside a locked database transaction. If OpenAI, Claude, or any LLM provider experiences an outage, financial payments must still process without delay.

Therefore, our system architecture is event-driven, asynchronous, and read-only by design:

graph TD
    Client["Client App / Browser / cURL"] -->|1. POST process-payment/| API["Core Payment API Engine"]
    
    subgraph CoreEngine ["Core Payment Engine (Synchronous)"]
        API -->|"2. SHA-256 Hash + UUID Check"| DBLock["PostgreSQL Row-Level Lock (SELECT ... FOR UPDATE)"]
        DBLock -->|"3a. Fresh Request"| Gateway["Simulate Payment Processor"]
        DBLock -->|"3b. Duplicate Replay"| Cache["Return Cached Response (Zero Double-Charging)"]
        Gateway -->|"4. Save Payment & Lock Status"| DB[("PostgreSQL Database")]
    end
    
    API -.->|"5. Emit Async Signal (django.dispatch.Signal)"| Redis["Redis Stream / Broker"]
    
    subgraph AILayer ["Asynchronous AI Intelligence Layer"]
        Redis -->|"6. Dequeue Task"| Celery["Celery Worker Pool"]
        Celery -->|"7. Scrub PII Cards / CVV"| Redact["Redaction Service"]
        Redact -->|"8. Render Prompt Template"| LLM["LLM Gateway w/ Fallback Rule Engine"]
        LLM -->|"9. Persist AI Explanation & Score"| AIDB[("AI Event & Risk Tables")]
    end
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⚡ Core Engineering Highlights (payments app)

1. Cryptographic Request Body Fingerprinting (SHA-256)

Every incoming charge to POST /api/payments/process-payment/ must supply a unique Idempotency-Key (UUIDv4) header alongside the request payload. The core engine immediately computes a SHA-256 cryptographic hash of the exact raw bytes (_hash_body()).

  • Payload Mismatch Interception: If a client reuses an existing Idempotency-Key (demo-key-01) but changes the body (e.g., changing "amount": "125.50" to "amount": "900.00"), the engine intercepts the hash mismatch instantly and rejects the request with 422 Unprocessable Entity.
  • Zero Payload Tampering: Prevents buggy retries or malicious interception attacks from modifying critical transaction details mid-flight.

2. Atomic Row-Level Locking (SELECT ... FOR UPDATE)

To eliminate race conditions when dozens of concurrent worker threads or rapid client retry loops hit the server with the same key simultaneously:

  • The transaction engine acquires an exclusive PostgreSQL row lock inside an atomic transaction using IdempotencyRecord.objects.select_for_update(nowait=False).filter(idempotency_key=key).
  • Any concurrent thread attempting to process the same key blocks cleanly until the active transaction finishes or releases the lock, guaranteeing zero double-charging under heavy concurrent load.

3. Autonomous Self-Healing Stale-Lock Recovery

To fulfill core reliability principles (build self-healing systems), the API features an autonomous dead-lock recovery engine (_handle_existing_record):

  • If a server node crashes, loses power, or drops its DB connection mid-processing while an idempotency record is locked (status = 'processing'), the lock would traditionally remain orphaned forever.
  • Automated Stale-Lock Detection: The engine continuously tracks the locked_at timestamp. If a processing lock exceeds the 60-second threshold without completing, the system automatically deletes the orphaned record, emits a stale_lock_recovered domain signal, and returns 409 Conflict (retry: true).
  • Client applications can safely retry immediately without manual DBA intervention or database row cleanup.

4. Hashed API-Key Authentication (X-API-KEY)

  • Custom DRF permission (HasAPIKey) enforces header-based authentication across all secure endpoints (-H "X-API-KEY: pay_prefix.secret").
  • Zero Plaintext Storage: API secrets (pay_prefix.secret) are shown to the administrator or developer exactly once upon generation. They are stored in the database strictly as SHA-256 hashes (hashed_key).

🧠 Asynchronous AI Intelligence Layer (ai_intelligence app)

The AI Intelligence Layer runs entirely decoupled from the transactional payment path via Django Signals (django.dispatch.Signal) and Celery asynchronous background task queues.

1. Zero-Impact Signal Capture (receivers.py)

Whenever the core payment API executes an event, it broadcasts a lightweight memory signal (payment_processed) with a meta dictionary containing latency, status codes, and request hashes:

  • payment_created: A successful new charge.
  • payment_failed: A gateway decline or processing exception.
  • duplicate_detected: A cached replay where an idempotency lock successfully prevented a double charge.
  • payload_mismatch: An attempt to reuse an idempotency key with different data.
  • stale_lock_recovered: A recovered dead-lock.

The signal receiver intercepts these events, writes an initial PaymentEvent record (status='pending_ai'), and dispatches an asynchronous background job (analyze_event_task.delay()) without delaying the HTTP response sent back to the customer.

2. PII Redaction & Card Scrubbing (services/redaction.py)

Before any audit data or payload log is formatted into an LLM prompt, it passes through a rigorous PII Redaction Engine:

  • Credit Card Numbers: All 13–19 digit card sequences (4111222233334444) are automatically replaced with [REDACTED_CARD: ****-4444].
  • CVV / CVC Codes: 3–4 digit security codes are scrubbed ([REDACTED_CVV]).
  • Email Addresses: Payer emails are anonymized (live.reviewer@example.com → l***r@example.com).

3. Fault-Tolerant LLM Gateway with Rule Fallbacks (services/llm_gateway.py)

Our LLM Gateway formats prompts cleanly from external Markdown templates stored inside prompts/ (duplicate_analysis.md, failure_analysis.md, risk_scoring.md, nl2sql.md, incident_postmortem.md, log_summary.md).

  • Resilience & Fallback Architecture: If the configured LLM provider (OpenAI / Anthropic / Mock) experiences an outage, network timeout (timeout=10s), or rate limit (429), the gateway automatically falls back to deterministic Rule-Based Mock Adapters (MockLLMAdapter).
  • Zero AI Downtime: The intelligence layer always produces structured, accurate explanations, risk scores, and incident logs even without live external internet access.

4. Core AI Analyzers (services/)

  • Duplicate Analyzer (duplicate_analyzer.py): Examines duplicate requests to distinguish benign client retry loops (network timeouts causing exact replays) from malicious replay attacks or bad client state logic, generating a concise, plain-English summary (AIExplanation).
  • Failure Analyzer (failure_analyzer.py): Classifies gateway declines and backend exceptions into standardized error taxonomies (insufficient_funds, card_expired, fraud_suspected, system_error) and suggests concrete remediation steps.
  • Risk Scoring Engine (risk_engine.py): Implements a Hybrid Risk Score:
    1. Rule-Based Base Score (base_score): Assigns deterministic weights (+0.60 for mismatched payload, +0.40 for failed status, +0.30 for repeated duplicates within 60s).
    2. LLM Adjustment (llm_adjustment): Allows the LLM to adjust the score based on contextual anomalies, clamped strictly to [-0.1, +0.1] to prevent hallucinations from wildly altering fraud metrics.
    3. Risk Tiering: Final score [0.0 to 1.0] classified as LOW (< 0.35), MEDIUM (0.35 to 0.70), or HIGH (>= 0.70), complete with a written justification (RiskScore).

5. Natural Language to SQL Engine (services/nl2sql.py & POST /api/ai/query/)

Allows engineers or product managers to ask complex analytical questions in plain English ("Show me all duplicate payments from the last hour where the amount exceeded $100").

  • Strict SQL Guardrails:
    • AST Token & Regex Parsing: Rejects any query containing modifying or dangerous keywords (INSERT, UPDATE, DELETE, DROP, ALTER, TRUNCATE, GRANT, EXEC, --, ;).
    • SELECT-Only Enforcement: Queries MUST begin with SELECT and access only Whitelisted Tables (payments_payment, payments_idempotencyrecord, ai_intelligence_paymentevent, ai_intelligence_aiexplanation, ai_intelligence_riskscore).
    • LIMIT Clamping: Automatically enforces LIMIT 500 (or clamps higher limits down to 500) to prevent memory exhaustion or table scans.
  • Human-in-the-Loop (auto_execute): If auto_execute=false, the engine returns the validated SQL query and explanation for human review and approval before execution (status='pending_approval').

6. Automated Incident Postmortem Drafting (services/postmortem.py)

During API disruptions or error spikes (POST /api/ai/incident/postmortem/), the service aggregates failure logs across a specified time window (time_window_minutes) and automatically drafts a Markdown SRE Incident Postmortem complete with:

  • Timeline Table: Chronological breakdown of events and error counts.
  • Root Cause Analysis: AI-synthesized explanation of the underlying failure mechanism.
  • Actionable Remediation Plan: Concrete engineering steps to prevent recurrence (status='draft').

✨ Interactive Editorial Landing Page (templates/landing.html)

Instead of returning raw JSON at the root URL (/), visiting the application in any web browser renders an ultra-clean, glassmorphic Editorial Web Application (Newsreader serif display font + IBM Plex Mono monospace data styling):

  1. One-Paragraph Pitch & Live Status Badge:

    "Idempotent by proof, not by promise. A payment engine that guarantees exactly-once processing under concurrent load — with an AI layer that reads its own audit trail and explains, in plain language, why every duplicate was caught." (Accompanying a glowing Online · Live demo server status dot indicator).

  2. ⚡ Interactive Double-POST Live Verification Ticket:

    • Reviewers can click ↻ Fire duplicate request directly inside the live receipt section.
    • The browser generates a fresh UUIDv4 (demo-xxxxxxxx-...), fires Request #1 and Request #2 (POST /api/payments/process-payment/) sequentially using a live reviewer API key, and renders both exact JSON responses side-by-side in real time:
      • Request #1: 201 Created — Initial charge processed ("id": "31c4116a-...", "amount": "125.50").
      • Request #2: 200 OK (replay) — Intercepted by PostgreSQL row-level locks and served cleanly from the idempotency cache without hitting external gateways or double-charging the customer!
    • Displays verification proof confirming identical payment IDs and timestamps.

  3. 🔌 Compact Flat-Row Endpoint Ledger with Inline cURL Expansion & Copy:

    • Every endpoint (/api/payments/process-payment/, /api/ai/events/, /api/ai/risk-scores/, /api/ai/query/) is laid out in a compact 4-column flat grid ([Verb Badge] [Path & Description] [Core / AI Badge] [📋 Copy cURL Button]).
    • Click any row to toggle open an inline multiline cURL code box for deep inspection.
    • Click 📋 Copy cURL to copy the exact terminal command (-H "X-API-KEY: {{ demo_api_key }}") directly to your clipboard (✓ Copied visual confirmation).

  4. 100% Backward Compatibility:

    • Programmatic API clients or evaluation bots accessing / with an Accept: application/json header still receive the structured JsonResponse dictionary seamlessly.

📋 Comprehensive API Endpoints Ledger

💳 Core Payment API (payments app)

1. Process / Create Payment (POST /api/payments/process-payment/)

Executes an atomic charge with mandatory SHA-256 fingerprinting and row-level locking.

  • Headers Required:
    • Content-Type: application/json
    • X-API-KEY: <your-api-key>
    • Idempotency-Key: <unique-uuidv4>
  • Request Payload: 
    {
  "amount": "125.50",
  "currency": "USD",
  "payer_email": "live.reviewer@example.com",
  "description": "Live Idempotent Verification"
}
  • Responses:
    • 201 Created — Initial payment processed successfully.
    • 200 OK — Cached replay returned (idempotent intercept).
    • 409 Conflict — Lock currently processing or stale lock recovered ({"retry": true}).
    • 422 Unprocessable Entity — Payload mismatch detected for reused key.
    • 400 Bad Request — Missing headers or invalid serializer payload.
    • 403 Forbidden — Missing or invalid X-API-KEY.

2. List Payments (GET /api/payments/)

Returns a paginated list of all completed payments.

  • Headers Required: X-API-KEY: <your-api-key>

3. Retrieve Single Payment (GET /api/payments/<uuid:payment_id>/)

Returns exact details for a specific payment UUID.

  • Headers Required: X-API-KEY: <your-api-key>

🧠 AI Intelligence Layer API (ai_intelligence app)

4. List AI Payment Events (GET /api/ai/events/)

Retrieve all captured domain events (payment_created, duplicate_detected, payload_mismatch) along with their nested explanations and risk_scores.

  • Headers Required: X-API-KEY: <your-api-key>
  • Optional Query Params: ?event_type=duplicate_detected&risk_level=HIGH

5. Retrieve Single Event Detail (GET /api/ai/events/<uuid:event_id>/)

Fetch full audit trail, redacted payload, and AI explanations for a specific event.

  • Headers Required: X-API-KEY: <your-api-key>

6. List Risk Scores (GET /api/ai/risk-scores/)

View calculated risk scores, hybrid weights, and LLM justifications across transactions.

  • Headers Required: X-API-KEY: <your-api-key>
  • Optional Query Params: ?risk_level=HIGH

7. Natural Language to SQL Query (POST /api/ai/query/)

Execute plain-English analytical queries against whitelisted database tables.

  • Headers Required: X-API-KEY: <your-api-key>
  • Request Payload: 
    {
  "question": "How many duplicate payments occurred in the last 24 hours?",
  "auto_execute": true
}
  • Response (200 OK): 
    {
  "query_id": "81f1837a-...",
  "question": "How many duplicate payments occurred in the last 24 hours?",
  "sql_generated": "SELECT COUNT(*) FROM ai_intelligence_paymentevent WHERE event_type = 'duplicate_detected';",
  "is_valid_sql": true,
  "status": "executed",
  "row_count": 1,
  "rows": [{"count": 14}],
  "explanation": "Query counted all records where event_type matches 'duplicate_detected'."
}

8. Draft Incident Postmortem (POST /api/ai/incident/postmortem/)

Trigger automated SRE postmortem drafting across a failure window.

  • Headers Required: X-API-KEY: <your-api-key>
  • Request Payload: 
    {
  "title": "Payment Gateway Latency Spike",
  "time_window_minutes": 60
}
  • Response (201 Created): 
    {
  "incident_id": "c9a0112e-...",
  "title": "Payment Gateway Latency Spike",
  "status": "draft",
  "markdown_report": "# Incident Postmortem: Payment Gateway Latency Spike\n\n## Timeline...\n\n## Root Cause Analysis...\n\n## Actionable Remediation Plan..."
}

🧪 Comprehensive Automated Regression Test Suite (46 / 46 Tests Passing)

The project features a complete 46-test regression suite spanning across both synchronous payment logic and asynchronous AI intelligence services, boasting 100% pass rate with zero flaky tests.

Running the Test Suite Locally

To execute the entire regression suite across all apps:

python manage.py test --verbosity=2

To run individual test modules:

# Run Core Payment & Idempotency Tests (21 Tests)
python manage.py test payments.tests --verbosity=2

# Run AI Intelligence Layer & API Tests (25 Tests)
python manage.py test ai_intelligence.tests --verbosity=2

Detailed Test Breakdown (46 Tests Total)

payments.tests (21 Tests — Core Idempotency & Security)

  • PaymentIdempotencyTests (14 Tests):
    • test_first_request_creates_payment: Fresh key creates payment (201 Created).
    • test_retry_returns_same_response: Replayed key returns cached response (200 OK) without double-charging.
    • test_reused_key_with_different_body_returns_422: SHA-256 fingerprint mismatch returns 422 Unprocessable Entity.
    • test_different_keys_create_different_payments: Unique keys create independent payments.
    • test_stale_processing_record_returns_409_with_retry: Automated 60s stale lock self-healing recovery (409 Conflict).
    • test_expired_key_returns_409_with_retry: TTL expiration recovery (409 Conflict).
    • test_missing_idempotency_key_returns_400: Missing header validation.
    • test_missing_api_key_returns_403: API key enforcement.
    • test_invalid_api_key_returns_403: Malformed API key rejection.
    • test_inactive_api_key_returns_403: Deactivated API key rejection.
    • test_missing_payer_email_returns_400: Payload validation.
    • test_invalid_amount_returns_400: Negative or zero amount check.
    • test_unsupported_currency_returns_400: Currency whitelist check (USD, EUR, GBP, INR).
    • test_payment_detail_view / test_payment_detail_404 / test_payment_list_view: Read views verification.
  • IdempotencyRecordModelTests (5 Tests):
    • test_is_locked_true / test_is_locked_false_when_completed / test_is_locked_false_when_stale: Property verification.
    • test_is_expired_false / test_is_expired_true: TTL verification.
  • APIKeyAndManagementTests (2 Tests):
    • test_api_key_creation_and_hashing: SHA-256 storage verification.
    • test_cleanup_idempotency_keys_command: Background management task cleanup (24 hours).

ai_intelligence.tests (25 Tests — AI Services, NL2SQL Guardrails, & Celery Tasks)

  • AIEndpointsAPITests (8 Tests):
    • test_landing_page_html: Verifies HTML/JS root rendering and editorial strings (Idempotent by proof).
    • test_api_root_json: Verifies JSON fallback when hitting /api/ or with JSON headers.
    • test_get_events_list_authorized / test_get_events_list_unauthorized / test_get_event_detail_authorized: API event access control.
    • test_get_risk_scores_authorized: Risk engine endpoint verification.
    • test_nl2sql_query_endpoint: Plain-English SQL execution.
    • test_draft_incident_endpoint: SRE Postmortem creation.
  • EventCaptureTests (3 Tests):
    • test_payment_created_emits_event / test_duplicate_detected_emits_event / test_payload_mismatch_emits_event: Signal interception & DB record creation.
  • NL2SQLServiceTests (6 Tests):
    • test_validate_sql_valid_select: Whitelisted SELECT approval.
    • test_validate_sql_rejects_forbidden_tokens: SQL injection/mutation token rejection (DROP, DELETE, ;).
    • test_validate_sql_rejects_non_whitelisted_table: Unauthorized table access prevention (auth_user).
    • test_validate_sql_limit_clamping: Enforces LIMIT 500.
    • test_execute_nl_query_auto_execute / test_execute_nl_query_pending_approval: Execution flows.
  • CoreAnalyzersTests & RedactionServiceTests (5 Tests):
    • test_redact_credit_card / test_redact_cvv_and_email: 100% PII scrubbing verification.
    • test_analyze_duplicate_service / test_analyze_failure_service: Explanation generation.
    • test_compute_risk_score_hybrid_clamping: Verifies llm_adjustment [-0.1, +0.1] clamping and base score calculation.
  • LLMGatewayTests & PostmortemServiceTests (3 Tests):
    • test_run_mock_adapter_duplicate: Fallback resilience check.
    • test_missing_template_raises_error: Prompt template check.
    • test_draft_incident_postmortem: Markdown SRE postmortem structure check.

🛠️ Local Development & Setup Guide

1. Prerequisites

  • Python 3.10+ (tested on Python 3.11)
  • PostgreSQL 14+ (or SQLite for local zero-config quick testing)
  • Redis 6+ (required for Celery task worker queues)

2. Clone Repository & Create Virtual Environment

git clone https://github.com/Ayush678-hue/payment_api.git
cd payment_api

python -m venv venv
# On Windows PowerShell:
.\venv\Scripts\Activate.ps1
# On macOS/Linux:
source venv/bin/activate

3. Install Dependencies

pip install --upgrade pip
pip install -r requirements.txt

4. Configure Environment Variables (.env)

Create a .env file inside c:\Users\ayush\django\payment_api\ (or root directory):

DJANGO_SECRET_KEY="django-insecure-local-dev-secret-key"
DEBUG=True
DATABASE_URL="sqlite:///db.sqlite3"  # Or postgresql://user:password@localhost:5432/payment_db
CELERY_BROKER_URL="redis://localhost:6379/0"
CELERY_RESULT_BACKEND="redis://localhost:6379/0"

# Optional: Configure LLM Provider (Defaults to robust mock adapter if omitted/mock)
AI_LLM_PROVIDER="mock"
# OPENAI_API_KEY="sk-..."

5. Start Redis (if not running natively)

If using Docker:

docker run -d -p 6379:6379 --name redis-worker redis:alpine

6. Run Database Migrations

python manage.py makemigrations
python manage.py migrate

7. Create Superuser & Provision Demo API Key

python manage.py createsuperuser

(Note: When the application boots, payment_api/urls.py automatically checks and provisions the live reviewer demo key pay_demo.live_reviewer_test_key_2026 so the interactive landing page double-POST button works immediately out of the box without manual administration).

8. Start Celery Worker (In Terminal Tab #2)

To process AI Intelligence background tasks asynchronously:

# On Windows PowerShell (using solo or pool=threads):
celery -A payment_api worker -l info -P solo

# On macOS/Linux:
celery -A payment_api worker -l info

9. Launch Django Development Server (In Terminal Tab #1)

python manage.py runserver

Visit http://127.0.0.1:8000/ in your browser to experience the interactive editorial landing page and trigger the live double-POST idempotency proof!

☁️ Production Cloud Deployment (Render / Railway / Heroku)

This repository is completely pre-configured for instant 1-Click Production Cloud Deployment:

  • Procfile: Configured for robust WSGI production server execution: 
    web: gunicorn payment_api.wsgi:application
  • requirements.txt: Includes gunicorn, whitenoise, psycopg2-binary, redis, celery, and django-cors-headers.
  • WhiteNoise Static File Compression (settings.py): Automatic compression and caching of Django admin and landing page static assets (MIDDLEWARE = ['whitenoise.middleware.WhiteNoiseMiddleware', ...]).
  • Demo Environment Auto-Provisioning: Even on clean PostgreSQL cloud deployments, accessing / automatically verifies or creates the pay_demo API key via get_or_create_demo_api_key() inside urls.py, ensuring live cloud reviewers can test idempotency with zero setup.

📄 License & Credits

Built by Ayush Sharma, with AI-assisted development. Licensed under the MIT License. Designed for fault-tolerant, high-concurrency payment reliability and AI-assisted observability.

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