Chapter 4

API Implementation Guide

Practical implementation examples and patterns for integrating public document APIs into applications, with complete code samples in TypeScript, Python, and Java demonstrating real-world usage scenarios.

SDK Architecture and Design

Software Development Kits (SDKs) provide developer-friendly interfaces to APIs, abstracting HTTP details and providing language-idiomatic patterns. Well-designed SDKs dramatically reduce integration effort, handle authentication complexity, implement retry logic, and manage errors gracefully. The WIA-SOCIAL standard provides reference SDKs in TypeScript, Python, and Java, covering the most common government technology stacks.

TypeScript/JavaScript SDK

The TypeScript SDK targets web applications, Node.js services, and cross-platform mobile apps built with React Native. TypeScript's strong typing provides compile-time safety while maintaining JavaScript's flexibility. The SDK uses modern async/await patterns for handling asynchronous operations cleanly. Axios provides the HTTP client foundation, offering request/response interceptors for authentication and error handling.

TypeScript SDK - Installation and Setup

// Install via npm
npm install @wia-social/public-document-sdk

// Initialize SDK
import { PublicDocumentClient } from '@wia-social/public-document-sdk';

const client = new PublicDocumentClient({
  baseUrl: 'https://api.gov.ee/v2',
  apiKey: process.env.GOV_API_KEY,
  timeout: 30000,
  retryAttempts: 3,
  logLevel: 'info'
});

// Authenticate with OAuth
await client.authenticate({
  clientId: 'your-client-id',
  clientSecret: 'your-client-secret',
  grantType: 'client_credentials',
  scope: 'read:documents write:documents'
});

SDK Core Components

SDKs are typically organized into modules corresponding to API resource types. A Documents module handles document CRUD operations. A Verification module provides authenticity checking. A Search module wraps query and filter capabilities. A Signatures module manages cryptographic operations. Each module encapsulates related functionality with a consistent interface pattern.

Configuration management allows customization of base URLs, timeouts, retry behavior, and logging. Authentication handlers abstract OAuth flows, token storage, and automatic refresh. Error handlers translate HTTP errors into language-idiomatic exceptions. Interceptors enable cross-cutting concerns like logging, metrics, and custom headers.

SDK Module Primary Operations Key Methods Return Types
Documents CRUD operations create(), get(), update(), delete(), list() Document, DocumentList
Verification Authenticity checking verify(), checkSignature(), getStatus() VerificationResult
Search Query and filter search(), filter(), facet() SearchResults
Signatures Cryptographic ops sign(), validateSignature(), getCertificate() Signature, Certificate
Templates Document generation getTemplate(), renderDocument() Template, RenderedDocument

Document Retrieval Implementation

Document retrieval is the most common operation, used by citizen portals, agency systems, and third-party applications. The implementation must handle authentication, error cases, caching, and format negotiation.

Basic Document Retrieval

TypeScript Example:
// Retrieve a single document
async function getDocument(documentId: string): Promise {
  try {
    const document = await client.documents.get(documentId);

    console.log(`Retrieved document: ${document.documentType}`);
    console.log(`Issued by: ${document.issuer.organizationName}`);
    console.log(`Status: ${document.status}`);

    return document;
  } catch (error) {
    if (error instanceof NotFoundError) {
      console.error(`Document ${documentId} not found`);
    } else if (error instanceof UnauthorizedError) {
      console.error('Authentication required or invalid');
    } else if (error instanceof ForbiddenError) {
      console.error('Insufficient permissions to access document');
    } else {
      console.error('Unexpected error:', error);
    }
    throw error;
  }
}

// Retrieve with specific format
const pdfDocument = await client.documents.get(documentId, {
  format: 'pdf'
});

// Save PDF to file
await fs.promises.writeFile('document.pdf', pdfDocument);

Batch Document Retrieval

For scenarios requiring multiple documents, batch operations reduce round trips and improve performance. The SDK should support batch retrieval with appropriate error handling for partial failures.

Python Example:
from wia_social import PublicDocumentClient
from wia_social.exceptions import DocumentError

client = PublicDocumentClient(
    base_url='https://api.gov.ee/v2',
    api_key=os.environ['GOV_API_KEY']
)

# Batch retrieval
document_ids = ['DOC-001', 'DOC-002', 'DOC-003']

results = client.documents.get_batch(document_ids)

for result in results:
    if result.success:
        doc = result.document
        print(f"✓ Retrieved {doc.document_id}: {doc.document_type}")
    else:
        print(f"✗ Failed to retrieve {result.document_id}: {result.error}")

# List documents with pagination
documents = client.documents.list(
    document_type='passport',
    status='active',
    issued_after='2024-01-01',
    limit=50
)

print(f"Found {documents.total_count} documents")
print(f"Retrieved {len(documents.items)} in this page")

# Iterate through all pages
for document in client.documents.iterate(filters={'type': 'passport'}):
    process_document(document)

Document Creation and Issuance

Document creation involves data validation, workflow orchestration, digital signature application, and secure storage. The SDK should provide high-level interfaces that handle complexity while allowing customization for specific document types.

Birth Certificate Issuance

Java Implementation Example

import com.wia.social.PublicDocumentClient;
import com.wia.social.model.*;
import java.time.LocalDate;
import java.time.LocalDateTime;

PublicDocumentClient client = new PublicDocumentClient.Builder()
    .baseUrl("https://api.gov.ee/v2")
    .apiKey(System.getenv("GOV_API_KEY"))
    .timeout(Duration.ofSeconds(30))
    .build();

// Create birth certificate
BirthCertificate certificate = BirthCertificate.builder()
    .documentType("BirthCertificate")
    .issuer(Issuer.builder()
        .countryCode("EE")
        .organizationName("Estonian Ministry of Interior")
        .organizationId("70000329")
        .signerName("Maria Tamm")
        .signerTitle("Chief Registrar")
        .build())
    .issuedDate(LocalDateTime.now())
    .validFrom(LocalDate.of(2025, 1, 10))
    .subject(Person.builder()
        .fullName("Anna Kask")
        .dateOfBirth(LocalDate.of(2025, 1, 10))
        .placeOfBirth("Tallinn, Estonia")
        .gender(Gender.FEMALE)
        .personalCode("52501104567")
        .build())
    .parents(Arrays.asList(
        Parent.builder()
            .fullName("Maria Kask")
            .personalCode("48905144321")
            .relationship(ParentRelationship.MOTHER)
            .build(),
        Parent.builder()
            .fullName("Jaan Kask")
            .personalCode("48702103214")
            .relationship(ParentRelationship.FATHER)
            .build()
    ))
    .birthDetails(BirthDetails.builder()
        .hospital("Tallinn Children's Hospital")
        .weight(3450)  // grams
        .length(51)    // centimeters
        .attendingPhysician("Dr. Eva Mets")
        .build())
    .build();

try {
    // Submit for issuance
    IssuanceRequest request = client.documents.submitForIssuance(certificate);

    System.out.println("Issuance request submitted: " + request.getRequestId());

    // Poll for completion (alternatively use webhooks)
    IssuanceStatus status;
    do {
        Thread.sleep(2000);
        status = client.documents.getIssuanceStatus(request.getRequestId());
        System.out.println("Status: " + status.getState());
    } while (!status.isTerminal());

    if (status.getState() == IssuanceState.COMPLETED) {
        Document issued = status.getDocument();
        System.out.println("Document issued: " + issued.getDocumentId());
        System.out.println("Signature: " + issued.getSignature().getSignatureValue());
    } else {
        System.err.println("Issuance failed: " + status.getErrorMessage());
    }

} catch (ValidationException e) {
    System.err.println("Validation failed:");
    e.getErrors().forEach(error ->
        System.err.println("  - " + error.getField() + ": " + error.getMessage())
    );
} catch (ApiException e) {
    System.err.println("API error: " + e.getMessage());
}

Document Verification Implementation

Verification confirms document authenticity, validity, and current status. This is crucial for relying parties like employers, educational institutions, border control, and financial services. Verification must be fast (sub-second for most cases) and reliable.

QR Code Verification

Modern documents include QR codes containing either full document data with cryptographic signatures or URLs to verification services. Mobile applications can scan QR codes and verify documents instantly, even offline if the document includes embedded signatures.

TypeScript Mobile Verification:
import { PublicDocumentClient } from '@wia-social/public-document-sdk';
import { QRScanner } from 'react-native-qr-scanner';

// Initialize verifier (can work offline with embedded public keys)
const verifier = new DocumentVerifier({
  trustedIssuers: await client.verification.getTrustedIssuers(),
  offlineMode: true
});

// Scan QR code
const qrData = await QRScanner.scan();

try {
  // Parse and verify
  const result = await verifier.verifyQRCode(qrData);

  if (result.valid) {
    console.log('✓ Document is authentic');
    console.log(`Type: ${result.document.documentType}`);
    console.log(`Holder: ${result.document.subject.fullName}`);
    console.log(`Issued by: ${result.document.issuer.organizationName}`);
    console.log(`Valid until: ${result.document.validUntil || 'No expiration'}`);

    // Check specific attributes
    if (result.document.documentType === 'DriversLicense') {
      const license = result.document as DriversLicense;
      console.log(`Vehicle classes: ${license.vehicleClasses.join(', ')}`);
      console.log(`Restrictions: ${license.restrictions || 'None'}`);
    }

    // Display verification UI
    showVerificationSuccess(result);

  } else {
    console.error('✗ Document verification failed');
    console.error(`Reason: ${result.failureReason}`);

    showVerificationFailure(result);
  }

} catch (error) {
  console.error('Verification error:', error);
  showVerificationError(error);
}

Online Verification with API

For maximum security and real-time status checking, online verification queries the issuing authority's systems. This detects revoked or suspended documents that might pass offline cryptographic checks.

Python API Verification:
from wia_social import PublicDocumentClient
from wia_social.models import VerificationLevel

client = PublicDocumentClient(api_key=os.environ['GOV_API_KEY'])

def verify_document(document_id: str, verification_level: str = 'full'):
    """
    Verify document authenticity and status.

    Verification levels:
    - 'basic': Check signature and format only
    - 'standard': Include revocation check
    - 'full': Include issuer confirmation and attribute validation
    """

    result = client.verification.verify(
        document_id=document_id,
        level=VerificationLevel[verification_level.upper()],
        include_history=True
    )

    print(f"Verification Level: {result.level}")
    print(f"Overall Status: {'VALID' if result.valid else 'INVALID'}")
    print(f"Confidence: {result.confidence_score}/100")
    print()

    # Check individual validation results
    checks = result.checks
    print("Validation Checks:")
    print(f"  Signature Valid: {'✓' if checks.signature_valid else '✗'}")
    print(f"  Not Revoked: {'✓' if checks.not_revoked else '✗'}")
    print(f"  Not Expired: {'✓' if checks.not_expired else '✗'}")
    print(f"  Issuer Confirmed: {'✓' if checks.issuer_confirmed else '✗'}")

    if not result.valid:
        print(f"\nFailure Reasons:")
        for reason in result.failure_reasons:
            print(f"  - {reason}")

    # Check verification history
    if result.verification_history:
        print(f"\nPrevious Verifications: {len(result.verification_history)}")
        for entry in result.verification_history[:5]:
            print(f"  {entry.timestamp}: {entry.verifier} - {entry.result}")

    return result

# Example usage
result = verify_document('EE-PASS-2024-987654', verification_level='full')

if result.valid:
    print("\n✓ Document is authentic and currently valid")
else:
    print("\n✗ Document verification failed")
    print("Do not accept this document")
Verification Level Checks Performed Response Time Use Case
Basic Signature, format validation <100ms Offline mobile verification
Standard Basic + revocation check <500ms General business verification
Full Standard + issuer confirmation + attributes <2s High-security applications
Forensic Full + biometric matching + audit trail <10s Law enforcement, border control

Error Handling and Retry Logic

Robust applications must handle various error conditions gracefully. Network failures, rate limiting, temporary service unavailability, and invalid data all require appropriate handling strategies.

Exponential Backoff

When encountering retryable errors (network timeouts, 503 Service Unavailable, 429 Too Many Requests), SDKs should implement exponential backoff with jitter. The delay between retries increases exponentially (1s, 2s, 4s, 8s), with random jitter preventing thundering herds when many clients retry simultaneously.

Circuit Breaker Pattern

For critical dependencies, circuit breakers prevent cascading failures. After a threshold of consecutive failures, the circuit "opens," immediately failing requests without attempting the operation. After a timeout, the circuit enters "half-open" state, allowing a test request. Success closes the circuit; failure reopens it. This prevents overwhelming failing services with repeated requests.

HTTP Status Code Meaning Retry Strategy Action Required
400 Bad Request Invalid request format Do not retry Fix request parameters
401 Unauthorized Authentication failed Do not retry Refresh access token, re-authenticate
403 Forbidden Insufficient permissions Do not retry Request proper authorization
404 Not Found Resource doesn't exist Do not retry Verify document ID or endpoint
429 Too Many Requests Rate limit exceeded Retry with backoff Respect Retry-After header
500 Internal Server Error Server error Retry with backoff Report if persistent
503 Service Unavailable Temporary unavailability Retry with backoff Implement circuit breaker
504 Gateway Timeout Upstream timeout Retry with backoff Consider increasing client timeout

TypeScript Retry with Exponential Backoff

async function retryWithBackoff(
  operation: () => Promise,
  maxAttempts: number = 3,
  baseDelay: number = 1000
): Promise {
  let lastError: Error;

  for (let attempt = 0; attempt < maxAttempts; attempt++) {
    try {
      return await operation();
    } catch (error) {
      lastError = error;

      // Don't retry on client errors (4xx except 429)
      if (error.status >= 400 && error.status < 500 && error.status !== 429) {
        throw error;
      }

      if (attempt < maxAttempts - 1) {
        // Calculate delay with exponential backoff and jitter
        const exponentialDelay = baseDelay * Math.pow(2, attempt);
        const jitter = Math.random() * 1000;
        const delay = exponentialDelay + jitter;

        console.log(`Attempt ${attempt + 1} failed, retrying in ${delay}ms...`);
        await sleep(delay);
      }
    }
  }

  throw new Error(`Operation failed after ${maxAttempts} attempts: ${lastError.message}`);
}

// Usage
const document = await retryWithBackoff(
  () => client.documents.get(documentId),
  maxAttempts: 3,
  baseDelay: 1000
);

Webhooks and Event Notifications

For long-running operations or event-driven workflows, webhooks provide real-time notifications without polling. When document status changes (issuance completed, revocation, expiration warning), the system posts notifications to client-specified URLs.

Webhook Registration

Event Type Trigger Payload Data Typical Use Case
document.issued New document created documentId, type, issuedDate, holder Send notification to document holder
document.revoked Document invalidated documentId, revocationDate, reason Update verification systems immediately
document.expiring 30 days before expiration documentId, expirationDate Remind holder to renew document
document.expired Document past expiration documentId, expiredDate Disable access, archive document
verification.completed Async verification finished verificationId, result, confidence Continue workflow after verification
biometric.matched Biometric verification success matchScore, biometricType Authorize high-security transaction
audit.flagged Suspicious activity detected activityType, riskScore, details Alert security team for review
Webhook Setup Example:
// Register webhook endpoint
await client.webhooks.register({
  url: 'https://myapp.com/webhooks/documents',
  events: [
    'document.issued',
    'document.revoked',
    'document.expiring',
    'verification.completed'
  ],
  secret: 'webhook-signing-secret',
  retryPolicy: {
    maxAttempts: 5,
    backoffMultiplier: 2
  }
});

// Webhook handler (Express.js example)
app.post('/webhooks/documents', async (req, res) => {
  const signature = req.headers['x-webhook-signature'];
  const payload = req.body;

  // Verify webhook signature
  if (!verifyWebhookSignature(payload, signature, webhookSecret)) {
    return res.status(401).json({ error: 'Invalid signature' });
  }

  // Process event
  switch (payload.event) {
    case 'document.issued':
      await handleDocumentIssued(payload.data);
      break;

    case 'document.revoked':
      await handleDocumentRevoked(payload.data);
      break;

    case 'document.expiring':
      await handleDocumentExpiring(payload.data);
      break;
  }

  // Acknowledge receipt
  res.status(200).json({ received: true });
});

Testing and Mocking

SDKs should provide testing utilities including mock clients, fixture data, and sandbox environments. This enables developers to build and test integrations without accessing production systems or real citizen data.

Mock Client for Unit Testing

Python Testing Example:
import pytest
from wia_social import PublicDocumentClient
from wia_social.testing import MockDocumentClient, create_mock_document

# Use mock client in tests
@pytest.fixture
def client():
    mock = MockDocumentClient()

    # Pre-load mock data
    mock.add_document(create_mock_document(
        document_id='TEST-001',
        document_type='BirthCertificate',
        status='active'
    ))

    return mock

def test_document_retrieval(client):
    # Test retrieves from mock
    doc = client.documents.get('TEST-001')

    assert doc.document_id == 'TEST-001'
    assert doc.document_type == 'BirthCertificate'
    assert doc.status == 'active'

def test_document_not_found(client):
    with pytest.raises(NotFoundError):
        client.documents.get('NON-EXISTENT')

def test_verification_success(client):
    result = client.verification.verify('TEST-001')

    assert result.valid == True
    assert result.confidence_score >= 95

Summary

Key Takeaways:

Review Questions

  1. Why are SDKs important for API adoption? What problems do they solve compared to direct HTTP calls?
  2. Explain the difference between batch retrieval and pagination. When would you use each?
  3. Describe the document issuance workflow. What validation occurs before a document is signed?
  4. Compare offline QR code verification to online API verification. What are the trade-offs?
  5. What is exponential backoff and why is jitter important? How does it prevent thundering herds?
  6. Explain the circuit breaker pattern. How does it protect systems from cascading failures?
  7. Why is webhook signature verification critical? What attack does it prevent?
  8. How do mock clients improve development velocity? What testing challenges do they address?