[5/7] Telemetry Export: DatabricksTelemetryExporter with Retry and Circuit Breaker

[samikshya-db]

Part 5 of 7-part Telemetry Implementation Stack

This layer adds HTTP export with retry logic and circuit breaker integration.

Summary

Implements DatabricksTelemetryExporter for reliable HTTP export to Databricks with retry logic, exponential backoff, and circuit breaker protection.

Components

DatabricksTelemetryExporter (lib/telemetry/DatabricksTelemetryExporter.ts)

HTTP export with resilience patterns:

Endpoints:

• /api/2.0/sql/telemetry-ext - Authenticated export (default)
• /api/2.0/sql/telemetry-unauth - Unauthenticated export (fallback)

Export Flow:

1. Check circuit breaker state (skip if OPEN)
2. Execute export with circuit breaker protection
3. Retry on retryable errors with exponential backoff
4. Circuit breaker tracks success/failure for endpoint health
5. All exceptions swallowed and logged at debug level

Retry Strategy:

• Max retries: 3 (configurable via telemetryMaxRetries)
• Backoff: Exponential with jitter (100ms * 2^attempt + random(0-100ms))
• Terminal errors → No retry (401, 403, 404, 400, AuthenticationError)
• Retryable errors → Retry with backoff (429, 500, 502, 503, 504, timeouts)
• Prevents thundering herd with jitter

Circuit Breaker Integration:

• Success → circuitBreaker.recordSuccess()
• Failure → circuitBreaker.recordFailure()
• Circuit OPEN → Skip export, log at debug, save bandwidth
• Automatic recovery via HALF_OPEN state testing

Payload Format:

{
  "workspace_id": "...",
  "session_id": "...",
  "driver_version": "...",
  "metrics": [...]
}

Critical Requirements ✅

• ✅ NEVER throws exceptions - All errors caught and swallowed
• ✅ Debug-only logging - No warn/error logs from telemetry
• ✅ No console logging - Uses IDBSQLLogger only
• ✅ Driver resilience - Driver continues when telemetry fails

Testing

• 24 comprehensive unit tests (96% statement, 84% branch coverage)
• Tests verify exception swallowing (CRITICAL)
• Tests verify retry logic with backoff
• Tests verify circuit breaker integration
• Tests verify authenticated vs unauthenticated endpoints
• TelemetryExporterStub for integration testing

Performance Characteristics

• Circuit breaker prevents wasted calls to failing endpoints
• Exponential backoff reduces server load during failures
• Jitter prevents synchronized retry storms
• Batch export amortizes HTTP overhead

Next Steps

This PR is followed by:

• [6/7] Integration: Wire into DBSQLClient/Operation/Session
• [7/7] Testing & Documentation

Dependencies

Depends on:

• [1/7] Telemetry Foundation ([1/7] Telemetry Foundation: Types, Config, and Exception Classifier #324)
• [2/7] Infrastructure ([2/7] Telemetry Infrastructure: CircuitBreaker and FeatureFlagCache #325) - Uses CircuitBreaker & ExceptionClassifier
• [3/7] Client Management ([3/7] Telemetry Client Management: TelemetryClient and Provider #326)
• [4/7] Event & Aggregation ([4/7] Telemetry Event Emission and Aggregation #327) - Exports aggregated metrics