feat(#293): implement retry logic with exponential backoff

Add retry capability with exponential backoff for HTTP requests.
- Implement withRetry utility with configurable retry logic
- Exponential backoff: 1s, 2s, 4s, 8s (max)
- Maximum 3 retries by default
- Retry on network errors (ECONNREFUSED, ETIMEDOUT, etc.)
- Retry on 5xx server errors and 429 rate limit
- Do NOT retry on 4xx client errors
- Integrate with connection service for HTTP requests

Fixes #293

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/federation/utils/retry.ts

@@ -0,0 +1,146 @@
+/**
+ * Retry Utility
+ *
+ * Provides retry logic with exponential backoff for HTTP requests.
+ */
+
+import { Logger } from "@nestjs/common";
+import type { AxiosError } from "axios";
+
+const logger = new Logger("RetryUtil");
+
+/**
+ * Configuration for retry logic
+ */
+export interface RetryConfig {
+  /** Maximum number of retry attempts (default: 3) */
+  maxRetries?: number;
+  /** Initial backoff delay in milliseconds (default: 1000) */
+  initialDelay?: number;
+  /** Maximum backoff delay in milliseconds (default: 8000) */
+  maxDelay?: number;
+  /** Backoff multiplier (default: 2 for exponential) */
+  backoffMultiplier?: number;
+}
+
+/**
+ * Default retry configuration
+ */
+const DEFAULT_CONFIG: Required<RetryConfig> = {
+  maxRetries: 3,
+  initialDelay: 1000, // 1 second
+  maxDelay: 8000, // 8 seconds
+  backoffMultiplier: 2,
+};
+
+/**
+ * Check if error is retryable (network errors, timeouts, 5xx errors)
+ * Do NOT retry on 4xx errors (client errors)
+ */
+export function isRetryableError(error: unknown): boolean {
+  // Check if it's a plain object (for testing) or Error instance
+  if (!error || (typeof error !== "object" && !(error instanceof Error))) {
+    return false;
+  }
+
+  const axiosError = error as AxiosError;
+
+  // Retry on network errors (no response received)
+  if (!axiosError.response) {
+    // Check for network error codes
+    const networkErrorCodes = [
+      "ECONNREFUSED",
+      "ETIMEDOUT",
+      "ENOTFOUND",
+      "ENETUNREACH",
+      "EAI_AGAIN",
+    ];
+
+    if (axiosError.code && networkErrorCodes.includes(axiosError.code)) {
+      return true;
+    }
+
+    // Retry on timeout
+    if (axiosError.message.includes("timeout")) {
+      return true;
+    }
+
+    return false;
+  }
+
+  // Retry on 5xx server errors
+  const status = axiosError.response.status;
+  if (status >= 500 && status < 600) {
+    return true;
+  }
+
+  // Retry on 429 (Too Many Requests) with backoff
+  if (status === 429) {
+    return true;
+  }
+
+  // Do NOT retry on 4xx client errors
+  return false;
+}
+
+/**
+ * Execute a function with retry logic and exponential backoff
+ */
+export async function withRetry<T>(
+  operation: () => Promise<T>,
+  config: RetryConfig = {}
+): Promise<T> {
+  const finalConfig: Required<RetryConfig> = {
+    ...DEFAULT_CONFIG,
+    ...config,
+  };
+
+  let lastError: Error | undefined;
+  let delay = finalConfig.initialDelay;
+
+  for (let attempt = 0; attempt <= finalConfig.maxRetries; attempt++) {
+    try {
+      return await operation();
+    } catch (error) {
+      lastError = error as Error;
+
+      // If this is the last attempt, don't retry
+      if (attempt === finalConfig.maxRetries) {
+        break;
+      }
+
+      // Check if error is retryable
+      if (!isRetryableError(error)) {
+        logger.warn(`Non-retryable error, aborting retry: ${lastError.message}`);
+        throw error;
+      }
+
+      // Log retry attempt
+      const errorMessage = lastError instanceof Error ? lastError.message : "Unknown error";
+      logger.warn(
+        `Retry attempt ${String(attempt + 1)}/${String(finalConfig.maxRetries)} after error: ${errorMessage}. Retrying in ${String(delay)}ms...`
+      );
+
+      // Wait with exponential backoff
+      await sleep(delay);
+
+      // Calculate next delay with exponential backoff
+      delay = Math.min(delay * finalConfig.backoffMultiplier, finalConfig.maxDelay);
+    }
+  }
+
+  // All retries exhausted
+  if (lastError) {
+    throw lastError;
+  }
+
+  // Should never reach here, but satisfy TypeScript
+  throw new Error("Operation failed after retries");
+}
+
+/**
+ * Sleep for specified milliseconds
+ */
+function sleep(ms: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}