In-flight request deduplication to prevent duplicate operations.

This module prevents duplicate requests from causing duplicate side effects (e.g., double charges, duplicate orders) by tracking in-flight requests and sharing responses with identical concurrent requests.

How It Works

1. Request Fingerprinting - Each request gets a hash based on method + URL + body, plus the auth-context headers (authorization, cookie, x-api-key) so responses are never shared across different credentials
2. In-Flight Tracking - First request executes normally, subsequent identical requests wait
3. Response Sharing - When the first request completes, all waiting requests receive the same response
4. Automatic Cleanup - Completed responses are cached briefly so late-arriving duplicates still share them, then swept; orphaned in-flight rows (owner crashed without completing) are reaped as a safety net

Use Cases

• Prevent double charges from double-clicks on payment buttons
• Prevent duplicate orders from retry storms or race conditions
• Ensure idempotency for critical mutations (POST/PUT/DELETE)

Configuration

# Global configuration
config :httpower, :deduplicate,
  enabled: true

# Per-request configuration
HTTPower.post(url,
  body: payment_data,
  deduplicate: true
)

# Or with options
HTTPower.post(url,
  body: payment_data,
  deduplicate: [
    enabled: true,
    wait_timeout: 60_000,   # How long waiters block for an in-flight request (default: 30s)
    key: "custom-dedup-key"  # Optional: override hash generation
  ]
)

States

• :in_flight - Request currently executing, other identical requests will wait
• :completed - Brief window (~0.5s) after completion so late duplicates still share the response

Thread Safety

Hot path (claim, wait, complete, cancel) is lock-free on a public ETS table via :ets.insert_new/2 and :ets.select_replace/2 CAS — no GenServer round-trip. The GenServer owns the table lifecycle and runs periodic cleanup only.