Which strategy ensures idempotent PATCH operations without duplicate writes?

Answer

The most reliable way to guarantee that a PATCH is applied only once is to let the server enforce idempotency. Each PATCH carries a client‑generated Idempotency-Key header, the server stores that key together with the final response in a dedicated table that has a UNIQUE constraint, and the entire update is wrapped in a single database transaction. On receipt of a repeat request with the same key, the server simply returns the cached response instead of re‑applying the changes. When a resource exposes a version field or an ETag, adding an If‑Match header further protects against concurrent modifications.

Why This Works

• Atomicity: The transaction ensures that the patch data and the idempotency key are written together, preventing partial writes that could be replayed.
• Uniqueness: The UNIQUE constraint on the key column guarantees that a duplicate key cannot be inserted again, causing a database error that the application can translate into a “duplicate” response.
• Cache‑On‑Retry: Returning the stored response on a duplicate key means the client sees the same result as the first attempt, satisfying idempotency semantics.
• Concurrent Guard: If‑Match or a version column ensures that two clients cannot simultaneously patch the same resource, avoiding lost updates.

Implementation Steps (example in pseudocode)

1. Receive PATCH
   

   PATCH /resource/123 HTTP/1.1
   Idempotency-Key: abc123
   If-Match: "v5"   # optional
   { "name": "New Name" }

2. Check key table
   

   SELECT response FROM idempotency WHERE key = 'abc123';

   If a row exists, return the stored response with the original status code.
3. Start transaction
   

   BEGIN;
   -- Verify If-Match if present
   SELECT version FROM resources WHERE id = 123;
   -- Apply patch
   UPDATE resources SET name = 'New Name', version = version + 1 WHERE id = 123;
   -- Store key and response
   INSERT INTO idempotency (key, response, status) VALUES ('abc123', , 200);
   COMMIT;

4. Return result
   

   HTTP/1.1 200 OK
   { "id": 123, "name": "New Name", "version": "v6" }

Cleaning Up

• Delete idempotency rows older than a configurable TTL (e.g., 24 h) to keep storage bounded.
• If the PATCH payload is large, stream it to the idempotency table before applying the update so that the key is captured even if the body is processed in chunks.

Missing Diagnostic Detail

Does your resource expose a version field or an ETag that the client can send in an If‑Match header? If so, incorporating that header can further protect against race conditions and should be part of the implementation plan.