HTTP API vs REST API: what it means for webhook events that arrive out of order

Why this matters for webhooks

Webhooks are just HTTP requests sent to your endpoint. Whether the sender calls its interface an “HTTP API” or a “REST API,” the transport semantics are the same for your receiver: HTTP is stateless and does not, by itself, provide cross-request ordering guarantees. REST explicitly embraces statelessness as a constraint, which also implies no built-in message ordering across separate requests (Codehooks: API vs REST).

HTTP does offer well-defined request/response headers and status codes that help you acknowledge, cache or negotiate representations, but it does not sequence messages across requests (MDN HTTP headers).

Practical patterns to handle out-of-order (and duplicate) webhook events

1) Treat delivery as at-least-once; add idempotency and dedupe

• Store a durable event log keyed by a unique event ID from the sender. If the sender doesn’t provide one, derive a deterministic idempotency key (e.g., a hash of immutable attributes) and reject replays that you’ve already processed. See our intro to idempotency keys.

2) Version or sequence your domain objects

• Persist a monotonically increasing version (or created_at + tiebreaker) per aggregate (e.g., per order, per user). On ingest, compare the incoming event’s version to the stored version and only apply if it is newer. This discards older, out-of-order updates safely.

3) Acknowledge quickly; process asynchronously when needed

• Respond fast with a success code (often 200 or 202) after persisting the event and enqueueing work. The HTTP Prefer header supports a respond-async preference so the client knows you’re handling work asynchronously (MDN HTTP headers (Prefer)).

4) Verify authenticity before you trust ordering or content

• Verify the webhook signature before processing so your dedupe/versioning isn’t applied to forged requests. See our primer on webhook signature verification.

5) Reconcile with the sender’s REST API when you detect gaps

• When sequence gaps are detected (e.g., missing version), backfill by pulling from the publisher’s REST API. Many APIs expose pagination via the Link response header (rel="next", rel="prev"), which your client can follow to fetch additional pages deterministically (GitHub REST pagination). This lets you resync state whenever you observe out-of-order or lost webhooks.

6) Keep handlers idempotent and side effects safe

• Apply business actions only after passing authenticity, idempotency and freshness checks (version is newer). All downstream calls should be safe to retry.

HTTP API vs. REST API in one page

• HTTP API: any interface over HTTP. May use arbitrary paths, methods or payload formats.
• REST API: an HTTP API that intentionally follows REST constraints such as statelessness, a uniform interface (resource URLs, representations negotiated by Accept/Content-Type), and standard methods (GET/POST/PUT/PATCH/DELETE) (Codehooks: API vs REST; MDN HTTP headers (Accept, Content-Type)).
• Implication for webhooks: regardless of style, neither guarantees cross-request ordering; your receiver must implement idempotency, versioning and reconciliation.

Put this into practice

Replay the same event and deliberately deliver related events out of order in a test environment. Record the duplicate-handling and ordering decision separately from the transport’s HTTP or REST label.

Webhook intake safety checklist

• Persist the raw event with its unique ID before any side effects
• Reject duplicates via an idempotency key or stored event ID
• Compare event version/timestamp to the current record; apply only if newer
• Verify webhook signatures before processing
• Return 200/202 promptly; process work asynchronously when needed (Prefer: respond-async)
• Backfill missing versions via the publisher’s REST API using Link-based pagination

Short FAQ

Does REST guarantee delivery order?

No. REST is an architectural style that emphasizes statelessness; it does not define delivery order across separate requests (Codehooks: API vs REST).

Can HTTP help with asynchronous webhook processing?

Yes. You can acknowledge quickly and process later. The HTTP Prefer header supports a respond-async preference to communicate asynchronous handling (MDN HTTP headers).

How do I safely fetch missed events?

Use the sender’s REST API and follow Link headers (rel="next", rel="prev") to paginate until you reach the needed range (GitHub REST pagination).

Related reading on Stack & Method: What is an idempotency key in APIs? and Webhook signature verification explained.