Heuristic:Fede1024 Rust rdkafka Commit Mode Sync Vs Async
| Knowledge Sources | |
|---|---|
| Domains | Messaging, Debugging |
| Last Updated | 2026-02-07 19:30 GMT |
Overview
Despite being an async-first library, offset commit APIs are not async in the Rust sense; `CommitMode::Sync` blocks the calling thread while `CommitMode::Async` only enqueues the commit without waiting for broker acknowledgement.
Description
The `Consumer::commit` method accepts a `CommitMode` enum that controls whether the calling thread blocks for the broker's response. Both modes enqueue the commit request internally; the difference is only in whether the caller waits for the Kafka broker's acknowledgement. Critically, neither mode is truly async (i.e., returning a `Future`). `CommitMode::Sync` blocks the OS thread, which can stall an async runtime if called from an async context without `spawn_blocking`. `CommitMode::Async` returns immediately but provides no notification of success or failure (except through the `commit_callback` on `ConsumerContext`).
Usage
Use this heuristic when deciding between Sync and Async commit modes in consumer applications. In async contexts (tokio, async-std), prefer `CommitMode::Async` to avoid blocking the runtime, and rely on the `commit_callback` for error monitoring. For the at-least-once pattern with `enable.auto.offset.store=false`, use auto-commit (which is async internally) rather than manual commits.
The Insight (Rule of Thumb)
- Action: In async applications, prefer `CommitMode::Async` or the auto-commit mechanism. Only use `CommitMode::Sync` when you need confirmation before proceeding.
- Value: `CommitMode::Async` has near-zero overhead on the calling thread. `CommitMode::Sync` blocks until the broker responds (typically 5-50ms on LAN).
- Trade-off: `Async` gives no inline error feedback; errors only surface in `commit_callback`. `Sync` gives immediate error feedback but blocks.
Reasoning
The limitation exists because librdkafka does not expose a callback-based commit API (see librdkafka issue #3212). The Rust wrapper faithfully reflects this limitation: there is no way to get a `Future` for a commit operation. The auto-commit mechanism (`enable.auto.commit=true`) handles this by performing async commits periodically in the background, which is the recommended approach for most applications.
Code Evidence
CommitMode documentation from `src/consumer/mod.rs:143-158`:
/// Regardless of the `CommitMode`, the commit APIs enqueue the commit request
/// in a local work queue. A separate worker thread picks up this commit request
/// and forwards it to the Kafka broker over the network.
///
/// The difference between [`CommitMode::Sync`] and [`CommitMode::Async`] is in
/// whether the caller waits for the Kafka broker to respond that it finished
/// handling the commit request.
///
/// Note that the commit APIs are not async in the Rust sense due to the lack of
/// a callback-based interface exposed by librdkafka. See
/// [librdkafka#3212](https://github.com/edenhill/librdkafka/issues/3212).