This example demonstrates how NATS JetStream handles message consumption concurrency using a callback method.
To run the example, install dependencies with bun or npm, then run with bun run index.ts or npx tsx index.ts.
You can edit MAX_CONCURRNECY and NUMBER_OF_CONCURRENT_MESSAGES_TO_PUBLISH to see how the behavior changes with different numbers of messages and concurrency.
This currently demonstrates two major issues:
- The concurrency limit is not respected. All messages are being processed within 10-20ms, despite multiple batches of max messages being sent and long wait times in the callback.
- The NATS connection is not draining properly, we have outstanding messages that are not done being processed.
Potential sources of the issues:
- NATS does not await a callback to finish, it simply continues when the function returns. I would consider this a NATS bug since the callback is supposed to be a promise. Will dig into their code to see if this is the case.