# 42912: Channel enumeration already in progress

Identifier: `rate_limit_channel_iteration_in_progress`



A request to enumerate the channels active in an app was rejected because another enumeration was already running. Only one channel enumeration can run at a time for an app.

## What you should do

Wait for the in-progress enumeration to finish before starting another, since only one can run at a time. If your application makes these requests from more than one place, coordinate them so they do not overlap, or retry after a short delay.

## Why it happens

Enumerating the channels active in an app is an expensive operation, so Ably allows only one to run at a time for a given app. A request that arrives while another enumeration is still in progress is rejected rather than queued.

## What you'll see

The request is rejected. The error is reported with code 42912 and HTTP status 429, with a message of the form `Channel iteration call already in progress. Only 1 call(s) may be executed concurrently`. "Iteration" here is the internal term for channel enumeration.