# Apex Publish Callback Limits

Keep in mind this limit for Apex publish callbacks.

| Description                                                                                                               | Limit |
| ------------------------------------------------------------------------------------------------------------------------- | ----- |
| Maximum cumulative usage of all publish callbacks in the last 30 minutes                                                  | 5 MB  |
| Maximum number of times a publish callback method implementation can call `EventBus.publish` with a callback recursively. | 10    |

The publish callback size used in the callback allocation is the size of the objects contained in a callback class instance, such as the class variable objects. It isn’t the length of the Apex class in characters, although overhead related to serializing and storing the class does contribute to the publish callback size. The publish callback size is calculated at event publish time, not at callback method invocation time. Therefore, each time that a callback class instance is passed into `EventBus.publish`, it counts against the limit, regardless of whether the callback methods are executed.

For example, in the `FailureCallbackWithCorrelation` class in [Example: Publish Callback Class That Correlates Callback Results with Event Messages](/docs/platform/platform-events/guide/platform-events-publish-callback-example-correlate.md), the objects that contribute to the size counted in the allocation are these class variables: `MAX_RETRIES`, `retryCounter`, and `uuidMap`. The overhead of the class instance also contributes to the size counted in the allocation. Whenever a `FailureCallbackWithCorrelation` class instance is passed into `EventBus.publish`, it counts against the limit even if the event is published successfully.

The cumulative usage is the sum of the sizes of callback instances that were executed in the last 30 minutes. If you hit the callback size limit, try to reduce the size of the objects stored in your callback class through the class variables. Alternatively, limit the number of retried callback executions or wait before using callbacks again. The callback limit is a rolling limit and counts usage in the last 30 minutes, so usage can decrease after some time has passed. Usage is updated every time you publish an event with a callback.

To monitor the usage of all publish callbacks in the last 30 minutes, make a REST API call to the `limits` resource, and inspect the `PublishCallbackUsageInApex` value in the returned response. Make a GET request to:

```nolang
/services/data/v68.0/limits
```

The `PublishCallbackUsageInApex` value in the returned response looks similar to this example. The `PublishCallbackUsageInApex` value returns the maximum usage and the remaining usage size in bytes.

```nolang
"PublishCallbackUsageInApex" : {
    "Max" : 5242880,
    "Remaining" : 4011396
  }

```

**See Also**

- [*REST API Developer Guide*: Limits](https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/resources_limits.htm)
- [*REST API Developer Guide*: List Organization Limits](https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_limits.htm)