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# Monitor Asynchronous Actions using Event Handlers

The fetch actions sent to a connector are asynchronous. Asynchronous actions do not run immediately, but are added to a queue. This means that the person or application that sends the action does not receive an immediate response. However, you can configure the connector to call an event handler when an asynchronous action starts, finishes, or encounters an error.

You can use an event handler to:

- return data about an event back to the application that sent the action.
- write event data to a text file, to log any errors that occur.

You can also use event handlers to monitor the size of asynchronous action queues. If a queue becomes full this might indicate a problem, or that applications are making requests to POP3 Connector faster than they can be processed.

POP3 Connector can call an event handler for the following events.

OnStart | The `OnStart` event handler is called when POP3 Connector starts processing an asynchronous action.
OnFinish | The `OnFinish` event handler is called when POP3 Connector successfully finishes processing an asynchronous action.
OnError | The `OnError` event handler is called when an asynchronous action fails and cannot continue.
OnQueueEvent | The `OnQueueEvent` handler is called when an asynchronous action queue becomes full, becomes empty, or the queue size passes certain thresholds. A `QueueFull` event occurs when the action queue becomes full. A `QueueFilling` event occurs when the queue size exceeds a configurable threshold (`QueueFillingThreshold`) and the last event was a `QueueEmpty` or `QueueEmptying` event. A `QueueEmptying` event occurs when the queue size falls below a configurable threshold (`QueueEmptyingThreshold`) and the last event was a `QueueFull` or `QueueFilling` event. A `QueueEmpty` event occurs when the action queue becomes empty.

POP3 Connector supports the following types of event handler:

- The `TextFileHandler` writes event data to a text file.
- The `HttpHandler` sends event data to a URL.
- The `LuaHandler` runs a Lua script. The event data is passed into the script.

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