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# Asynchronous functions vs. synchronous functions

For a list of protocols that support asynchronous communication, see 64-bit recording, Async, and IPv6 support.

There are several functions that enable Vuser scripts to emulate asynchronous communication. These asynchronous functions differ from the other API functions in the following ways:

- The network traffic that the asynchronous functions generate runs in parallel – simultaneously – with the main flow in the Vuser script. This means that the asynchronous communication can continue even when the synchronous steps end.
- The asynchronous communication continues even during execution of non-web functions (e.g. lr_think_time).
- Some of the asynchronous communication API functions use callback functions. The user needs to specify callbacks that are scheduled by the replay engine when a predefined event occurs. For details on using callbacks with asynchronous functions, see Implement callbacks.

## See also

- Function Reference

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