# Request-Callback

Availability-note[Deprecated](https://help.talend.com/r/en-US/Cloud/labels-on-talend-help-center/availability-deprecated)

This content is deprecated.

When a consumer sends a request to a provider, the consumer normally waits for the response to return before it can continue with its work. In cases where the response returns immediately, a request-response message exchange pattern is sufficient. But there can be scenarios when request processing can be delayed by few hours or even days. In such cases, it is useful for the consumer to be able to continue its work and handle the response later. The Request-Callback message exchange pattern supports exactly this scenario. The consumer can continue its processing without interruptions and is notified when the response is returned at a later point in time.

![](https://help.qlik.com/talend/en-US/esb-service-developer-guide/Resources/images/request-callback.png)

### In this section

* [WSDL for Request-Callback Message Exchange](https://help.qlik.com/talend/en-US/esb-service-developer-guide/8.0/wsdl-for-request-callback-message-exchange)
* [Relation between Request and Callback Messages](https://help.qlik.com/talend/en-US/esb-service-developer-guide/8.0/relation-between-request-and-callback-messages)
* [Defining Participants](https://help.qlik.com/talend/en-US/esb-service-developer-guide/8.0/defining-participants)
* [Project dependencies](https://help.qlik.com/talend/en-US/esb-service-developer-guide/8.0/project-dependencies)