Integration Server generates the Callback service for each callback operation defined under a REST operation. The format of the callback service signature is the same as defined under the Service section. You must provide the implementation in these callback services to send the notification back to the client. You can use the pub.client:http service if the callback URL uses http or https. For instructions about using this service, see the webMethods Integration Server Built-In Services Reference. You can invoke this callback service by passing the inputs whenever you need to send a callback to the client.