The transport handlers are those closest to the actual transport, hence the name.

On the client side, during a remote function invocation, they are the last handlers run before the request is sent. If there is a response from the server, the same transport handlers are invoked in reverse order, to process and transform the response message.

On the server side, transport handlers are the first handlers invoked after an incoming request is received. If a response is needed, the transport handlers are invoked in reverse order before the response message is sent back to the calling client

The HydraExpress framework guarantees that the transport handlers are invoked on both incoming and outgoing messages (whether the message is a request or response). This is true even when message handling is interrupted, as described in Aborting Handler Processing.

Since the SOAP message is treated as a simple string payload when a transport handler is run, transport handlers are most suitable for tasks that have to be performed on the entire message. This includes tasks such as encryption/decryption, compression/decompression, etc. For tasks that need to access specific parts of a SOAP message, it is better to create a request or a response handler.