Message assemblers are pluggable transport elements which define the transport's wire format and protocol. For example, rather than requiring you to hand build HTTP messages, the HTTP message assembler takes in an abstract description of an HTTP request and converts it to a series of bytes which it then sends using the transport's mechanisms.

Similarly, transports such as SMTP require several interactions between sender and receiver to transmit one logical message. Message assemblers encode this behavior.

Assemblers can be used to coordinate with higher-level notions of sending and receiving. For example, the HTTP assembler allows the contents of an outgoing message to be a Block. When the assembler goes to output the contents, rather than writing it to the TCP socket it evaluates the block and supplies a stream as an argument. Using this mechanism, you can send large messages--such as files--efficiently and effect the lazy creation of message content.

The basic protocol for message assemblers is outlined below: