Qpid Design - Framing

Frame Classes

The framing definition in the protocol specification maps quite nicely to an object-oriented representation. The class diagram is shown below:

The AMQDataBlock at the root of the hierarchy defines a writePayload method that subclasses implement in order to be able to transform themselves into bytes. This is called by the encoder, documented below. The decoding (from bytes into objects) is slightly more complex since it involves factories for the instantiation of the correct objects (again documented below).

An AMQFrame is the basic unit transmitted over the network, and contains a body which is the real payload. There are numerous method frames, which are subclasses of AMQMethodBody. The method body subclasses are all code generated from the protocol specification. The ContentHeaderBody can support different types of content properties or metadata (examples being file or stream in addition to basic which is standard JMS-style messaging).

ContentBody is a lightweight wrapper for message data.

Encoding

Encoding is a straightforward process. The AMQDataBlock class has only two method: getSize() and writePayloadToBuffer(ByteBuffer). The encoder simply needs to ask the data block its size, allocate a buffer of that size, then ask the data block to write itself into the buffer.

Decoding

The classes involved in decoding are illustrated in this UML class diagram:

The AMQDataBlockDecoder has only two methods: decodable() in which it attempts to read enough information from the supplied buffer to determine whether it has all the data and whether it appears to represent a known data block. If it needs more data, it return false. If the frame appears to be invalid it throws an exception.

The decoder stores the factories for HeartbeatBody, ContentHeaderBody and ContentBody frame types in an array, indexed on type. The AMQMethodBody factory is version specific and retrieved from the current session. The decoder constructs an AMQFrame, passing in the factory the appropriate factory. The result of that call is either a fully populated frame or an exception being thrown if data is invalid or inconsistent.

The MethodBodyDecoderRegistry is generated from the protocol XML. Each method is registered by protocol class and protocol method and when looked up by the AMQMethodBodyFactory an instance of the appropriate method body is returned. The generated code for the methods handles the reading and writing of the bytes to and from ByteBuffers as well as calculation of the size of the populated method bodies.