Apama 10.3 | Apama Documentation | Connecting Apama Applications to External Components | Working with Connectivity Plug-ins | Developing Connectivity Plug-ins | Chain components and messages
Chain components and messages
Connectivity chains consist of zero or more codecs and one transport.
Codecs perform some transformation of events, processing events going to and from the correlator, and passing them on to the next component in the chain. Examples of codecs include:
*Translating events from structured form into a serialized form such as JSON or XML.
*Changing the name of a field to translate between different naming conventions.
*Removing unnecessary fields.
*Filtering, merging or splitting events.
Transports are the end of a chain. They may deliver the events to an external system that is not part of the chain and may be out of process, or may perform some operation and send the result to back to the correlator. Examples of transports include:
*Sending HTTP requests and receiving responses.
*Receiving events from a message bus.
*Performing operations on a file system.
Codecs and transports may be written in Java or C++, and chains can contain a mixture of C++ and Java plug-ins. Messages will be automatically converted between C++ and Java forms. The language bindings of any libraries required by a plug-in and familiarity with the programming environment should be the primary factors when deciding on the language in which to write a new plug-in. Note that conversions between C++ and Java forms are copies and there are overheads in performing these conversions. As the Apama host plug-ins are implemented in C++ (as is the core of the correlator), a chain consisting of only C++ plug-ins will perform better. In particular, avoid mixing many interleaved Java and C++ plug-ins in the same chain. If possible, put the C++ plug-ins on the host side of the chain. Where adjacent plug-ins in a chain are of the same type (C++/Java), messages are passed by reference/pointer (they are not copied).
Plug-ins communicate with each other and the correlator (also referred to as the host) by sending batches of messages. Messages are converted to/from the events within a correlator. When a chain sends a message to the host, the host plug-in converts it to an event and sends it into the correlator. When the correlator emits an event to a channel on which a chain is listening, then the host plug-in converts it from an event to a message, and delivers it to the chain. The plug-ins in a chain may do conversions, for example, a codec may convert a map to a single string (for example, a JSON codec), but that can be passed within a message. When it gets to a transport, it may be taken from the message and delivered in some other form (for example, an HTTP request). A message consists of a payload (which can be of different types according to the needs of the plug-in) and a metadata map of strings. For more information, see the Message class in the API Reference for Java (Javadoc) or API Reference for C++ (Doxygen).
Metadata holds data about the event, such as the channel on which the event was delivered or the type of the event. Plug-ins can use metadata to pass extra information about events that are not part of the event (for example, latency through a chain could be measured by adding timestamps to metadata and comparing that with the time needed for processing an event).
The message payload can be null. This means that the message is not a real event. This can be useful for passing non-event information between plug-ins; many plug-ins will ignore this. For example, a request to terminate a connection could be sent from one codec to signal the transport to disconnect, and intermediate codecs that perform transformations such as JSON encoding would ignore the event.
Messages are passed in batches so that transports (and codecs) can take advantage of amortizing costs if operating at high throughput. Most plug-ins can be written by subclassing AbstractSimpleCodec or AbstractSimpleTransport classes (see Requirements of a plug-in class) and only need to process a single Message at a time. The delineation of messages into batches does not carry any significance beyond the events are all available at the same time. This is only an opportunity for optimizations, not for passing extra semantic information. Codecs are not required to maintain the same batches and can re-batch messages if desired.
Messages are not copied between plug-ins and do not perform any locking or synchronization. If a codec wants to keep hold of a pristine copy of a message, it should store a copy of the message.
Every chain will need to work with one of the supplied host plug-ins. Most chains will use the apama.eventMap plug-in which allows events to be sent without needing to know the exact event definition. See also Map contents used by the apama.eventMap host plug-in.

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