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Networking and messaging
Corda uses AMQP/1.0 over TLS between nodes which is currently implemented using Apache Artemis, an embeddable message queue broker. Building on established MQ protocols gives us features like persistence to disk, automatic delivery retries with backoff and dead-letter routing, security, large message streaming and so on.
Artemis is hidden behind a thin interface that also has an in-memory only implementation suitable for use in unit tests and visualisation tools.
Note
A future version of Corda will allow the MQ broker to be split out of the main node and run as a separate server. We may also support non-Artemis implementations via JMS, allowing the broker to be swapped out for alternative implementations.
There are multiple ways of interacting with the network. When writing an application you typically won't use the messaging subsystem directly. Instead you will build on top of the protocol framework <protocol-state-machines>, which adds a layer on top of raw messaging to manage multi-step protocols and let you think in terms of identities rather than specific network endpoints.
Messaging types
Every Message object has an associated topic and may have a session ID. These are wrapped in a TopicSession. An implementation of MessagingService can be used to create messages and send them. You can get access to the messaging service via the ServiceHub object that is provided to your app. Endpoints on the network are identified at the lowest level using SingleMessageRecipient which may be e.g. an IP address, or in future versions perhaps a routing path through the network.
Network Map Service
Supporting the messaging layer is a network map service, which is responsible for tracking public nodes on the network.
Nodes have an internal component, the network map cache, which contains a copy of the network map (which is just a document). When a node starts up its cache fetches a copy of the full network map, and requests to be notified of changes. The node then registers itself with the network map service, and the service notifies subscribers that a new node has joined the network. Nodes do not automatically deregister themselves, so (for example) nodes going offline briefly for maintenance are retained in the network map, and messages for them will be queued, minimising disruption.
Nodes submit signed changes to the map service, which then forwards update notifications on to nodes which have requested to be notified of changes.
The network map currently supports:
- Looking up nodes by service
- Looking up node for a party
- Suggesting a node providing a specific service, based on suitability for a contract and parties, for example suggesting an appropriate interest rates oracle for a interest rate swap contract. Currently no recommendation logic is in place. The code simply picks the first registered node that supports the required service.