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gossip.md

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Gossip

Connects different nodes over the network with efficient message broadcast and unicast. This is the main method nodes in the network use to communicate among themselves.

Inside a node: A pub/sub model where any service subscribes with Gossip to a specific topic (group of messages, all topics are described here).

Currently a single instance per virtual chain per node.

Interacts with services

  • None - Passive, just provides services to others upon request.

 

Network Requirements

  • If the number of nodes is small:
    • Fully connected, every node is connected to every other node.
    • All communication is direct.
      • Optimization: Forwarding scheme for large messages like PRE_PREPARE.
  • Network topology is known in advance and configurable.
    • Currently assume all services are found on all nodes.
    • Currently assume single instance of a service on a node.
  • No special support for retransmission except standard TCP stack.
  • Recommended: Socket connections should be signed by nodes and authenticated when opened (via TLS).
    • Not a hard requirement because all communication is signed in the application level.
  • Reconnect to topology peers when disconnected with a configurable polling interval.
  • Calls OnMessageReceived when a gossip message is received.
  • Wire format encoding is Protobuf over TCP.

 

Data Structures

Network address map

  • Maps between node id (public key) to an active socket to the Gossip service of this node.
  • Used for inter node communication.
  • Assumes every node has a single gossip instance.
  • Initialized based on the public gossip endpoints from node topology configuration.

Topic subscription table

  • Map between topic to list of service ids that are subscribed to this topic.
  • Used for intra node communication.
  • Assumes a single instance of every service on every node.
  • Generated dynamically by calls to TopicSubscribe.

 

Init (flow)

  • Initialize the configuration.
  • Connect to the gossip endpoints of all relevant peer nodes.

 

TopicSubscribe (method)

Used by services to subscribe to a specific topic.

  • Add this service id to the the topic subscription table for the requested topic.

 

TopicUnsubscribe (method)

Used by services to unsubscribe from a specific topic.

  • Remove this service id from the the topic subscription table for the requested topic.

 

OnMessageReceived (method)

Called internally when a gossip message is received from another node (inter node).

  • Lookup the the list of subscribed services for this topic in the topic subscription table.
  • For each service id:
    • Rely on service topology configuration to locate the service endpoint by service id.
    • Call Target.GossipMessageReceived with message content.
    • The target service is responsible to identify the message type and process the message accordingly.

 

SendMessage (method)

Sends an inter node message, the message will be forwarded to the services subscribed to the topic on the receiving node.

  • The gossip message holds the list of destination nodes ids (public keys).
    • Setting inverse_recipients sends to all nodes in the virtual chain except the destination node ids.
    • NULL value for the list means broadcast to all nodes in the virtual chain.
  • For each node id:
    • Rely on the network address map to locate the relevant socket.
    • Send the message on the socket.