>EPMD: register(nodeb, Port) NodeA->>EPMD: lookup(nodeb) EPMD-->>NodeA: Port NodeA->>NodeB: connect on Port EPMD itself carries no message traffic between nodes — it's purely a directory service, running on a well-known port (4369) so nodes only need to know a hostname, not a specific port, to find each other. Losing EPMD after nodes are already connected doesn't drop those connections; it only affects new nodes trying to discover an existing one."> >EPMD: register(nodeb, Port) NodeA->>EPMD: lookup(nodeb) EPMD-->>NodeA: Port NodeA->>NodeB: connect on Port EPMD itself carries no message traffic between nodes — it's purely a directory service, running on a well-known port (4369) so nodes only need to know a hostname, not a specific port, to find each other. Losing EPMD after nodes are already connected doesn't drop those connections; it only affects new nodes trying to discover an existing one." /> >EPMD: register(nodeb, Port) NodeA->>EPMD: lookup(nodeb) EPMD-->>NodeA: Port NodeA->>NodeB: connect on Port EPMD itself carries no message traffic between nodes — it's purely a directory service, running on a well-known port (4369) so nodes only need to know a hostname, not a specific port, to find each other. Losing EPMD after nodes are already connected doesn't drop those connections; it only affects new nodes trying to discover an existing one." />

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Erlang / Erlang Advanced Interview questions

What is EPMD and what role does it play in distributed Erlang?

EPMD (Erlang Port Mapper Daemon) is a small, lightweight process that runs once per host and acts as a name-to-port lookup service for Erlang nodes on that machine. Each node, on startup, registers its name and the TCP port it's listening on with the local EPMD; other nodes trying to connect first ask EPMD "what port is node X on?" before opening the actual inter-node connection.

sequenceDiagram
    participant NodeA
    participant EPMD
    participant NodeB
    NodeB->>EPMD: register(nodeb, Port)
    NodeA->>EPMD: lookup(nodeb)
    EPMD-->>NodeA: Port
    NodeA->>NodeB: connect on Port

EPMD itself carries no message traffic between nodes — it's purely a directory service, running on a well-known port (4369) so nodes only need to know a hostname, not a specific port, to find each other. Losing EPMD after nodes are already connected doesn't drop those connections; it only affects new nodes trying to discover an existing one.

What does EPMD actually do for a distributed Erlang cluster?
What happens to already-established node connections if EPMD stops running?

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