EPMDPXY simulates the basic functionality of EPMD, just enough to make a local Erlang cluster work. With one exception though, instead of replying the listener port when handling PORT_PLEASE2_REQ, it spawns an internal listener and replies the new (random) port number instead. The newly spawned listener process accepts one single connection and connects to the 'real' listener port of the remote node, acting as a proxy between the two Erlang nodes.
If a distributed Erlang node is started with the -name or -sname argument it will first try to connect to the EPMD. In case the EPMD is not running on that local machine yet it gets started as a separate OS process. The EPMD normally listens on the 4369 TCP port, this can be adjusted using the environment variable ERL_EPMD_PORT. A distributed Erlang node connects to the EPMD and registers its node name together with a random listener port, that is used for accepting connections from other nodes in the cluster (ALIVE2_REQ). If a node wants to communicate with an other cluster node it hasn't seen yet, it contacts the EPMD and asks for the listening port of the other node (PORT_PLEASE2_REQ).
It's all on Distributed Erlang and Erlang Distribution Protocol
Open a new Erlang shell, without the -name or -sname arguments.
%% start the epmdpxy listening on the default EPMD port 4369.
%% be sure that no other EPMD is running on this port.
epmdpxy:start(). If you prefer to use a different EPMD port use:
%% start the epmdpxy listening on a non standard port.
epmdpxy:start(43690). If you go this way, you have to set the ERL_EPMD_PORT=43690 environment variable before starting
your cluster nodes.
EPMDPXY currently only implements one feature, namely the simulation of network partitions in an Erlang cluster.
Assuming your cluster consists of five nodes node1, node2, node3, node4, node5. Simulating a
netsplit resulting in two islands [node1, node2] and [node3, node4, node5] can be done
using:
epmdpxy:cut_cables([node1, node2], [node3, node4, node5]).In this situation the proxy blocks all the traffic between the two islands.
To fix the network partition call:
epmdpxy:fix_cables([node1, node2], [node3, node4, node5]).- it's pretty alpha..
- we'll be using it at Erlio GmbH, so it will eventually mature
- of course this is not a replacement for real hardware damage