Use PostgreSQL's NOTIFY/LISTEN for event pub/sub #100
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Include #100 in CHANGELOG
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Using PostgreSQL for pub/sub removes need for EventStore to form a cluster of nodes. It can be used on a single node, or multiple nodes, but they don't need to be directly connected. Instead the database will be used to distribute events. This simplifies running EventStore on a cluster.
Listener module is now a cluster singleton so only one PostgreSQL `LISTEN` connection is used per event store cluster.
Create PostgreSQL connection for notify test. Allow short delay for distributed subscription tests. Increase assert receive and refute timeouts in `distributed` env. Use latest Elixir and Erlang versions in Travis CI config. Use default logger format in test envs. Additional subscription logging to help identify intermittent test failure in `:distributed` env.
Include #100 in CHANGELOG
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Using PostgreSQL for pub/sub removes the requirement for EventStore to form a cluster of nodes.
It can be used on a single node or multiple nodes, but they don't need to be directly connected. The database will be used to distribute events instead. This simplifies running EventStore on a cluster and removes the dependency upon Swarm.
A single listener process subscribes to event notifications using PostgreSQL's
LISTENcommand. Whenever events are appended to storage aNOTIFYcommand is executed by a table trigger. The notification payload contains the first and last event number of the appended events (e.g.1,2). These events are then read from storage and published to interested subscribers. Elixir'sGenStageis used to read and forward events to subscriptions.PG2 groups are now used for event pub/sub when run in distributed mode.