# Expected Version and Concurrency

Note: Applies to both the message writer and the message data writer.

The expected_version argument is an optimistic concurrency (opens new window) protection. It can also be used to assure that a message written to a stream is the first message in the stream.

In the typical handler workflow, a stream's version is retrieved along with the stream's projected entity at the start of a handler. That retrieved version is then included as the value of the expected_version argument to the writer.



handle Something do |something|
  account, version = store.fetch(something.id, include: :version)

  # ...

  write.(some_message, some_stream, expected_version: version)

If the expected version and the stream version no longer match at the time of the write, the MessageStore::ExpectedVersion::Error is raised.

# Concurrency


Except when running multiple instances of a component for hot fail-over, concurrent writing to an event stream is considered an anomaly. It's not expected that two separate writers would be writing to the same event stream concurrently as this would violate the authority of a component over its streams. Only one instance of a hosted component should be empowered to write to a stream. If two instances of the same component are writing to the same event stream, then appropriate measures must be taken to retry the writes.