Commanded

v0.16.0

Commanded v0.16.0 Commanded.Aggregates.Aggregate View Source

Aggregate is a GenServer process used to provide access to an instance of an event sourced aggregate.

It allows execution of commands against an aggregate instance, and handles persistence of created events to the configured event store. Concurrent commands sent to an aggregate instance are serialized and executed in the order received.

The Commanded.Commands.Router module will locate, or start, an aggregate instance when a command is dispatched. By default, an aggregate process will run indefinitely once started. Its lifespan may be controlled by using the Commanded.Aggregates.AggregateLifespan behaviour.

Snapshotting

You can configure state snapshots for an aggregate in config. By default snapshots are not taken for an aggregate. The following options are available to enable snapshots:

  • snapshot_every - snapshot aggregate state every so many events. Use nil to disable snapshotting, or exclude the configuration entirely.

  • snapshot_version - a non-negative integer indicating the version of the aggregate state snapshot. Incrementing this version forces any earlier recorded snapshots to be ignored when rebuilding aggregate state.

Example

In config/config.exs enable snapshots for ExampleAggregate after every ten events:

config :commanded, ExampleAggregate
  snapshot_every: 10,
  snapshot_version: 1

Link to this section Summary

Functions

child_spec(arg)

Returns a specification to start this module under a supervisor

execute(aggregate_module, aggregate_uuid, context, timeout \\ 5000)

Execute the given command against the aggregate

init(state)

Invoked when the server is started. start_link/3 or start/3 will block until it returns

start_link(aggregate_module, aggregate_uuid, opts \\ [])

Link to this section Functions

Link to this function child_spec(arg) View Source

Returns a specification to start this module under a supervisor.

See Supervisor.

Link to this function execute(aggregate_module, aggregate_uuid, context, timeout \\ 5000) View Source

Execute the given command against the aggregate.

  • aggregate_module - the aggregate’s module (e.g. BankAccount).
  • aggregate_uuid - uniquely identifies an instance of the aggregate.
  • context - includes command execution arguments (see Commanded.Aggregates.ExecutionContext for details).
  • timeout - an integer greater than zero which specifies how many milliseconds to wait for a reply, or the atom :infinity to wait indefinitely. The default value is five seconds (5,000ms).

Return values

Returns {:ok, aggregate_version, events} on success, or {:error, reason} on failure.

  • aggregate_version - the updated version of the aggregate after executing the command.
  • events - events produced by the command, can be an empty list.
Link to this function init(state) View Source

Invoked when the server is started. start_link/3 or start/3 will block until it returns.

args is the argument term (second argument) passed to start_link/3.

Returning {:ok, state} will cause start_link/3 to return {:ok, pid} and the process to enter its loop.

Returning {:ok, state, timeout} is similar to {:ok, state} except handle_info(:timeout, state) will be called after timeout milliseconds if no messages are received within the timeout.

Returning {:ok, state, :hibernate} is similar to {:ok, state} except the process is hibernated before entering the loop. See c:handle_call/3 for more information on hibernation.

Returning :ignore will cause start_link/3 to return :ignore and the process will exit normally without entering the loop or calling c:terminate/2. If used when part of a supervision tree the parent supervisor will not fail to start nor immediately try to restart the GenServer. The remainder of the supervision tree will be (re)started and so the GenServer should not be required by other processes. It can be started later with Supervisor.restart_child/2 as the child specification is saved in the parent supervisor. The main use cases for this are:

  • The GenServer is disabled by configuration but might be enabled later.
  • An error occurred and it will be handled by a different mechanism than the Supervisor. Likely this approach involves calling Supervisor.restart_child/2 after a delay to attempt a restart.

Returning {:stop, reason} will cause start_link/3 to return {:error, reason} and the process to exit with reason reason without entering the loop or calling c:terminate/2.

Callback implementation for GenServer.init/1.

Link to this function start_link(aggregate_module, aggregate_uuid, opts \\ []) View Source