Chronik v0.1.10 Chronik.Projection behaviour View Source
Chronik.Projection
is a read-only model connected to Chronik.PubSub
Client code has to implement the Chronik.Projection.init/1
function
and the state transition Chronik.Projection.handle_event/2
.
Example
defmodule DomainEvents do
defmodule CounterCreated do
defstruct [:id]
end
defmodule CounterIncremented do
defstruct [:id, :increment]
end
end
defmodule CounterState do
@behaviour Chronik.Projection
alias DomainEvents.{CounterCreated, CounterIncremented}
alias Chronik.Projection
def start_link(opts), do: Projection.start_link(__MODULE__, opts)
def init(_opts), do: {nil, []}
def handle_event(%CounterCreated{}, nil), do: 0
def handle_event(%CounterIncremented{increment: increment}, value) do
value + increment
end
end
Link to this section Summary
Functions
Returns a specification to start this module under a supervisor
Invoked when the server is started. start_link/3
or start/3
will
block until it returns
Start a new projection
Return the current state for projection_id
Callbacks
The handle_event
function is executed each time an event record is
received on the Chronik.PubSub
and is responsible of the
projection state transition
The init
function defines the intial state of an projection and
some options
Link to this section Types
The state
represents the state of an projection.
Link to this section Functions
Returns a specification to start this module under a supervisor.
See Supervisor
.
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 callingSupervisor.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
.
Start a new projection
Return the current state for projection_id
Link to this section Callbacks
The handle_event
function is executed each time an event record is
received on the Chronik.PubSub
and is responsible of the
projection state transition.
The return value is a new state
for the received record
The init
function defines the intial state of an projection and
some options.
The accepted options
:
:version
start replaying events fromversion
and up.:all
indicates that the replay should be from the begining of times while:current
means that no re-play should be made and start feeding domain events from current version and on.:consistency
indicates how the projection should subscribe to theChronik.PubSub
. Possible values are:eventual
(default) and:strict
.