Chronik v0.1.10 Chronik.Aggregate behaviour View Source
The Chronik.Aggregate
is the base for all aggregates in Chronik.
The module that implements the Chronik.Aggregate
behaviour
module can be configured with a number of options:
shutdown_timeout
indicatesChronik
to shutdown the aggregate GenServer after a number of milliseconds. Defualt value is 15 minutes.snapshot_every
indicates that a snapshot must be done on theChronik.Store
everysnapshot_every
domain events processed. Default value is 100. This configuration is looked up in the:chronik
app under the given module.
Example
defmodule DomainEvents do
defmodule CounterCreated do
defstruct [:id]
end
defmodule CounterIncremented do
defstruct [:id, :increment]
end
end
defmodule Counter do
@behaviour Chronik.Aggregate
alias Chronik.Aggregate
alias DomainEvents.CounterCreated
alias DomainEvents.CounterIncremented
defstruct [:id, value: 0]
# Public API
def create(id), do: Aggregate.command(__MODULE__, id, {:create, id})
def increment(id, increment),
do: Aggregate.command(__MODULE__, id, {:increment, increment})
# Command handlers
def handle_command({:create, id}, nil) do
%CounterCreated{id: id}
end
def handle_command({:create, _id}, _state) do
raise "counter alredy created"
end
def handle_command({:increment, increment}, %Counter{id: id}) do
%CounterIncremented{id: id, increment: increment}
end
# Event handlers
def handle_event(%CounterCreated{id: id}, nil) do
%Counter{id: id}
end
def handle_event(%CounterIncremented{increment: i}, %Counter{} = state) do
update_in(state.value, &(&1 + i))
end
end
The application code must implement the handle_command
and handle_event
callbacks.
Link to this section Summary
Types
The state
represents the state of an aggregate
An aggregate is identified by its module and an id
Functions
Returns a specification to start this module under a supervisor
The command
function is the entry point to Chronik aggregate.
It sends the cmd
request to the Aggregate identifed by module
and id
.
The timeout
is either :infinity
or a number of milliseconds (defaults
to 5000
)
Invoked when the server is started. start_link/3
or start/3
will
block until it returns
Start a Chronik.Aggregate
with callbacks on module
with id id
The state(module, id)
function returns the current aggregate state
Callbacks
The handle_command
is the entry point for commands on an aggregate
The handle_event
is the transition function for the aggregate. After
command validation, the aggregate generates a number of domain events
and then the aggregate state is updated for each event with this function
Link to this section Types
The state
represents the state of an aggregate.
Is used in the to validate a command (in handle_command
) and
in handle_event
callback.
An aggregate is identified by its module and an id.
Link to this section Functions
Returns a specification to start this module under a supervisor.
See Supervisor
.
command( module :: module(), id :: Chronik.id(), cmd :: term(), timeout :: :infinity | non_neg_integer() ) :: :ok | {:error, String.t()}
The command
function is the entry point to Chronik aggregate.
It sends the cmd
request to the Aggregate identifed by module
and id
.
The timeout
is either :infinity
or a number of milliseconds (defaults
to 5000
).
The results is either :ok
or {:error, reason}
in case of failure.
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_link(module :: module(), id :: Chronik.id()) :: {:ok, pid()} | {:error, reason :: String.t()}
Start a Chronik.Aggregate
with callbacks on module
with id id
.
state(module(), Chronik.id()) :: Chronik.Aggregate.state()
The state(module, id)
function returns the current aggregate state.
This should only be used for debugging purposes.
Link to this section Callbacks
handle_command(cmd :: Chronik.command(), state :: state()) :: [Chronik.domain_event()] | no_return()
The handle_command
is the entry point for commands on an aggregate.
The command format is application dependend. Throughout Chronik
,
commands are tagged tuples where the first element is an atom
indicating the command to execute and the remaining elements are arguments
to the command. E.g: {:add_item, 13, "Elixir Book", "$15.00"}
Example
def handle_command({:add_item, id, book, price}, %Cart{}) do
%ItemsAdded{id: id, book: book, price: price}
end
This handle_command
validate the command. If the command is valid on the
given state, the function should return a list (or a single) of domain events.
If the command is invalid the handle_command
should raise an exception.
handle_event(event :: Chronik.domain_event(), state :: state()) :: state()
The handle_event
is the transition function for the aggregate. After
command validation, the aggregate generates a number of domain events
and then the aggregate state is updated for each event with this function.
Note that this function can not fail since the domain event where generated by a valid command.