Stateful grains are backed by persistent storage and referenced by a primary key set by the grain. An activation of a grain is a single Erlang process in on an Erlang node (silo) in an Erlang cluster. Activation placement is handled by Erleans and communication is over standard Erlang distribution. If a grain is sent a message and does not have a current activation one is spawned.

Grain state is persisted through a database provider with an always increasing change id or etag. If the change id or etag has been by another activation the activation attempting to save state will stop.

Activations are registered through lasp_pg.

Stateless grains have no restriction on the number of activations and do not persist state to a database.

Stateless grain activations are pooled through sbroker while being counted by a gproc resource counter. This allows for the use of sbroker to select an activation if available and to create a new activation if none were available immediately and the number currently activated is less than the max allowed.

Timers that are associated with a grain, meaning if a grain is not active but a reminder for that grain ticks the grain is activated at that time and the reminder is delivered.

Processes can subscribe to grains to receive notifications for grain specific events. If a grain supports observers a group is created through lasp_pg that observers are added to and to which notifications are sent.

Interface that must be implemented for any persistent store to be used for grains.

Streams have a provider type as well for providing a pluggable stream layer.

Grains can subscribe to streams and implement callbacks for handling records as they are published. Each erleans node runs a configurable number of stream agents which are responsible for the actual fetching off a stream and forwarding to the subscribed grains. Stream providers implement how to read from and publish to a given stream backend. On arrival of new events on the stream the agents makes a call to each subscriber with the new event, thus blocking for as long as the slowest subscriber takes to handle the event, before fetching any new events.

A grain must explicitly unsubscribe from a stream or it will continue to receive calls even after being deactivated. This is because subscriptions are based on the grain reference, not the activation, so if a subscribed grain has deactivated it will be reactivated on the next event on the stream.

No starting or linking, a grain is activated when it is sent a request if an activation is not currently running.

• prefer_local: If an activation does not exist this causes the new activation to be on the same node making the request.
• random: Picks a random node to create any new activation of a grain.
• stateless: Stateless grains are always local. If no local activation to the request exists one is created up to a default maximum value.
• {stateless, Max :: integer()}: Allows for up to Max number of activations for a grain to exist per node. A new activation, up until Max exist on the node, will be created for a request if an existing activation is not currently busy.

The grain implementation test_grain is found in test/:

-module(test_grain).

-behaviour(erleans_grain).

...

placement() ->
    prefer_local.

provider() ->
    ets_provider.

deactivated_counter(Ref) ->
    erleans_grain:call(Ref, deactivated_counter).

activated_counter(Ref) ->
    erleans_grain:call(Ref, activated_counter).

node(Ref) ->
    erleans_grain:call(Ref, node).

state(_) ->
    #{activated_counter => 0,
      deactivated_counter => 0}.

activate(_, State=#{activated_counter := Counter}) ->
    {ok, State#{activated_counter => Counter+1}, #{}}.

$ rebar3 as test shell
...
> Grain1 = erleans:get_grain(test_grain, <<"grain1">>).
> test_grain:activated_counter(Grain1).
{ok, 1}

Because the tests rely on certain configurations of apps it is easiest to run the tests with the alias test:

$ epmd -daemon
$ rebar3 test

Note that the distributed tests will only work on a node with hostname fanon. This will be easily avoidable in the next release of OTP but until need to figure out another work around. Running rebar3 ci will run all the tests but dist tests.