JournaledGrain Basics

Article
07/03/2024

Journaled grains derive from JournaledGrain<TGrainState,TEventBase>, with the following type parameters:

The TGrainState represents the state of the grain. It must be a class with a public default constructor.
TEventBase is a common supertype for all the events that can be raised for this grain, and can be any class or interface.

All state and event objects should be serializable (because the log-consistency providers may need to persist them, and/or send them in notification messages).

For grains whose events are POCOs (plain old C# objects), JournaledGrain<TGrainState> can be used as a shorthand for JournaledGrain<TGrainState,TEventBase>.

Reading the grain state

To read the current grain state, and determine its version number, the JournaledGrain has properties

GrainState State { get; }
int Version { get; }

The version number is always equal to the total number of confirmed events, and the state is the result of applying all the confirmed events to the initial state. The initial state, which has version 0 (because no events have been applied to it), is determined by the default constructor of the GrainState class.

Important: The application should never directly modify the object returned by State. It is meant for reading only. Rather, when the application wants to modify the state, it must do so indirectly by raising events.

Raise events

Raising events is accomplished by calling the RaiseEvent function. For example, a grain representing a chat can raise a PostedEvent to indicate that a user-submitted a post:

RaiseEvent(new PostedEvent()
{
    Guid = guid,
    User = user,
    Text = text,
    Timestamp = DateTime.UtcNow
});

Note that RaiseEvent kicks off a write to storage access, but does not wait for the write to complete. For many applications, it is important to wait until we have confirmation that the event has been persisted. In that case, we always follow up by waiting for ConfirmEvents:

RaiseEvent(new DepositTransaction()
{
    DepositAmount = amount,
    Description = description
});
await ConfirmEvents();

Note that even if you don't explicitly call ConfirmEvents, the events will eventually be confirmed - it happens automatically in the background.

State transition methods

The runtime updates the grain state automatically whenever events are raised. There is no need for the application to explicitly update the state after raising an event. However, the application still has to provide the code that specifies how to update the state in response to an event. This can be done in two ways.

(a) The GrainState class can implement one or more Apply methods on the StateType. Typically, one would create multiple overloads, and the closest match is chosen for the runtime type of the event:

class GrainState
{
    Apply(E1 @event)
    {
        // code that updates the state
    }

    Apply(E2 @event)
    {
        // code that updates the state
    }
}

(b) The grain can override the TransitionState function:

protected override void TransitionState(
    State state, EventType @event)
{
   // code that updates the state
}

The transition methods are assumed to have no side effects other than modifying the state object, and should be deterministic (otherwise, the effects are unpredictable). If the transition code throws an exception, that exception is caught and included in a warning in the Orleans log, issued by the log-consistency provider.

When, exactly, the runtime calls the transition methods depends on the chosen log consistency provider and its configuration. Applications shouldn't rely on a particular timing, except when specifically guaranteed by the log consistency provider.

Some providers, such as the Orleans.EventSourcing.LogStorage log-consistency provider, replay the event sequence every time the grain is loaded. Therefore, as long as the event objects can still be properly deserialized from storage, it is possible to radically modify the GrainState class and the transition methods. But for other providers, such as the Orleans.EventSourcing.StateStorage log-consistency provider, only the GrainState object is persisted, so developers must ensure that it can be deserialized correctly when read from storage.

Raise multiple events

It is possible to make multiple calls to RaiseEvent before calling ConfirmEvents:

RaiseEvent(e1);
RaiseEvent(e2);
await ConfirmEvents();

However, this is likely to cause two successive storage accesses, and it incurs a risk that the grain fails after writing only the first event. Thus, it is usually better to raise multiple events at once, using

RaiseEvents(IEnumerable<EventType> events)

This guarantees that the given sequence of events is written to storage atomically. Note that since the version number always matches the length of the event sequence, raising multiple events increases the version number by more than one at a time.

Retrieve the event sequence

The following method from the base JournaledGrain class allows the application to retrieve a specified segment of the sequence of all confirmed events:

Task<IReadOnlyList<EventType>> RetrieveConfirmedEvents(
    int fromVersion,
    int toVersion);

However, it is not supported by all log consistency providers. If not supported, or if the specified segment of the sequence is no longer available, a NotSupportedException is thrown.

To retrieve all events up to the latest confirmed version, one would call

await RetrieveConfirmedEvents(0, Version);

Only confirmed events can be retrieved: an exception is thrown if toVersion is larger than the current value of the property Version.

Since confirmed events never change, there are no races to worry about, even in the presence of multiple instances or delayed confirmation. However, in such situations, the value of the property Version may be larger by the time the await resumes than at the time RetrieveConfirmedEvents is called, so it may be advisable to save its value in a variable. See also the section on Concurrency Guarantees.

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