Feat: Pointers

[the-avid-engineer]

It would be useful for the ISnapshotRepository, IEntityRepository, and IProjectionRepository instances to be able to get entities at any version instead of always pulling the latest version.

To that end, introduce a new value object, the pointer:

public record Pointer(Id Id, VersionNumber VersionNumber);

For ISnapshotRepository, mark the existing GetSnapshot method as obsolete and forward the call to the new GetSnapshotOrDefault method, which accepts a Pointer instead of an Id

For IEntityRepository and IProjectionRepository, mark the existing GetCurrent and GetAtVersion methods as obsolete and forward the call to the new GetSnapshot method, which accepts a Pointer instead of an Id

Affected Snapshot Repository Implementations

Redis

The Redis snapshot key will be changed from {keyNamespace}#{snapshotId} to {keyNamespace}#{snapshotPointer.Id}@{snapshotPointer.VersionNumber}

In Memory

The in-memory snapshot repository will simply use Pointer as the dictionary key instead of Id

Additional Changes

Transaction Subscribers

The TransactionSubscriber has been buffed in non-test mode. Instead of calling Task.Run and ignoring the result, it will now add the result to a BufferBlock. (Note: Test mode still uses Task.Run(...).Wait() so that transactions are processed synchronously.) In addition, the transaction subscriber will be registered as a BackgroundService and will process the BufferBlock asynchronously. This processor is also try-catched, so any failed processes will not cause the background service to stop.

Entity & Projection Snapshot Transaction Subscribers

The entity snapshot subscriber and projection snapshot subscriber have been refactored to implement ITransactionProcessor and are registered using TransactionProcessorSubscriber. The two processors have a common base class which abstracts some redundant work and also provides a caching mechanism to avoid hitting the database multiple times.

You also must provide a transaction session options name now, as it is possible to hit the database if snapshots don't have all of the information needed.

Entity & Projection Repositories

These repositories will no longer return the default snapshot state - if the expected pointer cannot be loaded, they will always throw SnapshotPointerDoesNotExistException (which replaces the EntityNotCreatedException that was originally thrown on the now-obsolete GetCurrent method)

Entity, Projection and Snapshot - Common Library Interfaces

Some common functionality between IEntity<> and IProjection<> (e.g., GetId, GetVersionNumber, Construct) have been moved to the ISnapshot<> interface, and these common library interfaces now extend ISnapshot<>

ISnapshot<> has two new method, ShouldRecord and ShouldRecordAsLatest, the latter of which replaces the now-obsolete ShouldReplace

Projections

The IProjectionStrategy interface has been removed - the functionality has been moved to the IProjection<> interface, more or less. To accomplish this, there are two methods:

1. GetProjectionIdOrDefault - this will statically map an object to an Id?
2. GetCommandQuery - this will dynamically map the projection and projection pointer to any query you want - it should load all of the commands which are needed to reach the projection pointer.

Because you now control the query, the GetEntityVersionNumber method has been removed.