Storage overview

OpStream persists two things:

What	Where	Interface
The op log (append-only)	IDocumentStore	AppendOpAsync / StreamOpsAsync
Snapshots (compacted state)	IDocumentStore	SaveSnapshotAsync / LoadSnapshotAsync

Plus an optional history store (IHistoryStore) for the history / milestone subsystem.

Supported backends

Backend	Best for	Setup
In-memory	Tests, demos	Default
EF Core	Existing EF Core code-first apps	UseEfCoreStorage<TContext>()
SQL Server	Microsoft-centric stacks	UseSqlServer(connStr)
PostgreSQL	OSS / cloud-native	UsePostgreSqlStorage(...)
MySQL	LAMP-style stacks	UseMySqlStorage(...)
SQLite	Embedded / desktop / single-process	UseSqliteStorage(...)
MongoDB	Document-first stacks	UseMongoDbStorage(...)
Redis	High-throughput, RAM-resident	UseRedisStorage(...)

Use* is singleton-style — calling it twice replaces the previous registration. See Builder API conventions.

Choosing a backend

• You already use EF Core → EF Core. Migrations land in your existing context.
• You want maximum write throughput → Redis. Sub-ms appends; persistence via AOF / RDB.
• You want maximum simplicity for a single-server deploy → SQLite. One file, no infrastructure.
• You want operational maturity in production → PostgreSQL or SQL Server.
• Your business data is in MongoDB → MongoDB keeps the op log next to it.

Implementing a custom backend

The interface is small:

public interface IDocumentStore
{
    Task AppendOpAsync(string documentId, StoredOp op, CancellationToken ct = default);
    IAsyncEnumerable<StoredOp> StreamOpsAsync(string documentId, long fromExclusive, CancellationToken ct = default);

    Task<DocumentSnapshot?> LoadSnapshotAsync(string documentId, CancellationToken ct = default);
    Task SaveSnapshotAsync(string documentId, DocumentSnapshot snapshot, CancellationToken ct = default);
}

Register your implementation with services.AddSingleton<IDocumentStore, MyStore>() after AddOpStream(), or via a custom builder extension method.

Default warnings

The default MemoryDocumentStore is registered as a fallback so AddOpStream() works out of the box. The DocumentRouter logs a warning at startup when this default is still active — that's your signal to plug in a real backend before going live.