.NET SDK

Installation#

To install the SDK, run:

dotnet add package EventSourcingDb

You can also find this SDK on NuGet.

Capabilities#

This SDK intentionally focuses on application-level use cases. It provides core functionality for writing, reading, and observing events, and deliberately excludes operational concerns such as backup, restore, or health monitoring.

For a detailed explanation of the specific requirements behind each capability, see the Compliance Criteria.

Essentials

✓ Initialize the Client
✓ Ping the Server
✓ Verify API Token

Writing Events

✓ Write Single Events
✓ Write Multiple Events
✓ Use isSubjectPristine
✓ Use isSubjectOnEventId
Requires EventSourcingDB 1.1✓ Use isEventQlQueryTrue
✓ Include OpenTelemetry Trace Context

Reading Events

✓ Read Events by Subject
✓ Read Recursively
✓ Read Chronological and Anti-Chronological
✓ Use Boundaries
✓ Use fromLatestEvent

Using EventQL

✓ Execute EventQL Queries

Observing Events

✓ Observe Events By Subject
✓ Observe Recursively
✓ Use Boundaries
✓ Use fromLatestEvent

Metadata and Discovery

✓ Register Event Schemas
✓ List Subjects
✓ List Event Types
Requires EventSourcingDB 1.1✓ List a Specific Event Type
✓ Verify Event Hashes
Requires EventSourcingDB 1.1✓ Verify Event Signatures

Testcontainers Support

✓ Start and Stop EventSourcingDB in Tests
✓ Use Custom Configuration (e.g., API Token, Port, …)

Getting Started#

Import the Client class and create an instance by providing the URL of your EventSourcingDB instance and the API token to use:

using EventSourcingDb;

var url = new Uri("http://localhost:3000");
var apiToken = "secret";

var client = new Client(url, apiToken);

Then call the PingAsync method to check whether the instance is reachable. If it is not, the method will throw an exception:

await client.PingAsync();

Optionally, you might provide a CancellationToken.

Note that PingAsync does not require authentication, so the call may succeed even if the API token is invalid.

If you want to verify the API token, call VerifyApiTokenAsync. If the token is invalid, the function will throw an exception:

await client.VerifyApiTokenAsync();

Optionally, you might provide a CancellationToken.

Serializing and Deserializing#

Basically, System.Text.Json is used for JSON serialization and deserialization.

You can override the default settings by either using JSON attributes on your types and properties or by providing your own JsonSerializerOptions when creating the Client both manually or via dependency injection.

Dependency Injection#

If you use Microsoft's dependency injection framework, you can call the AddEventSourcingDb extension method to register the Client as a scoped service:

var builder = WebApplication.CreateBuilder(args);
builder.services.AddEventSourcingDb(builder.Configuration);

The client is configured according to your IConfiguration state, e.g. by setting environment variables or using the EventSourcingDb section in appsettings.json:

{
  "EventSourcingDb": {
    "BaseUrl": "http://localhost:3000",
    "ApiToken": "secret"
  }
}

To configure the client at runtime, provide a configuration action:

builder.Services.AddEventSourcingDb(builder.Configuration, options =>
{
    options.BaseUrl = new Uri("http://localhost:3000");
    options.ApiToken = "secret";
});

Writing Events#

Call the WriteEventsAsync method and provide a collection of events. You do not have to set all event fields – some are automatically added by the server.

Specify Source, Subject, Type, and Data according to the CloudEvents format.

For Data, you may provide any object that is serializable to JSON.

The method returns a list of written events, including the fields added by the server:

var @event = new EventCandidate(
    Source: "https://library.eventsourcingdb.io",
    Subject: "/books/42",
    Type: "io.eventsourcingdb.library.book-acquired",
    Data: new {
        title = "2001 – A Space Odyssey",
        author = "Arthur C. Clarke",
        isbn = "978-0756906788"
    }
);

var writtenEvents = await client.WriteEventsAsync(new[] { @event });

Optionally, you might provide a CancellationToken.

Using the IsSubjectPristine precondition#

If you only want to write events in case a subject (such as /books/42) does not yet have any events, use the IsSubjectPristinePrecondition:

var writtenEvents = await client.WriteEventsAsync(
    new[] { @event },
    new[] { Precondition.IsSubjectPristinePrecondition("/books/42") }
);

Using the IsSubjectOnEventId precondition#

If you only want to write events in case the last event of a subject (such as /books/42) has a specific ID (e.g., "0"), use the IsSubjectOnEventIdPrecondition:

var writtenEvents = await client.WriteEventsAsync(
    new[] { @event },
    new[] { Precondition.IsSubjectOnEventIdPrecondition("/books/42", "0") }
);

Note that according to the CloudEvents standard, event IDs must be of type string.

Using the IsEventQlQueryTrue preconditionRequires EventSourcingDB 1.1#

If you want to write events depending on an EventQL query, use the IsEventQlQueryTruePrecondition:

var writtenEvents = await client.WriteEventsAsync(
    new[] { @event },
    new[] { Precondition.IsEventQlQueryTruePrecondition("FROM e IN events WHERE e.type == 'io.eventsourcingdb.library.book-borrowed' PROJECT INTO COUNT() < 10") }
);

Note that the query must return a single row with a single value, which is interpreted as a boolean.

Reading Events#

To read all events of a subject, call the ReadEventsAsync method and pass the subject and an options object. Set Recursive to false to ensure that only events of the given subject are returned, not events of nested subjects.

The method returns an async stream, which you can iterate over using await foreach:

await foreach (var @event in client.ReadEventsAsync(
    "/books/42",
    new ReadEventsOptions(Recursive: false)))
{
    // Handle event
}

If an error occurs, the stream will terminate with an exception.

Optionally, you might provide a CancellationToken.

Deserializing Event Data#

Each event contains a Data property, which holds the event payload as JSON. To deserialize this payload into a strongly typed object, call GetData<T>():

var book = @event.GetData<BookAcquired>();

Alternatively, you can use the non-generic overload GetData(Type) to resolve the type at runtime:

var type = typeof(BookAcquired);
var book = (BookAcquired)@event.GetData(type)!;

If you prefer to work directly with the JSON structure, access the Data property as a JsonElement:

var title = @event.Data.GetProperty("title").GetString();

Reading from subjects recursively#

If you want to read not only all events of a subject, but also the events of all nested subjects, set Recursive to true:

await foreach (var @event in client.ReadEventsAsync(
    "/books/42",
    new ReadEventsOptions(Recursive: true)))
{
    // ...
}

This also allows you to read all events ever written by using / as the subject.

Reading in anti-chronological order#

By default, events are read in chronological order. To read in anti-chronological order, use the Order option:

await foreach (var @event in client.ReadEventsAsync(
    "/books/42",
    new ReadEventsOptions(
        Recursive: false,
        Order: Order.Antichronological)))
{
    // ...
}

Note that you can also use Order.Chronological to explicitly enforce the default order.

Specifying bounds#

If you only want to read a range of events, set the LowerBound and UpperBound options — either one of them or both:

await foreach (var @event in client.ReadEventsAsync(
    "/books/42",
    new ReadEventsOptions(
        Recursive: false,
        LowerBound: new Bound("100", BoundType.Inclusive),
        UpperBound: new Bound("200", BoundType.Exclusive))))
{
    // ...
}

Starting from the latest event of a given type#

To start reading from the latest event of a specific type, set the FromLatestEvent option:

await foreach (var @event in client.ReadEventsAsync(
    "/books/42",
    new ReadEventsOptions(
        Recursive: false,
        FromLatestEvent: new ReadFromLatestEvent(
            Subject: "/books/42",
            Type: "io.eventsourcingdb.library.book-borrowed",
            IfEventIsMissing: ReadIfEventIsMissing.ReadEverything))))
{
    // ...
}

Note that FromLatestEvent and LowerBound cannot be used at the same time.

Running EventQL Queries#

To run an EventQL query, call the RunEventQlQueryAsync<TRow> method and provide the query as an argument. The method returns an async stream, which you can iterate over using await foreach:

await foreach (var row in client.RunEventQlQueryAsync<Event>(
    "FROM e IN events PROJECT INTO e"))
{
    // ...
}

Each row is deserialized automatically and returned as TRow, according to your projection. Ensure your projection matches the shape of TRow.

Optionally, you might provide a CancellationToken.

Observing Events#

To observe all future events of a subject, call the ObserveEventsAsync method and pass the subject and an options object. Set Recursive to false to observe only the events of the given subject.

The method returns an async stream:

await foreach (var @event in client.ObserveEventsAsync(
    "/books/42",
    new ObserveEventsOptions(Recursive: false)))
{
    // Handle event
}

If an error occurs, the stream will terminate with an exception.

Optionally, you might provide a CancellationToken.

Deserializing Event Data#

Each event contains a Data property, which holds the event payload as JSON. To deserialize this payload into a strongly typed object, call GetData<T>():

var book = @event.GetData<BookAcquired>();

Alternatively, you can use the non-generic overload GetData(Type) to resolve the type at runtime:

var type = typeof(BookAcquired);
var book = (BookAcquired)@event.GetData(type)!;

If you prefer to work directly with the JSON structure, access the Data property as a JsonElement:

var title = @event.Data.GetProperty("title").GetString();

Observing from subjects recursively#

If you want to observe not only the events of a subject, but also events of all nested subjects, set Recursive to true:

await foreach (var @event in client.ObserveEventsAsync(
    "/books/42",
    new ObserveEventsOptions(Recursive: true)))
{
    // ...
}

This also allows you to observe all events ever written by using / as the subject.

Specifying bounds#

If you want to start observing from a certain point, set the LowerBound option:

await foreach (var @event in client.ObserveEventsAsync(
    "/books/42",
    new ObserveEventsOptions(
        Recursive: false,
        LowerBound: new Bound("100", BoundType.Inclusive))))
{
    // ...
}

Starting from the latest event of a given type#

To observe starting from the latest event of a specific type, use the FromLatestEvent option:

await foreach (var @event in client.ObserveEventsAsync(
    "/books/42",
    new ObserveEventsOptions(
        Recursive: false,
        FromLatestEvent: new ObserveFromLatestEvent(
            Subject: "/books/42",
            Type: "io.eventsourcingdb.library.book-borrowed",
            IfEventIsMissing: ObserveIfEventIsMissing.ReadEverything))))
{
    // ...
}

Note that FromLatestEvent and LowerBound cannot be used at the same time.

Registering an Event SchemaChanged in 1.3#

To register an event schema, call the RegisterEventSchemaAsync method and hand over an event type and the desired schema:

await client.RegisterEventSchemaAsync(
    "io.eventsourcingdb.library.book-acquired",
    new Dictionary<string, object>
    {
        ["type"] = "object",
        ["properties"] = new Dictionary<string, object>
        {
            ["title"] = new Dictionary<string, object> { ["type"] = "string" },
            ["author"] = new Dictionary<string, object> { ["type"] = "string" },
            ["isbn"] = new Dictionary<string, object> { ["type"] = "string" }
        },
        ["required"] = new[] { "title", "author", "isbn" },
        ["additionalProperties"] = false
    }
);

Optionally, you might provide a CancellationToken.

Listing Event Types#

To list all event types, call the ReadEventTypesAsync method. The method returns an async stream:

await foreach (var eventType in client.ReadEventTypesAsync())
{
    // ...
}

Listing a Specific Event TypeRequires EventSourcingDB 1.1#

To list a specific event type, call the ReadEventTypeAsync method with the event type as an argument. The method returns the detailed event type, which includes the schema:

var eventType = await client.ReadEventTypeAsync("io.eventsourcingdb.library.book-acquired");

Listing Subjects#

To list all subjects, call the ReadSubjectsAsync method with / as the base subject. The method returns an async stream:

await foreach (var eventType in client.ReadSubjectsAsync("/"))
{
    // ...
}

If you only want to list subjects within a specific branch, provide the desired base subject instead:

await foreach (var eventType in client.ReadSubjectsAsync("/books"))
{
    // ...
}

Optionally, you might provide a CancellationToken.

Verifying an Event's Hash#

To verify the integrity of an event, call the VerifyHash method on the event instance. This recomputes the event's hash locally and compares it to the hash stored in the event. If the hashes differ, the function returns an error:

event.VerifyHash();

Note that this only verifies the hash. If you also want to verify the signature, you can skip this step and call VerifySignature directly, which performs a hash verification internally.

Verifying an Event's SignatureRequires EventSourcingDB 1.1#

To verify the authenticity of an event, call the VerifySignature method on the event instance. This requires the public key that matches the private key used for signing on the server.

The function first verifies the event's hash, and then checks the signature. If any verification step fails, it returns an error:

var verificationKey = // an ed25519 public key

event.VerifySignature(verificationKey);

Using Testcontainers#

Import the Container class, create an instance, call the StartAsync method to run a test container, get a client, run your test code, and finally call the StopAsync method to stop the test container:

using EventSourcingDb;

var container = new Container();
await container.StartAsync();

var client = container.GetClient();

// ...

await container.StopAsync();

Optionally, you might provide a CancellationToken to the StartAsync and StopAsync methods.

To check if the test container is running, call the IsRunning method:

var isRunning = container.IsRunning();

Configuring the Container Instance#

By default, Container uses the latest tag of the official EventSourcingDB Docker image. To change that, call the WithImageTag method:

var container = new Container()
    .WithImageTag("1.0.0");

Similarly, you can configure the port to use and the API token. Call the WithPort or the WithApiToken method respectively:

var container = new Container()
    .WithPort(4000)
    .WithApiToken("secret");

If you want to sign events, call the WithSigningKey method. This generates a new signing and verification key pair inside the container:

var container = new Container()
    .WithSigningKey();

Changed in 1.8You can retrieve the public key (for verifying signatures) once the container has been started. If the container was started without a signing key, GetVerificationKey throws an InvalidOperationException:

var verificationKey = container.GetVerificationKey();

The verificationKey can be passed to VerifySignature when verifying events read from the database.

Configuring the Client Manually#

In case you need to set up the client yourself, use the following methods to get details on the container:

• GetHost() returns the host name
• GetMappedPort() returns the port
• GetBaseUrl() returns the full URL of the container
• GetApiToken() returns the API token

Further Resources#

• Repository: thenativeweb/eventsourcingdb-client-dotnet
• Registry: NuGet
• License (MIT): License file
• Issue tracker: GitHub Issues