WebSocketeer

A thin, intuitive, idiomatic and high-performance API for Azure Web PubSub protobuf subprotocol.

What

Azure Web PubSub protobuf subprotocol is super awesome and general purpose and I can see endless applications for this new service from Azure. The message-based nature of its “API” is not very intuitive or idiomatic for a dotnet developer though, I think.

I wanted to create a super thin layer on top that didn’t incur unnecessary allocations or buffer handling or extra threads, since that would detract from the amazing work on performance that .NET 6 brings to the table. I use the best practices for sending binary payloads using low-level (and quite new!) protobuf APIs for avoiding unnecessary buffer creation/handling.

In order to also squeeze every bit of performance, this project uses the protobuf subprotocol exclusively, even though there is support in the service for JSON payloads.

The actual binary payloads you send/receive can of course be decoded into any format you need, including JSON if you just encode/decode it as UTF8 bytes.

Usage

First acquire a proper client access URI for Azure Web PubSub using the official client API, such as:

var serviceClient = new WebPubSubServiceClient([WEB_PUB_SUB_CONNECTION_STRING], [HUB_NAME]);
var serviceUri = serviceClient.GenerateClientAccessUri(
    userId: Guid.NewGuid().ToString("N"),
    roles:  new[]
    {
        "webpubsub.joinLeaveGroup",
        "webpubsub.sendToGroup"
    });

Next simply connect the WebSocketeer:

await using IWebSocketeer socketeer = WebSocketeer.ConnectAsync(serviceUri);

NOTE: the IWebSocketeer interface implements both IAsyncDisposable, which allows the await using pattern above, but also the regular IDisposable interface. The former will perform a graceful WebSocket disconnect/close. The latter will simply dispose the underlying WebSocket.

At this point, the socketeer variable contains a properly connected Web PubSub client, and you can inspect its ConnectionId and UserId properties, for example.

Next step is perhaps to join some groups:

IWebSocketeerGroup group = await socketeer.JoinAsync("MyGroup");

The IWebSocketeerGroup is an observable of ReadOnlyMemory<byte>, exposing the incoming messages to that group, and it also provides a SendAsync(ReadOnlyMemory<byte> message) method to post messages to the group.

To write all incoming messages for the group to the console, you could write:

using var subscription = group.Subscribe(bytes => 
    Console.WriteLine(Encoding.UTF8.GetString(bytes.Span)));

In order to start processing incoming messages, though, you need to start the socketeer “message loop” first. This would typically be done on a separate thread, using Task.Run, for example:

var started = Task.Run(() => socketeer.RunAsync());

The returned task from RunAsync will remain in progress until the socketeer is disposed, or the underlying WebSocket is closed (either by the client or the server), or when an optional cancellation token passed to it is cancelled.

You can also send messages to a group you haven’t joined (provided the roles specified when opening the connection allow it) via the IWebSocketeer.SendAsync method too:

await socketeer.SendAsync("YourGroup", Encoding.UTF8.GetBytes("Hello World"));

Advanced Scenarios

Accessing Joined Group

Sometimes, it’s useful to perform group join up-front, but at some later time you might also need to get the previously joined group from the same IWebSocketeer instance.

IWebSocketeer socketeer = /* connect, join some groups, etc. */;

// If group hasn't been joined previously, no incoming messages would arrive in this case.
IWebSocketeerGroup group = socketeer.Joined("incoming");
group.Subscribe(x => /* process incoming */);

Handling the WebSocket

You can alternatively handle the WebSocket yourself. Instead of passing the service Uri to ConnectAsync, you can create and connect a WebSocket manually and pass it to the WebSocketeer.ConnectAsync(WebSocket, CancellationToken) overload.

In this case, it’s important to remember to add the protobuf.webpubsub.azure.v1 required subprotocol:

using Devlooped.Net;

var client = new ClientWebSocket();
client.Options.AddSubProtocol("protobuf.webpubsub.azure.v1");

await client.ConnectAsync(serverUri, CancellationToken.None);

await using var socketeer = WebSocketeer.ConnectAsync(client);

Split Request/Response Groups

You may want to simulate request/response communication patterns over the socketeer. In cases like this, you would typically do the following:

• Server joined to a client-specific group, such as SERVER_ID-CLIENT_ID (with a [TO]-[FROM] format, so, TO=server, FROM=client)
• Server replying to requests in that group by sending responses to CLIENT_ID-SERVER_ID (TO=client, FROM=server);
• Client joined to the responses group CLIENT_ID-SERVER_ID and sending requests as needed to SERVER_ID-CLIENT_ID.

Note that the client must not join the SERVER_ID-CLIENT_ID group because otherwise it would also receive its own messages that are intended for the server only. Likewise, the server cannot join the CLIENT_ID-SERVER_ID group either. This is why this pattern might be more common than it would otherwise seem.

Server-side:

IWebSocketeer socketeer = ...;
var serverId = socketeer.UserId;

// Perhaps from an initial exchange over a shared channel
var clientId = ...;

await using IWebSocketeerGroup clientChannel = socketeer.Split(
    await socketeer.JoinAsync($"{serverId}-{clientId}"), 
    $"{clientId}-{serverId}");

clientChannel.Subscribe(async x => 
{
    // do some processing on incoming requests.
    ...
    // send a response via the outgoing group
    await clientChannel.SendAsync(response);
});

Client-side:

IWebSocketeer socketeer = ...;
var clientId = socketeer.UserId;

// Perhaps a known identifier, or looked up somehow
var serverId = ...;

await using IWebSocketeerGroup serverChannel = socketeer.Split(
    await socketeer.JoinAsync($"{clientId}-{serverId}""),
    $"{serverId}-{clientId}");

serverChannel.Subscribe(async x => /* process responses */);
await serverChannel.SendAsync(request);

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