Chapter 14. Server-Sent Events (SSE) Support
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## Chapter 14. Server-Sent Events (SSE) Support

## 14.1. What are Server-Sent Events

In a standard HTTP request-response scenario a client opens a connection, sends a HTTP request to the server (for example a HTTP `GET` request), then receives a HTTP response back and the server closes the connection once the response is fully sent/received. The initiative always comes from a client when the client requests all the data. In contrast, Server-Sent Events (SSE) is a mechanism that allows server to asynchronously push the data from the server to the client once the client-server connection is established by the client. Once the connection is established by the client, it is the server who provides the data and decides to send it to the client whenever new "chunk" of data is available. When a new data event occurs on the server, the data event is sent by the server to the client. Thus the name Server-Sent Events. Note that at high level there are more technologies working on this principle, a short overview of the technologies supporting server-to-client communication is in this list:

Polling

  With polling a client repeatedly sends new requests to a server. If the server has no new data, then it send appropriate indication and closes the connection. The client then waits a bit and sends another request after some time (after one second, for example).

Long-polling

  With long-polling a client sends a request to a server. If the server has no new data, it just holds the connection open and waits until data is available. Once the server has data (message) for the client, it uses the connection and sends it back to the client. Then the connection is closed.

Server-Sent events

  SSE is similar to the long-polling mechanism, except it does not send only one message per connection. The client sends a request and server holds a connection until a new message is ready, then it sends the message back to the client while still keeping the connection open so that it can be used for another message once it becomes available. Once a new message is ready, it is sent back to the client on the same initial connection. Client processes the messages sent back from the server individually without closing the connection after processing each message. So, SSE typically reuses one connection for more messages (called events). SSE also defines a dedicated media type that describes a simple format of individual evnets sent from the server to the client. SSE also offers standard javascript client API implemented most modern browsers. For more information about SSE, see the SSE API specification.

WebSocket

  WebSocket technology is different from previous technologies as it provides a real full duplex connection. The initiator is again a client which sends a request to a server with a special HTTP header that informs the server that the HTTP connection may be "upgraded" to a full duplex TCP/IP WebSocket connection. If server supports WebSocket, it may choose to do so. Once a WebSocket connection is established, it can be used for bi-directional communication between the client and the server. Both client and server can then send data to the other party at will whenever it is needed. The communication on the new WebSocket connection is no longer based on HTTP protocol and can be used for example for for online gaming or any other applications that require fast exchange of small chunks of data in flowing in both directions.

## 14.2. When to use Server-Sent Events

As explained above, SSE is a technology that allows clients to subscribe to event notifications that originate on a server. Server generates new events and sends these events back to the clients subscribed to receive the notifications. In other words, SSE offers a solution for a one-way publish-subscribe model.

A good example of the use case where SSE can be used is a simple message exchange RESTful service. Clients `POST` new messages to the service and subscribe to receive messages from other clients. Let's call the resource `messages`. While `POST`ing a new message to this resource involves a typical HTTP request-response communication between a client and the `messages` resource, subscribing to receive all new message notifications would be hard and impractical to model with a sequence of standard request-response message exchanges. Using Server-sent events provides a much more practical approach here. You can use SSE to let clients subscribe to the `messages` resource via standard `GET` request (use a SSE client API, for example javascript API or Jersey Client SSE API) and let the server broadcast new messages to all connected clients in the form of individual events (in our case using Jersey Server SSE API). Note that with Jersey a SSE support is implemented as an usual JAX-RS resource method. There's no need to do anything special to provide a SSE support in your Jersey/JAX-RS applications, your SSE-enabled resources are a standard part of your RESTful Web application that defines the REST API of your application. The following chapters describes SSE support in Jersey in more details.

### Important

Note, that while SSE in Jersey is supported with standard JAX-RS resources, Jersey SSE APIs are not part of the JAX-RS specification. SSE support and related APIs are a Jersey specific feature that extends JAX-RS.

## 14.3. Jersey Server-Sent Events API

This chapter briefly describes the Jersey support for SSE. Details and examples will be covered in chapters below.

Jersey contains support for SSE for both - server and client. SSE in Jersey is implemented as an extension supporting a new media type, which means that SSE really treated as just another media type that can be returned from a resource method and processed by the client. There is only a minimal additional support for "chunked" messages added to Jersey which could not be implemented as standard JAX-RS media type extension.

Before you start working with Jersey SSE, in order to add support for SSE you need to include the dependency to the SSE media type module:

Example 14.1. Add `jersey-media-sse` dependency.

    <dependency>
        <groupId>org.glassfish.jersey.media</groupId>
        <artifactId>jersey-media-sse</artifactId>
    </dependency>

### Note

Prior to Jersey 2.8, you had to manually register SseFeature in your application. (The `SseFeature` is a feature that can be registered for both, the client and the server.) Since Jersey 2.8, the feature gets automatically discovered and registered when Jersey SSE module is put on the application's classpath. The automatic discovery and registration of SSE feature can be suppressed by setting DISABLE_SSE property to `true`. The behavior can also be selectively suppressed in either client or server runtime by setting DISABLE_SSE_CLIENT or DISABLE_SSE_SERVER property respectively.

`SseFeature` adds new supported entity (representation) Java types, namely OutboundEvent for the outbound server events and InboundEvent for inbound client events. These types are serialized by `OutboundEventWriter` and de-serialized by `InboundEventReader`. There is no restriction for a media type used in individual event messages; however the media type used for a SSE stream as whole is "text/event-stream" and this media type should be set on messages that are used to serve SSE events (for example on the server side using @Produces on the method that returns an `EventOutput` - see below). The `InboundEvent` and `OutboundEvent` contain all the fields needed for composing and processing individual SSE events. These entities represent the chunks sent or received over an open server-to-client connection that is represented by an ChunkedOutput on the servers side and ChunkedInput on the client side (if you are not familiar with `ChunkedOutput` and `ChunkedInput`, see the Async chapter first for more details). In other words, our resource method that is used to open a SSE connection to a client does not return individual `OutboundEvent`s. Instead, a new instance of EventOutput is returned. `EventOutput` is a typed extension of `ChunkedOutput<OutboundEvent>`. Similarly, to receive `InboundEvent`s on a client side, Jersey SSE API provides a `EventInput` that represents a typed extension of `ChunkedInput<InboundEvent>`.

The Jersey server SSE API also contains a SseBroadcaster utility, that provides a convenient way of grouping multiple `EventOutput` instances that represent individual client connections into a group, and exposes methods for broadcasting new events to all the client connections grouped in the broadcaster. The `SseBroadcaster` inherits from Broadcaster which is the generic broadcaster implementation of the Jersey chunked message processing facility. On the he client side, the Jersey SSE API contains additional EventSource and EventListener classes that further improve convenience of processing new inbound SSE events.

## 14.4. Implementing SSE support in a JAX-RS resource

### 14.4.1. Simple SSE resource method

Firstly you need to add a Jersey SSE module dependency to your project as shown in the earlier section and register the SseFeature from this module in your Application or ResourceConfig. Once done, you are ready to add SSE support to your resource:

Example 14.2. Simple SSE resource method

    ...
    import org.glassfish.jersey.media.sse.EventOutput;
    import org.glassfish.jersey.media.sse.OutboundEvent;
    import org.glassfish.jersey.media.sse.SseFeature;
    ...

    @Path("events")
    public static class SseResource {

        @GET
        @Produces(SseFeature.SERVER_SENT_EVENTS)
        public EventOutput getServerSentEvents() {
            final EventOutput eventOutput = new EventOutput();
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        for (int i = 0; i < 10; i++) {
                            // ... code that waits 1 second
                            final OutboundEvent.Builder eventBuilder
                            = new OutboundEvent.Builder();
                            eventBuilder.name("message-to-client");
                            eventBuilder.data(String.class,
                                "Hello world " + i + "!");
                            final OutboundEvent event = eventBuilder.build();
                            eventOutput.write(event);
                        }
                    } catch (IOException e) {
                        throw new RuntimeException(
                            "Error when writing the event.", e);
                    } finally {
                        try {
                            eventOutput.close();
                        } catch (IOException ioClose) {
                            throw new RuntimeException(
                                "Error when closing the event output.", ioClose);
                        }
                    }
                }
            }).start();
            return eventOutput;
        }
    }

The code above defines the resource deployed on URI `"/events"`. This resource has a single @GET resource method which returns as an entity EventOutput - an extension of generic Jersey ChunkedOutput API for output chunked message processing.

### Note

If you are not familiar with `ChunkedOutput` and `ChunkedInput`, see the Async chapter first for more details.

After the `eventOutput` is returned from the method, the Jersey runtime recognizes that this is a `ChunkedOutput` extension and does not close the client connection immediately. Instead, it writes the HTTP headers to the response stream and waits for more chunks (SSE events) to be sent. At this point the client can read headers and starts listening for individual events.

### Note

Since Jersey runtime does not implicitly close the connection to the client (similarly to asynchronous processing), closing the connection is a responsibility of the resource method or the client listening on the open connection for new events (see following example).

In the Example 14.2, “Simple SSE resource method”, the resource method creates a new thread that sends a sequence of 10 events. There is a 1 second delay between two subsequent events as indicated in a comment. Each event is represented by `OutboundEvent` type and is built with a helpf of an outbound event `Builder`. The `OutboundEvent` reflects the standardized format of SSE messages and contains properties that represent `name` (for named events), `comment`, `data` or `id`. The code also sets the event data media type using the `mediaType(MediaType)` method on the `eventBuilder`. The media type, together with the data type set by the `data(Class, Object>` method (in our case `String.class`), is used for serialization of the event data. Note that the event data media type will not be written to any headers as the response `Content-type` header is already defined by the `@Produces` and set to `"text/event-stream"` using constant from the `SseFeature`. The event media type is used for serialization of event `data`. Event data media type and Java type are used to select the proper MessageBodyWriter<T> for event data serialization and are passed to the selected writer that serializes the event `data` content. In our case the string `"Hello world " + i + "!"` is serialized as `"text/plain"`. In event `data` you can send any Java entity and associate it with any media type that you would be able to serialize with an available `MessageBodyWriter<T>`. Typically, you may want to send e.g. JSON data, so you would fill the `data` with a JAXB annotated bean instance and define media type to JSON.

### Note

If the event media type is not set explicitly, the `"text/plain"` media type is used by default.

Once an outbound event is ready, it can be written to the `eventOutput`. At that point the event is serialized by internal `OutboundEventWriter` which uses an appropriate `MessageBodyWriter<T>` to serialize the `"Hello world " + i + "!"` string. You can send as many messages as you like. At the end of the thread execution the response is closed which also closes the connection to the client. After that, no more messages can be send to the client on this connection. If the client would like to receive more messages, it would have to send a new request to the server to initiate a new SSE streaming connection.

A client connecting to our SSE-enabled resource will receive the following data from the entity stream:

    event: message-to-client
    data: Hello world 0!

    event: message-to-client
    data: Hello world 1!

    event: message-to-client
    data: Hello world 2!

    event: message-to-client
    data: Hello world 3!

    event: message-to-client
    data: Hello world 4!

    event: message-to-client
    data: Hello world 5!

    event: message-to-client
    data: Hello world 6!

    event: message-to-client
    data: Hello world 7!

    event: message-to-client
    data: Hello world 8!

    event: message-to-client
    data: Hello world 9!

Each message is received with a delay of one second.

### Note

If you have worked with streams in JAX-RS, you may wonder what is the difference between ChunkedOutput and StreamingOutput.

`ChunkedOutput` is Jersey-specific API. It lets you send "chunks" of data without closing the client connection using series of convenient calls to `ChunkedOutput.write` methods that take POJO + chunk media type as an input and then use the configured JAX-RS `MessageBodyWriter<T>` providers to figure out the proper way of serializing each chunk POJO to bytes. Additionally, `ChunkedOutput` writes can be invoked multiple times on the same outbound response connection, i.e. individual chunks are written in each write, not the full response entity.

`StreamingOutput` is, on the other hand, a low level JAX-RS API that works with bytes directly. You have to implement `StreamingOutput` interface yourself. Also, its `write(OutputStream)` method will be invoked by JAX-RS runtime only once per response and the call to this method is blocking, i.e. the method is expected to write the entire entity body before returning.

### 14.4.2. Broadcasting with Jersey SSE

Jersey SSE server API defines SseBroadcaster which allows to broadcast individual events to multiple clients. A simple broadcasting implementation is shown in the following example:

Example 14.3. Broadcasting SSE messages

    ...
    import org.glassfish.jersey.media.sse.SseBroadcaster;
    ...

    @Singleton
    @Path("broadcast")
    public static class BroadcasterResource {

        private SseBroadcaster broadcaster = new SseBroadcaster();

        @POST
        @Produces(MediaType.TEXT_PLAIN)
        @Consumes(MediaType.TEXT_PLAIN)
        public String broadcastMessage(String message) {
            OutboundEvent.Builder eventBuilder = new OutboundEvent.Builder();
            OutboundEvent event = eventBuilder.name("message")
                .mediaType(MediaType.TEXT_PLAIN_TYPE)
                .data(String.class, message)
                .build();

            broadcaster.broadcast(event);

            return "Message '" + message + "' has been broadcast.";
        }

        @GET
        @Produces(SseFeature.SERVER_SENT_EVENTS)
        public EventOutput listenToBroadcast() {
            final EventOutput eventOutput = new EventOutput();
            this.broadcaster.add(eventOutput);
            return eventOutput;
        }
    }

Let's explore the example together. The `BroadcasterResource` resource class is annotated with @Singleton annotation which tells Jersey runtime that only a single instance of the resource class should be used to serve all the incoming requests to `/broadcast` path. This is needed as we want to keep an application-wide single reference to the private `broadcaster` field so that we can use the same instance for all requests. Clients that want to listen to SSE events first send a `GET` request to the `BroadcasterResource`, that is handled by the `listenToBroadcast()` resource method. The method creates a new `EventOutput` representing the connection to the requesting client and registers this `eventOutput` instance with the singleton `broadcaster`, using its `add(EventOutput)` method. The method then returns the `eventOutput` which causes Jersey to bind the `eventOutput` instance with the requesting client and send the response HTTP headers to the client. The client connection remains open and the client is now waiting ready to receive new SSE events. All the events are written to the `eventOutput` by `broadcaster` later on. This way developers can conveniently handle sending new events to all the clients that subscribe to them.

When a client wants to broadcast new message to all the clients listening on their SSE connections, it sends a `POST` request to `BroadcasterResource` resource with the message content. The method `broadcastMessage(String)` is invoked on `BroadcasterResource` resource with the message content as an input parameter. A new SSE outbound event is built in the standard way and passed to the broadcaster. The broadcaster internally invokes `write(OutboundEvent)` on all registered `EventOutput`s. After that the method just return a standard text response to the `POST`ing client to inform the client that the message was successfully broadcast. As you can see, the `broadcastMessage(String)` resource method is just a simple JAX-RS resource method.

In order to implement such a scenario, you may have noticed, that the Jersey `SseBroadcaster` is not mandatory to complete the use case. individual EventOutputs can be just stored in a collection and iterated over in the `broadcastMessage` method. However, the `SseBroadcaster` internally identifies and handles also client disconnects. When a client closes the connection the broadcaster detects this and removes the stale connection from the internal collection of the registered EventOutputs as well as it frees all the server-side resources associated with the stale connection. Additionally, the `SseBroadcaster` is implemented to be thread-safe, so that clients can connect and disconnect in any time and `SseBroadcaster` will always broadcast messages to the most recent collection of registered and active set of clients.

## 14.5. Consuming SSE events with Jersey clients

On the client side, Jersey exposes APIs that support receiving and processing SSE events using two programming models:

Pull model - pulling events from a EventInput, or
Push model - listening for asynchronous notifications of `EventSource`

Both models will be described.

### 14.5.1. Reading SSE events with `EventInput`

The events can be read on the client side from a EventInput. See the following code:

    Client client = ClientBuilder.newBuilder()
            .register(SseFeature.class).build();
    WebTarget target = client.target("http://localhost:9998/events");

    EventInput eventInput = target.request().get(EventInput.class);
    while (!eventInput.isClosed()) {
        final InboundEvent inboundEvent = eventInput.read();
        if (inboundEvent == null) {
            // connection has been closed
            break;
        }
        System.out.println(inboundEvent.getName() + "; "
            + inboundEvent.readData(String.class));
    }

In this example, a client connects to the server where the `SseResource` from the Example 14.2, “Simple SSE resource method” is deployed. At first, a new JAX-RS/Jersey `client` instance is created with a `SseFeature` registered. Then a WebTarget instance is retrieved from the `client` and is used to invoke a HTTP request. The returned response entity is directly read as a `EventInput` Java type, which is an extension of Jersey `ChunkedInput` that provides generic support for consuming chunked message payloads. The code in the example then process starts a loop to process the inbound SSE events read from the `eventInput` response stream. Each chunk read from the input is a `InboundEvent`. The method `InboundEvent.readData(Class)` provides a way for the client to indicate what Java type should be used for the event data de-serialization. In our example, individual events are de-serialized as `String` Java type instances. This method internally finds and executes a proper MessageBodyReader<T> which is the used to do the actual de-serialization. This is similar to reading an entity from the Response by `readEntity(Class)`. The method `readData` can also throw a ProcessingException.

The `null` check on `inboundEvent` is necessary to make sure that the chunk was properly read and connection has not been closed by the server. Once the connection is closed, the loop terminates and the program completes execution. The client code produces the following console output:

    message-to-client; Hello world 0!
    message-to-client; Hello world 1!
    message-to-client; Hello world 2!
    message-to-client; Hello world 3!
    message-to-client; Hello world 4!
    message-to-client; Hello world 5!
    message-to-client; Hello world 6!
    message-to-client; Hello world 7!
    message-to-client; Hello world 8!
    message-to-client; Hello world 9!

### 14.5.2. Asynchronous SSE processing with `EventSource`

The main Jersey SSE client API component used to read SSE events asynchronously is EventSource. The usage of the `EventSource` is shown on the following example.

Example 14.4. Registering `EventListener` with `EventSource`

    Client client = ClientBuilder.newBuilder()
                            .register(SseFeature.class).build();
    WebTarget target = client.target("http://example.com/events");
    EventSource eventSource = EventSource.target(target).build();
    EventListener listener = new EventListener() {
            @Override
            public void onEvent(InboundEvent inboundEvent) {
                System.out.println(inboundEvent.getName() + "; "
                        + inboundEvent.readData(String.class));
            }
        };
    eventSource.register(listener, "message-to-client");
    eventSource.open();
    ...
    eventSource.close();

In this example, the client code again connects to the server where the `SseResource` from the Example 14.2, “Simple SSE resource method” is deployed. The Client instance is again created and initialized with SseFeature. Then the WebTarget is built. In this case a request to the web target is not made directly in the code, instead, the web target instance is used to initialize a new EventSource.Builder instance that is used to build a new `EventSource`. The choice of `build()` method is important, as it tells the `EventSource.Builder` to create a new `EventSource` that is not automatically connected to the `target`. The connection is established only later by manually invoking the `eventSource.open()` method. A custom EventListener implementation is used to listen to and process incoming SSE events. The method readData(Class) says that the event data should be de-serialized from a received InboundEvent instance into a `String` Java type. This method call internally executes MessageBodyReader<T> which de-serializes the event data. This is similar to reading an entity from the Response by `readEntity(Class)`. The method `readData` can throw a ProcessingException.

The custom event source listener is registered in the event source via `EventSource``.register(EventListener, String)` method. The next method arguments define the names of the events to receive and can be omitted. If names are defined, the listener will be associated with the named events and will only invoked for events with a name from the set of defined event names. It will not be invoked for events with any other name or for events without a name.

### Important

It is a common mistake to think that unnamed events will be processed by listeners that are registered to process events from a particular name set. That is NOT the case! Unnamed events are only processed by listeners that are not name-bound. The same limitation applied to HTML5 Javascript SSE Client API supported by modern browsers.

After a connection to the server is opened by calling the `open()` method on the event source, the `eventSource` starts listening to events. When an event named `"message-to-client"` comes, the listener will be executed by the event source. If any other event comes (with a name different from `"message-to-client"`), the registered listener is not invoked. Once the client is done with processing and does not want to receive events anymore, it closes the connection by calling the `close()` method on the event source.

The listener from the example above will print the following output:

    message-to-client; Hello world 0!
    message-to-client; Hello world 1!
    message-to-client; Hello world 2!
    message-to-client; Hello world 3!
    message-to-client; Hello world 4!
    message-to-client; Hello world 5!
    message-to-client; Hello world 6!
    message-to-client; Hello world 7!
    message-to-client; Hello world 8!
    message-to-client; Hello world 9!

When browsing through the Jersey SSE API documentation, you may have noticed that the EventSource implements EventListener and provides an empty implementation for the `onEvent(InboundEvent inboundEvent)` listener method. This adds more flexibility to the Jersey client-side SSE API. Instead of defining and registering a separate event listener, in simple scenarios you can also choose to derive directly from the `EventSource` and override the empty listener method to handle the incoming events. This programming model is shown in the following example:

Example 14.5. Overriding `EventSource.onEvent(InboundEvent)` method

    Client client = ClientBuilder.newBuilder()
                            .register(SseFeature.class).build();
    WebTarget target = client.target("http://example.com/events");
    EventSource eventSource = new EventSource(target) {
        @Override
        public void onEvent(InboundEvent inboundEvent) {
            if ("message-to-client".equals(inboundEvent.getName())) {
                System.out.println(inboundEvent.getName() + "; "
                        + inboundEvent.readData(String.class));
            }
        }
    };
    ...
    eventSource.close();

The code above is very similar to the code in Example 14.4, “Registering `EventListener` with `EventSource`”. In this example however, the `EventSource` is constructed directly using a single-parameter constructor. This way, the connection to the SSE endpoint is by default automatically opened at the event source creation. The implementation of the `EventListener` has been moved into the overridden `EventSource.onEvent(...)` method. However, this time, the listener method will be executed for all events - unnamed as well as with any `name`. Therefore the code checks the name whether it is an event with the name "message-to-client" that we want to handle. Note that you can still register additional `EventListener`s later on. The overridden method on the event source allows you to handle messages even when no additional listeners are registered yet.

#### 14.5.2.1. `EventSource` reconnect support

Starting in Jersey 2.3, the `EventSource` implementation supports automated recuperation from a connection loss, including negotiation of delivery of any missed events based on the last received SSE event `id` field value, provided this field is set by the server and the negotiation facility is supported by the server. In case of a connection loss, the last received SSE event `id` field value is send in the `Last-Event-ID` HTTP request header as part of a new connection request sent to the SSE endpoint. Upon a receipt of such reconnect request, the SSE endpoint that supports this negotiation facility is expected to replay all missed events.

### Note

Note, that SSE lost event negotiation facility is a best-effort mechanism which does not provide any guaranty that all events would be delivered without a loss. You should therefore not rely on receiving every single event and design your client application code accordingly.

By default, when a connection the the SSE endpoint is lost, the event source will use a default delay before attempting to reconnect to the SSE endpoint. The SSE endpoint can however control the client-side retry delay by including a special `retry` field value in the any send event. Jersey `EventSource` implementation automatically tracks any received SSE event `retry` field values set by the endpoint and adjusts the reconnect delay accordingly, using the last received `retry` field value as the new reconnect delay.

In addition to handling the standard connection losses, Jersey `EventSource` automatically deals with any `HTTP 503 Service Unavailable` responses received from the SSE endpoint, that include a `Retry-After` HTTP header with a valid value. The `HTTP 503 + Retry-After` technique is often used by HTTP endpoints as a means of connection and traffic throttling. In case a `HTTP 503 + Retry-After` response is received in return to a connection request from SSE endpoint, Jersey `EventSource` will automatically schedule a reconnect attempt and use the received `Retry-After` HTTP header value as a one-time override of the reconnect delay.

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