> ## Documentation Index > Fetch the complete documentation index at: https://mintlify.com/ably/docs/llms.txt > Use this file to discover all available pages before exploring further. # Server-Sent Events (SSE) API The Ably Server-Sent Events (SSE) API provides a simple way to receive real-time messages over HTTP using the [Server-Sent Events](https://www.w3.org/TR/eventsource/) standard. This is useful for browser clients that support SSE and scenarios where the full Ably client library SDK is not suitable. ## Overview The SSE API enables you to: * Receive real-time messages over HTTP * Subscribe to one or more channels * Use standard SSE libraries and the browser EventSource API * Automatically reconnect on connection loss ## When to Use SSE The SSE API is ideal for: * Browser applications that need simple real-time updates * Environments with limited WebSocket support * Read-only real-time data consumption * Lightweight integrations For full bidirectional communication and advanced features, use the [Realtime SDK](/docs/api/realtime-sdk). ## Getting Started ### Basic Usage Connect to the SSE endpoint and subscribe to a channel:


  ```javascript theme={null}
  const eventSource = new EventSource(
    'https://realtime.ably.io/sse?channels=my-channel&v=1.2&key={{API_KEY}}'
  );

  eventSource.onmessage = (event) => {
    const message = JSON.parse(event.data);
    console.log('Message:', message);
  };
  ```

  ```python theme={null}
  import sseclient
  import requests

  url = 'https://realtime.ably.io/sse?channels=my-channel&v=1.2&key={{API_KEY}}'
  response = requests.get(url, stream=True)
  client = sseclient.SSEClient(response)

  for event in client.events():
      message = json.loads(event.data)
      print('Message:', message)
  ```

## Endpoint The SSE endpoint is:


  ```text theme={null}
  GET https://realtime.ably.io/sse
  ```

### Parameters One or more channel names, separated by commas. Non-URL-safe characters should be URL-encoded. Example: `channels=channel1,channel2` API version. Use `v=1.2` for the current version. Your Ably API key for Basic authentication. An Ably auth token for Token authentication. Event ID to resume from when reconnecting. Number of recent messages to retrieve on connection. Maximum 100. Example: `rewind=10` retrieves the last 10 messages. If `true`, message data is wrapped in a Message object. If `false`, returns raw message payload. If `true`, sends explicit heartbeat events. If `false`, uses newlines as keepalive. Custom separator for channel names (useful when channel names contain commas). Example: `separator=|&channels=ch1|ch2` ## Message Format Messages are delivered as SSE events:


  ```text theme={null}
  id: cbfKayrzgAXDWM:1556806691343-0
  event: message
  data: {
    "id":"YqigX7VFsR:0:0",
    "name":"event-name",
    "timestamp":1556806691341,
    "channel":"my-channel",
    "data":"{\"foo\":1}"
  }
  ```

### Event Types The SSE API sends the following event types: A message published to the channel. A presence event on the channel. An error occurred (for example, token expired). Keepalive heartbeat (when `heartbeats=true`). ## Connection Management ### Automatic Reconnection The browser EventSource API automatically reconnects on connection loss. Use the `lastEvent` parameter to resume from the last received message:


  ```javascript theme={null}
  const eventSource = new EventSource(
    'https://realtime.ably.io/sse?channels=my-channel&v=1.2&key={{API_KEY}}'
  );

  let lastEventId;

  eventSource.onmessage = (event) => {
    lastEventId = event.lastEventId;
    // Process message
  };

  eventSource.onerror = () => {
    eventSource.close();
    // Reconnect with lastEventId
    const newSource = new EventSource(
      `https://realtime.ably.io/sse?channels=my-channel&v=1.2&key={{API_KEY}}&lastEvent=${lastEventId}`
    );
  };
  ```

### Keepalive The SSE connection sends keepalive packets to maintain the connection: * By default, newline characters (`\n`) are sent * With `heartbeats=true`, explicit heartbeat events are sent ## Authentication ### Basic Authentication Use your API key in the URL:


  ```javascript theme={null}
  const eventSource = new EventSource(
    'https://realtime.ably.io/sse?channels=my-channel&v=1.2&key={{API_KEY}}'
  );
  ```

### Token Authentication Use an Ably Token:


  ```javascript theme={null}
  const eventSource = new EventSource(
    'https://realtime.ably.io/sse?channels=my-channel&v=1.2&accessToken={{TOKEN}}'
  );
  ```

## Examples ### Subscribe to Multiple Channels


  ```javascript theme={null}
  const eventSource = new EventSource(
    'https://realtime.ably.io/sse?channels=channel1,channel2,channel3&v=1.2&key={{API_KEY}}'
  );

  eventSource.onmessage = (event) => {
    const message = JSON.parse(event.data);
    console.log(`Channel: ${message.channel}, Data: ${message.data}`);
  };
  ```

### Retrieve Recent Messages


  ```javascript theme={null}
  const eventSource = new EventSource(
    'https://realtime.ably.io/sse?channels=my-channel&v=1.2&key={{API_KEY}}&rewind=10'
  );

  eventSource.onmessage = (event) => {
    const message = JSON.parse(event.data);
    console.log('Message:', message);
  };
  ```

### Handle Errors


  ```javascript theme={null}
  const eventSource = new EventSource(
    'https://realtime.ably.io/sse?channels=my-channel&v=1.2&key={{API_KEY}}'
  );

  eventSource.addEventListener('error', (event) => {
    if (event.data) {
      const error = JSON.parse(event.data);
      console.error('Error:', error.message);
    }
  });
  ```

## Limitations ### Read-Only The SSE API is read-only. To publish messages, use: * [REST API](/docs/api/rest-api) * [REST SDK](/docs/api/rest-sdk) * [Realtime SDK](/docs/api/realtime-sdk) ### No Presence While you can receive presence events, you cannot enter presence using the SSE API. Use the [Realtime SDK](/docs/api/realtime-sdk) for full presence functionality. ### Connection Limits Browsers typically limit concurrent SSE connections to 6 per domain. ## Related Resources * [Realtime SDK](/docs/api/realtime-sdk) * [REST API](/docs/api/rest-api) * [EventSource API (MDN)](https://developer.mozilla.org/en-US/docs/Web/API/EventSource) * [Server-Sent Events Specification](https://www.w3.org/TR/eventsource/)