# Server-Sent Events (SSE)

Pipedream supports Server-sent events (opens new window) (SSE) as a destination, enabling you to send events from a workflow directly to a client subscribed to the event stream.

# What is SSE?

Server-sent Events (opens new window) (SSE) is a specification that allows servers to send events directly to clients that subscribe to those events, similar to WebSockets (opens new window) and related server to client push technologies.

Unlike WebSockets, SSE enables one-way communication from server to clients (WebSockets enable bidirectional communication between server and client, allowing you to pass messages back and forth). Luckily, if you only need a client to subscribe to events from a server, and don't require bidirectional communication, SSE is simple way to make that happen.

# What can I do with the SSE destination?

SSE is typically used by web developers to update a webpage with new events in real-time, without forcing a user to reload a page to fetch new data. If you'd like to update data on a webpage in that manner, you can subscribe to your workflow's event stream and handle new events as they come in.

Beyond web browsers, any program that's able to create an EventSource interface (opens new window) can listen for server-sent events delivered from Pipedream. You can run a Node.js script or a Ruby on Rails app that receives server-sent events, for example.

# Sending data to an SSE Destination in workflows

You can send data to an SSE Destination in Node.js code steps using the $.send.sse() function.

Add a new step to your workflow, then search for "Code":

Then add this code to that step:

defineComponent({
  async run({ steps, $ }) {
    $.send.sse({
      channel: "events",
      payload: {
        name: "Luke Skywalker"
      }
    });
  })
});

See this workflow (opens new window) for an example of how to use $.send.sse().

Send a test event to your workflow, then review the section on Receiving events to see how you can setup an EventSource to retrieve events sent to the SSE Destination.

$.send.sse() accepts an object with the following properties:

defineComponent({
  async run({ steps, $ }) {
    $.send.sse({
      channel, // Required, corresponds to the event in the SSE spec
      payload // Required, the event payload
    });
  })
});

Again, it's important to remember that Destination delivery is asynchronous. If you iterate over an array of values and send an SSE for each:

defineComponent({
  async run({ steps, $ }) {
    const names = ["Luke", "Han", "Leia", "Obi Wan"];
    names.forEach(name => {
      $.send.sse({
        channel: "names",
        payload: {
          name
        }
      });
    });
  })
});

you won't have to await the execution of the SSE Destination requests in your workflow. We'll collect every $.send.sse() call and defer those requests, sending them after your workflow finishes.

# Using $.send.sse in component actions

If you're authoring a component action, you can send events to an SSE destination using $.send.sse.

$.send.sse functions the same as $.send.sse in workflow code steps:

defineComponent({
  async run({ steps, $ }) {
    $.send.sse({
      channel: "events",
      payload: {
        name: "Luke Skywalker"
      }
    });
  })
});

# Receiving events

Once you've sent events to an SSE Destination, you can start receiving a stream of those events in a client by configuring an EventSource (opens new window) that connects to the Pipedream SSE stream.

# Retrieving your workflow's event stream URL

First, it's important to note that all events sent to an SSE destination within a workflow are sent to an SSE event stream specific to that workflow. The event stream is tied to the workflow's ID, which you can find by examining the URL of the pipeline in the Pipedream UI. For example, the p_aBcDeF in this URL is the pipeline ID:

New code step

Note that the p_ prefix is part of the workflow ID.

Once you have the workflow ID, you can construct the event source URL for your SSE destination. That URL is of the following format:

http://sdk.m.pipedream.net/pipelines/[YOUR WORKFLOW ID]/sse

In the example above, the URL of our event stream would be:

http://sdk.m.pipedream.net/pipelines/p_aBcDeF/sse

You should be able to open that URL in your browser. Most modern browsers support connecting to an event stream directly, and will stream events without any work on your part to help you confirm that the stream is working.

If you've already sent events to your SSE destination, you should see those events here! We'll return the most recent 100 events delivered to the corresponding SSE destination immediately. This allows your client to catch up with events previously sent to the destination. Then, any new events sent to the SSE destination while you're connected will be delivered to the client.

# Sample code to connect to your event stream

It's easy to setup a simple webpage to console.log() all events from an event stream. You can find a lot more examples of how to work with SSE on the web, but this should help you understand the basic concepts.

You'll need to create two files in the same directory on your machine: an index.html file for the HTML, and an sse.js file to keep the JavaScript that connects to the event stream.

index.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <title>SSE test</title>
    <script src="sse.js"></script>
  </head>
  <body>
    <div id="app"></div>
  </body>
</html>

sse.js

const eventSource = new EventSource(
  "http://sdk.m.pipedream.net/pipelines/[YOUR WORKFLOW ID]/sse"
);

eventSource.addEventListener("[YOUR CHANNEL]", function(e) {
  console.log("New event from cron test event stream: ", e);
});

Make sure to add your workflow ID and the name of your channel you specified in your SSE Destination. Then, open the index.html page in your browser. In your browser's developer tools JavaScript console, you should see new events appear as you send them.

Note that the addEventListener code will listen specifically for events sent to the cron_test channel specified in our SSE destination. You can listen for multiple types of events at once by adding multiple event listeners on the client.

Try triggering more test events from your workflow while this page is open to see how this works end-to-end.

# :keepalive messages

The SSE spec (opens new window) notes that

Legacy proxy servers are known to, in certain cases, drop HTTP connections after a short timeout. To protect against such proxy servers, authors can include a comment line (one starting with a ':' character) every 15 seconds or so.

Roughly every 15 seconds, we'll send a message with the :keepalive comment to keep open SSE connections alive. These comments should be ignored when you're listening for messages using the EventSource interface.