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I am writing the server-side portion of an application in NodeJS with Express, which will use Server-Sent-Events (SSE). I have a solution that seems to work, but need to understand whether there is a better way of doing it in Node.

The relevant portion of code currently looks like this:

const headers = {
    'Content-Type': 'text/event-stream',
    'Connection': 'keep-alive',
    'Cache-Control': 'no-cache',
    'Access-Control-Allow-Origin': '*',
    'Access-Control-Allow-Credentials': 'true'
};
res.writeHead(200, headers);

// Do more stuff [...]

where res is the response object.

Instead of calling res.writeHead(), I have seen other examples use res.flushHeaders():

res.setHeader('Content-Type', 'text/event-stream');
res.setHeader('Connection', 'keep-alive');
res.setHeader('Cache-Control', 'no-cache');
res.setHeader('Access-Control-Allow-Origin', '*');
res.setHeader('Access-Control-Allow-Credentials', 'true');
res.flushHeaders();

Is there a difference between the two and is one preferable to the other, with respect to SSE and keeping a connection open to send messages?

Widor
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  • Do you get chunks of data at client side when you do res.writeHead ? Or do you get chunks of data at client side with res.flushHeaders ? – Shiv Dec 29 '20 at 09:54

2 Answers2

2

I had the same question. In addition to Walters response I found the following:

writeHead and flushHeaders are both methods expressjs uses from nodejs and you don't need to use one of those two to properly implement SSE. For passing headers you'll need writeHead or setHeader.

response.flushHeaders() is used when you need to bypass buffering the request headers. From testing, I can confirm it's not needed for SSE to work properly. So using it will be based on your specific application communication needs, not the needs of SSE.

Using response.writeHead() is a preferred alternative to using setHeader. If progressive population of headers is desired with potential future retrieval and modification, use response.setHeader() instead.

Neither writeHead or flushHeaders are used to keep the connection open. The connection will stay open by default until the client disconnects or you close it on the server with response.end() or response.send() which will call end. The exception to that is the HTTP default timeout.

Explicitly specifying a statusCode code is based on your usage but not needed (unless you use response.writeHead()). For example you can use a response code to tell a client to stop reconnecting by using a HTTP 204 No Content response code.

Note: For expressjs users using compression, make sure you call the res.flush() method (provided by the compression middleware) after your res.write() per the docs.

Here is a repo with complete example. For the server sse portion see this file.

jtlindsey
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1

They are practically equivalent because res.write() flushes the headers along with its first chunk. For SSE you're very likely needing an immediate response, at the very least to indicate successful connection. res.setHeader() can be useful if you later need to access or modify header state, but that doesn't seem relevant to SSE.

See nodejs.org documentation for response.write() and response.setHeader().

Walter Monroe
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