Support for Sticky Session

Questions / Help
1

Hi,

I’m currently running my app that requires session affinity as described here:

This works very well but I’m curious if there are any plans to support sticky session in the same region eventually.

Thanks, Simon

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2

They said it was on their todo list almost 4 years ago, so I wouldn’t hold your breath.

You could use fly-force-instance-id to force a client to connect to a specific instance.

You’ll need to set the instance-id in your response header via some middleware, then in your client, wrap all API calls to set this header, then reupdate the value in case one of the instance no longer exists and was rerouted.

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3

Thanks for your answer.
I can intercept the call on the server but I’m unfortunatly not able to change anything on the clien because I’m using a framework.

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4

What framework is that? Unless it’s doing some wizardry, you can override the window.fetch and wrap it w/ the extra logic.

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5
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6

Would you be able to set a cookie with the value of process.env.FLY_MACHINE_ID, and then reply with a Fly-Replay header if you get a request where the machine id does not match?

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7

There’s probably a way to do so, everything uses fetch. Look at their docs.

8

I wrote up a blueprint describing both approaches:

3 Likes
9

In the redirect if block, do you need to set the Location headers? Or will excluding it tell the client to redirect to same URL?

10

Short answer: no; that example is complete as is.

Longer answer: the way this works is that a request comes into an edge machine (we have slightly more regions that can receive requests than those that can host a machine). From there the request is routed to the nearest available machine that you have running.

If that machine responds with a Fly-Replay header, the original request complete with original headers is sent to the machine indicated by the Fly-Replay header. Any response from that machine is sent directly to the original client.

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11

To be fair, it’s still on the todo list. :smiley:

We’re working on some improvements to the proxy right now that might make this easier to implement.

One hack you can use for now is to run one instance of your app in 2+ regions. That will keep people “pinned” to the region closest to them, and failover gracefully if something goes wrong.

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12

The hack is what the link in my initial question describes :wink:

13

You’d think I’d know our own docs, haha.

14

And to finally answer my initial question I take your statement above

15

To be fair, that’s a brand new doc @kurt.

@simasch here’s how you achieve this with Meteor specifically:

click for code 
import { WebApp } from 'meteor/webapp';
import cookieParser from "cookie-parser";


// Middleware to parse cookies
WebApp.connectHandlers.use(cookieParser());

// Make sessions sticky
WebApp.connectHandlers.use('/',(request, response, next) => {
  if (!process.env.FLY_MACHINE_ID) {
    next();
  } else if (!request.cookies["fly-machine-id"]) {
    const maxAge = 24 * 60 * 60 * 1000; // 24 hours
    response.cookie("fly-machine-id", process.env.FLY_MACHINE_ID, { maxAge });
    next();
  } else if (request.cookies["fly-machine-id"] !== process.env.FLY_MACHINE_ID) {
    response.set('Fly-Replay', `instance=${request.cookies["fly-machine-id"]}`)
    response.status(307)
    response.send()
  } else {
    next();
  }
});

Import that server-side and you should be off to the races. It’s working in the demo app. If you inspect the cookies (or visit the /hello endpoint) you’ll see the Machine ID that you’re pinned to.

Edit: not sure if OP is actually using Meteor, but the solution here should be adaptable to any code base.

16

I don’t use Meteor but have a stateful application

17

This works great but there might be an edge case that fly should consider addressing:
if a machine that was set on the user’s cookie is destroyed, the request from that user will hang indefinitely. It looks like Fly replay isn’t making sure that the machine exists before replaying the request.

After about a minute or so, the request errors w/ 503. I’m guessing the Fly edge should also also expire the cookie, but that cookie key is user defined, so it’s not as straightforward.

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18

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