# Problem routing to correct instance via fly-force-instance-id

**URL:** <https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941>\
**Category:** General\
**Tags:** troubleshooting, machines, streams, proxy, login-only\
**Created:** [December 6, 2024, 12:28pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941 "2024-12-06T12:28:04Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![Panuchka](https://sea1.discourse-cdn.com/fly1/user_avatar/community.fly.io/panuchka/32/40778_2.png) [@Panuchka](https://community.fly.io/u/Panuchka)\
**Post date:** [December 6, 2024, 12:28pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941/1 "2024-12-06T12:28:04Z")

</div>

Hi everyone!

I have a quite a pickle in my game server routing handling. I’m creating fly machines on demand using the machines api. They are created fine, and everything works really well… If I have only one machine running. I’m using `fly-replay` and `fly-force-instance-id` to route my requests to the correct game server instance. There are basically two requests that happen:

- A regular http get request that gets the game map for the user. This is routed to random game server as well.
- A ws upgrade request. This is routed to random game server.

This is my gateway application, it is really simple and as I said, it works, but the problem is only that the replay goes to wrong machine ☹ Any ideas what the problem could be?

```ts
import { Hono } from 'hono'
import { cors } from 'hono/cors'
import { csrf } from 'hono/csrf'

const app = new Hono()
app.use(
  '*',
  cors({
    origin: [
      // Origins
    ],
  })
)
app.use(
  '*',
  csrf({
    origin: [
      // Origins
    ],
  })
)
app.onError((error, c) => {
  console.error(`${error}`)
  return c.json({ error, message: error.message || 'Custom Error Message' }, 500)
})

app.get('*', (c) => {
  const region = c.req.header('region') ?? c.req.query('region')
  const instance = c.req.header('instance') ?? c.req.query('instance')
  if (!region || !instance) {
    throw new Error('Machine not found')
  }
  console.log(
    'replaying',
    `region=${region};app=floating-islands-game-server;fly-force-instance-id=${instance}`
  )
  c.header(
    'fly-replay',
    `region=${region};app=floating-islands-game-server;fly-force-instance-id=${instance}`
  )
  return c.body('OK')
})

export default {
  fetch: app.fetch,
  port: 8080,
  hostname: '0.0.0.0',
}

```

I’m getting seemingly ok logs, but they seem to not go to the right machine:

```auto
2024-12-06T12:26:31.547 app[48e5376b3d9128] ams [info] replaying region=ams;app=floating-islands-game-server;fly-force-instance-id=9185954bd43518
2024-12-06T12:26:32.899 app[48e5376b3d9128] ams [info] replaying region=ams;app=floating-islands-game-server;fly-force-instance-id=9185954bd43518

```

---

<div class="post-metadata">

**Author:** ![pavel](https://avatars.discourse-cdn.com/v4/letter/p/a4c791/32.png) [@pavel](https://community.fly.io/u/pavel)\
**Post date:** [December 6, 2024, 1:11pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941/2 "2024-12-06T13:11:59Z")

</div>

Hey @Panuchka

This doesn’t look like correct `fly-replay` syntax.  
In order to target a specific instance with `fly-replay` your app needs to respond with `instance=<machine id>` value:

```bash
region=${region};app=floating-islands-game-server;instance=${instance}

```

> **[Dynamic Request Routing](https://fly.io/docs/networking/dynamic-request-routing/#replay-requests-to-specific-machines)**
>
> Documentation and guides from the team at Fly.io.

---

<div class="post-metadata">

**Author:** ![Panuchka](https://sea1.discourse-cdn.com/fly1/user_avatar/community.fly.io/panuchka/32/40778_2.png) [@Panuchka](https://community.fly.io/u/Panuchka)\
**Post date:** [December 6, 2024, 1:16pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941/3 "2024-12-06T13:16:49Z")

</div>

Do you mean that I need to also include the `instance` parameter with `fly-force-instance-id`? I really need to make sure that the request goes to exact machine and it is not routed to any other machine in any case.

---

<div class="post-metadata">

**Author:** ![pavel](https://avatars.discourse-cdn.com/v4/letter/p/a4c791/32.png) [@pavel](https://community.fly.io/u/pavel)\
**Post date:** [December 6, 2024, 1:31pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941/4 "2024-12-06T13:31:24Z")

</div>

> [@Panuchka](#):
>
> Do you mean that I need to also include the `instance` parameter with `fly-force-instance-id`?

No, instead of it. `fly-force-instance-id` is not a valid syntax for `fly-replay` response. `instance=<machine id>` is the valid syntax.

> [@Panuchka](#):
>
> I really need to make sure that the request goes to exact machine and it is not routed to any other machine in any case.

Yeah, that’s how `fly-replay` to specific instance works.

---

<div class="post-metadata">

**Author:** ![Panuchka](https://sea1.discourse-cdn.com/fly1/user_avatar/community.fly.io/panuchka/32/40778_2.png) [@Panuchka](https://community.fly.io/u/Panuchka)\
**Post date:** [December 6, 2024, 3:40pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941/6 "2024-12-06T15:40:20Z")

</div>

Oooh I didn’t realize they were different headers altogether, I just thought they were options in the fly-replay header…

So as I want to make absolutely sure that the correct instance is being hit, I should use the `fly-force-instance-id` ? Do I still need the `fly-replay` header?

Like:

```auto
  c.header('fly-replay', `region=${region};app=floating-islands-game-server;instance=${instance};`)
  c.header('fly-force-instance-id', instance)

```

---

<div class="post-metadata">

**Author:** ![pavel](https://avatars.discourse-cdn.com/v4/letter/p/a4c791/32.png) [@pavel](https://community.fly.io/u/pavel)\
**Post date:** [December 6, 2024, 3:52pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941/7 "2024-12-06T15:52:57Z")

</div>

`fly-force-instance-id` is a request header. The clients of your app can set it to make sure their requests are routed to a specific machine, but the machine needs to be a part of the app that received this request.

`fly-replay` is a response header that instructs the proxy to re-route the original request to a different app/region/instance.

So if you control the clients as well you can use `fly-force-instance-id` and avoid the round-trip to your app (for `fly-replay`). Though, keep in mind that if a client asks to route its requests to a machine that, for example, no longer exists, the request will just fail.

So I’d say it’s better to rely on `fly-replay` as your app can keep track of instances and re-route requests to the ones that are definitely valid.

---

<div class="post-metadata">

**Author:** ![system](https://global.discourse-cdn.com/fly1/original/3X/6/e/6e8955d00cb218e186ebbe3a6ab06fefdce15290.svg) [@system](https://community.fly.io/u/system)\
**Post date:** [December 13, 2024, 3:53pm UTC](https://community.fly.io/t/problem-routing-to-correct-instance-via-fly-force-instance-id/22941/8 "2024-12-13T15:53:03Z")

</div>

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