[Miniflare 3] Add outboundService option for customising global fetch() target

[mrbbot]

workerd exposes a globalOutbound option for customising which service global fetch() calls should be dispatched to. For tests, it's useful to be able to mock responses to certain routes or assert certain requests are made. This new option allows fetch() call to be sent to another configured Worker. See the added test for an example.

Note support for undici's MockAgent API in Miniflare 3 is still planned. This is a lower-level primitive for customising fetch() responses.

/cc @vlovich

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[vlovich]

Omg. I added so much custom code to emulate this. If I'd know I would have asked for this sooner. LGTM

[kentonv]

Note that this setting affects more than just fetch() -- it will also affect where connect() goes, and would presumably affect similar functions that we add in the future. Hence why the setting in workerd is called globalOutbound and not globalFetch.

What you want to name it is up to you but just wanted to make sure this was understood.

[vlovich]

@mrbbot is there a way to make it easier to have a globalOutbound option that takes a function & sets up the requisite magic worker to tunnel out and invoke the function?

[mrbbot]

Note that this setting affects more than just fetch()

Ah good to know, thanks! Will rename to outboundService. 👍

---

@vlovich something like...

new Miniflare({
  async outboundService(request) {
    return new Response(...)
  }
})

Could definitely make it accept any service designator similar to the serviceBindings option 👍

[penalosa]

Looks good! Total bike shedding, but why the change in name from globalOutbound?

[mrbbot]

@penalosa wanted the name to include service to make it clear what the option value was, and match up with serviceBindings since the accepted service designators are the same.

[limzykenneth]

Has this feature landed in the latest release yet? The documentation on NPM already contains the outboundService option but it does not seem to work yet.

[mrbbot]

Hey! 👋 This should've been released as part of [email protected]. Which version are you using?

[limzykenneth]

I doubled checked and turns out I was missing some bindings and fetch is just failing instead.

Sorry another question though, is it possible to filter which request get passed to the outboundService while the rest will route as usual, for example only intercept requests to a certain hostname but for everything else make the network request as usual?

Edit: NVM, I can just return a direct fetch response from the outboundService function. It's not fully the same as coming from Miniflare itself but I'll chalk up any difference to problems with API client libraries I'm using.