fastify-http-proxy: setting up dynamic upstream and dynamic authentication - fastify

Question

I would like to use fastify-http-proxy plugin with fastify-3.x.
However I am facing a few difficulties when trying to use it per my application.
Let me explain my requirement:
In my application, user is going to be presented a set of environments. When user selects a particular environment (say env-1), based on that I need to make a REST API call to another service, passing the env-1 ID to get the IP address of my upstream and the authorization token (Basic auth of username:password) for that upstream.
When user selects another environment (say env-2), I should "re-set" the new upstream and new authorization token based on env-2.
For that I would like to dynamically register the upstream and the auth token. I saw the examples of fastify-http-proxy testcases. However all the examples just adding the replyOptions during the server.register(proxy, ...) step.
I understand somehow I need to override the getUpstream and rewriteRequestHeader. However not able to get an idea how can I defer it to the later stage where based on environment selection, I can write logic by calling different services' APIs to get the upstream endpoint IP address and upstream authorization information.
Along with that, if I want to just log some information (say some telemetry information .. how much time it to proxy the request to upstream server), what is the best way to do it.
Just to demonstrate it in a code snippet:
const server = Fastify()
server.register(proxy, {
upstream: '',
replyOptions: {
getUpstream: function (original, base) {
// Here I want to pass the selected environment ID
// Based on the environment ID, will make other services' API call to get the upstream
return `<...>`
}
rewriteRequestHeaders: (originalRequest, headers) => {
// Here I want to pass the selected environment ID
// Based on the environment ID, will make other services' API call to get the envAuth
return {
...headers,
"Authorization": envAuth
};
}
}
})

Answer 1

The getUpstream and rewriteRequestHeaders parameters are sync functions, so you can't execute async operations such as an HTTP call.
One option to do so is a local cache you can use in those parameters
Pseudo code:
const cache = {}
server.post('/select-env', async (request, reply) => {
const data = await datasource.load(request.userId, request.body.env)
cache[request.userId] = { data }
return 'loaded'
})
server.register(proxy, {
upstream: '',
replyOptions: {
getUpstream: function (original, base) {
const userId = original.headers['user-id']
return cache[userId].data.upstream
},
rewriteRequestHeaders: (originalRequest, headers) => {
// Here I want to pass the selected environment ID
// Based on the environment ID, will make other services' API call to get the envAuth
const userId = originalRequest.headers['user-id']
return {
...headers,
Authorization: cache[userId].data.auth,
}
},
},
})
The other options are:
implement this feature and send a PR to https://github.com/fastify/fastify-reply-from
note that you can't access the Fastify's Request object using the origin argument. You could access it by adding this feature too to the module.