Instrumentation middleware cannot work with async requests

[avmnu-sng]

Reference: lostisland/faraday-retry#2
Adapter: Typhoeus

Problem Statement

The current implementation is:

def call(env)
  @instrumenter.instrument(@name, env) do
    @app.call(env)
  end
end

Let's define the connection and subscribe to the default event:

faraday = Faraday.new('https://httpstat.us') do |builder|
  builder.request :instrumentation
  builder.adapter :typhoeus
end

ActiveSupport::Notifications.subscribe('request.faraday') do |_, start, finish, _, env|
  puts format(
    '[%<host>s] %<method>s %<request_uri>s [%<start>i -> %<finish>i = %<duration>.3f]',
    host: env[:url].host,
    method: env[:method].to_s.upcase,
    request_uri: env[:url].request_uri,
    start: start.to_i,
    finish: finish.to_i,
    duration: finish - start
  )
end

Consider the following sync request that works perfectly fine:

faraday.get('200?sleep=100')

# [httpstat.us] GET /200?sleep=100 (200) [1642094481 -> 1642094482 = 0.393]

Now, let's make async (in_parallel) requests:

faraday.in_parallel(Typhoeus::Hydra.new(max_concurrency: 1)) do
  9.times { |i| faraday.get("/200?sleep=#{(i + 1) * 100}") }
end

# [httpstat.us] GET /200?sleep=100 () [1642094450 -> 1642094450 = 0.000]
# [httpstat.us] GET /200?sleep=200 () [1642094450 -> 1642094450 = 0.000]
# [httpstat.us] GET /200?sleep=300 () [1642094450 -> 1642094450 = 0.000]
# [httpstat.us] GET /200?sleep=400 () [1642094450 -> 1642094450 = 0.000]
# [httpstat.us] GET /200?sleep=500 () [1642094450 -> 1642094450 = 0.000]
# [httpstat.us] GET /200?sleep=600 () [1642094450 -> 1642094450 = 0.000]
# [httpstat.us] GET /200?sleep=700 () [1642094450 -> 1642094450 = 0.001]
# [httpstat.us] GET /200?sleep=800 () [1642094450 -> 1642094450 = 0.000]
# [httpstat.us] GET /200?sleep=900 () [1642094450 -> 1642094450 = 0.000]

Notice that there is no response because these requests are added to the Hydra queue only and will be performed later.

Flawed Solution

Since the notification is published before the response is completed, we should try to delay the notification until there is a response, so we can look at an updated implementation:

def call(request_env)
  @instrumenter.start(@name, request_env)

  @app.call(request_env).on_complete do |response_env|
    @instrumenter.finish(@name, response_env)
  end
end

Let's make the requests:

faraday.in_parallel(Typhoeus::Hydra.new(max_concurrency: 1)) do
  9.times { |i| faraday.get("/200?sleep=#{(i + 1) * 100}") }
end

# [httpstat.us] GET /200?sleep=100 (200) [1642095297 -> 1642095299 = 1.593]
# [httpstat.us] GET /200?sleep=200 (200) [1642095297 -> 1642095299 = 2.053]
# [httpstat.us] GET /200?sleep=300 (200) [1642095297 -> 1642095300 = 2.726]
# [httpstat.us] GET /200?sleep=400 (200) [1642095297 -> 1642095301 = 3.406]
# [httpstat.us] GET /200?sleep=500 (200) [1642095297 -> 1642095302 = 4.356]
# [httpstat.us] GET /200?sleep=600 (200) [1642095297 -> 1642095303 = 5.369]
# [httpstat.us] GET /200?sleep=700 (200) [1642095297 -> 1642095304 = 6.334]
# [httpstat.us] GET /200?sleep=800 (200) [1642095297 -> 1642095305 = 7.431]
# [httpstat.us] GET /200?sleep=900 (200) [1642095297 -> 1642095306 = 8.577]

We have fixed the response issue, seems like there is a problem with the duration, well not exactly, because technically, we are now measuring the total time to finish the nth request since we submitted the requests.

There is a bigger problem, consider the following requests:

faraday.in_parallel(Typhoeus::Hydra.new(max_concurrency: 1)) do
  9.times do |i|
    faraday.get("/200?sleep=#{(i + 1) * 100}")
    sleep(1)
  end
end

# [httpstat.us] GET /200?sleep=100 (200) [1642095933 -> 1642095935 = 2.261]
# [httpstat.us] GET /200?sleep=200 (200) [1642095932 -> 1642095935 = 3.712]
# [httpstat.us] GET /200?sleep=300 (200) [1642095931 -> 1642095936 = 5.259]
# [httpstat.us] GET /200?sleep=400 (200) [1642095930 -> 1642095937 = 7.818]
# [httpstat.us] GET /200?sleep=500 (200) [1642095929 -> 1642095938 = 9.668]
# [httpstat.us] GET /200?sleep=600 (200) [1642095928 -> 1642095939 = 11.511]
# [httpstat.us] GET /200?sleep=700 (200) [1642095927 -> 1642095940 = 13.456]
# [httpstat.us] GET /200?sleep=800 (200) [1642095926 -> 1642095941 = 15.497]
# [httpstat.us] GET /200?sleep=900 (200) [1642095925 -> 1642095942 = 17.645]

Note that the start times are in reverse order because ActiveSupport::Notifications::Fanout maintains a time stack.

Conclusion

We cannot easily figure out the exact start time of the requests and building a system across all the adapters to get the start times would be a tedious and error-prone process. A better solution is to provide a way to access additional response information from the adapters that may contain timing and other information.

[avmnu-sng]

@iMacTia Even after providing a way to access response information from the adapters, we still need to fix the inaccuracy in the start time. For that, we need to patch the ActiveSupport::Notifications::Fanout to use a Concurrent::Map and store the start time based on request-id.

Changing the existing behavior entirely is not a good idea, so, we can introduce start_faraday and finish_faraday to measure the times correctly.

def initialize(app, name = 'request.faraday')
  super(app)

  @name = name
  @notifier = ActiveSupport::Notifications.notifier
  @instrumenter_id = ActiveSupport::Notifications.instrumenter.id
end

def call(request_env)
  context = request_env.request.context || {}
  context[:request_id] = SecureRandom.hex(10)
  request_env.request.context = context

  @notifier.start_faraday(@name, @instrumenter_id, request_env)

  @app.call(request_env).on_complete do |response_env|
    @notifier.finish_faraday(@name, @instrumenter_id, response_env)
  end
end

With these customizations, we can accurately measure the duration:

faraday.in_parallel(Typhoeus::Hydra.new(max_concurrency: 1)) do
  9.times do |i|
    faraday.get("/200?sleep=#{(i + 1) * 100}")
    sleep(1)
  end
end

# [httpstat.us] GET /200?sleep=100 (200) [1642159338 -> 1642159348 = 10.312]
# [httpstat.us] GET /200?sleep=200 (200) [1642159339 -> 1642159349 = 9.964]
# [httpstat.us] GET /200?sleep=300 (200) [1642159340 -> 1642159350 = 9.508]
# [httpstat.us] GET /200?sleep=400 (200) [1642159341 -> 1642159350 = 9.160]
# [httpstat.us] GET /200?sleep=500 (200) [1642159342 -> 1642159351 = 9.051]
# [httpstat.us] GET /200?sleep=600 (200) [1642159343 -> 1642159352 = 9.186]
# [httpstat.us] GET /200?sleep=700 (200) [1642159344 -> 1642159353 = 9.145]
# [httpstat.us] GET /200?sleep=800 (200) [1642159345 -> 1642159354 = 9.199]
# [httpstat.us] GET /200?sleep=900 (200) [1642159346 -> 1642159356 = 9.450]

[iMacTia]

@avmnu-sng first of all, thanks for re-raising this issue together with a detailed explanation and steps to reproduce, it helped me having a much better understanding as to what's is going on (which is hard, as I'm not really a parallel user that much myself).

Your proposed solution looks similar to the one from @BuonOmo, although you both pointed out it sadly doesn't work exactly as expected.
I'm wondering at this point if the "fault" here might be on ActiveSupport::Notifications and if there's a chance we could fix it there. For example, looking at the start and finish implementation, it doesn't look like they use the id at all, something that would potentially all us to distinguish between requests:

def start(name, id, payload)
  # id is provided, but not used to fetch the time, so this works well with
  # nested measurements, but not for concurrent ones.
  # 
  # A better implementation could be:
  # timestack = Thread.current[:_timestack] ||= Hash.new { Array.new }
  # timestack[key_from(name, id)].push Time.now
  timestack = Thread.current[:_timestack] ||= []
  timestack.push Time.now
end

def finish(name, id, payload)
  # same here as for start
  timestack = Thread.current[:_timestack]
  started = timestack.pop
  @delegate.call(name, started, Time.now, id, payload)
end

A better solution is to provide a way to access additional response information from the adapters that may contain timing and other information.

Should we not be able to find a way to better measure, then this would eventually be necessary, although it would require explicit implementation from each adapter via some sort of new internal API. That will take much longer to document and then we'd need to wait for each adapter to implement that as well, so I'd keep it as a last resort

[iMacTia]

@avmnu-sng thanks for the extra comment on how to fix ActiveSupport::Notifications, that is exactly what I was aiming for, although your solution with the Concurrent::Map is obviously better.

Changing the existing behavior entirely is not a good idea

Why do you think they wouldn't want to support that?

[avmnu-sng]

@iMacTia ActiveSupport::Notifications is synchronized instrumentation that works at either a Thread or Fiber isolation. The id that you see is the instrumenter' id. What we need is a unique identifier for what is being instrumented not who is instrumenting it.

Basically, there is nothing wrong with the Rails implementation. We are trying to use something designed for sync behavior which won't work by default.

[avmnu-sng]

If we want to use the existing start and finish APIs, we need to file a feature request to have something like:

# event_id is a unique identifier of the block getting executed
def start(event_name, event_id, payload); end

And therefore, the subscribers APIs need change:

# Use event_id to find the correct start time
def start(event_name, event_id, instrumenter_id, payload); end

[avmnu-sng]

• For the ActiveSupport::Notifications, I suggest, we inject additional APIs start_faraday and finish_faraday which use Concurrent::Map and has the correct time data.
• For getting additional response information, we should look into two milestones:
  • Update the adapter save_response to send additional information:

  def save_response(env, status, body, headers = nil, reason_phrase = nil, informations = nil)
    env.status = status
    env.body = body
    env.reason_phrase = reason_phrase && reason_phrase.to_s.strip
    env.response_headers = Utils::Headers.new.tap do |response_headers|
      response_headers.update headers unless headers.nil?
      yield(response_headers) if block_given?
    end
    env.response_informations = informations
  end

  • Start updating the adapters to send the additional information. For example, Typhoeus could be updated to:

  def request(env)
    read_body env
  
    req = typhoeus_request(env)
  
    configure_ssl     req, env
    configure_proxy   req, env
    configure_timeout req, env
    configure_socket  req, env
  
    req.on_complete do |resp|
      if resp.timed_out?
        env[:typhoeus_timed_out] = true
        unless parallel?(env)
          raise Faraday::Error::TimeoutError, "request timed out"
        end
      elsif (resp.response_code == 0) || ((resp.return_code != :ok) && !resp.mock?)
        env[:typhoeus_connection_failed] = true
        env[:typhoeus_return_message] = resp.return_message
        unless parallel?(env)
          raise Faraday::Error::ConnectionFailed, resp.return_message
        end
      end
  
      informations = [
        :primary_ip, :request_size, :return_code, :return_message,
        :total_time, :name_lookup_time, :connect_time, :start_transfer_time,
        :app_connect_time, :pretransfer_time, :redirect_time, :redirect_count
      ].each_with_object({}) { |key, result| result[key] = resp.public_send(key) }
  
      save_response(env, resp.code, resp.body, nil, nil, informations) do |response_headers|
        response_headers.parse resp.response_headers
      end
  
      # in async mode, :response is initialized at this point
      env[:response].finish(env) if parallel?(env)
    end
  
    req
  end

I am running both these patches in my local environment.

ActiveSupport::Notifications.subscribe('request.faraday') do |_, start, finish, _, env|
  puts format(
    '[%<host>s] %<method>s %<request_uri>s (%<status>s) [%<start>i -> %<finish>i = %<duration>.3f] %<information>s',
    host: env.url.host,
    method: env.method.to_s.upcase,
    request_uri: env.url.request_uri,
    status: env.status,
    start: start.to_i,
    finish: finish.to_i,
    duration: finish - start,
    information: env.response_informations
  )
end; nil

• Sync request

  faraday.get('200')
  
  # [httpstat.us] GET /200 (200) [1642162573 -> 1642162574 = 1.011] {:primary_ip=>"20.40.202.3", :request_size=>82, :return_code=>:ok, :return_message=>"No error", :total_time=>1.009126, :name_lookup_time=>0.002421, :connect_time=>0.270699, :start_transfer_time=>1.008614, :app_connect_time=>0.768444, :pretransfer_time=>0.768517, :redirect_time=>0.0, :redirect_count=>0}

• Async requests

  faraday.in_parallel(Typhoeus::Hydra.new(max_concurrency: 1)) do
    9.times do |i|
      faraday.get("/200?sleep=#{(i + 1) * 100}")
      sleep(1)
    end
  end
  
  # [httpstat.us] GET /200?sleep=100 (200) [1642162721 -> 1642162732 = 10.502] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>1.45959, :name_lookup_time=>0.002013, :connect_time=>0.410836, :start_transfer_time=>1.459373, :app_connect_time=>0.907401, :pretransfer_time=>0.90748, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=200 (200) [1642162722 -> 1642162732 = 9.962] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>0.463959, :name_lookup_time=>3.9e-05, :connect_time=>3.9e-05, :start_transfer_time=>0.463728, :app_connect_time=>3.9e-05, :pretransfer_time=>7.6e-05, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=300 (200) [1642162723 -> 1642162733 = 9.542] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>0.581223, :name_lookup_time=>3.6e-05, :connect_time=>3.6e-05, :start_transfer_time=>0.580916, :app_connect_time=>3.6e-05, :pretransfer_time=>8.5e-05, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=400 (200) [1642162724 -> 1642162733 = 9.197] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>0.656018, :name_lookup_time=>4.3e-05, :connect_time=>4.3e-05, :start_transfer_time=>0.655717, :app_connect_time=>4.4e-05, :pretransfer_time=>8.8e-05, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=500 (200) [1642162725 -> 1642162734 = 8.977] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>0.783188, :name_lookup_time=>3.7e-05, :connect_time=>3.8e-05, :start_transfer_time=>0.783018, :app_connect_time=>3.8e-05, :pretransfer_time=>8.6e-05, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=600 (200) [1642162726 -> 1642162735 = 8.816] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>0.841132, :name_lookup_time=>5.5e-05, :connect_time=>5.5e-05, :start_transfer_time=>0.840889, :app_connect_time=>5.5e-05, :pretransfer_time=>9.5e-05, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=700 (200) [1642162727 -> 1642162736 = 8.773] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>0.959293, :name_lookup_time=>6.0e-05, :connect_time=>6.1e-05, :start_transfer_time=>0.958201, :app_connect_time=>6.1e-05, :pretransfer_time=>0.000131, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=800 (200) [1642162728 -> 1642162737 = 8.811] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>1.04154, :name_lookup_time=>3.6e-05, :connect_time=>3.6e-05, :start_transfer_time=>1.041346, :app_connect_time=>3.6e-05, :pretransfer_time=>8.0e-05, :redirect_time=>0.0, :redirect_count=>0}
  # [httpstat.us] GET /200?sleep=900 (200) [1642162729 -> 1642162738 = 8.954] {:primary_ip=>"20.40.202.3", :request_size=>92, :return_code=>:ok, :return_message=>"No error", :total_time=>1.144223, :name_lookup_time=>4.8e-05, :connect_time=>4.8e-05, :start_transfer_time=>1.143428, :app_connect_time=>4.8e-05, :pretransfer_time=>9.9e-05, :redirect_time=>0.0, :redirect_count=>0}

[iMacTia]

Basically, there is nothing wrong with the Rails implementation. We are trying to use something designed for sync behavior which won't work by default.

I think this is the key thing here. We use ActiveSupport::Notifications as the default instrumenter because it's the most common one and because it works in the majority of applications. But when you go for parallel requests, it obviously start showing its limits.

That said, the instrumentation middleware does allow you to provide your own instrumenter, which at this point might be the right thing to do for those planning to run parallel requests.
Rather than monkey-patching an instrumenter that is not fit-for-purpose, should we instead provide an alternative one that is capable of supporting parallel requests? I did a quick search and couldn't find any obvious candidate, but I guess you might know one having worked more in a parallel context.

Another option we can look into is to ship this as a new middleware.
I originally planned to remove the instrumenter middleware from Faraday as we did with retry and multipart, but for some reason it ended up staying. That doesn't mean we can't have a new Instrumentation middleware shipped as a separate gem. This would give us more implementation freedom and could be the obvious choice for those using a parallel-capable adapter.

For getting additional response information, we should look into two milestones:

• Update the adapter save_response to send additional information
• Start updating the adapters to send the additional information. For example, Typhoeus could be updated to

Thanks for explaining this in more details, I still believe this is not the right way forward, as it will require too many changes across too many repositories for this to work. Having a solution that just works, without having to rely on adapters, would be much more powerful.

[avmnu-sng]

I guess we are confusing the use-case when we use parallel. Let me re-iterate:

• Currently, when we instrument connection.in_parallel, it immediately returns after enqueuing the requests, but the instrumentation is correct because ActiveSupport::Notifications measures the time to execute the block then sends the notification.
• Here, what we want from Faraday is to be a bit smarter and get the instrumenter to send notifications when each of the requests is complete, i.e., request.on_complete should be the trigger point for the notifications.
• Now, when we try to use the start and finish APIs explicitly from ActiveSupport::Notifications, these are not designed to be used explicitly. But note that there is nothing about parallel. But can explicitly instrument using start and finish or similar APIs.

Regarding the additional response details, I say to use an adapter because these details are part of the response, and currently, Faraday uses the adapters to get the response.

[iMacTia]

Thanks for going through the details again @avmnu-sng, I agree with everything you said.
I believe I should also attempt to articulate my thoughts a bit more clearly.

Preamble

The Faraday instrumentation middleware is supposed to fire instrumentation metric events so that you can instrument your requests. This can be done independently from the adapter you pick, in accordance with Faraday's spirit of abstracting the HTTP backend library.
Although the middleware allows you to pick the instrumenter you prefer, it currently defaults to ActiveSupport::Notifications as this is the most used one and available out-of-the-box in Rails applications.

Problems

There are mainly 2 issues as you correctly highlighted already:

1. The current implementation of the instrumentation middleware is block-based, which doesn't work well with parallel requests. We can solve this by switching to the start/stop interface instead.
2. ActiveSupport::Notifications have not been designed to work with parallel events, where the start event is called from the same thread, since it relies on an events stack. So if we fix problem 1, we're still going to experience issues here.

Solutions

Assuming that we already solved problem 1 by fixing the middleware and switching to the start/stop interface, there are a few solutions that we've discussed already:

1. Monkey-patch ActiveSupport::Notifications with ad-hoc faraday_start/stop methods: this would eventually work, but at that point we're bringing the maintenance burden of that monkey-patch into Faraday, and we'd need to test it across different ActiveSupport::Notifications versions to ensure it doesn't break. And that's simply because we chose it as the default instrumenter, so we're ignoring the fact that users can still change that, causing the middleware to behave unexpectedly again.
2. Rely on the adapter to provide instrumentation info: as I explained above, this would definitely work but would push the responsibility on the adapters and would potentially take a long time before they all support this, causing issues for developers who need to check this before swapping adapters.

Alternative solutions

As neither solutions above make me extremely happy, I proposed a couple of alternative solutions in my last comment which I'd be personally happy with:

1. Create a new instrumentation middleware as a separate gem (can be called ParallelInstrumentation, or something similar). This could explicitly depend on ActiveSupport::Notifications and apply the monkey-patch from solution 1, but would be maintained separately and possibly by someone else.
2. Let's assume for a moment that a better alternative to ActiveSupport::Notifications exists, an instrumenter built for concurrency that supports the start/stop interface (we'll call this imaginary instrumenter ParallelInstrumenter). One could instruct the existing middleware to use that instead with the following:

conn = Faraday.new(...) do |f|
  f.request :instrumentation, instrumenter: ParallelInstrumenter
  ...
end

Now, since I don't personally work much with concurrent requests, I'm unaware of such an instrumenter, but I'd expect something like this to exist already and potentially being known by someone that works with concurrent ruby on a daily basis.

Open for feedback

I hope the above is clearer and my reasoning makes sense. I do want to support this, but not at the expense of additional maintenance burden on the team, or entirely relying on the adapters.

Do any of the alternative solutions make sense and look like a viable option?
Are you aware of any alternative instrumenter that supports parallel instrumentation?

[avmnu-sng]

@iMacTia I agree with you on the instrumentation part that it makes more sense to have an alternative to ActiveSupport::Notifications or monkey-patch it. However, I think we have a different perspective on the additional response data. I am not considering this as instrumentation information.

Assuming we have ParallelInstrumenter or I would rather call it InstrumenterX for now. The time taken here is the moment the request is queued to when the response is received. It is different in the case of a sync request, where we measure the time since the request is executed to the response is received. It is hard to achieve the same with connection.in_parallel because can't actually know when the request starts executing, but definitely get the time when the request finishes. This is where the additional response data would be handy and you get fine-grained data, but it is on top of the changes to instrumentation we are talking about, it is a must.

Another option is if we have request.on_start then we actually know when a request starts executing. It again requires changes on the adapters level.

[iMacTia]

I'm finally seeing what you mean, and apologies if it took me so long.
I was under the wrong assumption that parallel requests would start immediately, but in your example you made it pretty clear that if you queue N requests and the max_concurreny is lower than N, then some time will lapse between the call to the instrumenter start and the parallel request actually being started.

With that in mind I can't but agree that this is a service the adapter should be exposing, as that's the only thing that can know when a request actually starts.

A way to get around this could be that the middleware checks for the adapter support of this "feature" and falls back to the inaccurate behaviour of start/stop if that's not the case.
I'll need to think about this more though, as middleware is not supposed to have a direct communication with the adapter, so we'll need some sort of middle abstraction layer to get around that.

Bonus thought

I'm also starting to think if this should not be managed by the underlying library entirely at this point.
Why use the faraday middleware and instead not request that Typhoeus or other libraries expose some sort of notifications interface one can register to? Do they do this already by any chance?

[BuonOmo]

@iMacTia sorry for the late reply, also I have to admit I use more Typhoeus on its own nowadays due to Faraday's lack in parallelism support and my lack of time to fix that...

Anyway, I've quickly read your conversation, hopefully it will lead to something great! Just here to say: Typhoeus does expose instrumentation (https://github.com/typhoeus/ethon/blob/master/lib/ethon/easy/informations.rb#L18), and some kind of notification (e.g: on_complete)

[iMacTia]

Thanks @BuonOmo, I believe that's the same informations @avmnu-sng mentioned in their comment earlier, and it sure looks like a lot of information! The only missing thing there is the integration with a notification system like ActiveSupport::Notifications, but that's something the middleware could provide.

I spent some more time thinking about this, and I believe the following step-by-step approach should work.
It's mostly based on things that @avmnu-sng already raised, but my brain needed a little extra help to put them together, so I hope this list will help others see them clearly as well:

1. We change the instrumentation middleware to use the start/stop interface. This could "replace" the existing block behaviour or, to be backwards-compatible, enrich it after a) checking if the instrumenter supports it; or b) check if we're running in_parallel. This will make things better, but will also introduce some inconsistencies as explained in the ticket description, which we can document and point out
2. We introduce a new field to the response object that allows adapters to provide additional information about the request. I don't really like informations 😄, but it will basically contain that info. Maybe instrumentation or something like that.
3. We update the instrumentation middleware once again so that, in the on_complete callback, it checks for this new field presence, and if found it uses that to enrich the notification payload, then it fires it.
4. Lastly, we go through each adapter (capable of parallel requests) and we add support for this new field. As I said this will potentially take some time, but it seems like we already know how to do it for Typhoeus.

@BuonOmo @avmnu-sng what do you think about the plan above? Any feedback would be appreciated.
Also, @BuonOmo, I appreciate the parallelism support have not been our main concern, mostly due to the Faraday core team inexperience on the topic, but I believe it could become a big focus now that v2.0 is out and we plan our road to v3.0.
It would be really useful to have a comprehensive list of current issues so that we can explore and tackle them as necessary.
May I suggest we start a new Discussion on the topic so we can coordinate there? How does that sound?

[avmnu-sng]

@iMacTia I am definitely in agreement 😄. Feel free to start a new discussion and let's plan and execute it.

[iMacTia]

#1386