Change HTTP2 max concurrent streams value

[dunkmann00]

Is it possible to change the max concurrent streams value for an HTTP2 connection? I was looking into this but it appears that the default value from the H2Connection class is used in the HTTP2Connection object, and cannot be changed. Is this intended to be not configurable, or am I missing something?

Thanks!

Note: I had originally opened an issue in encode/httpcore/issues/598 but if this is the more appropriate forum to discuss this, I will close that.

[lovelydinosaur]

Is it possible to change the max concurrent streams value for an HTTP2 connection?

Currently, no.

[dunkmann00]

I am trying to update a library that helps with sending Apple Push Notifications, PyAPNs2. It was written with hyper as a dependency, but since that is no longer maintained (for quite some time as you are probably aware) I figured I would try replacing it with httpx.

I've gotten most of this working but had to stop at the part about handling multiple streams. I know this has come up before, but I'll include a snippet from Apple's docs about concurrent streams:

APNs allows multiple concurrent streams for each connection, but don’t assume a specific number of streams. The exact number varies based on server load and whether you use a provider certificate or an authentication token. For example, when using an authentication token, APNs allows only one stream until you post a request with a valid authentication token. APNs ignores HTTP/2 PRIORITY frames, so don’t send them on your streams.

Source: Sending Notification Requests to APNs

1. So to work with this, I could initially just set max concurrent streams to 1, and then after sending a single request, check the remote value, and assuming it has changed to greater than 1, update my local value to that.

2. Unless changing my local value because the remote value changed is not a good pattern. In this case it would be nice to set the local value to some higher value (say 1000), and then httpx could determine how many concurrent streams are allowed based on both the remote and local max concurrent streams values (i.e. it would only send at most, the minimum of the two values).

3. If that is not something that httpx is going to handle, then it would be nice to at least raise the local max concurrent streams value, and then I could manually, in the PyAPNs2 library, make sure to only send up to the remote's max concurrent streams value.

I think for my use case (and maybe a general case?) it would be great if httpx just handled max concurrent streams like I described in 2. That way I could set the local max concurrent streams to some value I'm comfortable with as an upper bound, and httpx will figure out how many streams it can send concurrently based on checking the remote max concurrent streams value. In this case, I think its important to note you would have to support checking for changes to the remote max concurrent streams.

Hopefully this is clear and coherent...I did my best! Thoughts?

[lovelydinosaur]

Okay. It looks to me like "can I change the max concurrent streams setting" isn't necessarily actually what's needed here...

I'm up for spending some time working through this.

had to stop at the part about handling multiple streams

Can you be more specific here...

• Do you see errors from the server, and if so what?
• Are you able to observe the HTTP/2 frames in each direction? (The "trace" extension is useful here)

There's more that could be said here, but let's try to take this step-by-step.

[dunkmann00]

I'm up for spending some time working through this.

I appreciate that, thank you!

had to stop at the part about handling multiple streams

I just meant there was no way for me to observe the remote max concurrent streams value (through the public parts of httpx anyway) and no way to change the local max concurrent streams value. So as far as updating PyAPNs2, I got it working with sending one stream at a time and had to stop. Although, I would like to be able to have it support sending up-to as many streams as the APNs server is willing to accept, which is how it worked with hyper. But I do not know what the remote server is willing to accept because that is not available in httpx, and even if I did know, there is currently no way to use that to change how many concurrent streams httpx will use for a connection.

Okay. It looks to me like "can I change the max concurrent streams setting" isn't necessarily actually what's needed here...

I'm not sure I understand what you mean?

[lovelydinosaur]

So as far as updating PyAPNs2, I got it working with sending one stream at a time and had to stop.

Okay, so the docs state "For example, when using an authentication token, APNs allows only one stream until you post a request with a valid authentication token"...

• Do all of your outgoing requests include an authentication token?
• What behaviour do you see if you attempt to send multiple concurrent requests?

Ideally you shouldn't need to be doing any stream concurrency on your side of the client at all, because httpx ought to be just handling that for you, establishing multiple connections if needed. I can see where there's a couple of possible gaps in how that handling currently works, but it'd be really helpful to observe any actual failure cases against a live server...

If you're able to hop onto a chat on https://gitter.im/encode/community sometime, we could see about working through this together.

[dunkmann00]

Do all of your outgoing requests include an authentication token?

In my testing, yes. The other option, rather than using an auth token, is to use a certificate. But you have to use one or the other and I've just been sticking with the auth token method as that is what I have configured with Apple.

What behaviour do you see if you attempt to send multiple concurrent requests?

I actually hadn't tried this until now. But the error I am getting when I tried to send 100 notifications concurrently:

httpcore.LocalProtocolError: Max outbound streams is 1, 77 open from here

which was raised because of:

h2.exceptions.TooManyStreamsError: Max outbound streams is 1, 77 open from here in h2

Here is an excerpt from the trace log (thanks for pointing trace out btw):

...
http2.send_request_headers.started {'request': <Request [b'POST']>, 'stream_id': 149}
http2.send_request_headers.complete {'return_value': None}
http2.send_request_body.started {'request': <Request [b'POST']>, 'stream_id': 63}
http2.send_request_headers.started {'request': <Request [b'POST']>, 'stream_id': 151}
http2.send_request_headers.started {'request': <Request [b'POST']>, 'stream_id': 153}
http2.send_request_headers.started {'request': <Request [b'POST']>, 'stream_id': 155}
http2.send_request_headers.failed {'exception': TooManyStreamsError('Max outbound streams is 1, 77 open')}
http2.response_closed.started {'stream_id': 155}
http2.response_closed.complete {'return_value': None}
http2.send_request_headers.complete {'return_value': None}
http2.send_request_body.started {'request': <Request [b'POST']>, 'stream_id': 65}
http2.send_request_headers.started {'request': <Request [b'POST']>, 'stream_id': 155}
http2.send_request_headers.failed {'exception': TooManyStreamsError('Max outbound streams is 1, 77 open')}
http2.response_closed.started {'stream_id': 155}
http2.send_request_headers.started {'request': <Request [b'POST']>, 'stream_id': 155}
http2.send_request_headers.failed {'exception': TooManyStreamsError('Max outbound streams is 1, 77 open')}
http2.response_closed.started {'stream_id': 155}
http2.send_request_body.complete {'return_value': None}
http2.receive_response_headers.started {'request': <Request [b'POST']>, 'stream_id': 11}
http2.response_closed.complete {'return_value': None}
http2.send_request_headers.started {'request': <Request [b'POST']>, 'stream_id': 155}
http2.send_request_headers.failed {'exception': TooManyStreamsError('Max outbound streams is 1, 77 open')}
http2.response_closed.started {'stream_id': 155}
http2.send_request_headers.failed {'exception': CancelledError()}
http2.send_request_body.failed {'exception': CancelledError()}
...

Afterwards it is just more CancelledError(). It does seem like a bunch will complete, not sure why h2 doesn't stop it right away. I tried it a second time and it failed at 48 open rather than 77 so I don't know what's going on there, guess it has something to do with the order the coroutines end up running?

Ideally you shouldn't need to be doing any stream concurrency on your side of the client at all, because httpx ought to be just handling that for you, establishing multiple connections if needed.

That makes sense to me. Originally I wasn't sure it was something httpx wanted to support, which is why I was trying to figure out how to handle it manually, but that being something invisible to the end user seems great.

Chatting on gitter sounds good. I should have time tomorrow so I'll try to be on as much as I can.

[tinycogio]

Hello!
Was there any progress on this issue? Like dunkman00, I'm trying to migrate to httpx for sending APNs and currently running into some perf issues with the max number of streams set to 1. I can try to play around with it more on our next push blast.

Thanks!

[dunkmann00]

Hello @tinycogio! Tom & I discussed this some more but I have unfortunately not gotten a chance to work on this. I should have time this week and plan to get back to this.

I will post back here when I have made some progress.

[dunkmann00]

@tomchristie Whenever you get a chance, I have opened encode/httpcore#652 to adrdess this issue.