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From Nick Muerdter <st...@nickm.org>
Subject TrafficServer 6, keep-alive, connection retries, and 502 Server Hangups
Date Sun, 04 Oct 2015 16:16:33 GMT
Hi,

I've observed some differences in how TrafficServer 6.0.0 behaves with
connection retrying and outgoing keep-alive connections. I believe the
changes in behavior might be related to this issue:
https://issues.apache.org/jira/browse/TS-3440 However, I wasn't sure if
the new behavior (specifically around keep-alive handling) was
intentional or not, so I thought I'd ping the mailing list.

What I'm seeing in 6.0.0 is that if TrafficServer has some backend
keep-alive connections already opened, but then one of the keep-alive
connections is closed, the next request to TrafficServer may generate a
502 Server Hangup response when attempting to reuse that connection.
Previously, I think TrafficServer was retrying when it encountered a
closed keep-alive connection, but that is no longer the case. So if you
have a backend that might unexpectedly close its open keep-alive
connections, the only way I've found to completely prevent these 502
errors in 6.0.0 is to disable outgoing keepalive
(proxy.config.http.keep_alive_enabled_out and
proxy.config.http.keep_alive_post_out settings).

For a slightly more concrete example of what can trigger this, this is
fairly easy to reproduce with the following setup:

- TrafficServer is proxying to nginx with outgoing keep-alive
connections enabled (the default).
- Throw a constant stream of requests at TrafficServer.
- While that constant stream of requests is happening, also send a
regular stream of SIGHUP commands to nginx to reload nginx.
- Eventually you'll get some 502 Server Hangup responses from
TrafficServer among your stream of requests.

SIGHUPs in nginx should result in zero downtime for new requests, but I
think what's happening is that TrafficServer may fail when an old
keep-alived connection is reused (it's not common, so it depends on the
timing of things and if the connection is from an old nginx worker that
has since been shut down). In TrafficServer 5.3.1 these connection
failures were retried, but in 6.0.0, no retries occur in this case.

Here's some debug logs that show the difference in behavior between
6.0.0 and 5.3.1. Note that differences seem to stem from how each
version eventually handles the "VC_EVENT_EOS" event following
"&HttpSM::state_send_server_request_header, VC_EVENT_WRITE_COMPLETE".

5.3.1:
https://gist.github.com/GUI/0c53a6c4fdc2782b14aa#file-trafficserver_5-3-1-log-L316
6.0.0:
https://gist.github.com/GUI/0c53a6c4fdc2782b14aa#file-trafficserver_6-0-0-log-L314

Interestingly, if I'm understand the log files correctly, it looks like
TraffficServer is reporting an odd empty response from these connections
("HTTP/0.9 0" in 5.3.1 and "HTTP/1.0 0" in 6.0.0). However, as far as I
can tell from TCP dumps on the system, nginx is not actually sending any
form of response.

So my basic question is whether the new behavior in 6.0.0 is correct or
not. Based on the discussion in
https://issues.apache.org/jira/browse/TS-3440 I'm unsure whether 5.3.1
retrying on these closed keep-alive connections was actually safe or
not. In these example cases the backend server isn't sending back any
data (at least as far as I can tell), so from what I understand, it
should be safe to retry. However, I'm not totally sure that this
situation with dead keep-alive connections can properly be distinguished
between other types of hangups or connection errors, so perhaps it isn't
safe.

If the 6.0.0 behavior is correct, is disabling outgoing keep-alive
connections the best option if I'm worried about backend services
unexpectedly killing off old keep-alive connections? Or is this a bug
with 6.0.0, and should TrafficServer retires technically be possible in
these cases?

Thanks!
Nick

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