Resolved Only some web requests getting 503 error

[varun33]

Issue
Some users are getting a 503 error (around 5-6 users a day) and the error does not go away with a refresh of the page. Once that error pops-up any page requested on the domain throws the same 503 error. There is no specific website or domain from the ones configured on the server where this error is being faced it is just random.

Errors in the log
(Apache)
AH01067: Failed to read FastCGI header
[proxy_fcgi:error] [pid 30805:tid 140427730589440](104)Connection reset by peer: [client xxx.xxx.xxx.xxx:41520] AH01075: Error dispatching request to :
(Syslog)
localhost kernel: [2043980.836123] php-fpm[65943]: segfault at 7fff2417ab40 ip 000056425a0108dc sp 00007fff2417ab20 error 6 in php-fpm[564259d59000+4c4000]

Server Environment
Ubuntu 18.04.1 LTS
CPU: 1 processor with 16 cores
RAM: 48G
Plesk Onyx Version 17.8.11 Update #35
PHP 7.2.13
FPM application served by Apache
SSL enabled on all domains
Nginx Proxy mode enabled along with smart static files processing
Linux+Nginx+Apache2+PHP-FPM-7.2+HTTP2+MySQL (mariadb)
15 domains configured with the above settings all running under a common wordpress multisite installation

Background
We recently migrated to the above server, before this we were using PHP 5.6.33 with FastCGI and did not have any such issues.

Our analysis
- No issues with the availability of CPU/memory as the average load is around 3-4 and there is at least 24G of memory available
- We are unable to replicate the issue and it just happens
- We have found that on the web browser if the issue occurs we can delete the wordpress cookie 'wordpress_logged_in_*******' and that fixes the issue and allows to open the requested page instead of a 503

We are still trying to find the cause for this issue but if any of you have faced this or a similar problem please share your version and the solution if you were able to find it.

Thank you.

 

[Brujo]

Well this issue was discussed on the forum many times and there are different approaches for it.
Also, there are several related KB articles where different reasons for this issue were described, you can just try for example to use phrase "AH01075: Error dispatching request to" for KB search.

maybe this one will be a starting point for you
WordPress website shows 503 Service Temporarily Unavailable on Plesk for Linux: AH01075: Error dispatching request to
WordPress website shows 503 Service Temporarily Unavailable

 

[varun33]

We have tried the above solutions but none of them worked for us.

 

[varun33]

Removing the PHP extension "ioncube_loader_lin_7.2" changes the error from 503 to 502.

 

[Brujo]

for interesting and perhaps to get different results, what happens when you install additionially php 7.1 from plesk additional components and switch one of the domain with the issue to php 7.1

How to install/remove additional PHP versions provided by Plesk

 

[varun33]

Switching to 7.1.26 gives a 503 error and after removing the php extension "ioncube_loader_lin_7.1" it turns into a 502 error.

Switching to 7.3.1 gives a 502 error.

 

[Brujo]

the errors have changed in the associated logfile or are still the same error messages when using different php versions?
one more question have you some special settings in the Apache & nginx Settings

I belive you also hav already checked and followed this knowledgebase entry How to set up a WordPress multisite on the Plesk server?

well if no one else has an idea I suggest you contact the Plesk support directly How to submit a request to Plesk support?

 

[varun33]

The errors have changed : Issue - Only some requests receiving 502 Bad Gateway

 

[varun33]

We finally managed to resolve the issue. The problem was with the wordpress plugin "iThemes Security", due to a bug in the plugin related to expired wordpress login keys there was an infinite loop being created which ultimately lead to the 502/503 error.

Thank you all for your help.