Varnish 3.0.2 to Apache2 sometimes return error 503

Question

Hey guys I hope you can help me out here.

I have an Ngingx parsing http and https to a varnish cache(3.0.2). From the varnish it is sent to apache2. Now I have for some time been tracking some strange 503 errors. But I cant seem to find the silver bullet.

Currently I am logging the 503 errors through varnish this way:

sudo varnishlog -c -m TxStatus:503 >> /home/rj/varnishlog503.log

and then referring to the apache access log to see if any 503 requests have been handled.

Today I had a health check from the firewall that failed:

20 SessionOpen  c 127.0.0.1 34319 :8081
20 ReqStart     c 127.0.0.1 34319 607335635
20 RxRequest    c HEAD
20 RxURL        c /health-check
20 RxProtocol   c HTTP/1.0
20 RxHeader     c X-Real-IP: 192.168.3.254
20 RxHeader     c Host: 192.168.3.189
20 RxHeader     c X-Forwarded-For: 192.168.3.254
20 RxHeader     c Connection: close
20 RxHeader     c User-Agent: Astaro Service Monitor 0.9
20 RxHeader     c Accept: */*
20 VCL_call     c recv lookup
20 VCL_call     c hash
20 Hash         c /health-check
20 VCL_return   c hash
20 VCL_call     c miss fetch
20 Backend      c 33 aurum aurum
20 FetchError   c http first read error: -1 11 (No error recorded)
20 VCL_call     c error deliver
20 VCL_call     c deliver deliver
20 TxProtocol   c HTTP/1.1
20 TxStatus     c 503
20 TxResponse   c Service Unavailable
20 TxHeader     c Server: Varnish
20 TxHeader     c Content-Type: text/html; charset=utf-8
20 TxHeader     c Retry-After: 5
20 TxHeader     c Content-Length: 879
20 TxHeader     c Accept-Ranges: bytes
20 TxHeader     c Date: Wed, 06 Jun 2012 12:35:12 GMT
20 TxHeader     c X-Varnish: 607335635
20 TxHeader     c Age: 60
20 TxHeader     c Via: 1.1 varnish
20 TxHeader     c Connection: close
20 Length       c 879
20 ReqEnd       c 607335635 1338986052.649786949 1338986112.648169994 0.000160217 59.997980356 0.000402689

Now the backend server (apache) does not have any 503 error in the access log at this point. So I am confused. Is this varnish throwing a 503 because it thinks apache is to slow? There is a lot traffic coming through at this point so I know the server is up and running.

I do have other 503 error codes with posts and gets so there is really no pattern. It seems to be at random times and random requests. Even in the morning when the server dosen't seem to be doing anything.

I do see another pattern in the log:

4 VCL_call     c recv pass
4 VCL_call     c hash
4 Hash         c /?id=412
4 VCL_return   c hash
4 VCL_call     c pass pass
4 FetchError   c no backend connection
4 VCL_call     c error deliver
4 VCL_call     c deliver deliver

Here fetcherror says "no backend connection". A summery of the FetchErrors in todays log:

16 FetchError   c http first read error: -1 11 (No error recorded)
 5 FetchError   c http first read error: -1 11 (No error recorded)
 4 FetchError   c http first read error: -1 11 (No error recorded)
19 FetchError   c http first read error: -1 11 (No error recorded)
 5 FetchError   c http first read error: -1 11 (No error recorded)
23 FetchError   c http first read error: -1 11 (No error recorded)
24 FetchError   c http first read error: -1 11 (No error recorded)
16 FetchError   c http first read error: -1 11 (No error recorded)
 6 FetchError   c http first read error: -1 11 (No error recorded)
 4 FetchError   c http first read error: -1 11 (No error recorded)
 5 FetchError   c http first read error: -1 11 (No error recorded)
 4 FetchError   c http first read error: -1 11 (No error recorded)
 4 FetchError   c http first read error: -1 11 (No error recorded)
22 FetchError   c http first read error: -1 11 (No error recorded)
 6 FetchError   c http first read error: -1 11 (No error recorded)
21 FetchError   c http first read error: -1 11 (No error recorded)
26 FetchError   c no backend connection
 4 FetchError   c no backend connection
20 FetchError   c http first read error: -1 11 (No error recorded)
39 FetchError   c http first read error: -1 11 (No error recorded)

I haven't changed the default timeout values for varnish. This is my configuration for one of the backend servers.

backend xenon {
    .host = "192.168.3.187";
    .port = "80";
    .probe = {
        .url = "/health-check/";
        .interval = 3s;
        .window = 5;
        .threshold = 2;
    }
}

I'm running prefork module on apache2 with this configuration

<IfModule mpm_prefork_module>
    StartServers          1
    MinSpareServers       2
    MaxSpareServers       5
    MaxClients           200
    MaxRequestsPerChild  75
</IfModule>

and only PHP files is sent to the server. Every other static file is handled by Nginx.

Any ideas?

------- EDIT --------------

Some more debuging information

I have run a varnishadm debug.health

Backend radon is Healthy
Current states  good:  5 threshold:  2 window:  5
Average responsetime of good probes: 0.002560
Oldest                                                    Newest
================================================================
4444444444444444444444444444444444444444444444444444444444444444 Good IPv4
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit
RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv
HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy
Backend xenon is Healthy
Current states  good:  5 threshold:  2 window:  5
Average responsetime of good probes: 0.002760
Oldest                                                    Newest
================================================================
4444444444444444444444444444444444444444444444444444444444444444 Good IPv4
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit
RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv
HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy
Backend iridium is Healthy
Current states  good:  5 threshold:  2 window:  5
Average responsetime of good probes: 0.000849
Oldest                                                    Newest
================================================================
4444444444444444444444444444444444444444444444444444444444444444 Good IPv4
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit
RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv
HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy
Backend aurum is Healthy
Current states  good:  5 threshold:  2 window:  5
Average responsetime of good probes: 0.002100
Oldest                                                    Newest
================================================================
4444444444444444444444444444444444444444444444444444444444444444 Good IPv4
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit
RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv
HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy

And I have been monitoring varnishstat from the two load balancers

 3224774         3.99         2.61 backend_conn - Backend conn. success
      27         0.00         0.00 backend_unhealthy - Backend conn. not attempted
      63         0.00         0.00 backend_fail - Backend conn. failures
  358798         0.00         0.29 backend_reuse - Backend conn. reuses
   21035         0.00         0.02 backend_toolate - Backend conn. was closed
  379834         0.00         0.31 backend_recycle - Backend conn. recycles
      26         0.00         0.00 backend_retry - Backend conn. retry

3217751         5.99         2.61 backend_conn - Backend conn. success
      32         0.00         0.00 backend_fail - Backend conn. failures
  364185         0.00         0.30 backend_reuse - Backend conn. reuses
   27077         0.00         0.02 backend_toolate - Backend conn. was closed
  391263         0.00         0.32 backend_recycle - Backend conn. recycles
      36         0.00         0.00 backend_retry - Backend conn. retry

Notice that none of them have reported backend_fail.

/Ronnie

Answer 1

I was running into this with Apache, and the solution was a combination of the following (note that I'm using Apache/2.4.7 (Ubuntu) + varnish 3.0.5-2 on Ubuntu 14.04 LTS in AWS EC2):

Please keep in mind that this was made for an M3.Medium instance on Amazon EC2 (1x Intel Xeon E5-2670 core + 3.75GB RAM). Adjust as necessary for your hardware!

1. In /etc/default/varnish, edit your start-up options:

    DAEMON_OPTS="-a :80 \
        -T localhost:6082 \
        -f /etc/varnish/default.vcl \
        -S /etc/varnish/secret \
        -p thread_pools=2 \
        -p thread_pool_max=600 \
        -p listen_depth=1024 \
        -p lru_interval=900 \
        -p connect_timeout=600 \
        -p max_restarts=6 \
        -s malloc,1G"
   

2. In /etc/varnish/default.vcl or whatever your VCL is, change the back-end timeouts (note that we're also setting these in /etc/default/varnish):

   backend default {
       .host = "127.0.0.1";
       .port = "8000";
       .connect_timeout = 600s;
       .first_byte_timeout = 600s;
       .between_bytes_timeout = 600s;
   }
   

3. Disable KeepAlives. This page has more information (varies depending on back-end web server software): http://www.feedthebot.com/pagespeed/keep-alive.html

For Apache, all I had to do was change line 92 in /etc/apache2/apache2.conf to the following:

    KeepAlive Off

What I think is going on here is that the KeepAlives, as implemented in the back-end web server software, are sending explicit connection resets, which Varnish doesn't work well with. There is probably more to this story, and I encourage you to dig into this and post your findings here for future generations to learn from.

Additional reading: - https://www.varnish-cache.org/trac/wiki/Future_Feature#Keepalivetimeoutonbackendconnections ( and a few more, but can't post the links. Some Googling for "varnish keepalive backend timeout" should surface what you want)

More debugging help: If you're still stuck, try doing the following: - start varnishlog -w err.log on your Varnish server - On your client, get Siege: http://www.joedog.org/siege-home/ and load it up with some of the URLs you've seen 503 (hint: urls.txt, use -i -b -c500 -r10 and it should be enough to trigger the 503s) - start varnishlog -r temp -c -m 'TxStatus:503' > err-parsed.txt. This will grab all the Varnish log entries where Varnish returned a 503. FWIW, here's the full text of one of my errors. TL;DR the error Varnish was reporting was FetchError c http first read error: -1 0 (Success) :

---

936 SessionOpen c 10.8.226.98 51895 :80 936 ReqStart c 10.8.226.98 51895 357447130 936 RxRequest c GET 936 RxURL c /ip/69.120.68.54 936 RxProtocol c HTTP/1.1 936 RxHeader c Host: 10.201.81.157 936 RxHeader c Accept: */* 936 RxHeader c Accept-Encoding: gzip 936 RxHeader c User-Agent: Mozilla/5.0 (apple-x86_64-darwin11.4.2) Siege/3.0.5 936 RxHeader c Connection: close 936 VCL_call c recv lookup 936 VCL_call c hash 936 Hash c /ip/69.120.68.54 936 Hash c 10.201.81.157 936 VCL_return c hash 936 HitPass c 357445183 936 VCL_call c pass pass 936 Backend c 103 default default 936 FetchError c http first read error: -1 0 (Success) 936 Backend c 269 default default 936 FetchError c http first read error: -1 0 (Success) 936 VCL_call c error deliver 936 VCL_call c deliver deliver 936 TxProtocol c HTTP/1.1 936 TxStatus c 503 936 TxResponse c Service Unavailable 936 TxHeader c Server: Varnish 936 TxHeader c Content-Type: text/html; charset=utf-8 936 TxHeader c Retry-After: 5 936 TxHeader c Content-Length: 418 936 TxHeader c Accept-Ranges: bytes 936 TxHeader c Date: Thu, 05 Jun 2014 23:05:48 GMT 936 TxHeader c X-Varnish: 357447130 936 TxHeader c Age: 0 936 TxHeader c Via: 1.1 varnish 936 TxHeader c Connection: close 936 Length c 418

Hope this helps!

Answer 2

If this is a busy server, which I assume it is since you state "There is a lot traffic coming through at this point so I know the server is up and running" Have you evaluated first your apache config to be able to handle the traffic influx? And second you are using nginx to proxy requests to varnish, have you set a retry value for the requests? For instance when using apache proxypass you can do something like this

ProxyPass / http://192.1.1.11:9001/ retry=3 timeout=5

This will have the proxy do n retries for those requests. Find the equivalent to this for nginx. This may help mitigate the amount of 503s however if it is a traffic issue you need to address that ultimately. Also you may want to consider haproxy rather than nginx for proxying in this way since that is what it is made for.

Answer 3

503 means no healthy backend available. Apache didnt respond to probe with in timeout or with 200

varnishadm backend.health

Can give backend health status. This is the reason why your Apache logs doesn't have 503 logged

Answer 4

I hope it is a typo, but you mentioned that there were no errors in the access logs? The errors would be in the error log (-: Check there, if you haven't already? The file is called error_log. Also, check your httpd.conf for error log level. Try setting it to debug, and restart to see more details in the error logs. I believe the default is warn. Note, there is a performance overhead with debug, so do did until you have the necessary data and set it back to warn.

Another item to consider is increasing/tuning some of your prefork settings. If you're seeing a lot of traffic" coming through, these are too low -- IMO. Here are defaults on my RHEL 6.1, apache 2.2:

<IfModule prefork.c>
    StartServers            8
    MinSpareServers         5
    MaxSpareServers        20
    ServerLimit           256
    MaxClients            256
    MaxRequestsPerChild  4000
</IfModule>

The optimal settings depend on your install of apache and the hardware you are running -- memory, CPUs etc. I would start by gently increasing the first three. See Apache MPM prefork for more info on these parameters.