Bad Gateway 502 error with Apache mod proxy and Tomcat

Question

We re running a web app on Tomcat 6 and Apache mod proxy 2 2 3   Seeing a lot of 502 errors like this      Bad Gateway    The proxy server received an invalid response from an upstream server       The proxy server could not handle the request GET  the page do       Reason  Error reading from remote server      If you think this is a server error  please contact the webmaster       Error 502    Tomcat has plenty of threads  so it s not thread-constrained  We re pushing 2400 users via JMeter against the app   All the boxes are sitting inside our firewall on a fast unloaded network  so there shouldn t be any network problems     Anyone have any suggestions for things to look at or try   We re heading to tcpdump next   UPDATE 10 21 08  Still haven t figured this out  Seeing only a very small number of these under load   The answers below haven t provided any magical answers   yet

User · Answer

I m guessing your using mod proxy http  or proxy balancer    Look in your tomcat logs  localhost log  or catalina log  I suspect your seeing an exception in your web stack bubbling up and closing the socket that the tomcat worker is connected to

User · Answer

I know this does not answer this question  but I came here because I had the same error with nodeJS server  I am stuck a long time until I found the solution  My solution just adds slash or  in end of proxyreserve apache    my old code is   ProxyPass   http   192 168 1 1 3001 ProxyPassReverse   http   192 168 1 1 3001   the correct code is   ProxyPass   http   192 168 1 1 3001  ProxyPassReverse   http   192 168 1 1 3001

User · Answer

So  answering my own question here   We ultimately determined that we were seeing 502 and 503 errors in the load balancer due to Tomcat threads timing out   In the short term we increased the timeout   In the longer term  we fixed the app problems that were causing the timeouts in the first place   Why Tomcat timeouts were being perceived as 502 and 503 errors at the load balancer is still a bit of a mystery

User · Answer

You can avoid global timeouts or having to virtual hosts by specifying the proxy timeouts in the ProxyPass directive as follows   ProxyPass  svc http   example com svc timeout 600 ProxyPassReverse  svc http   example com svc timeout 600   Notice timeout 600 seconds   However this does not always work when you have load balancer  In that case you must add the timeouts in both the places  tested in Apache 2 2 31   Load Balancer example    lt Proxy  balancer   mycluster  gt       BalancerMember  http   member1 8080 svc  timeout 600      BalancerMember  http   member2 8080 svc  timeout 600  lt  Proxy gt    ProxyPass  svc  balancer   mycluster  timeout 600 ProxyPassReverse  svc  balancer   mycluster  timeout 600   A side note  the timeout 600 on ProxyPass was not required when Chrome was the client  I don t know why  but without this timeout on ProxyPass Internet Explorer  11  aborts saying connection reset by server   My theory is that the    ProxyPass timeout is used between the client browser  and the Apache   BalancerMember timeout is used between the Apache and the backend   To those who use Tomcat or other backed you may also want to pay attention to the HTTP Connector timeouts

User · Answer

you should be able to get this problem resolved through a timeout and proxyTimeout parameter set to 600 seconds   It worked for me after battling for a while

User · Answer

You can use    proxy-initial-not-pooled  See http   httpd apache org docs 2 2 mod mod proxy http html       If this variable is set no pooled connection will be reused if the client connection is an initial connection  This avoids the  proxy  error reading status line from remote server  error message caused by the race condition that the backend server closed the pooled connection after the connection check by the proxy and before data sent by the proxy reached the backend  It has to be kept in mind that setting this variable downgrades performance  especially with HTTP 1 0 clients    We had this problem  too  We fixed it by adding  SetEnv proxy-nokeepalive 1 SetEnv proxy-initial-not-pooled 1   and turning keepAlive on all servers off   mod proxy http is fine in most scenarios but we are running it with heavy load and we still got some timeout problems we do not understand    But see if the above directive fits your needs

User · Answer

So  answering my own question here   We ultimately determined that we were seeing 502 and 503 errors in the load balancer due to Tomcat threads timing out   In the short term we increased the timeout   In the longer term  we fixed the app problems that were causing the timeouts in the first place   Why Tomcat timeouts were being perceived as 502 and 503 errors at the load balancer is still a bit of a mystery

User · Answer

Just to add some specific settings  I had a similar setup  with Apache 2 0 63 reverse proxying onto Tomcat 5 0 27    For certain URLs the Tomcat server could take perhaps 20 minutes to return a page   I ended up modifying the following settings in the Apache configuration file to prevent it from timing out with its proxy operation  with a large over-spill factor in case Tomcat took longer to return a page    Timeout 5400 ProxyTimeout 5400     Some backgound  ProxyTimeout alone wasn t enough  Looking at the documentation for Timeout I m guessing  I m not sure  that this is because while Apache is waiting for a response from Tomcat  there is no traffic flowing between Apache and the Browser  or whatever http client  - and so Apache closes down the connection to the browser   I found that if I left the Timeout setting at its default  300 seconds   then if the proxied request to Tomcat took longer than 300 seconds to get a response the browser would display a  502 Proxy Error  page  I believe this message is generated by Apache  in the knowledge that it s acting as a reverse proxy  before it closes down the connection to the browser  this is my current understanding - it may be flawed    The proxy error page says      Proxy Error      The proxy server received an invalid   response from an upstream server  The   proxy server could not handle the   request GET       Reason  Error reading from remote server      which suggests that it s the ProxyTimeout setting that s too short  while investigation shows that Apache s Timeout setting  timeout between Apache and the client  that also influences this

User · Answer

Most likely you should increase Timeout parameter in apache conf  default value 120 sec

User · Answer

Sample from apache conf    Default value is 2 minutes   Timeout 600   ProxyRequests off ProxyPass  app balancer   MyApp stickysession JSESSIONID lbmethod bytraffic nofailover On ProxyPassReverse  app balancer   MyApp ProxyTimeout 600  lt Proxy balancer   MyApp gt      BalancerMember http   node1 8080  route node1 retry 1 max 25 timeout 600                lt  Proxy gt

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If you want to handle your webapp s timeout with an apache load balancer  you first have to understand the different meaning of timeout  I try to condense the discussion I found here  http   apache-http-server 18135 x6 nabble com mod-proxy-When-does-a-backend-be-considered-as-failed-td5031316 html    It appears that mod proxy considers a backend as failed only when the transport layer connection to that backend fails  Unless failonstatus failontimeout is used       So  setting failontimeout is necessary for apache to consider a timeout of the webapp  e g  served by tomcat  as a fail  and consecutively switch to the hot spare server   For the proper configuration  note the following misconfiguration   ProxyPass   balancer   localbalance  failontimeout on timeout 10 failonstatus 50  This is a misconfiguration because   You are defining a balancer here  so the timeout parameter relates to the balancer  like the two others   However for a balancer  the timeout parameter is not a connection timeout  like the one used with BalancerMember   but the maximum time to wait for a free worker member  e g  when all the workers are busy or in error state  the default being to not wait    So  a proper configuration is done like this  set timeout at the BalanceMember level     lt Proxy balancer   mycluster gt       BalancerMember http   member1 8080 svc timeout 6       more BalanceMembers here  lt  Proxy gt    set the failontimeout on the balancer  ProxyPass  svc balancer   mycluster failontimeout on  Restart apache

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