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We run two separate tomcat-servers (tomcat 7) behind a load-balancer and an Apache (v. 2.2) working as proxy (on the same host). Every server runs about 70 microservices which are accessed from a remote Liferay system and another server-cluster containing services with business-logic.

The problem: About every one to two days, the apache that is accepting the requests from the load-balancer to pass it on to the tomcat suddenly seems not to be able to connect and is hence spawning workers until it reaches it's limit. The tomcat then is completely unreachable.

The error-message in the httpd/error.log looks like this (repeating many times):

[.... 2018] [error] (70007)The timeout specified has expired: 
ajp_ilink_receive() can't receive header

We also observed several hundred TCP connections in the status CLOSE-WAIT. Strangely we do not observe any error-message in the catalina.out. The CPU and memory-usage of the servers is within save limits.

We think the problem might be related to one of the microservices not handling connections correctly (as we observed in the thread-dump many threads using the HttpClient in the status WAITING).

Anyhow, the question remains if this could also be a configuration problem of the Tomcat or Apache, or if it is possible to change the configuration somehow to make the server more resilient.

Any suggestions? Relevant configuration information below:

tomcat server.xml

<Connector port="8080"
protocol="HTTP/1.1"
connectionTimeout="20000"
compression="on"
noCompressionUserAgents="gozilla, traviata"
compressableMimeType="text/html,text/xml,text/css,text/javascript,text/plain"
redirectPort="8443"
URIEncoding="UTF-8"
maxThreads="200"
acceptCount="800"
maxHttpHeaderSize="8192"
maxKeepAliveRequests="-1" />

<Connector port="8009" protocol="AJP/1.3" redirectPort="8443"/>

apache httpd.conf

Timeout 60
RLimitCPU max
RLimitMEM max
KeepAlive Off
MaxKeepAliveRequests 100
KeepAliveTimeout 5

<IfModule prefork.c>
StartServers       16
MinSpareServers    8
MaxSpareServers   20
ServerLimit      350
MaxClients       350
MaxRequestsPerChild  4000
</IfModule>

apache vhosts.conf

ProxyPass / ajp://localhost:8009/ keepalive=on retry=0 timeout=60 ping=120 
connectiontimeout=120
  • it looks like you are never closing connections maxKeepAliveRequests= -1 – Sum1sAdmin Apr 19 '18 at 13:13
  • @Sum1sAdmin Hi, thanks for your answer... as far as I understand it, the maxKeepAliveReguest just means that consecutive requests can be send using the same HTTP connection. It does not mean that idle connections won't timeout. The other way around: If the connection is somehow blocked locally from closing, changing the maxKeepAliveRequests wouldn't change that. (As explained here). – jannis Apr 20 '18 at 6:58
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  • ajp is port 8009, so don't look at http port 8080 – kubanczyk Apr 20 '18 at 7:24
  • @kubanczyk thanks... I added the configuration info. As there is a timeout defined on apache-side, configuration seems okay to me... – jannis Apr 20 '18 at 8:37

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