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Apache 2.2 ships with advanced mod_proxy set of modules that have some of the mod_jk capabilities namely AJP protocol and integrated load balancer.
AJP (Apache Jserv Protocol) with it's current version 1.3 is constant binary protocol. Constant means that the connection between web server and application server is presumed to stay open once established for the system lifetime.
This in essence makes web server and application server a single system from the users point of view.

Since there is always a chance that one of the points in the system can fail for various reasons, the mod_proxy and mod_jk as well as the application server must have some sort of connection and transport channel error detection, and act accordingly.

One of the main technological advances in mod_jk during the past few years was made in that area, and there are various techniques for communication channel error detection and recovery. At present date mod_jk is far more advanced in that area compared with mod_proxy.


The foundation of mod_jk is AJP protocol, that is sometimes known as binary http protocol. The point beside the custom binary protocol is that the client request data is already decoded inside web server, so there is no need for doubling that procedure in the application server as well. Next, the standard request and response headers are passed as atoms in a form of two byte sequence instead as strings, thus lowering down the network traffic between web and application server.

However there is one major limitation of the AJP protocol, and that is it's maximum packet size. The packet size is limited to little bit less then 8K. With latest mod_jk and Tomcat versions the packet size can be enlarged to 64K, but it's still limited. Mod_proxy still has no such capability, so it's maximum packet size is 8K. This can be problem with large client requests, especially with some custom SSO modules that store huge session data inside cookies or custom headers. In case there is a need to support the huge client requests, the only solution is to use http in favor of AJP protocol.

Encryption and SSL support

AJP protocol is not encrypted, so it should not be used with public network infrastructure. In case there is a need for securing the data transfer between web and application server because the transport media could be sniffed by outside world, then some sort of SSL tunnel must be used. The other option is to use the https protocol with mod_proxy. However using https protocol makes things a little bit more complex because one must assure to write the custom Filter in application server so that client certificates get passed transparently to the application server. AJP protocol on the other hand handles this automatically, but with the consequence of passing decrypted data between web and application server. In essence for SSL, the AJP protocol behaves like caching SSL accelerator.
This offers much higher performance because data is only decrypted once. Securing the network between web and application server by using a different network card and set of firewalls and routers is the most secure solution.
One other option is to put the web and application server on the same physical box in which case the in-memory communication will be used thus increasing the security of the entire system.

Load balancing

Recent versions of mod_jk have much advanced load balancer when compared to mod_proxy_balancer. Mod_jk has additional 'by busyness' method that load balance according to the actual application server response time.
Mod_jk also has so called load balancer maintenance to be able to handle the burst load more effectively or to decay the load to the node that was down for maintenance.
In cases where there is a need for large amount of application servers and session replication mod_jk has so called 'Domain Model Clustering' that supports new JBoss Cache's buddy replication. In essence it allows to lower down the session replication data transfer by grouping the nodes in the clusters, and doing the replication only between the members of the cluster.

Apache Httpd versions

New mod_proxy is present only with Apache httpd versions 2.2 and up. This means that the web server upgrade will be needed in case there is a need for a new mod_proxy.
Use worker-mpm with Apache httpd. Using worker mpm both mod_proxy and mod_jk have the option to tune the connection pool size between web and application server. This is needed in deployments where the Apache httpd is used to deliver some other content beside just fronting application server. For example delivering static content etc. In those cases the actual number of requests to the application server can be lower then the total number of client connections that needs to be handled by the web server. In this case the worker mpm allow to have the connection pool size lower then the number of MaxThreadsPerChild.
Windows and Netware Apache httpd versions are completely threaded, so their mpm and connection pool size can be handled in much wider range.

mod_proxy vs. mod_jk

So what to use when? It depends on your topology. In case you already have or need Apache 2.2 functionality you have the choice to choose between mod_proxy and mod_jk. Mod_jk works very well on Apache 2.2, so it all depends on the functionality needed:

  • Pros:
    • No need for a separate module compilation and maintenance. mod_proxy, mod_proxy_http, mod_proxy_ajp and mod_proxy_balancer comes as part of standard Apache 2.2+ distribution
    • Ability to use http https or AJP protocols, even within the same balancer.
  • Cons:
    • mod_proxy_ajp does not support large 8K+ packet sizes.
    • Basic load balancer
    • Does not support Domain model clustering

  • Pros:
    • Advanced load balancer
    • Advanced node failure detection
    • Support for large AJP packet sizes
  • Cons:
    • Need to build and maintain a separate module


My personal suggestion is to use the mod_jk if you can, and if you have the staff to maintain the module binary versions. Mod_proxy is still in active development and misses some of the features from mod_jk. However if there is a need to use the https or a simple load balancing scenario use the mod_proxy.
Mladen Turk

Apache Tomcat PMC

Posted by Mladen Turk Jun 18, 2007
After 72 hours of vote I was elected as third Tomcat PMC Chair just about yesterday.
It really means lot to me and I'll do my best to fulfill my duties. Being a Project Management Committee Chair for a project like Tomcat is a huge liability. Just take a look at the latest graphical presentation of the access.log from the ASF. In March we had more then 600000 downloads. It's not an typo, this is really 600K without counting any mirror sites.
Something like that makes me a little bit scary. We had more downloads then all other ASF projects together had. We have a strong developer and user community, and it seems that we were right when pushing Tomcat to the Apache TLP two years ago.
I won't for sure acting as a PMC Chair influence the way how we did things till now because it seems we are on the right track. However I'll do my best to further push the Tomcat to the user community. Things we certainly need are better documentation and better PR, but IMHO we are on the right track. As a footnote we had almost entire track on the last ApacheCon EU 2007 in Amsterdam, and beside the fact that it was Friday and the last day of the conference we had all talks crowded with people sitting on the floor. I hope we'll do something similar for next ApacheCon US this winter.
So wish me luck :)

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