CONCEPTS
As always some concepts are needed before the entry goes into the substance. Mainly Application Servers have three different clustered solutions.
Load Balancing + Stickyness
The first and more easy clustered solution is just a setup with two or more applications servers which receive clients in a sticky way. When I say stickyness I refer to the fact that when a user request has first sent towards a specified server (and the java session has been created in this server) the load balancing element always sends his future requests to the same server. This way the session is maintained in only one server but everything works as expected.
In this scenario there is no special session treatment and, therefore, if one application server dies all its sessions (clients) are lost. The balancer will detect the server has fallen and all new requests will be redirected against another server but, as sessions are stored individually, the user will have to re-login and previous work (in a shopping cart application for instance) could be lost.
As you see this solution is very very simple. There are lots of load balancers solutions (software or hardware) and almost all of them support stickyness (using jsessionid tracking cookie) and server status checks. Stickyness can also be used in the other two solutions and, in fact, it is usually recommended.
Load Balancing + Session Replication
The second solution tries to solve the session lost problem of the previous scenario. Many application servers (all famous ones at least) implement a in-memory session replication. This way when a request modifies any attribute in the session these changes are sent to the rest of servers and, consequently, session is always up to date in all of them.
This solution solves the main problem but it is not absolutely safe. Some problems are the following: all sessions are in all serves (this is the main one, think about a ten server cluster with thousands and thousands of sessions), replication is not immediately done, performance drops when a lot of sessions are involved or they are very big and, if all servers die unexpectedly, they are also lost.
Load Balancing + HA Storage
The final solution is to use another persistent element to store sessions (Application Server can save sessions only in the repository or in both sites, its own heap and in the new repository). This solution has two main problems, the High Availability feature is moved from Application Servers to the external repository and the performance penalty of storing and retrieving any session may be major.
DEMO
The third solution presented before used to be not very common, mainly because the repository for storing sessions traditionally was a database, which represents a severe performance impact and a clustered database was also needed (you know, the box was worth more than its contents). So usually AppServer HA solutions were reduced to the first (when session lost was not a decisive penalty) or the second scenario (when a session-aware cluster was really needed). Nevertheless, after memcached, the in-memory backends are getting more and more popular and this kind of external repositories can solve, in theory, all the typical problems of JDBC stores. When I heard about this idea (as I commented in the beginning of the entry some commercial Application Servers are starting to offer this solution) I checked if there was any open source implementation for doing the same and I found the memcached-session-manager or MSM project. MSM honors its name and it is exactly that, a session manager for tomcat (6 and 7) and memcached. So the rest of this chapter I am going to explain how to setup the previous three scenarios using tomcat.
lenglui
http://www.linkedin.com/groups/Measure-tomcat-Performance-with-JMeter-3863637.S.64805236
Maria Gabriela
Measure tomcat Performance with JMeter
The advantage of using JMeter is that you can use it to create a graph which is visually more appealing to many administrators. If you wanted to use JMeter on a Linux system you can install it by looking in the repository for the specific distribution. In this example with Debian 6 it was easily installed on a GUI desktop system.
how to use jmeter on debian6
http://www.google.com.my/search?q=how+to+use+jmeter+on+debian6&sugexp=chrome,mod=6&sourceid=chrome&ie=UTF-8
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