Apache Jakarta and Beyond: A Java Programmeramp;#039;s Introduction [Electronic resources] نسخه متنی

7.2. Distributed Testing

Load testing inadvertently puts stress on two computers. Not only do the incoming requests push the limits of the server, but all the test threads and data verification and result presentation puts a heavy burden on the machine doing the testing. One implication of this is that the testing machine may run short of resources before the server does, leading to inaccurate results. This is especially true if the server is being hosted on a production-class machine with lots of memory and multiple processors.

The solution is to run the same test from multiple computers all testing against the same server. This reduces the burden on any one computer while allowing a more rigorous overall test. This also provides a more realistic test, as in the real world multiple users from many computers will be hitting the server simultaneously. It is not at all inconceivable that a system will exhibit different behavior when handling requests from many computers than it would when handling the same number of requests from a single computer.

Manually starting a test from several computers is a burden, even if a system like X windows or PC Anywhere is used to allow all control to happen from one computer. Besides the cumbersome need to start the tests on all these machines, the resulting logs and other reports will need to be gathered and combined once the test has completed. JMeter addresses these problems by providing for distributed testing.

To use distributed testing one or more instances of JMeter will be started in

server mode, which is done by adding the -s option on the command line. These are the instances of JMeter, which will perform the tests, so calling them "server instances" may be slightly misleading. They act as clients against the Web application to be tested, but they act as servers to a master instance of JMeter, which will tell them when to start and stop testing.

Server instances may be left running indefinitely. When they are not engaged in testing, they will consume very few resources on their host computer.

The JMeter instance from which the tests will be controlled must be made aware of the other instances. This can be done from the command line by use of the -Jremote_hosts= option, which takes a comma-separated list of computer names or IP addresses. Alternately, the list of JMeter.servers can be placed in the jmeter.properties file, which lives in the bin directory of the JMeter