Gatling Load Test Tricks and Debug

Introduction:

Gatling is a light weighted load generator. Though it is 'load test as code', sometimes it is hard to debug. Especially when there is lack of documentation and tutorial. I run into an issue when upgrading from 2.3.1 to 3.0.2. The error message is not very helpful.

Here I will show this case to illustrate an upgrading issue, as well is tricks to print out some debug information.

Some Debug Tricks:

According to gatling document, you can print out debug information in 2 ways.

• change conf/logback.xml to DEBUG/TRACE for logger name="io.gatling.http.engine.response"

This generates too much information and my tests times out.

•  Use println in exec. This is more flexible and can prints the exact information I need. How ever I would like to put the debug messages in a separate file. Here is an example:

val file = System.getProperty("file", "/tmp/debug_cookie.out")
 val scn = scenario(myReadScenario) 
 .during (myDuration) { feed(userData)
 .exec(http(myReadRequest)
 .get("something that returns pagedResultsCookie at the end")
 .queryParam("_pageSize", "1")
 .check(jsonPath("$.pagedResultsCookie").saveAs("pageCookie")))
 .asLongAs(some-condition) {
 exec(session => {
 scala.tools.nsc.io.File(file).appendAll("debug====")
 scala.tools.nsc.io.File(file).appendAll(session("pageCookie").as[String]+"\n")
 session
 })
 .exec(http(myReadRequest)
 .get("something that returns pagedResultsCookie at the end")
 .queryParam("_pageSize", "50")
 .queryParam("_pagedResultsCookie", "${pageCookie}")
 .check(jsonPath("$.pagedResultsCookie").saveAs("pageCookie")))
}

The code won't work in 3.0.2, but as an example how to print debug information tailored to my case.

It works in 2.3.1 But not in 3.0.2:

I have a test that go over each page of pagination. In 2.3.1, I used something like:

val scn = scenario(myReadScenario)
.during (myDuration) {
    feed(userData)
   .exec(http(myReadRequest)
   .get("something that returns pagedResultsCookie at the end")
   .queryParam("_pageSize", "50")
   .check(jsonPath("$.pagedResultsCookie").saveAs("pageCookie")))
   .asLongAs(session => session("pageCookie").as[String] != null) {
           exec(http(myReadRequest)
       .get("/openidm/managed/role/${name}/members")
       .queryParam("_pageSize", "50")
       .queryParam("_pagedResultsCookie", "${pageCookie}")
       .check(jsonPath("$.pagedResultsCookie").saveAs("pageCookie")))
   }
}

In 3.0.2, It throws an exception 'j.l.ClassCastException: value is null'

I have to read the gatling session source code to understand why the exception is thrown. The session("key") returns asOption. When the response has '"PagedResultsCookie": null' the asOption tries to cast the value null into String. That is the short version of the explanation. But I do not understand why it works in 2.3.1.
My current workaround is use:

.asLongAs(session => session.attributes.getOrElse("pageCookie", null) != null )

This will by pass the wrapper for session. Maybe there is a better solution, I'd be happy to know it.

References:

https://gatling.io/docs/current/whats_new/3.0/?highlight=transformresponse

https://gatling.io/docs/current/migration_guides/2.3-to-3.0/