How do we run multi concurrent sessions Websocket testing using JMeter? - websocket

Question

I was using the JMeter WebSocket Samplers to perform WebSocket latency testing. My test plan contains 4 thread groups.
Firstly, here is the order of operations I was producing, each step is grouped in a thread group. And the connection for the 1st. operation on Presenter was cached before executing the 2nd. operation because the 3rd. operation, the same Presenter, is gonna continue to use this connection.
def connection = sampler.threadLocalCachedConnection
props.put('presenterConnection', connection.get())
Same idea for the 2nd & 4th connection for Participant. The participant at step 4 is gonna continue to use the connection created at step 2.
Presenter connects to WS, creates session & saves the session id
Participant connects to WS & subscribes to the saved session
Presenter sends out commands
Participant reads the commands that were sent by Presenter
It worked as expected with single session. However I ran into an issue with the WebSocket Single Read Sampler at the 4th operation of reading commands that were sent from Presenter when I was running 2 concurrent sessions.
I changed the "Number of Threads" to 2 in the thread properties of each thread group. The presenter was able to create 2 session IDs at the 1st. operation. And I added the "Inter-Thread Communication PostProcessor" to store the session IDs in a queue such that it's able to be passed to other thread group. It worked for the first 3 operations until it went to the WebSocket Single Read Sampler in the last operation of reading the commands that were sent from the Presenter.
I had a JSR223 PreProcessor in place to use the cached connection at step 2.
def connection = props.get('Participant1Connection')
sampler.threadLocalCachedConnection.set(connection)
The issue was found in the response body of the 2 threads of the last operation. The first session ID was not being used. The first user at step 4 was using the 2nd session ID and the 2nd response body was empty because the 2 users were using the same session. What shall I do to fix this issue to have 2 users reading data from 2 sessions created accordingly?

Answer 1

This is something which normally happens when people copypaste the code without understanding what it is doing. Moreover if you need to pass data between thread groups most probably there is a problem with your test design.
Whatever.
As per JMeter Documentation:
Properties are not the same as variables. Variables are local to a thread; properties are common to all threads
So when you have > 1 thread (virtual user) the second thread overwrites the value of the presenterConnection property so no matter how many threads you have they all are trying to use single instance of the presenterConnection
My expectation is that you want a connection per thread, in this case you should add current thread number as a prefix/postfix for the property key so:
1st thread would use presenterConnection1
2nd thread would use presenterConnection2
etc.
Amendment to "your" code would be something like:
props.put('presenterConnection' + ctx.getThreadNum(), connection.get())
similarly for accessing the connection.
In the above example ctx stands for JMeterContext class instance, see Top 8 JMeter Java Classes You Should Be Using with Groovy article for more information on this and other JMeter API shorthands available for the JSR223 Test Elements.