A minimal Erlang script for load testing

Recently I became interested in Erlang, a functional programming language that excels at developing concurrent applications. From a SQA perspective, concurrency immediately made me think of performance and load testing. I am not the first one to think this - see the open source application Tsung to see just how far this idea has been taken.

I set about to see how minimal an Erlang program I could write that would implement a bear-bones load tester for web apps. Here is what I came up with:

 1: -module(lmin).
 2: -export([example/0, ramp/4, vclient/2]).
 3:
 4: vclient(Supervisor, Script) ->
 5:    inets:start(),
 6:    Total = vclient_acc(Script, 0),
 7:    Supervisor ! {self(), Total / 1000000}.
 8:
 9: vclient_acc([Cmd|Remaining_cmds], Total_time) ->
10:   Fetch_time = timed_request(Cmd),
11: vclient_acc(Remaining_cmds, Fetch_time + Total_time);
12: vclient_acc([], Total_time) -> Total_time.
13:
14: timed_request({Delay, Url}) ->
15:    timer:sleep(Delay + random:uniform(Delay)),
16:    Start_time = now(),
17:    {ok, _Result} = http:request(Url),
18:    timer:now_diff(now(), Start_time).
19:
20: ramp(Nstart, Nend, _Nstep, _Script) when Nstart > Nend -> done;
21: ramp(Nstart, Nend, Nstep, Script) ->
22:    launch_vclients(Nstart, Script),
23:    collect_results(Nstart),
24:    timer:sleep(1000),
25:    ramp(Nstart+Nstep, Nend, Nstep, Script).
26:
27: launch_vclients(0, _Script) -> true;
28: launch_vclients(N, Script) ->
29:    spawn(?MODULE, vclient, [self(), Script]),
30:    launch_vclients(N-1, Script).
31:
32: collect_results(Nexpected) ->
33:    {Sum, _Results} = collect_results(Nexpected, Nexpected, [], 0),
34:    io:format("~p: ~.2f average: ~.2f~n",
35:    Nexpected, Sum/Nexpected]).
36:
37: collect_results(0, _Nexpected, Results, Sum) -> {Sum, Results};
38: collect_results(Nremaining, Nexpected, Results, Sum) ->
39:    receive
40:       {Vclient, Time} ->
41:          collect_results(Nremaining-1, Nexpected,
42:          [{Vclient, Time}|Results], Sum+Time)
43:    end.

I am not going to write a Erlang tutorial here; I am a complete Erlang newbie and would not do it justice. The Erlang documentation section is a good place to start for those who are interested. What I will do is provide a brief overview of this application.

• Lines 4-18 are the virtual clients that will send requests to the SUT (System Under Test). timed_request (lines 14-18) makes the actual request to the server; vclient_acc accumulates the response times for each command in the script. A script is a list of commands of the format {Delay, URL}. For each command in the script, the virtual client waits for a random time in the range Delay..2*Delay and then makes a request for URL.
  
• Lines 2o-30 launch the the virtual clients. Each client runs the same script and starts executing immediately when it is spawned. The key is ramp/4, which in turn hits the SUT with Nstart, (Nstart+Nstep), (Nstart+2*Nstep)... Nend virtual clients.
  
• Lines 32-42 collect and report the results.

So is this proof of concept load tester useful for anything? See my previous article, "Results using a simple Erlang load tester" to find out.