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.
