Testing Applications Instead of Libraries

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The Zen of Application Test Suites

Serious testing for serious software (work in progress)


This document is about testing applications — it's not about how to write tests. Application test suites require a different, more disciplined approach than library test suites. I describe common misfeatures experienced in large application test suites and follow with recommendations on best practices. Much of what I describe below is generic and applies to test suites written in any programming language, despite many examples being written in Perl.


I often speak with developers who take a new job and they describe a Web site built out of a bunch of separate scripts scattered randomly through directories, lots of duplicated code, poor use of modules, with embedded SQL and printing HTTP headers and HTML directly. The developers shake their head in despair, but grudgingly admit an upside: job security. New features are time-consuming to add, changes are difficult to implement and may have wide-ranging side-effects, and reorganizing the codebase to have a proper separation of concerns, to make it cheaper and safer to hack on, will take lots and lots of time.

A bunch of randomly scattered scripts, no separation of concerns, lots of duplicated code, poor use of modules, SQL embedded directly in them? Does this sound familiar? It's your standard test suite. We're horrified by this in the code, but don't bat an eyelash at the test suite.

Part of this is because much, if not most, of the testing examples we find focus on testing distributions, not applications. If you were to look at the tests for my module DBIx::Class::EasyFixture, you'd see the following tests:


These tests were added one by one, as I added new features to DBIx::Class::EasyFixture and each *.t file represents (more or less) a different feature.

For a small distribution, this isn't too bad because it's very easy to keep it all in your head. With only nine files, it's trivial to glance at them, or grep them, to figure out where the relevant tests are. Applications, however, are a different story. This is the number of files from one of my customer's test suites:

$ find t -type f | wc -l

That's actually fairly small. One codebase I worked on had close to a million lines of code with thousands of test scripts. You couldn't hold the codebase in your head, you're couldn't glance at the tests to figure out what went where, nor was grepping necessarily going to tell you as tests for particular sections of code were often scattered around multiple test scripts. And, of course, I regularly heard the lament that I've heard at many shops with larger codebases: where are the tests for feature X? Instead of just sitting down and writing code, the developers are hunting for the tests, wondering if there are any tests for the feature they're working on and, if not, trying to figure out where to put their new tests.

Unfortunately, this disorganization is only the start of the problem.

Large-scale test suites

I've worked with many companies with large test suites and they tend to share some common problems. I list them in below in the order I try to address these problems (in other words, roughly easiest to hardest).

  • Tests often emit warnings
  • Tests often fail ("oh, that sometimes fails. Ignore it.")
  • There is little evidence of organization
  • Much of the testing code is duplicated
  • Testing fixtures are frequently not used (or poorly used)
  • Code coverage is spotty
  • They take far too long to run

Problems are one thing, but what features do we want to see in large-scale test suites?

  • Tests should be very easy to write and run
  • They should run relatively quickly
  • The order in which tests run should not matter
  • Test output should be clean
  • It should be obvious where to find tests for a particular piece of code
  • Testing code should not be duplicated
  • Code coverage should be able to analyze different aspects of the system

Let's take a look at some of the problems and try to understand their impacts. While it's good to push a test suite into a desirable state, often this is risky if the underlying problems are ignored. I will offer recommendations for resolving each problem, but it's important to understand that these are recommendations. They may not apply to your situation.

Tests often emit warnings

This seems rather innocuous. Sure, code emits warnings and we're used to that. Unfortunately, we sometimes forget that warnings are warnings: there might very well be something wrong. In my time at the BBC, one of the first things I did was try to clean up all of the warnings. One was a normal warning about use of an undefined variable, but it was unclear to me from the code if this should be an acceptable condition. Another developer looked at it with me and realized that the variable should never be undefined: this warning was masking a very serious bug in the code, but the particular condition was not explicitly tested. By rigorously eliminating all warnings, we found it easier to make our code more correct, and in those places where things were dodgy, comments were inserted into the code to explain why warnings were suppressed. In short: the code became easier to maintain.

Another issue with warnings in the test suite is that they condition developers to ignore warnings. We get so used to them that we stop reading them, even if something serious is going on (on a related note, I often listed to developers complain about stack traces, but a careful reading of a stack trace will often reveal the exact cause of the exception). New warnings crop up, warnings change, but developers conditioned to ignore them often overlook serious issues with their code.

Recommendation: Eliminate all warnings from your test suite, but investigate each one to understand if it reflects a serious issue. Also, some tests will capture STDERR, effectively hiding warnings. Making warnings fatal while running tests can help to overcome this problem.

Tests often fail ("oh, that sometimes fails. Ignore it.")

For one client, their hour-long test suite had many failing tests. When I first started working on it, I had a developer walk me through all of the failures and explain why they failed and why they were hard to fix. Obviously this is a far more serious problem than warnings, but in the minds of the developers, they were under constant deadline pressures and as far as management was concerned, the test suite was practically a luxury, not "serious code." As a result, developers learned to recognize these failures and consoled themselves with the thought that they understood the underlying issues.

Of course, that's not really how it works. The developer explaining the test failures admitted that he didn't understand some of them and with longer test suites that routinely fail, more failures tend to crop up, but developers conditioned to accept failures tend not to notice them. They kick off the test suite, run and grab some coffee and later glance over results to see if they look reasonable (that's assuming they run all of the tests, something which often stops happening at this point). What's worse, continuous integration tools are often built to accomodate this. From the Jenkin's xUnit Plugin page:


  • Records xUnit tests
  • Mark the build unstable or fail according to threshold values

In other words, there's an "acceptable" level of failure. What's the acceptable level of failure when you debit someone's credit card, or you're sending their medical records to someone, or you're writing embedded software that can't be easily updated?

Dogmatism aside, you can make a case for acceptable levels of test failure, but you need to understand the risks and be prepared to accept them. However, for the purposes of this document, we'll assume that the acceptable level of failure is zero.

If you absolutely cannot fix a particular failure, you should at least mark the test as TODO so that the test suite can pass. Not only does this help to guide you to a clean test suite, the TODO reason is generally embedded in the test, giving the next developer a clue what's going on.

Recommendation: Do not allow any failing tests. If tests fail which do not impact the correctness of the application (such as documentation or "coding style" tests), they should be separated from your regular tests in some manner and your systems should recognize that it's OK for them to fail.

There is little evidence of organization

As mentioned previously, a common lament amongst developers is the difficulty of finding tests for the code they're working on. Consider the case of HTML::TokeParser::Simple. The library is organized like this:

└── HTML
    └── TokeParser
        ├── Simple
        │   ├── Token
        │   │   ├── Comment.pm
        │   │   ├── Declaration.pm
        │   │   ├── ProcessInstruction.pm
        │   │   ├── Tag
        │   │   │   ├── End.pm
        │   │   │   └── Start.pm
        │   │   ├── Tag.pm
        │   │   └── Text.pm
        │   └── Token.pm
        └── Simple.pm

There's a class in there named HTML::TokeParser::Simple::Token::ProcessInstruction. Where, in the following tests, would you find the tests for process instructions?

├── constructor.t
├── get_tag.t
├── get_token.t
├── internals.t
└── munge_html.t

You might think it's in the get_token.t test, but are you sure? And what's that strange munge_html.t test? Or the internals.t test? As mentioned, for a small library, this really isn't too bad. However, what if we reorganized our tests to reflect our library hierarchy?

└── tests/
    └── html/
        └── tokeparser/
            ├── simple/
            │   ├── token/
            │   │   ├── comment.t
            │   │   ├── declaration.t
            │   │   ├── tag/
            │   │   │   ├── end.t
            │   │   │   └── start.t
            │   │   ├── tag.t
            │   │   └── text.t
            │   └── token.t
            └── simple.t 

It's clear that the tests for HTML::TokeParser::Simple::Token::Tag::Start are in t/tests/html/tokeparser/simple/token/tag/start.t. And you can see easily that there is no file for processinstruction.t. This test organization not only makes it easy to find where your tests are, it's also easy to program your editor to automatically switch between the code and the tests for the code. For large test suites, this saves a huge amount of time. When I reorganized the test suite of the BBC's central metadata repository, PIPs, I followed a similar pattern and it made our life much easier.

(Note: the comment about programming your editor is important. Effective use of your editor/IDE is one of the most powerful tools in a developer's toolbox.)

Of course, your test suite could easily be more complicated and your top-level directories inside of your test directory may be structured differently:

├── unit/
├── integration/
├── api/
└── web/

Recommendation: Organize your test files to have a predictable, discoverable structure. The test suite should be much easier to work with.

Much of the testing code is duplicated

We're aghast that that people routinely cut-n-paste their application code, but we don't even notice when people do this in their test code. More than once I've worked on a test suite with a significant logic change and I've had to find this duplicated code and either change it many places or try to refactor it so that it's in a single place and then change it. We already know why duplicated code is bad, I'm unsure why we tolerate this in test suites.

Much of my work in tests has been to reduce this duplication. For example, many test scripts lists the same set of modules at the top. I did a heuristic analysis of tests on the CPAN and chose the most popular testing modules and that allowed me to change this:

use strict;
use warnings;
use Test::Exception;
use Test::Differences;
use Test::Deep;
use Test::Warn;
use Test::More tests => 42;

To this:

use Test::Most tests => 42;

You can easily use similar strategies to bundle up common testing modules into a single testing module that all of your tests use. Less boilerplate and you can easily dive into testing.

Or as a more egregious example, I often see something like this (a silly example just for illustration purposes):

my $object = Object->new($id);

is $object->attr1, $expected1, 'attr1 works';
is $object->attr2, $expected2, 'attr2 works';
is $object->attr3, $expected3, 'attr3 works';
is $object->attr4, $expected4, 'attr4 works';
is $object->attr5, $expected5, 'attr5 works';

And then a few lines later:

$object = Object->new($new_id);

is $object->attr1, $new_expected1, 'attr1 works';
is $object->attr2, $new_expected2, 'attr2 works';
is $object->attr3, $new_expected3, 'attr3 works';
is $object->attr4, $new_expected4, 'attr4 works';
is $object->attr5, $new_expected5, 'attr5 works';

And then a few lines later, the same thing ...

And in another test file, the same thing ...

Put that in its own test function and wrap those attribute tests in a loop. If this pattern is repeated in different test files, put it in a custom test library:

sub test_fetching_by_id {
    my ( $class, $id, $tests ) = @_;
    my $object = $class->new($id);

    # this causes diagnostics to display the file and line number of the
    # caller on failure, rather than reporting *this* file and line number
    local $Test::Builder::Level = $Test::Builder::Level + 1;

    foreach my $test (@$tests) {
        my ( $attribute, $expected ) = @$test;
        is $object->$attribute, $expected, "$attribute works for $class $id";

And then you call it like this:

my @id_tests = (
    { id => $id,
      tests => [
        [ attr1 => $expected1 ],
        [ attr2 => $expected2 ],
        [ attr3 => $expected3 ],
        [ attr4 => $expected4 ],
        [ attr5 => $expected5 ],
    { id => $new_id,
      tests  => [
        [ attr1 => $new_expected1 ],
        [ attr2 => $new_expected2 ],
        [ attr3 => $new_expected3 ],
        [ attr4 => $new_expected4 ],
        [ attr5 => $new_expected5 ],

for my $test ( @id_tests ){
      test_fetching_by_id( 'Object', $test->{id}, $tests->{test} );

This is a cleanly refactored data-driven approach. By not repeating yourself, if you need to test new attributes, you can just add an extra line to the data structures and the code remains the same. Or, if you need to change the logic, you only have one spot in your code where this is done. Once a developer understands the test_fetching_by_id() function, they can reuse this understanding in multiple places. Further, it makes it easier to find patterns in your code and any competent programmer is always on the lookout for patterns because those are signposts leading to cleaner designs.

Recommendation: Keep your test code as clean as your application code.

Testing fixtures are frequently not used (or poorly used)

One difference between your application code and the test suite is in an application, we often have no idea what the data will be and we try to have a clean separation of data and code.

In your test suite, we also want a clean separation of data and code (in my experience, this is very hit-or-miss), but we often need to know the data we have. We set up data to run tests against to ensure that we can test various conditions. Can we give a customer a birthday discount if they were born February 29th? Can a customer with an overdue library book check out another? If our employee number is no longer in the database, is our code properly deleted, along with the backups and the git history erased? (kidding!)

When we set up the data for these known conditions under which to test, we call the data a test fixture. Test fixtures, when properly designed, allow us generate clean, understandable tests and make it easy to write tests for unusual conditions that may otherwise be hard to analyze.

There are several common anti-patterns I see in fixtures.

  • Hard to set up and use
  • Adding them to the database and not rolling them back
  • Loading all your test data at once with no granularity

In reviewing various fixture modules on the CPAN and for clients I have worked with, much of the above routinely holds true. On top of that, documentation is often rather sparse or non-existent. Here's a (pseudo-code) example of an almost undocumented fixture system for one client I worked with and it exemplified common issues in this area.

    database => 'sales',
    client   => $client_id,
    datasets => [qw/ customers orders items order_items referrals /],

This had several problems, all of which could be easily corrected as code, but they built a test suite around these problems and had backed themselves into a corner, making their test suite dependent on bad behavior.

The business case is that my client had a product serving multiple customers and each customer would have multiple separate databases. In the above, client $client_id connects to their sales database and we load several test datasets and run tests against them. However, loading of data was not done in a transaction, meaning that there was no isolation between different test cases in the same process. More than once I caught issues where running an individual test case would often fail because it depended on data loaded by a different test case, but it wasn't always clear which test cases were coupled with which.

Another issue is that fixtures were not fine-tuned to address particular test cases. Instead, if you loaded "customers" or "referrals", you got all of them in the database. Do you need a database with a single customer with a single order and only one order item on it to test that obscure bug that occurs when a client first uses your software? There really wasn't any clean way of doing that; data was loaded in an "all or nothing" context. Even if you violated the paradigm and tried to create fine-tuned fixtures, it was very, very hard to write them due to the obscure, undocumented format needed to craft the data files for them.

Because transactions were not used and changes could not be rolled back, each *.t file would rebuild its own test database, a very slow process. Further, due to lack of documentation about the fixtures, it was often difficult to figure out which combination of fixtures to load to test a given feature. Part of this is simply due to the complex nature of the business rules, but the core issues stemmed from a poor understanding of fixtures. This client now has multiple large, slow test suites, spread across multiple repositories, all of which constantly tear down and set up databases and load large amounts of data. The test suites are both slow and fragile The time and expense to fix this problem is considerable due to how long they've pushed forward with this substandard setup.

What you generally want is the ability to easily create understandable fixtures which are loaded in a transaction, tests are run, and then changes are rolled back. The fixtures need to be fine-grained so you can tune them for a particular test case.

One attempt I've made to fix this situation is releasing DBIx::Class::EasyFixture, along with a tutorial. It does rely on DBIx::Class, the most popular ORM for Perl. This will likely make it unsuitable for some use cases.

Using them is very simple:

my $fixtures = DBIx::Class::EasyFixture->new(schema => $schema);

# run your tests

$fixtures->unload; # also unloads when out of scope

For the customer's code, we could satisfy the different database requirements by passing in different schemas. Other (well-documented) solutions, particularly those which are pure DBI based are welcome in this area.

Recommendation: Fine-grained, well-documented fixtures which are easy to create and easy to clean up.

Code coverage is poorly understood

Consider the following code:

float recip(float number) {
    return 1.0 / number;

And a sample test:

assert recip(2.0) returns .5;

Congratulations! You now have 100% code coverage of that function.

For a statically typed language, I'm probably going to be moderately comfortable with that test. Alas, for dynamically typed languages we're fooling ourselves. An equivalent function in Perl will pass that test if we use recip("2 apples") as the argument. And what happens if we pass a file handle? And would a Unicode number work? What happens if we pass no arguments? Perl is powerful and lets us write code quickly, but there's a price: it expects us to know what we're doing and passing unexpected kinds of data is a very common source of errors, but one that 100% code coverage will never (no pun intended) uncover. This can lead to false confidence.

To work around false confidence in your code, always assume that you write applications to create things and you write tests to destroy them. Testing is, and should be, an act of violence. If you're not breaking anything with your tests, you're probably doing it wrong.

Or what if you have that code in a huge test suite, but it's dead code? We tend to blindly run code coverage over our entire test suite, never considering whether or not we're testing dead code. This is because we slop our unit, integration, API and other tests all together.

Or consider the following test case:

sub forum : Tests(1) {
    my $self = shift;
    my $site = $self->test_website;
    $site->login($user, $pass);
    $site->follow_link( text => 'Off Topic' );
        title => 'What is this?',
        body  => 'This is a test'.
    }, 'We should be able to post to the forum');

Devel::Cover doesn't know which code is test code and which is not. Devel::Cover merely tells you if your application code was exercised in your tests. You can annotate your code with "uncoverable" directives to tell Devel::Cover to ignore the following code, but that potentially means sprinkling your code with annotations all over the place.

There are multiple strategies to deal with this. One of the simplest is to merely run your code coverage tools over the public-facing portions of your code, such as web or API tests. If you find uncovered code, you either have code that is not fully tested (in the sense that you don't know if your API can really use that code) or, if you cannot write an API test to reach that code, investigate if it is dead code.

You can do this by grouping your tests into subdirectories:


Alternatively, if you use Test::Class::Moose, you can tag your tests and only run coverage over tests including the tags you wish to test:

  include_tags => [qw/api/],

If you start tagging your tests by the subsystems they are testing, you can then start running code coverage on specific subsystems to determine which ones are poorly tested.

Recommendation: Run coverage over public-facing code and on different subsystems to find poor coverage.

They take far too long to run

The problem with long-running test suites is well known, but it's worth covering this again here. These are problems that others have discussed and that I have also personally experienced many times.

Perl's version of waiting for a compile

With apologies to XKCD

In the best case scenario for developers who always run that long-running test suite, expensive developer time is wasted while the test suite is running. When they launch that hour-long (or more) test suite, they frequently take a break, talk to (read: interrupt) other developers, check their Facebook, or do any number of things which equate to "not writing software." Yes, some of those things involve meetings or research, but meetings don't conveniently schedule themselves when we run tests and for mature products (those which are more likely to have long-running test suites), there's often not that much research we really need to do.

Here are some of the issues with long-running test suites:

  • Expensive developer time is wasted while the test suite runs
  • Developers often don't run the entire test suite
  • Expensive code coverage is not generated as a result
  • Code is fragile as a result

What I find particularly curious is that we accept this state of affairs. Even a back-of-the-envelope calculation can quickly show significant productivity benefits that will pay off in the long run by taking care of our test suite. I once reduced a BBC test suite's run time from one hour and twenty minutes down to twelve minutes (Note: today I use a saner approach that results in similar or greater performance benefits). We had six developers on that team. When the test suite took over an hour to run, they often didn't run the test suite. They would run tests on their section of code and push their code when they were comfortable with it. This led to other developers finding buggy code and wasting time trying to figure out how they broken it when, in fact, someone else broke the code.

But let's assume each developer was running the test suite at least once a day (I'm careful about testing and often ran mine twice a day). By cutting test suite run time by over an hour, we reclaimed a full day of developer productivity every day! Even if it takes a developer a month to increase perfomance by that amount it pays for itself many times over very quickly. Why would you not do this? As a business owner, wouldn't you want your developers to save time on their test suite so they can create features faster for you?

There are several reasons why this is difficult. Tasking a developer with a block of time to speed up a test suite means the developer is not creating user-visible features during that time. For larger test suites, it's often impossible to know in advance just how much time you can save or how long it will take you to reach your goal. In most companies I've worked with, the people who can make the decision to speed up the test suite are often not the people feeling the pain. Productivity and quality decrease slowly over time, leading to the boiling frog problem.

What's worse: in order to speed up your test suite without affecting behavior, the test suite often has to be "fixed" (eliminating warnings, failures, and reducing duplication) to ensure that no behavior has been changed during the refactor.

Finally, some developers simply don't have the background necessary to implement performance optimizations. While performance profiles such as Perl's Devel::NYTProf can easily point out problem areas in the code, it's not always clear how to overcome the discovered limitations.

The single biggest factor in poor test suite performance for applications is frequently I/O. In particular, working with the database tends to be a bottleneck and there's only so much database tuning that can be done. After you've profiled your SQL and optimized it, several database-related optimizations which can be considered are:

  1. Using transactions to clean up your database rather than rebuilding the database
  2. Only connect to the database once per test suite (hard when you're using a separate processe per test file)
  3. If you must rebuild the database, maintain a pool of test databases and assign them as needed, rebuilding used ones in the background
  4. Use smaller database fixtures instead of loading everything at once

After you've done all you can to improve your database access, you may find that your test suite is "fast enough", but if you wish to go further, there are several steps you can take.

Use Test::Aggregate

Test::Aggregate can often double the speed of your test suite (I've had it speed up test suites by around 65%). It does this by taking your separate *.t files and runs them in a single process. Not all tests can be run this way (tests that munge global state without cleaning up are prime examples), but it's the easiest way to get a quick boost to test suite performance.

Aggressively search for and remove duplicated tests.

For poorly organized test suites, developers sometimes make the mistake of putting tests for something in a new *.t file or add them to a different *.t file, even if related tests already exist. This strategy can be time-consuming and often does not result in quick wins.

Use Devel::NYTProf aggressively.

For one test suite, I found that we were using a pure Perl implementation of JSON. As the test suite used JSON extensively, switching to JSON::XS gave us a nice performance boost. We may not have noticed that if we hadn't been profiling our code.

Look for code with "global" effects

On one test suite, I ensured that Universal::isa and Universal::can cannot be loaded. It was a quick fix and sped up the test suite by 2% (several small accumulations of improvements can add up quickly).

Inline "hot" functions.

Consider the following code which runs in about 3.2 seconds on my computer:

#!/usr/bin/env perl

use strict;
use warnings;
no warnings 'recursion';

for my $i ( 1 .. 40 ) {
    for my $j ( 1 .. $i**2 ) {
        my $y = factorial($j);

sub factorial {
    my $num = shift;
    return 1 if $num <= 1;
    return $num * factorial($num - 1);

By rewriting the recursive function as a loop, the code takes about .87 seconds:

sub factorial {
    my $num = shift;
    return 1 if $num <= 1;
    $num *= $_ for 2 .. $num - 1;
    return $num;

By inlining the calculation, the code completes in .69 seconds:

for my $i ( 1 .. 40 ) {
    for my $j ( 1 .. $i**2 ) {
        my $y = $j;
        if ( $y > 1 ) {
            $y *= $_ for 2 .. $y - 1;

In other words, in our trivial example, the inlined behavior is roughly 20% faster than the iterative function and 80% faster than the recursive function.

Recompile your Perl

You may wish to recompile your Perl to gain a performance improvement. Many Linux distributions ship with a threaded Perl by default. Depending on the version of Perl you ship with, you can gain performance improvements of up to 30% by recompiling without threads. Of course, if you use threads, you'll feel very stupid for doing this. However, if you don't make heavy use of threads, switching to a forking model for the threaded code may make the recompile worth it. Naturally, you'll need to heavily benchmark your code (preferably under production-like loads) to understand the trade-offs here.

Preload modules

If your codebase makes heavy use of modules that are slow to load, such as Moose, Catalyst, DBIx::Class and others, preloading them might help. forkprove is a utility written by Tatsuhiko Miyagawa that allows you to preload slow-loading modules and then forks off multiple processes to run your tests. Using this tool, I reduced one sample test suite's run time from 12 minutes to about a minute. Unfortunately, forkprove doesn't allow schedules, a key component often needed for larger test suites. I'll explain that in the next section.

Parallel tests

Running tests in parallel is tricky. Some tests simply can't be run with other tests. Usually these are tests which alter global state in some manner that other processes will pick up, or might cause resource starvation of some kind.

Or some tests can be run in parallel with other tests, but if several tests are updating the same records in the database at the same time, locking behavior might slow down the tests considerably.

Or maybe you're running 4 jobs, but all of your slowest tests are grouped in the same job: not good.

To deal with this, you can create a schedule that assigns different tests to different jobs, based on a set of criteria, and then puts tests which cannot run in parallel in a single job that runs after the others have completed.

You can use TAP::Parser::Scheduler to create an effective parallel testing setup. You can use this with TAP::Parser::Multiplexer to create your parallel tests. Unfortunately, as of this writing there's a bug in the Multiplexer whereby it uses select in a loop to read the parser output. If one parser blocks, none of the other output is read. Further, the schedule must be created prior to loading your test code, meaning that if your tests would prefer a different schedule, you're out of luck. Also, make test currently doesn't handle this well. There is work being done by David Golden to alleviate this problem.

My preferred solution is to use Test::Class::Moose. That has built-in parallel testing and writing schedules is very easy. Further, different test cases can simply use a Tags(noparallel) attribute to ensure that they're run sequentially after the parallel tests.

Aside from the regular benefits of Test::Class::Moose, an interesting benefit of this module is that it loads all of your test and application code into a single process and then forks off subprocesses. As a result, your code is loaded once and only once. Alternate strategies which try to fork before loading your code might still cause the code to be loaded multiple times.

I have used this strategy to reduce a 12 minute test suite to 30 seconds.

Distributed tests

Though I haven't used this module, Alex Vandiver has written TAP::Harness::Remote. This module allows you to rsync directory trees to multiple servers and run tests on those servers. Obviously, this requires multiple servers.

If you want to roll your own version of this, I've also released TAP::Stream, a module that allows you to take streams (the text, actually) of TAP from multiple sources and combine them into a single TAP document.


There is yet another interesting strategy: only run tests that exercise the code that you're changing. Johan Lindström wrote Devel::CoverX::Covered. This modules is used in conjunction with Paul Johnson's Devel::Cover to identify all the places in your tests which cover a particular piece of code. In the past, I've written tools for vim to read this data and only run relevant tests. This is a generally useful approach, but there are a couple of pitfalls.

First, if you test suite takes a long time to run, it will take much, much longer to run with Devel::Cover. As a result, I recommend that this be used with a special nightly "cover build" and have the results synched back to the developers.

Second, when changing code, it's easy to change which tests cover your code, leading to times when this technique won't cover your actual changes thoroughly. In practice, this hasn't been a problem for me, but I've not used it enough to say that with confidence.

Recommendation: Don't settle for slow test suites. Pick a goal and work to achieving that goal (it's easy to keep optimizing for too long and start getting diminishing marginal returns).


If you start creating a large Web site, do you start writing a bunch of individual scripts, each designed to handle one URL and each handling their own database access and printing their output directly to STDOUT? Of course not. Today, professional developers reach for Sinatra, Seaside, Catalyst, Ruby on Rails or other Web frameworks. They take a bit more time to set up and configure, but we know they generally save more time in the long run. Why wouldn't you do that with your test suite?

If you're using Perl, many of the problems listed in this document can be avoided by switching to Test::Class::Moose. This is a testing framework I designed to make it very easy to test applications. Once you understand it, it's actually easy to use for testing libraries, but it really shines for application testing.

Note that I now regret putting Moose in the name. Test::Class::Moose is a rewrite of Test::Class using Moose, but it's not limited to testing Moose applications. It uses Moose because internally it relies on the Moose meta-object protocol for introspection.

Out of the box you get:

  • Reporting
  • Parallel tests (which optionally accepts a custom schedule)
  • Tagging tests (slice and dice your test suite!)
  • Test inheritance (xUnit for the win!)
  • Full Moose support
  • Test control methods (startup, setup, teardown, shutdown)
  • Extensibility
  • All the testing functions and behavior from Test::Most

To learn about xUnit testing in Perl, you may wish to read a five-part tutorial I published at Modern Perl Books:

  1. Organizing test suites with Test::Class
  2. Reusing test code
  3. Making your testing life easier
  4. Using test control methods
  5. Working with Test::Class test suites

That tutorial is slightly out of date (I wrote it five years ago), but it explains effective use of Test::Class and some common anti-patterns when using it.

Doug Bell has started a tutorial for Test::Class::Moose. That also needs updating, but between those and reading the Test::Class::Moose documentation, you should be able to get up to speed fairly quickly.

About The Author

For those of you who may be reading this and are not familiar with me, I am Curtis "Ovid" Poe. I authored the test harness that ships with the Perl programming language. I wrote the well-reviewed book Beginning Perl and and am one of the authors of Perl Hacks (how's that for a redundant title?). I also sit on the Board of Directors of the Perl Foundation and am one of the people behind All Around The World, a company offering software consulting, training, and international IT recruiting.

If you'd like to hire me to fix your test suite or write software for you, drop me a line at jobs@allaroundtheworld.fr.