Tag Archives: make

Erlang Release Handling with Fab and Reltools

You’ve already got a first target system installed, and now you’ve written some new code and want to deploy it. This article will show you how to setup make and fab commands that use reltools to build & install new releases.


Your code should be part of an OTP application structure. Additionally, you will need an appup file in the ebin/ directory for each application you want to upgrade. There’s a lot you can do in an appup file:

Once you’ve updated app files with the newest version and configuration and created appup files with all the necessary commands, you’re ready to create a new release.

Note: The app configuration will always be updated to the newest version, even if you have no appup commands.


To create a new release, you’ll need a new rel file, which I’ll refer to as NAME-VSN.rel. VSN should be greater than your previous release version. My usual technique is to copy my latest rel file to NAME-VSN.rel, then update the release VSN and all the application versions.

Note: reltools assumes that the rel file will be in $ROOTDIR/releases/, where $ROOTDIR defaults to code:root_dir(). This path is also used below in the make and fab commands. You can pass a different value for $ROOTDIR, but releases/ is hard coded. This may change in the future, but for now your rel files must be in $ROOTDIR/releases/ if you want to use reltools.


Before you finalize the new release, make sure reltools is in your code path. There 2 ways to do this:

  1. Make a copy of reltools and add it to your application.
  2. Clone elib and add it to your code path with erl -pa PATH/TO/elib/ebin.

If you choose option 1, be sure to include reltools in your app modules, and add it to your appup file with {add_module, reltools}.

But I’ll assume you want option 2 because it provides cleaner code separation and easier release handling. Keeping elib external means you can easily pull new code, and only need to add the elib application to your rel file with the latest vsn.

Make Upgrade

Now that you have a new release defined, and elib is in your code path, you’re ready to build release upgrade packages. Below is the make command I use to call reltools:make_upgrade("NAME-VSN"). Be sure to update PATH/TO/ to your particular code paths.

ERL=erl # use default erl command, but can override path on command line

src: FORCE
    @$(ERL) -pa lib/*/ebin -make # requires an Emakefile

upgrade: src
    @$(ERL) -noshell \                          # run erlang with no shell
        -pa lib/*/ebin \                        # include your local code repo
        -pa PATH/TO/elib/ebin \                 # include elib
        -pa PATH/TO/erlang/lib/*/ebin \         # include local erlang libs
        -run reltools make_upgrade $(RELEASE) \ # run reltools:make_upgrade
        -s init stop                            # stop the emulator when finished

FORCE: # empty rule to force run of erl -make

Using the above make rules, you can do make upgrade RELEASE=PATH/TO/releases/NAME-VSN to build a release upgrade package. Once you can do this locally, you can use fab to do remote release builds and installs. But in order to build a release remotely, you need to get the code onto the server. There are various ways to do this, the simplest being to clone your repo on the remote server(s), and push your updates to each one.

fab release build install

Below is an example fabfile.py for building and installing releases remotely using fab. Add your own hosts and roles as needed.

PATH/TO/TARGET should be the path to your first target system.

release is a separate command so that it you are only asked for NAME-VSN once, no matter how many hosts you build and install on.

build will run make upgrade RELEASE=releases/NAME-VSN on the remote system, using the target system’s copy of erl. Theoretically, you could build a release package once, then distribute it to each target system’s releases/ directory. But that requires each target system being exactly the same, with all the same releases and applications installed. If that’s the case, modify the above recipe to run build on a single build server, have it put the release package into all the other node’s releases/ directory, then run install on each node.

install uses _rpcall to run rpc:call(NODE@HOST, reltools, install_release, ["NAME-VSN"]). I’ve kept _rpcall separate so you can see how to define your own fab commands by setting env.mfa.

from fabric.api import env, prompt, require, run

env.erl = 'PATH/TO/TARGET/bin/erl'

def release():
    '''Prompt for release NAME-VSN. rel file must be in releases/.'''
    prompt('Specify release as NAME-VERSION:', 'release',

def build():
    '''Build upgrade release package.'''
    run('cd PATH/TO/REPO && hg up && make upgrade ERL=%s RELEASE=releases/%s' % (env.erl, env.release))

def install():
    '''Install release to running node.'''
    env.mfa = 'reltools,install_release,["%s"]' % env.release

def _rpccall():
    evalstr = 'io:format(\"~p~n\", [rpc:call(NODE@%s, %s)])' % (env.host, env.mfa)
    # NOTE: local user must have same ~/.erlang.cookie as running nodes
    run("%s -noshell -sname fab -eval '%s' -s init stop" % (env.erl, evalstr))


Once you’ve updated your Makefile and created fabfile.py, your workflow can be something like this:

  1. Write new application code.
  2. Update the app and appup files for each application to upgrade.
  3. Create a new rel file as releases/NAME-VSN.rel.
  4. Commit and push your changes.
  5. Run fab release build install.
  6. Enter NAME-VSN for your new release.
  7. Watch your system hot upgrade in real-time 🙂


Sometimes reltools:install_release(NAME-VSN) can fail, usually when the release_handler can’t find an older version of your code. In this case, your new release will be unpacked but not installed. You can see the state of all the known releases using release_handler:which_releases().. This can usually be fixed by removing old releases and trying again. Shell into your target system and do something like this (where OLDVSN is the VSN of a release marked as old):

See the release_handler manual for more information.

release_handler:remove_release("OLDVSN"). % repeat as necessary

Test Driven Development in Python

One of my favorite aspects of Python is that it makes practicing TDD very easy. What makes it so frictionless is the doctest module. It allows you to write a test at the same time you define a function. No setup, no boilerplate, just write a function call and the expected output in the docstring. Here’s a quick example of a fibonacci function.

def fib(n):
        '''Return the nth fibonacci number.
        >>> fib(0)
        >>> fib(1)
        >>> fib(2)
        >>> fib(3)
        >>> fib(4)
        if n == 0:
                return 0
        elif n == 1:
                return 1
                return fib(n - 1) + fib(n - 2)

If you want to run your doctests, just add the following three lines to the bottom of your module.

if __name__ == '__main__':
        import doctest

Now you can run your module to run the doctests, like python fib.py.

So how well does this fit in with the TDD philosophy? Here’s the basic TDD practices.

  1. Think about what you want to test
  2. Write a small test
  3. Write just enough code to fail the test
  4. Run the test and watch it fail
  5. Write just enough code to pass the test
  6. Run the test and watch it pass (if it fails, go back to step 4)
  7. Go back to step 1 and repeat until done

And now a step-by-step breakdown of how to do this with doctests, in excruciating detail.

1. Define a new empty method

def fib(n):
'''Return the nth fibonacci number.'''

if __name__ == '__main__':
import doctest

2. Write a doctest

def fib(n):
        '''Return the nth fibonacci number.
        >>> fib(0)

3. Run the module and watch the doctest fail

python fib.py
File "fib1.py", line 3, in __main__.fib
Failed example:
Got nothing
1 items had failures:
   1 of   1 in __main__.fib
***Test Failed*** 1 failures.

4. Write just enough code to pass the failing doctest

def fib(n):
        '''Return the nth fibonacci number.
        >>> fib(0)
        return 0

5. Run the module and watch the doctest pass

python fib.py

6. Go back to step 2 and repeat

Now you can start filling in the rest of function, one test at time. In practice, you may not write code exactly like this, but the point is that doctests provide a really easy way to test your code as you write it.

Unit Tests

Ok, so doctests are great for simple tests. But what if your tests need to be a bit more complex? Maybe you need some external data, or mock objects. In that case, you’ll be better off with more traditional unit tests. But first, take a little time to see if you can decompose your code into a set of smaller functions that can be tested individually. I find that code that is easier to test is also easier to understand.

Running Tests

For running my tests, I use nose. I have a tests/ directory with a simple configuration file, nose.cfg


Then in my Makefile, I add a test command so I can run make test.

        @nosetests --config=tests/nose.cfg tests PACKAGE1 PACKAGE2

PACKAGE1 and PACKAGE2 are optional paths to your code. They could point to unit test packages and/or production code containing doctests.

And finally, if you’re looking for a continuous integration server, try Buildbot.

Static Analysis of Erlang Code with Dialyzer

Dialyzer is a tool that does static analysis of your erlang code. It’s great for identifying type errors and unreachable code. Here’s how to use it from the command line.


Pretty simple! PATH/TO/APP should be an erlang application directory containing your ebin/ and/or src/ directories. PATH/TO/INCLUDE should be a path to a directory that contains any .hrl files that need to be included. The -I is optional if you have no include files. You can have as many -r and -I options as you need. If you add -q, then dialyzer runs more quietly, succeeding silently or reporting any errors found.

If you have a test/ directory with Common Test suites, then you’ll want to add “-I /usr/lib/erlang/lib/test_server*/include/” and “-I /usr/lib/erlang/lib/common_test*/include/”. I’ve actually set this up in my Makefile to run as make check. It’s been great for catching bad return types, misspellings, and wrong function parameters.

Unit Testing with Erlang's Common Test Framework

One of the first things people look for when getting started with Erlang is a unit testing framework, and EUnit tends to be the framework of choice. But I always had trouble getting EUnit to play nice with my code since it does parse transforms, which screws up the handling of include files and record definitions. And because Erlang has pattern matching, there’s really no reason for assert macros. So I looked around for alternatives and found that a testing framework called common_test has been included since Erlang/OTP-R12B. common_test (and test_server), are much more heavy duty than EUnit, but don’t let that scare you away. Once you’ve set everything up, writing and running unit tests is quite painless.

Directory Setup

I’m going to assume an OTP compliant directory setup, specifically:

  1. a top level directory we’ll call project/
  2. a lib/ directory containing your applications at project/lib/
  3. application directories inside lib/, such as project/lib/app1/
  4. code files are in app1/src/ and beam files are in app1/ebin/

So we end up with a directory structure like this:


Test Suites

Inside the app1/ directory, create a directory called test/. This is where your test suites will go. Generally, you’ll have 1 test suite per code module, so if you have app1/src/module1.erl, then you’ll create app1/test/module1_SUITE.erl for all your module1 unit tests. Each test suite should look something like this: (unfortunately, wordpress doesn’t do syntax highlighting for erlang, so it looks kinda crappy)


% easier than exporting by name

% required for common_test to work

%% common test callbacks %%

% Specify a list of all unit test functions
all() -> [test1, test2].

% required, but can just return Config. this is a suite level setup function.
init_per_suite(Config) ->
    % do custom per suite setup here

% required, but can just return Config. this is a suite level tear down function.
end_per_suite(Config) ->
    % do custom per suite cleanup here

% optional, can do function level setup for all functions,
% or for individual functions by matching on TestCase.
init_per_testcase(TestCase, Config) ->
    % do custom test case setup here

% optional, can do function level tear down for all functions,
% or for individual functions by matching on TestCase.
end_per_testcase(TestCase, Config) ->
    % do custom test case cleanup here

%% test cases %%

test1(Config) ->
    % write standard erlang code to test whatever you want
    % use pattern matching to specify expected return values

test2(Config) -> ok.

Test Specification

Now the we have a test suite at project/app1/test/module1_SUITE.erl, we can make a test specification so common_test knows where to find the test suites, and which suites to run. Something I found out that hard way is that common_test requires absolute paths in its test specifications. So instead of creating a file called test.spec, we’ll create a file called test.spec.in, and use make to generate the test.spec file with absolute paths.


{logdir, "@PATH@/log"}.
{alias, app1, "@PATH@/lib/app1"}.
{suites, app1, [module1_SUITE]}.


    erl -pa lib/*/ebin -make

test.spec: test.spec.in
    cat test.spec.in | sed -e "s,@PATH@,$(PWD)," > $(PWD)/test.spec

test: test.spec src
    run_test -pa $(PWD)/lib/*/ebin -spec test.spec

Running the Tests

As you can see above, I also use the Makefile for running the tests with the command make test. For this command to work, run_test must be installed in your PATH. To do so, you need to run /usr/lib/erlang/lib/common_test-VERSION/install.sh (where VERSION is whatever version number you currently have). See the common_test installation instructions for more information. I’m also assuming you have an Emakefile for compiling the code in lib/app1/src/ with the make src command.

Final Thoughts

So there you have it, an example test suite, a test specification, and a Makefile for running the tests. The final file and directory structure should look something like this:


Now all you need to do is write your unit tests in the form of test suites and add those suites to test.spec.in. There’s a lot more you can get out of common_test, such as code coverage analysis, HTML logging, and large scale testing. I’ll be covering some of those topics in the future, but for now I’ll end with some parting thoughts from the Common Test User’s Guide:

It’s not possible to prove that a program is correct by testing. On the contrary, it has been formally proven that it is impossible to prove programs in general by testing.

There are many kinds of test suites. Some concentrate on calling every function or command… Some other do the same, but uses all kinds of illegal parameters.

Aim for finding bugs. Write whatever test that has the highest probability of finding a bug, now or in the future. Concentrate more on the critical parts. Bugs in critical subsystems are a lot more expensive than others.

Aim for functionality testing rather than implementation details. Implementation details change quite often, and the test suites should be long lived.

Aim for testing everything once, no less, no more