12. Buildout I

Buildout composes your application for you, according to your rules.

To compose your application you must define the eggs you need, which version, what configuration files Buildout has to generate for you, what to download and compile, and so on. Buildout downloads the eggs you requested and resolves all dependencies. You might need five different eggs, but in the end, Buildout has to install 300 eggs, all with the correct version in order to resolve all the dependencies.

Buildout does this without touching your system Python or affecting any other package. The commands created by buildout bring all the required packages into into the Python environment. Each command it creates my use different libraries or even different versions of the same library.

Plone needs folders for log files, databases and configuration files. Buildout assembles all of this for you.

You will need a lot of functionality that Buildout does not provide out of the box, so you’ll need several extensions. Some extensions provide new functionality, like mr.developer, the best way to manage your checked out sources.

Syntax

The syntax of Buildout configuration files is similar to classic ini files. You write a parameter name, an equals sign and the value. If you enter another value in the next line and indent it, Buildout understands that both values belong to the parameter name, and the parameter stores all values as a list.

A Buildout consists of multiple sections. Sections start with the section name in square brackets. Each section declares a different part of your application. As a rough analogy, your Buildout file is a cookbook with multiple recipes.

There is a special section, called [buildout]. This section can change the behavior of Buildout itself. The variable parts defines, which of the existing sections should actually be used.

Recipes

Buildout itself has no idea how to install Zope. Buildout is a plugin based system, it comes with a small set of plugins to create configuration files and download eggs with their dependencies and the proper version. To install a Zope site, you need a third-party plugin. The plugin provide new recipes that you have to declare and configure in a section.

One example is the section

[instance]
recipe = plone.recipe.zope2instance
user = admin:admin

This uses the python-package plone.recipe.zope2instance to create and configure the Zope 2 instance which we use to run Plone. All the lines after recipe = xyz are the configuration of the used recipe.

References

Buildout allows you to use references in the configuration. A variable declaration may not only hold the variable value, but also a reference to where to look for the variable value.

If you have a big setup with many Plone sites with minor changes between each configuration, you can generate a template configuration, and each site references everything from the template and overrides just what needs to be changed.

Even in smaller buildouts this is a useful feature. We are using collective.recipe.omelette. A very practical recipe that creates a virtual directory that eases the navigation to the source code of each egg.

The omelette-recipe needs to know which eggs to reference. We want the same eggs as our instance uses, so we reference the eggs of the instance instead of repeating the whole list.

Another example: Say you create configuration files for a web server like nginx, you can define the target port for the reverse proxy by looking it up from the zope2instance recipe.

Configuring complex systems always involves a lot of duplication of information. Using references in the buildout configuration allows you to minimize these duplications.

A real life example

Let us walk through the buildout.cfg for the training and look at some important variables:

[buildout]
extends =
    http://dist.plone.org/release/4.3.10/versions.cfg

# We add our own versions
    versions.cfg

versions = versions

extensions = mr.developer
# Tell mr.developer to ask before updating a checkout.
always-checkout = true
show-picked-versions = true
sources = sources

# Put checkouts in src-mrd. We keep our own package in src
sources-dir = src-mrd

# The directory this buildout is in. Modified when using vagrant.
buildout_dir = ${buildout:directory}

# We want to checkouts these eggs directly from github
auto-checkout =
#    ploneconf.site_sneak
#    starzel.votable_behavior
#    ploneconf.site

parts =
    checkversions
    codeintel
    instance
    mrbob
    packages
#    robot
    test
    zopepy
#    zopeskel

eggs =
    Plone
    Pillow

# development tools
    z3c.jbot
    plone.api
    plone.reload
    Products.PDBDebugMode
    plone.app.debugtoolbar
    Products.PrintingMailHost

# 3rd party addons
    Products.PloneFormGen
    collective.plonetruegallery
    collective.js.datatables
    eea.facetednavigation
    collective.behavior.banner

# dexterity default types
    plone.app.contenttypes

# The addon we develop in the training
#    ploneconf.site

# Voting on content
#    starzel.votable_behavior

zcml =

test-eggs +=
#    ploneconf.site [test]

[instance]
recipe = plone.recipe.zope2instance
user = admin:admin
http-address = 8080
debug-mode = on
verbose-security = on
deprecation-warnings = on
eggs = ${buildout:eggs}
zcml = ${buildout:zcml}
file-storage = ${buildout:buildout_dir}/var/filestorage/Data.fs
blob-storage = ${buildout:buildout_dir}/var/blobstorage

[test]
recipe = zc.recipe.testrunner
eggs = ${buildout:test-eggs}
defaults = ['--exit-with-status', '--auto-color', '--auto-progress']

[packages]
recipe = collective.recipe.omelette
eggs = ${buildout:eggs}
location = ${buildout:buildout_dir}/packages

[codeintel]
recipe = corneti.recipes.codeintel
eggs = ${buildout:eggs}

[checkversions]
recipe = zc.recipe.egg
eggs = z3c.checkversions [buildout]

[zopepy]
recipe = zc.recipe.egg
eggs = ${buildout:eggs}
interpreter = zopepy
scripts = zopepy

[zopeskel]
recipe = zc.recipe.egg
eggs =
    ZopeSkel
    Paste
    PasteDeploy
    PasteScript
    zopeskel.diazotheme
    zopeskel.dexterity
    zest.releaser
    ${buildout:eggs}

[mrbob]
recipe = zc.recipe.egg
eggs =
    mr.bob
    bobtemplates.plone

[sources]
# ploneconf.site = fs ploneconf.site full-path=${buildout:directory}/src/ploneconf.site
starzel.votable_behavior = git https://github.com/collective/starzel.votable_behavior.git pushurl=git@github.com:collective/starzel.votable_behavior.git

When you run ./bin/buildout without any arguments, Buildout will look for this file.

Let us look closer at some variables.

extends =
    http://dist.plone.org/release/4.3.10/versions.cfg
    versions.cfg

This line tells Buildout to read more configuration files. You can refer to configuration files on your computer or to configuration files on the Internet, reachable via http. You can use multiple configuration files to share configurations between multiple Buildouts, or to separate different aspects of your configuration into different files. Typical examples are version specifications, or configuration that differ between different environments.

eggs =
    Plone
    Pillow
    z3c.jbot
    plone.api
    plone.reload
    Products.PDBDebugMode
    plone.app.debugtoolbar
    Paste
    Products.PloneFormGen
    collective.plonetruegallery
    collective.js.datatables
    eea.facetednavigation
    collective.behavior.banner
    plone.app.contenttypes
#    ploneconf.site
#    starzel.votable_behavior

This is the list of eggs that we configure to be available for Zope. These eggs are put in the python-path of the script bin/instance with which we start and stop Plone.

The egg Plone is a wrapper without code. Among its dependencies is Products.CMFPlone which is the egg that is at the center of Plone.

The rest are addons we already used or will use later. The last eggs are commented out so they will not be installed by Buildout.

The file versions.cfg that is included by the extends = ... statement hold the version-pinnings:

[versions]
# dev tools
z3c.jbot = 0.7.2
plone.api = 1.1.0
plone.app.debugtoolbar = 1.0a3
...

This is another special section. It has become a special section by declaration. In our [buildout] section we set a variable versions = versions. This told buildout, that there is a section named versions, containing version information. When Buildout installs eggs it will use the version defined in this section.

Hello mr.developer!

There are many more important things to know, and we can’t go through them in all the detail but I want to focus on one specific feature: mr.developer

With mr.developer you can declare which packages you want to check out from which version control system and which repository URL. You can check out sources from git, svn, bzr, hg and maybe more. Also, you can say that some source are in your local file system.

mr.developer comes with a command, ./bin/develop. You can use it to update your code, to check for changes and so on. You can activate and deactivate your source checkouts. If you develop your extensions in eggs with separate checkouts, which is a good practice, you can plan releases by having all source checkouts deactivated, and only activate them, when you write changes that require a new release. You can activate and deactivate eggs via the develop command or the Buildout configuration. You should always use the Buildout way. Your commit serves as documentation.

Extensible

You might have noticed that most if not all functionality is only available via plugins. One of the things that Buildout excels at without any plugin, is the dependency resolution. You can help Plone in dependency resolution by declaring exactly which version of an egg you want. This is only one use case. Another one is much more important: If you want to have a repeatable Buildout, one that works two months from now also, you must declare all your egg versions. Else Buildout might install newer versions.

Be McGuyver

As you can see, you can build very complex systems with Buildout. It is time for some warnings. Be selective in your recipes. Supervisor is a program to manage running servers, its pretty good. There is a recipe for it.

The configuration for this recipe is more complicated than the supervisor configuration itself! By using this recipe, you force others to understand the recipes specific configuration syntax and the supervisor syntax. For such cases, collective.recipe.template is a better match.

Another problem is error handling. Buildout tries to install a weird dependency you do not actually want? Buildout will not tell you, where it is coming from.

If there is a problem, you can always run Buildout with -v, to get more verbose output, sometimes it helps.

$ ./bin/buildout -v

If strange egg versions are requested, check the dependencies declaration of your eggs and your version pinnings.

Some parts of Buildout interpret egg names case-sensitive, others won’t. This can result in funny problems.

Always check out the ordering of your extends, always use the annotate command of Buildout to see if it interprets your configuration differently than you. Restrict yourself to simple Buildout files. You can reference variables from other sections, you can even use a whole section as a template. We learned that this does not work well with complex hierarchies and had to abandon that feature.

In the chapter Buildout II: Getting ready for deployment we will have a look at a production-ready for Plone that has many useful features.