Meta-recipes

Buildout recipes provide reusable Python modules for common configuration tasks. The most widely used recipes tend to provide low-level functions, like installing eggs or software distributions, creating configuration files, and so on. The normal recipe framework is fairly well suited to building these general components.

Full-blown applications may require many, often tens, of parts. Defining the many parts that make up an application can be tedious and often entails a lot of repetition. Buildout provides a number of mechanisms to avoid repetition, including merging of configuration files and macros, but these, while useful to an extent, don’t scale very well. Buildout isn’t and shouldn’t be a programming language.

Meta-recipes allow us to bring Python to bear to provide higher-level abstractions for buildouts.

A meta-recipe is a regular Python recipe that primarily operates by creating parts. A meta recipe isn’t merely a high level recipe. It’s a recipe that defers most or all of it’s work to lower-level recipes by manipulating the buildout database.

A presentation at PyCon 2011 described early work with meta recipes.

A simple meta-recipe example

Let’s look at a fairly simple meta-recipe example. First, consider a buildout configuration that builds a database deployment:

[buildout]
parts = ctl pack

[deployment]
recipe = zc.recipe.deployment
name = ample
user = zope

[ctl]
recipe = zc.recipe.rhrc
deployment = deployment
chkconfig = 345 99 10
parts = main

[main]
recipe = zc.zodbrecipes:server
deployment = deployment
address = 8100
path = /var/databases/ample/main.fs
zeo.conf =
   <zeo>
      address ${:address}
   </zeo>
   %import zc.zlibstorage
   <zlibstorage>
     <filestorage>
        path ${:path}
     </filestorage>
   </zlibstorage>

[pack]
recipe = zc.recipe.deployment:crontab
deployment = deployment
times = 1 2 * * 6
command = ${buildout:bin-directory}/zeopack -d3 -t00 ${main:address}

This buildout doesn’t build software. Rather it builds configuration for deploying a database configuration using already-deployed software. For the purpose of this document, however, the details are totally unimportant.

Rather than crafting the configuration above every time, we can write a meta-recipe that crafts it for us. We’ll use our meta-recipe as follows:

[buildout]
parts = ample

[ample]
recipe = com.example.ample:db
path = /var/databases/ample/main.fs

The idea here is that the meta recipe allows us to specify the minimal information necessary. A meta-recipe often automates policies and assumptions that are application and organization dependent. The example above assumes, for example, that we want to pack to 3 days in the past on Saturdays.

So now, let’s see the meta recipe that automates this:

class Recipe:

    def __init__(self, buildout, name, options):

        buildout.parse('''
            [deployment]
            recipe = zc.recipe.deployment
            name = %s
            user = zope
            ''' % name)

        buildout['main'] = dict(
            recipe = 'zc.zodbrecipes:server',
            deployment = 'deployment',
            address = 8100,
            path = options['path'],
            **{
              'zeo.conf': '''
                <zeo>
                  address ${:address}
                </zeo>

                %import zc.zlibstorage

                <zlibstorage>
                  <filestorage>
                    path ${:path}
                  </filestorage>
                </zlibstorage>
                '''}
            )

        buildout.parse('''
            [pack]
            recipe = zc.recipe.deployment:crontab
            deployment = deployment
            times = 1 2 * * 6
            command =
              ${buildout:bin-directory}/zeopack -d3 -t00 ${main:address}

            [ctl]
            recipe = zc.recipe.rhrc
            deployment = deployment
            chkconfig = 345 99 10
            parts = main
            ''')

    def install(self):
        pass

    update = install

The meta recipe just adds parts to the buildout. It does this by setting items and calling the parse method. The parse method just takes a string in buildout configuration syntax. It’s useful when we want to add static, or nearly static part data. The setting items syntax is useful when we have non-trivial computation for part data.

The order that we add parts is important. When adding a part, any string substitutions and other dependencies are evaluated, so the referenced parts must be defined first. This is why, for example, the pack part is added after the main part.

Note that the meta recipe supplied an integer for one of the options. In addition to strings, it’s legal to supply integer values.

There are a few things to note about this example:

  • The install and update methods are empty.

    While not required, this is a very common pattern for meta recipes. Most meta recipes, simply invoke other recipes.

  • Setting a buildout item or calling parse, adds any sections with recipes as parts.

  • An exception will be raised if a section already exists.

Testing

Now, let’s test our meta recipe. We’ll test it without actually running buildout. Rather, we’ll use a specialized buildout provided by the zc.buildout.testing module.

>>> import zc.buildout.testing
>>> buildout = zc.buildout.testing.Buildout()

The testing buildout is intended to be passed to recipes being tested:

>>> _ = Recipe(buildout, 'ample', dict(path='/var/databases/ample/main.fs'))

After running the recipe, we should see the buildout database populated by the recipe:

>>> buildout.print_options(base_path='/sample-buildout')
[ctl]
chkconfig = 345 99 10
deployment = deployment
parts = main
recipe = zc.recipe.rhrc
[deployment]
name = ample
recipe = zc.recipe.deployment
user = zope
[main]
address = 8100
deployment = deployment
path = /var/databases/ample/main.fs
recipe = zc.zodbrecipes:server
zeo.conf =

                  <zeo>
                    address 8100
                  </zeo>

                  %import zc.zlibstorage

                  <zlibstorage>
                    <filestorage>
                      path /var/databases/ample/main.fs
                    </filestorage>
                  </zlibstorage>

[pack]
command = /sample-buildout/bin/zeopack -d3 -t00 8100
deployment = deployment
recipe = zc.recipe.deployment:crontab
times = 1 2 * * 6