zc.metarecipe 0.2.1

============

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 of it’s work to lower-level recipe by manipulating the buildout database.

Unfortunately, buildout doesn’t yet provide a high-level API for creating parts. It has a private low-level API which has been promoted to public (meaning it won’t be broken by future release), and it’s straightforward to write the needed high-level API, but it’s annoying to repeat the high-level API in each meta recipe.

This small package provides the high-level API needed for meta recipes and a simple testing framework. It will be merged into a future buildout release.

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

Contents

• A simple meta-recipe example

• Testing

• Changes

  • 0.2.1 (2014-01-24)

  • 0.2.0 (2012-09-24)

  • 0.1.0 (2012-05-31)

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:

import zc.metarecipe

class Recipe(zc.metarecipe.Recipe):

    def __init__(self, buildout, name, options):
        super(Recipe, self).__init__(buildout, name, options)

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

        self['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>
                '''}
            )

        self.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
            ''')

The meta recipe just adds parts to the buildout. It does this by calling inherited __setitem__ and parse methods. The parse method just takes a string in ConfigParser syntax. It’s useful when we want to add static, or nearly static part data. The setitem 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 and unicode values.

Testing

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

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

>>> _ = Recipe(buildout, 'ample', dict(path='/var/databases/ample/main.fs'))
[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 ${:address}
                  </zeo>
<BLANKLINE>
                  %import zc.zlibstorage
<BLANKLINE>
                  <zlibstorage>
                    <filestorage>
                      path ${:path}
                    </filestorage>
                  </zlibstorage>
[ctl]
chkconfig = 345 99 10
deployment = deployment
parts = main
recipe = zc.recipe.rhrc
[pack]
command = ${buildout:bin-directory}/zeopack -d3 -t00 ${main:address}
deployment = deployment
recipe = zc.recipe.deployment:crontab
times = 1 2 * * 6

When we call our recipe, it will add sections to the test buildout and these are simply printed as added, so we can verify that the correct data was generated.

That’s pretty much it.

Changes

0.2.1 (2014-01-24)

• Fixed: When parsing configuration text, sections were input and

  evaluated at the same time in section sorted order. This caused problems if a section that sorted early referred to a section that sorted late.

0.2.0 (2012-09-24)

• When setting option values, unicode and int values will be converted to strings. Other non-string values are rejected. Previously, it was easy to get errors from buildout when setting options with values read from ZooKeeper trees, which are unicode due to the use of JSON.

• Fixed: When using the meta-recipe parse method, the order that resulting sections were added was non-deterministic, due to the way ConfigParser works. Not sections are added to a buildout in sortd order, by section name.

0.1.0 (2012-05-31)

Initial release