Deploying multiple apps with different versions using a single Ansible command

Problem description

In this article, we will look at how to deal with the following scenario. Our system consists of several applications. Let’s imagine we need to deploy some (or all) of those apps with a single command. We also need to specify a common version for all apps or different versions for each app. The desired command could look like this:

ansible-playbook -i prod app-1.yml app-2.yml app-3.yml -e app_1_version=1.2.3 -e app_version=3.2.1

Such a command should result in the installation of app1 in version
1.2.3 and remaining apps in version 3.2.1.

The main difficulty here is that we use the same tasks (e.g. common role) for deploying all apps, so the app version variable is dynamic. Additionally, our app names can contain dashes (which is an invalid character in variable names) so we need to replace it with an underscore.

How to get a dynamic variable

First things first. We need to have access to all variables if the names of those variables are dynamic. Variables must be passed in a form that allows us to retrieve their value using a dynamic variable name. This is where a vars variable comes to the rescue.

vars is a special variable in the form of a dictionary, where we can get a specific value using the syntax of vars[key]. Now, if we have our app name in a variable app_name we can get that variable using the syntax vars[app_name | replace('-', '_') + '_version']. Alternative syntax for this is lookup('vars', app_name | replace('-', '_') + '_version'), but the first one is more pleasant to us. Anyway, not bad, is it?

Default app version

Here comes our additional requirement – a default app version. That’s
a piece of cake – we just have to check if a variable is defined and
take the default version, right? Both solutions above come with some
default support.

vars[spring_app_name | replace('-', '_') + '_version'] | default(app_version) }}
lookup('vars', spring_app_name | replace('-', '_') + '_version', default = app_version)

All we need to do now is to register it in a fact (like
target_app_version) and we are good to go. This is not very
readable though, with all those braces and filters. Can we do it
better?

Custom filter plugin

We can create our custom filter plugin. Let’s see the usage first and focus on the implementation afterwards. In order to get the app version using a custom filter, we can use
something like this:

app_name | ver(vars)

To create a filter, we must add it in the folder filter_plugins of our role. Filters are written in python, so let’s add the following content to ROLE_NAME/filter_plugins/ver.py:

class FilterModule(object):
    def filters(self):
        return {
            'ver': self.ver
        }

    @staticmethod
    def ver(app_name, vars):
        app_name_version = app_name.replace('-', '_') + '_version'
        if app_name_version in vars:
            return vars[app_name_version]
        elif 'app_version' in vars:
            return vars['app_version']
        else:
            raise Exception('No ' + app_name_version + ' or app_version defined')

This solution also has an additional advantage – it raises a meaningful and descriptive error, which is much better than in previous solutions.

Summary

What we achieved is the ability to retrieve the app version from the command line parameter even if multiple apps were deployed at once. Additionally, we are able to specify the default version if a specific version for the app is not defined. Finally, we simplified playbooks code by moving complicated formulas to a custom plugin, providing a descriptive error as well.

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Inconsistent Dependency Injection to domains with Grails

I've encountered strange behavior with a domain class in my project: services that should be injected were null. I've became suspicious as why is that? Services are injected properly in other domain classes so why this one is different?

Constructors experiment

I've created an experiment. I've created empty LibraryService that should be injected and Book domain class like this:

class Book {
def libraryService

String author
String title
int pageCount

Book() {
println("Finished constructor Book()")
}

Book(String author) {
this()
this.@author = author
println("Finished constructor Book(String author)")
}

Book(String author, String title) {
super()
this.@author = author
this.@title = title
println("Finished constructor Book(String author, String title)")
}

Book(String author, String title, int pageCount) {
this.@author = author
this.@title = title
this.@pageCount = pageCount
println("Finished constructor Book(String author, String title, int pageCount)")
}

void logInjectedService() {
println(" Service libraryService is injected? -> $libraryService")
}
}
class LibraryService {
def serviceMethod() {
}
}

Book has 4 explicit constructors. I want to check which constructor is injecting dependecies. This is my method that constructs Book objects and I called it in controller:

class BookController {
def index() {
constructAndExamineBooks()
}

static constructAndExamineBooks() {
println("Started constructAndExamineBooks")
Book book1 = new Book().logInjectedService()
Book book2 = new Book("foo").logInjectedService()
Book book3 = new Book("foo", 'bar').logInjectedService()
Book book4 = new Book("foo", 'bar', 100).logInjectedService()
Book book5 = new Book(author: "foo", title: 'bar')
println("Finished constructor Book(Map params)")
book5.logInjectedService()
}
}

Analysis

Output looks like this:

Started constructAndExamineBooks
Finished constructor Book()
Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2
Finished constructor Book()
Finished constructor Book(String author)
Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2
Finished constructor Book(String author, String title)
Service libraryService is injected? -> null
Finished constructor Book(String author, String title, int pageCount)
Service libraryService is injected? -> null
Finished constructor Book()
Finished constructor Book(Map params)
Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2

What do we see?

  1. Empty constructor injects dependencies.
  2. Constructor that invokes empty constructor explicitly injects dependencies.
  3. Constructor that invokes parent's constructor explicitly does not inject dependencies.
  4. Constructor without any explicit call declared does not call empty constructor thus it does not inject dependencies.
  5. Constructor provied by Grails with a map as a parameter invokes empty constructor and injects dependencies.

Conclusion

Always explicitily invoke empty constructor in your Grail domain classes to ensure Dependency Injection! I didn't know until today either!