4. How generic app and an example works using configman

4.1. The minimum app

To illustrate the example, let’s look at an example of an app that uses generic_app to leverage configman to run. Let’s look at weeklyReportsPartitions.py

As you can see, it’s a subclass of the socorro.app.generic_app.App class which is a the-least-you-need wrapper for a minimal app. As you can see, it takes care of logging and executing your main function.

4.2. Connecting and handling transactions

Let’s go back to the weeklyReportsPartitions.py cron script and take a look at what it does.

It only really has one configman option and that’s the transaction_executor_class. The default value is TransactionExecutorWithBackoff which is the class that’s going to take care of two things:

  1. execute a callable that accepts an opened database connection as first and only parameter
  2. committing the transaction if there are no errors and rolling back the transaction if an exception is raised
  3. NB: if an OperationalError or InterfaceError exception is raised, TransactionExecutorWithBackoff will log that and retry after configurable delay

Note that TransactionExecutorWithBackoff is the default transaction_executor_class but if you override it, for example by the command line, with TransactionExecutor no exceptions are swallowed and it doesn’t retry.

Now, connections are created and closed by the ConnectionContext class. As you might have noticed, the default database_class defined in the TransactionExecutor is socorro.external.postgresql.connection_context.ConnectionContext as you can see here

The idea is that any external module (e.g. Boto, PostgreSQL, etc) can define a ConnectionContext class as per this model. What its job is is to create and close connections and it has to do so in a contextmanager. What that means is that you can do this:

connector = ConnectionContext()
with connector() as connection:  # opens a connection
    do_something(connection)
# closes the connection

And if errors are raised within the do_something function it doesn’t matter. The connection will be closed.

4.3. What was the point of that?!

For one thing, this app being a configman derived app means that all configuration settings are as flexible as configman is. You can supply different values for any of the options either by the command line (try running --help on the ./weeklyReportsPartitions.py script) and you can control them with various configuration files as per your liking.

The other thing to notice is that when writing another similar cron script, all you need to do is to worry about exactly what to execute and let the framework take care of transactions and opening and closing connections. Each class is supposed to do one job and one job only.

configman uses not only basic options such as database_password but also more complex options such as aggregators. These are basically invariant options that depend on each other and uses functions in there to get its stuff together.

4.4. How to override where config files are read

generic_app supports multiple ways of picking up config files. The most basic option is the –admin.conf= option. E.g.:

python myapp.py --admin.conf=/path/to/my.ini

The default if you don’t specify a --admin.conf is that it will look for a .ini file with the same name as the app. So if app_name='myapp' and you start it like this:

python myapp.py

it will automatically try to read config/myapp.ini and if you want to override the directory it searches in you have to set an environment variable called DEFAULT_SOCORRO_CONFIG_PATH like this:

export DEFAULT_SOCORRO_CONFIG_PATH=/etc/socorro
python myapp.py

Which means it will try to read /etc/socorro/myapp.ini.

NOTE: If you specify a DEFAULT_SOCORRO_CONFIG_PATH that directory must exist and be readable or else you will get an IOError when you try to start your app.