Writing Operations

Operations are, in general, simple Python functions. They are passed the current deploy state and a target host, along with any operation arguments. The function reads state from the host, comparing it to the arguments, and yields commands.

Input: reserved arguments

The following keyword arguments are reserved for controlling how operations deploy, and cannot be used within operations: sudo, sudo_user, ignore_errors, serial, run_once, timeout, env, name, op, get_pty. In addition to this, the first argument cannot accept set objects, as these will be removed for use as the operation name.

Output: commands

Operations are generator functions and yield three types of command:

# Shell commands, simply represented by a string
yield 'echo "Shell!"'

# File uploads represented by a tuple
yield (filename_or_io, remote_filename)

# Python functions represented by a tuple
yield (function, args_list, kwargs_dict)

# Additionally, commands can be wrapped in a dict, overriding sudo/sudo_user
yield {
    'command': 'echo "Shell!"',
    'sudo': True
}

Example: managing files

This is a simplified version of the files.file operation, which will create/remove a remote file based on the present kwargs:

@operation
def file(state, host, name, present=True):
    '''
    Manage the state of files.

    + name: name/path of the remote file
    + present: whether the file should exist
    '''

    info = host.fact.file(name)

    # Not a file?!
    if info is False:
        raise OperationError('{0} exists and is not a file'.format(name))

    # Doesn't exist & we want it
    if info is None and present:
        yield 'touch {0}'.format(name)

    # It exists and we don't want it
    elif info and not present:
        yield 'rm -f {0}'.format(name)