Metric collecting charms
Adding metrics to a charm is simple and straightforward with the reactive framework. For a general overview of metrics see Application metrics.
layer:metrics to the charm’s
layer.yaml. This layer provides the
collect-metrics hook, and allows metric collection to be defined completely by
Declare the metrics to be collected in your charm’s
metrics: users: type: gauge description: Number of users tokens: type: gauge description: Number of active tokens
type: gauge fully supports operational use-cases. Other types are experimental.
Gauge metrics are a snapshot value reading at a point in time, as a positive decimal number.
Absolute metrics track the quantity since the last measurement, as a positive decimal number. Future releases of Juju will track the cumulative aggregate of absolutes, providing a more useful indicator to operators.
Built-in metric `juju-units`
There is also a built-in metric, which has no type or description, named
juju-units. When declared, this metric sends a “1” for each unit.
When charming with
layer:metrics, add a
command: attribute to each metric in
metrics.yaml, containing a command line that measures the value when executed.
layer:metrics will then execute this command in the
collect-metrics hook for you automatically. Continuing with the example above:
metrics: users: type: gauge description: Number of users command: scripts/count_users.py tokens: type: gauge description: Number of active tokens command: scripts/count_tokens.py
Commands can use any script or executable in your charm or installed elsewhere on the workload. The current working directory for this command will be the charm directory (
charmhelpers.core.hookenv.charm_dir). The command must write only the metric value to standard output, and terminate with exit code 0 in order for the measurement to be to be counted valid.
Continuing with the metrics example above, a charm that relates to a PostgreSQL database probably stores its “users” and “tokens” in database tables. These can be counted with a simple SQL query.
scripts/count_users.py in such a charm might read as:
#!/usr/bin/env python3 # Python packages will have been installed by the charm. import configparser import psycopg2 if __name__ == '__main__': # Read the application's configuration file, which will have been written # by the charm's relation hooks. with open('/opt/sso-auth/config.ini') as f: config_str = f.read() config = configparser.ConfigParser(strict=False) config.read_string(config_str) # Build a database connection string from configuration. dbname = config['database']['NAME'] user = config['database']['USER'] password = config['database']['PASSWD'] hostport = config['database']['HOST'] host, port = hostport.split(':') conn_str = 'dbname=%s user=%s password=%s host=%s port=%s' % ( dbname, user, password, host, port) conn = psycopg2.connect(conn_str) try: cur = conn.cursor() try: # For sake of example, let's say we don't want to include the # default admin user account in the count. cur.execute("SELECT COUNT(1) FROM users WHERE name != 'admin';") row, = cur.fetchone() print(row) # Print the measurement to standard output, for Juju finally: cur.close() finally: conn.close()
Note that this command will not have access to the normal lifecycle hook environment. Refer to the
collect-metrics documentation for more information.