Client¶

class hermes.client.Client(dsn, watch_path=None, failover_files=None)¶

Bases: hermes.log.LoggerMixin, multiprocessing.process.Process, watchdog.events.FileSystemEventHandler

Responsible for Listener and Processor components. Provides functions to start/stop both itself and its components. In addition, it is also capable of receiving file-system events via the ‘watchdog’ library.

General procedure:

Starts both the Process and Listener components.

Listen and act upon exit/error notifications from components

Listen for file-system events and acts accordingly.

To make the client listen for Postgres ‘recovery.conf, recovery.done’ events:

from hermes.client import Client

dsn = {'database': 'example_db',
       'host': '127.0.0.1',
       'port': 5432,
       'user': 'example',
       'password': 'example'}

watch_path = '/var/lib/postgresql/9.4/main/'
failover_files = ['recovery.done', 'recovery.conf']

client = Client(dsn, watch_path, failover_files)

# Add processor and listener
...

# Start the client
client.start()

Or, if you decide you don’t want to use a file watcher, then you can omit those parameters. However, the Client will still perform master/slave checks if a problem is encountered:

from hermes.client import Client

dsn = {'database': 'example_db',
       'host': '127.0.0.1',
       'port': 5432,
       'user': 'example',
       'password': 'example'}

client = Client(dsn)

# Add processor and listener
...

# Start the client
client.start()

Parameters:	dsn – A Postgres-compatible DSN dictionary watch_path – The directory to monitor for filechanges. If None, then file monitoring is disabled. failover_files – A list of files which, when modified, will cause the client to call `execute_role_based_procedure()`

add_listener(listener)¶

Parameters:	listener – A `Component` object which will listen for notifications from Postgres and pass an event down a queue.
Raises:	`InvalidConfigurationException` if the provided listener is not a subclass of `Component`

add_processor(processor)¶

Parameters:	processor – A `Component` object which will receive notifications and run the `execute()` method.
Raises:	`InvalidConfigurationException` if the provided processor is not a subclass of `Component`

execute_role_based_procedure()¶

Starts or stops components based on the role (Master/Slave) of the Postgres host.

Implements a binary exponential backoff up to 32 seconds if it encounters a FATAL connection error.

on_any_event(event)¶

Listens to an event passed by ‘watchdog’ and checks the current master/slave status

Parameters:	event – A `FileSystemEvent`

object passed by ‘watchdog’ indicating an event change within the specified directory.

run()¶: Performs a select() on the components’ error queue. When a notification is detected, the client will log the message and then calculate if the Postgres server is still a Master - if not, the components are shutdown.

start()¶

Starts the Client, its Components and the directory observer

Raises:	`InvalidConfigurationException`