Refactor Charm for Sandbox support

hooks/config-changed

 #!/usr/bin/env python2
 # -*- python -*-
 
 # Copyright 2012 Canonical Ltd.  This software is licensed under the
 # GNU Affero General Public License version 3 (see the file LICENSE).
 
 import sys
 
 from charmhelpers import (
     get_config,
     log,
-    service_control,
-    STOP,
 )
 from shelltoolbox import (
     DictDiffer,
-    su,
 )
 
 from utils import (
-    AGENT,
     config_json,
-    fetch_api,
-    fetch_gui,
-    get_staging_dependencies,
-    HAPROXY,
-    IMPROV,
-    legacy_juju,
     log_hook,
-    NGINX,
-    save_or_create_certificates,
-    setup_gui,
-    start_agent,
-    start_gui,
-    start_improv,
 )
 
-
-def handle_config_changes(config, diff):
-    # Handle all configuration file changes.
-    log('Updating configuration.')
-
-    added_or_changed = diff.added_or_changed
-    in_staging = config.get('staging')
-    is_legacy_juju = legacy_juju()
-
-    # The gui_source_changed and api_branch_changed variables control whether
-    # we restart the GUI and the API, respectively, at the end of the function.
-    gui_source_changed = False
-    api_branch_changed = False
-
-    # Fetch new sources?
-    if 'juju-gui-source' in added_or_changed:
-        gui_source_changed = True
-        release_tarball = fetch_gui(
-            config['juju-gui-source'], config['command-log-file'])
-        setup_gui(release_tarball)
-    if is_legacy_juju and ('juju-api-branch' in added_or_changed):
-        api_branch_changed = True
-        fetch_api(config['juju-api-branch'])
-
-    # Handle changes to SSL certificates.
-    ssl_properties = set(
-        ['ssl-cert-path', 'ssl-cert-contents', 'ssl-key-contents'])
-    ssl_changed = added_or_changed & ssl_properties
-    if ssl_changed:
-        save_or_create_certificates(
-            config['ssl-cert-path'], config.get('ssl-cert-contents'),
-            config.get('ssl-key-contents'))
-
-    # Handle changes to the improv server configuration. This step is currently
-    # skipped if the charm is deployed in a juju-core environment where the
-    # staging server is not supported.
-    if is_legacy_juju:
-        if in_staging:
-            staging_properties = set(['staging', 'staging-environment'])
-            staging_changed = added_or_changed & staging_properties
-            if staging_changed or ssl_changed or api_branch_changed:
-                if 'staging' in added_or_changed:
-                    # 'staging' went from False to True, so the agent server is
-                    # running and must be stopped.
-                    current_api = AGENT
-                    # We need to make sure we have staging dependencies.
-                    get_staging_dependencies()
-                else:
-                    # Only staging parameters changed, so the existing staging
-                    # server must be stopped and later restarted.
-                    current_api = IMPROV
-                log('Stopping %s.' % current_api)
-                service_control(current_api, STOP)
-                # Now the improv server can be cleanly started.
-                log('Starting or restarting staging.')
-                start_improv(config.get('staging-environment'),
-                             config['ssl-cert-path'])
-        else:
-            agent_changed = 'staging' in added_or_changed
-            if agent_changed or ssl_changed or api_branch_changed:
-                if agent_changed:
-                    # If 'staging' transitions to False we need to stop the
-                    # backend and start the agent.
-                    current_api = IMPROV
-                else:
-                    # The agent is still running but the configuration has been
-                    # updated -- bounce it.
-                    current_api = AGENT
-                service_control(current_api, STOP)
-                log('Starting or restarting Juju API agent.')
-                start_agent(config['ssl-cert-path'])
-
-    # Handle changes to the juju-gui configuration.
-    gui_properties = set([
-        'juju-gui-console-enabled', 'login-help', 'read-only', 'serve-tests',
-        'secure'])
-    gui_changed = added_or_changed & gui_properties
-    # Changes on the "staging" option are only relevant in PyJuju environments.
-    staging_changed = is_legacy_juju and ('staging' in added_or_changed)
-    if gui_changed or staging_changed or ssl_changed or gui_source_changed:
-        with su('root'):
-            service_control(HAPROXY, STOP)
-            service_control(NGINX, STOP)
-        console_enabled = config.get('juju-gui-console-enabled')
-        login_help = config['login-help']
-        readonly = config['read-only']
-        serve_tests = config['serve-tests']
-        ssl_cert_path = config['ssl-cert-path']
-        secure = config['secure']
-        start_gui(
-            console_enabled, login_help, readonly, in_staging, ssl_cert_path,
-            serve_tests, secure=secure)
+from backend import Backend
 
 
 def main():
     config = get_config()
     prev_config = config_json.get()
     diff = DictDiffer(config, prev_config)
 
     if not diff.modified:
         log("No configuration changes, exiting.")
         sys.exit(0)
-    handle_config_changes(config, diff)
+
+    log('Updating configuration.')
+    backend = Backend(config, prev_config)
+
+    # Woot
+    if prev_config:
+        # Stop whatever the old config was.
+        prev_backend = Backend(prev_config)
+        prev_backend.stop()

[gary.poster, 2013/04/17 18:41:52]

I haven't commented on how much I like the readability of the new approach. 
It's very nice.  This change is the only one that I'd argue regresses the
behavior of the charm.  It used to be that, when you changed the config (such as
to upgrade the revision of the GUI), the charm would still serve the source. 
Once everything was installed, we'd stop and start, minimizing the actual
downtime.  I think this is a very good behavior for a production
environment--assuming that the old deployment works while the new one is being
installed.  Practically, it seemed to.

Would it be a big problem to move backend.install() to *before*
prev_backend.stop()?

[bcsaller, 2013/04/17 19:34:41]

I think it should be fine in practice (and will test to that effect prior to
re-proposing) but covering all the cases here are difficult.

Hooks are supposed to be (at least in spirit) idempotent. That change increases
the likelihood that this isn't true but I think in the sense that it should
allow forward progress in the event of partial failure so I'm willing to try.

Also removed my overly self-satisfied 'Woot' comment ;)

+    backend.install()
+    backend.start()
+
+    # Record new configuration
     config_json.set(config)
 
 
 if __name__ == '__main__':
     with log_hook():
         main()