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    execution. This article is to dive into its source code in order to understand its execution logic and design philosophy.

    In this article we will study charm's reactive framework to get a firm understanding of what they mean by writing an reactive charm.


    Decorator: @hook Definition: charm.reactive/charms/reactive/

    Hooks give one a dedicated sequence that execution is guaranteed in that order and developer can use @hook to define a function block to run.

    Sequence of charm hooks

    Hooks have a few limitations which affect the usefulness of this mechanism:

    1. Hooks are really meant to be used for internal workflow transitions. There are only 8 of them and they represent a specific stage in the life cycle of a charm. There is no mechanism to extend or inherit these built-ins.

      1. install
      2. config-changed
      3. start
      4. upgrade-charm
      5. stop
      6. update-status
      7. leader-elected
      8. leader-settings-changed
    2. Hook sequence is hardcoded. This is defined in juju/worker/uniter/operation/runhook.go. In other words, the transition from install to upgrade-charm is both guaranteed and mandatory. This limits how we can design our workflows.


    Decorator: @not_unless, @only_once, @when, @when_all, @when_any, @when_file_changed, @when_none, @when_not, @when_not_all Definition: charm.reactive/charms/reactive/

    States were probably designed to fix the limitation that Hooks present. States can be defined using arbitrary string except two reserved words juju and jujud. Further, workflow of states are not fixed. States are evaluated iteratively and a true condition will execute associated function block.

    When a state is true?

    What defines a true? The code is defined in class StateWatch in file charm.reactive/charms/reactive/ The value changed is true when there are states to monitor (set(states) is not empty) and there are changes (data['changes] is not empty).

    class StateWatch(object):
        def watch(cls, watcher, states):
            data = cls._get()
            iteration = data['iteration']
            changed = bool(set(states) & set(data['changes']))
            return iteration == 0 or changed

    What defines the values in data['changes']? There are only two places to set this value: set_state(...) and remove_state(...). Looking at set_state reveals that if the state is already in old_states list, it will not be set to changed, therefore the watch will fail to identify this state and will not trigger an execution. In other words, if a state becomes true, it will not be re-evaluated until it is reset to false first.

    Reactive charm triggers only on state transitions: False->True (@when) or True->False (@when_not).

    def set_state(state, value=None):
        Set the given state as active, optionally associating with a relation.
        old_states = get_states()
        unitdata.kv().update({state: value}, prefix='reactive.states.')
        if state not in old_states:


    There isn't really a namespace concept in charm. I'm borrowing the term to illustrate state boundaries. The question is that if I can define arbitrary states, in which scope are they visible? Can in charm A trigger an action in charm B? or in another layer? or in another unit? or even a bundle?

    To answer this, we need to examine how states are stored. The key information can be found in charmhelpers/core/ — this is the storage class that is used by charm to store states. Clearly the backend is a sqlite3 database.

    class Storage(object):
        """Simple key value database for local unit state within charms.
        Modifications are not persisted unless :meth:`flush` is called.
        To support dicts, lists, integer, floats, and booleans values
        are automatically json encoded/decoded.
        def __init__(self, path=None):
            self.db_path = path
            if path is None:
                if 'UNIT_STATE_DB' in os.environ:
                    self.db_path = os.environ['UNIT_STATE_DB']
                    self.db_path = os.path.join(
                        os.environ.get('CHARM_DIR', ''), '.unit-state.db')
            self.conn = sqlite3.connect('%s' % self.db_path)
            self.cursor = self.conn.cursor()

    Function _init(self) reveals the table schema in this database — three tables: kv, kv_revisions and hooks. kv is the primary store as can be seen in the set_state function above (unitdata.kv().update(....).

    def _init(self):
            create table if not exists kv (
               key text,
               data text,
               primary key (key)
            create table if not exists kv_revisions (
               key text,
               revision integer,
               data text,
               primary key (key, revision)
            create table if not exists hooks (
               version integer primary key autoincrement,
               hook text,
               date text

    What is not obvious is that each unit has its own DB. Therefore, the boundary of states are per charm unit. In other words, states are visible inside a unit. Using layers will package states inside a single charm, but in run time it is the unit boundary that matters.

    States do not go across charms. Using the same charm, states do not go across units either.


    Reading function dispatch in charms.reactive/charms/reactive/ is interesting because there is certainly something no document has mentioned. Dispatch is done in two phases: hooks and other.

    Hooks are run in the hooks phase. Registered hook will run its test() so this scan will test all hooks.

    def _test(to_test):
        return list(filter(lambda h: h.test(), to_test))
    unitdata.kv().set('reactive.dispatch.phase', 'hooks')
    hook_handlers = _test(Handler.get_handlers())

    States are run in the other phase. The magic number 100 for-loop highlights an underline assumption that states can converge within these iterations. Otherwise, state watch is reset and will be count from 0 again during next iteration → state effect can then ripple through one single iteration.

    unitdata.kv().set('reactive.dispatch.phase', 'other')
    for i in range(100):
        other_handlers = _test(Handler.get_handlers())
        if not other_handlers:

    — by Feng Xia


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