Lifecycle events

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Event > List of events > Lifecycle events

The bulk of the events a charm will see during its lifetime are “lifecycle events”: broadly defined as: events that don’t fit into any other specific category.

Contents:

Complete list of lifecycle events

Lifecycle event triggers

All lifecycle events are triggered in a predictable pattern following a specific action by the administrator (with the exception of update-status, which triggers on an interval, and Kubernetes container events, such as <container >-pebble-ready, which can occur at any time). For example:

Scenario Example Command Resulting Events
Deploy juju deploy ./hello-operator.charm install -> config-changed -> start -> <container>-pebble-ready
Scale juju add-unit -n 2 hello-operator install -> config-changed -> start -> <container>-pebble-ready
Configure juju config hello-operator thing=foo config-changed
Upgrade juju upgrade-charm hello-operator upgrade-charm -> config-changed -> <container>-pebble-ready
Remove juju remove-application hello-operator stop -> remove

Exception to the “all lifecycle events are fired in a predictable pattern following cloud admin actions” rule is <container>-pebble-ready. As kubernetes pods can churn autonomously (outside of juju’s control), it can happen that the container comes and goes at unpredictable times. When that happens, the charm will receive again <container>-pebble-ready. See the page on pebble-ready for more details.

Lifecycle events in Ops

In ops, all lifecycle events are accessible via CharmBase’s on attribute. So you typically want to observe these events by doing:

# in MyCharm(CharmBase)  __init__():
self.framework.observe(self.on.start, self._on_start)
self.framework.observe(self.on.install, self._on_install)
...
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Is “PebbleReady” considered a life-cycle event?

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Sure is! I didn’t document this here, as I was trying to cover only those events that all charms would see :slight_smile: I can make adjustments if that’s misleading! :slight_smile:

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Hello Jon!

For a side-car charm would be great if we have the table with the lifecycles events including the Pebble ones.
I am no sure if this information should be in this section, or maybe in the Running Workloads?

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Hi, here are some thoughts on how we can improve this page along with some questions that came up while reading it.

  • The config-change event seems overloaded. Is there a way to find out what caused this event to trigger?

  • “Invocations of associated callbacks should be idempotent and should not make changes to the environment, or restart services, unless there is a material change to the charm’s configuration.” This sentence needs more explanation, especially the unless part. What is a material change?

  • Are there any side effects in deferring the upgrade-charm event?

  • Should we discuss the “Remove” event? Based on the table “Example Lifecycles” there should be one such event.

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  • Charm authors should assume that anything and everything could change with config-change. Pebble is really a sidecar, but when healthchecks are introduced, a failing check may lead to killing a pod for a Charmed Operator which would break any “smarter” configuration. In keeping with idempotency, a config-changed event should assume that the entire application configuration should be re-written from scratch.

  • Updated with a couple of examples for what a “material change” may be.

  • “Remove” seems to be covered by “stop” to me.

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@tmihoc - This documentation is not complete either. @jnsgruk is missing alot of events/hooks like, storage-attached, storage-departed, etc.

Having complete documentation is what you taught me from the masterful workshop =)

1 Like
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Does the start hook fire after a machine reboots?

What are the sequence of events that happens when a machine comes back after a reboot?

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probably too late to be useful, but I wrote a little tool to see in real-time which events are fired on which unit. can be useful to see for yourself what event sequence occurs.

Try jhack utils tail name-of-unit/0 https://github.com/PietroPasotti/jhack

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Is config-changed fired after a juju attach-resource? Or there are other hooks as well, like upgrade?

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