Initial best practices work.

Author: tonyandrewmeyer

docs/howto/index.md

@@ -4,6 +4,7 @@
 ```{toctree}
 :maxdepth: 2
 
+Manage charms <manage-charms>
 Manage logs <manage-logs>
 Run workloads with a charm machines <run-workloads-with-a-charm-machines>
 Run workloads with a charm Kubernetes <run-workloads-with-a-charm-kubernetes>
@@ -25,6 +26,4 @@ Write unit tests for a charm <write-scenario-tests-for-a-charm>
 Write integration tests for a charm <write-integration-tests-for-a-charm>
 Write legacy unit tests for a charm <write-unit-tests-for-a-charm>
 Turn a hooks-based charm into an ops charm <turn-a-hooks-based-charm-into-an-ops-charm>
-
 ```
-

docs/howto/manage-charms.md

@@ -0,0 +1,123 @@
+(how-to-manage-charms)=
+# How to manage charms
+
+> See first: {external+juju:ref}`Juju | How to manage charms or bundles <manage-charms>`
+
+The primary programming language charms are written in is Python, and the
+primary framework for developing charms is the Python Operator Framework, or
+`ops`.
+
+`charmcraft init` provides you with a `src/charm.py` file that demonstrates the
+basis of using the library.
+
+```python
+# Import ops and use names from there.
+import ops
+
+# Charms always have a class that inherits from ops.CharmBase that define the
+# charm's functionality.
+class MyCharm(ops.CharmBase):
+    # There's always an __init__ method that sets up observers.
+    def __init__(self, framework: ops.Framework):
+        super().__init__(framework)
+        framework.observe(...)
+
+    # There will always be one or more event handlers that will be called in
+    # response to a Juju, ops, or custom event.
+    # Note that this has a leading underscore. These methods should only be
+    # called by the ops framework, so should not be public.
+    def _my_event_handler(self, event: ops.EventBase):  # or a more specific type
+        ...
+        self.unit.status = ops.ActiveStatus()
+
+# Finally, the charm.py file always ends with a call to ops.main, passing in the
+# charm class.
+if __name__ == "__main__":
+    ops.main(MyCharm)
+```
+
+## Interacting with the workload
+
+For simple interactions with an application or service or when a high quality
+Python binding is not available, subprocess or API calls should be used to
+perform the required operations on the application or service.
+
+TODO: Create a `src/{workload}.py` file for each workload or service that your charm
+is managing, and ... For example:
+
+```python
+import subprocess
+
+try:
+    # Comment to explain why subprocess is used.
+    result = subprocess.run(
+        # Array based execution.
+        ["/usr/bin/echo", "hello world"],
+        capture_output=True,
+        check=True,
+    )
+    logger.debug("Command output: %s", result.stdout)
+except subprocess.CalledProcessError as err:
+    logger.error("Command failed with code %i: %s", err.returncode, err.stderr)
+    raise
+```
+
+```{admonition} Best Practice
+:class: hint
+Limit the use of shell scripts and commands as much as possible in favour of
+writing Python for charm code. Examples where it could be reasonable to use a
+script include: extracting data from a machine or container which can't be
+obtained through Python; or issuing commands to applications that do not have
+Python bindings (such as starting a process on a machine).
+```
+
+## Base / Python
+
+Charms have access to the default Python version in the Ubuntu release defined
+as the base.
+
+Your code should be compatible with the operating system and Juju versions it
+will be executed on. For example, if your charm is to be deployed with Juju 3.6,
+its Python code should be compatible with Python 3.8.
+
+TODO: put in the relevant bits in pyproject.toml to have tools recognise this.
+
+TODO: can we be more detailed, or link to something here that says which Ubuntu
+has which Python, and which base you should support?
+
+## Dependencies
+
+TODO: put in instructions for dependencies from the spec (pyproject, generate requirements.txt)
+
+TODO: instructions on putting libs in charmcraft.yaml and running fetch-libs and then keeping that updated.
+
+> See more: charmcraft.yaml, charmcraft fetch-libs
+
+```{admonition} Best Practice
+:class: hint
+Including an external dependency in a charm is a significant choice. It can help
+with reducing the complexity and development cost. However, every additional
+dependency increases the risk for supply chain security issues, the maintenance
+burden of having to manage the dependencies, and makes it more difficult to
+completely understand all of the code involved in the charm.
+```
+
+> See more: [The software dependency issue](https://research.swtch.com/deps)
+
+TODO: is that still the article we want to link to? There are a lot on this problem.
+
+## Events
+
+TODO: There are Juju events, ops events, and custom events. A bit more about how to
+respond to each of these, probably mostly linking out.
+
+> See also: Juju events, something about the ops events.
+
+```{admonition} Best Practice
+:class: hint
+Custom events are defined and emitted by charm libraries. Charms should never
+define or emit custom events themselves.
+```
+
+> See more: manage libraries
+

docs/howto/manage-libraries.md

@@ -1,7 +1,7 @@
 (manage-libraries)=
 # Manage libraries
-> See first: {external+charmcraft:ref}`Charmcraft | Manage libraries <manage-libraries>`
 
+> See first: {external+charmcraft:ref}`Charmcraft | Manage libraries <manage-libraries>`
 
 ## Write a library
 
@@ -30,6 +30,12 @@ class DatabaseRequirer(ops.framework.Object):
             self.on.ready.emit()
 ```
 
+```{admonition} Best Practice
+:class: hint
+Libraries should never change the status of a unit or application.
+Use return values, or raise exceptions and let them bubble back up to the charm
+for the charm author to handle as they see fit.
+```
 
 ## Write tests for a library
 

docs/howto/manage-logs.md

@@ -3,13 +3,14 @@
 
 > See first: {external+juju:ref}`Juju | Log <log>`, {external+juju:ref}`Juju | Manage logs <manage-logs>`
 
-<!--
-> 
-> - **tl;dr:** <br>
-The default logging level for a Juju model is `INFO`. To see, e.g., `DEBUG` level messages,  you should change the model configuration: `juju model-config logging-config="<root>=DEBUG"`. 
--->
+```{tip}
+The default logging level for a Juju model is `INFO`. To see, for example,
+`DEBUG` level messages, change the model configuration:
+`juju model-config logging-config="<root>=DEBUG"`. 
+```
 
-To log a message in a charm, import Python's `logging` module, then use the `getLogger()` function with the desired level. For example:
+To log a message in a charm, import Python's `logging` module, then create a
+module-level logger object using `getLogger()`. For example:
 
 ```python
 import logging
@@ -27,38 +28,32 @@ class HelloOperatorCharm(ops.CharmBase):
             self._stored.things.append(current)
 ```
 
-> See more: 
-> - [`logging`](https://docs.python.org/3/library/logging.html), [`logging.getLogger()`](https://docs.python.org/3/library/logging.html#logging.getLogger)
-> - [`logging.getLogger().critical()`](https://docs.python.org/3/library/logging.html#logging.Logger.critical)
-> - [`logging.getLogger().error()`](https://docs.python.org/3/library/logging.html#logging.Logger.error)
-> - [`logging.getLogger().warning()`](https://docs.python.org/3/library/logging.html#logging.Logger.warning)
-> - [`logging.getLogger().info()`](https://docs.python.org/3/library/logging.html#logging.Logger.info)
-> - [`logging.getLogger().debug()`](https://docs.python.org/3/library/logging.html#logging.Logger.debug)
-
-Juju automatically picks up logs from charm code that uses the Python [logging facility](https://docs.python.org/3/library/logging.html), so we can use the Juju [`debug-log` command](https://juju.is/docs/juju/juju-debug-log) to display logs for a model. Note that it shows logs from the charm code (charm container), but not the workload container. Read ["Use `juju debug-log`"](https://juju.is/docs/sdk/get-logs-from-a-kubernetes-charm#heading--use-juju-debug-log) for more information.
-
-Besides logs, `stderr` is also captured by Juju. So, if a charm generates a warning, it will also end up in Juju's debug log. This behaviour is consistent between K8s charms and machine charms.
+```{admonition} Best Practice
+:class: hint
+Avoid spurious logging, ensure that log messages are clear and meaningful,
+ and provide the information a user would require to rectify any issues.
+```
 
-**Tips for good practice:**
+```{admonition} Best Practice
+:class: hint
+Never log credentials or other sensitive information.
+```
 
-- Note that some logging is performed automatically by the Juju controller, for example when an event handler is called. Try not to replicate this behaviour in your own code. 
+> See more: 
+> - [`logging`](https://docs.python.org/3/library/logging.html), [`logging.getLogger()`](https://docs.python.org/3/library/logging.html#logging.getLogger)
 
--  Keep developer specific logging to a minimum, and use `logger.debug()` for such output. If you are debugging a problem, ensure you comment out or remove large information dumps (such as config files, etc.) from the logging once you are finished.
+Juju automatically picks up logs from charm code that uses the Python logging
+package and any warnings generated by the warnings module, so we can use the
+Juju `debug-log` command to display logs for a model.
+Note that it shows logs from the charm code (charm container), but not the
+workload container.
 
-- When passing messages to the logger, do not build the strings yourself. Allow the logger to do this for you as required by the specified log level. That is:
+> See more: {external+juju:ref}`juju CLI commands > juju debug-log <command-juju-debug-log>`, [`warnings`](https://docs.python.org/3/library/warnings.html)
 
-<!--
-| DON'T &#10060; | DO :white_check_mark: | 
-|-|-|
-| `logger.info("Got some information {}".format(info))`| `logger.info("Got some information %s", info)` |
-|`logger.info(f"Got some more information {more_info}")`| |
--->
+Besides logs, `stderr` is also captured by Juju. This behaviour is consistent
+between K8s charms and machine charms.
 
-```python
-# Do this!
-logger.info("Got some information %s", info)
-# Don't do this
-logger.info("Got some information {}".format(info))
-# Or this ...
-logger.info(f"Got some more information {more_info}")
+```{note}
+Some logging is performed automatically by the Juju controller, for example when
+an event handler is called. Try not to replicate this behaviour in your own code.
 ```

docs/howto/manage-stored-state.md

@@ -11,25 +11,31 @@ the charm machine or (for Kubernetes charms) container - for state that should
 have the same lifetime as the machine or container, and storing state in a Juju
 peer relation - for state that should have the same lifetime as the application.
 
+```{admonition} Best Practice
+:class: hint
+Use state sparingly -- where possible, write your charm to be stateless. For
+sharing state between units of the same application, use peer relation data bags.
+```
+
 ## Storing state for the lifetime of the charm container or machine
 
 Where some state is required, and the state should share the same lifetime as
 the machine or (for Kubernetes charms) container, `ops` provides
 [](ops.StoredState) where data is persisted to the `ops` unit database in the
 charm machine or container.
 
-[caution]
+```{caution}
 Note that for Kubernetes charms, container recreation is expected: even if there
 are no errors that require the container to be recreated, the container will be
 recreated with every charm update.
-[/caution]
+```
 
-[note]
+```{note}
 In Kubernetes charms that use the older 'podspec' model, rather than the sidecar
 pattern, or when the `use_juju_for_storage` option is set, this data will be
 stored in Juju instead, and will persist for the life of the application.
 Avoid using `StoredState` objects in these situations.
-[/note]
+```
 
 A `StoredState` object is capable of persisting simple data types, such as
 integers, strings, or floats, and lists, sets, and dictionaries containing those