Add continuous writes test charm

[marceloneppel]

Issue

• Jira issue: DPE-533
• For at least some HA shakedown integration tests we need to ensure, along other requirements, that the PostgreSQL HA deployment correctly continues to handle incoming writes after some unexpected situations happen, like the DB process is killed, freezes, is restarted and so on.

Solution

• Add a charm that perform continuous writes to the PostgreSQL charm during for shakedown tests.

Context

• database_requires.py is an imported library and can be ignored while reviewing this PR.

• You can FOCUS on the code from the following files while reviewing this PR:

  • charm.py:

    • has event handlers to get the connections details from the PostgreSQL from the relation data (postgresql_client interface).

    • has action handlers to start and stop the continuous writes process from continuous_writes.py and also for erasing the written data.

  • continuous_writes.py:

    • this is a Python script that continuously writes data to the PostgreSQL database. It was adapted to PostgreSQL based on the amazing code that Mia implemented on Charmed MongoDB Operator.

    • it reconnects again to the database in every loop iteration because the current established connection can fail when the primary changes (Patroni sometimes take some seconds to reassign the primary role; this happens due to some values in some Patroni settings that are needed to avoid more frequent split brain scenarios).

Testing

• This charm will be used in later PRs (like this not ready yet for review PR) to support the HA shakedown integration tests.

Release Notes

• Add continuous writes test charm (for HA shakedown tests).

[codecov-commenter]

Codecov Report

Merging #36 (e9c2353) into main (ceeba02) will increase coverage by 0.20%.
The diff coverage is 61.39%.

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   60.11%   60.32%   +0.20%     
==========================================
  Files           6        6              
  Lines         692      809     +117     
  Branches      103      119      +16     
==========================================
+ Hits          416      488      +72     
- Misses        256      293      +37     
- Partials       20       28       +8     

Impacted Files	Coverage Δ
src/cluster.py	55.94% <55.17%> (+1.46%)	⬆️
src/charm.py	53.25% <60.33%> (+1.08%)	⬆️
src/constants.py	100.00% <100.00%> (ø)

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[MiaAltieri]

This looks awesome! Initially when I did mine I wanted to do it as an application charm but I was too lazy. This is really awesome to see!

[MiaAltieri]

Neat way of doing this

[MiaAltieri]

Nit add '# stop any writes that might be going'

[marceloneppel]

Nice. Added on e9c2353.

[shayancanonical]

lgtm!

[marceloneppel]

Thanks for the reviews!

[marceloneppel]

This looks awesome! Initially when I did mine I wanted to do it as an application charm but I was too lazy. This is really awesome to see!

You made most part of the work needed on this, Mia! It was your amazing implementation that made it easy to create the charm.