Relationship between Release and Config / Build is the wrong way around

[Cryptophobia]

From @helgi on June 8, 2016 14:54

deis/controller#792 has uncovered some inconsistencies in our data model on Release.

Problem

Currently things are set up so that Release is the child object of Config and Build via ForeignKey with CASCADE delete set.

Each Config and Build object can be re-used across multiple Release objects, if nothing changes on the respective object. Such as: User A does 50 releases but they are only config changes, then the build object will always stay the same.

In many parts of the code when doing deploy and that fails we only delete the Release object, thinking the ForeignKey relationship is going to take care of it but that doesn't happen since Release is a child object, not a parent.

Why is this a problem?

If only the Release object is remove then the Config object stays behind. When creating new releases it seems we are doing things like selecting the latest Config from the App or something... If the latest Config object is poisoned in a way that is hard to fix then.

Basically the User ends up in a situation where it becomes pretty hard / impossible to recover from

Experiment

I did a quick experiment and used the DB directly after creating a few releases with combinations of config and build changes - First I tried deleting a release by hand, no config / build objects went away, and then I went and deleted a config object attached to 2 releases and both releases went away.

Potential solutions

I reviewed a few solutions and how they would affect us

• Flip the FK relationship around
  • This would require the biggest rewrite as Config and Build objects can't be created without Release
• Start programmatically handling the "when to delete config / build as well" and such
  • Brittle as I will also have to look at all the latest() usages and try to reconcile things
• Create a 1:1 between Release and Config/Build and severing the Config/Build tie with the app_id making Release the only connection, turning a Release into a ledger of sorts
  • Allows us to treat the Release as an artefact but will make the DB bigger

Copied from original issue: deis/controller#798

[Cryptophobia]

From @gabrtv on June 8, 2016 14:59

@helgi I see your point about strange behavior after deleting a Release, but the Release object is not intended to be deleted. I don't believe we exposed an API for that. Where are we seeing this come up? Rollbacks maybe?

[Cryptophobia]

From @helgi on June 8, 2016 15:1

We delete Release objects ourselves if are in the middle of a deploy and that bombs - There's at least 2 if not 3 places that do that. This is not an end user triggerable thing beyond messing up their deploy in some way (see #792 for intentional mess up)

[Cryptophobia]

From @helgi on June 8, 2016 15:1

Sorry, #785 has the intentional mess up story that triggers this

[Cryptophobia]

From @jchauncey on June 8, 2016 15:51

I think the third option seems the cleanest and most reasonable. If I understand correctly, every time a config value changes or a build changes we create a new release object. This is actually a pretty reasonable thing in my opinion. If you were doing releases manually and you needed to change config values that would more than likely result in a new release.