Repo must tell client if command applied successfully

[enikao]

Once a client sent a command to the repository, which kind of information does it get back?

Categories of delta errors:

1. Protocol error (e.g. invalid message)
2. Technically broken model (unknown node id, containment circle)
3. Change taken into account, result might be different (e.g. because of conflict resolution, client assumes different state)
4. Meta-model incompliant (we don't check that: #300)

Alternatives

Option A: No information (fire-and-forget)

Pro:

• Efficient communication
• Easy to implement for repo

Con:

• Client needs potentially complex logic to coordinate its commands with updates received some time later
• Client needs to keep local history to be able to do this correlation

Option B: Repo MAY report back on success

Repo can chose which kind of problems it reports back as failure.

Pro:

• Less complexity for repo

Con:

• Almost useless for client, as it cannot rely on the kind of reported errors

Option C: Repo MUST report on protocol errors and technically broken model

Repo does not report on potential failures (e.g. command not applied due to conflict, both target and resolveInfo of a reference are null)

Pro:

• Client knows if command just cannot work
• Easier to spec potential failures (as in: we only need to distinguish them from clear-cut errors, don't need details)

Con:

• Client still needs complex logic to figure out what happened to its "intent" (as in option B)

Option D: Repo MUST report back whether successful

Repo MUST report failure/success for every command

Pro:

• Very helpful for client

Con:

• Repo needs to correlate command with update
• Repo can only answer command once it's completely processed
• Spec more complex on potential failures

Option E: Repo tells exactly what it did

Repo exactly describes in its response the effect of the command

Pro:

• Enables very smart clients
• Enables #299 option B
• Repo knows anyways

Con:

• Client would still need to sync own changes with changes from other clients
• Potentially complex to spec

[enikao]

Decision: Option C

Client sees whether the intent of the command was successful by the event that carries its commandId