How can we define SLO and SLA in DPDS?

[andrea-gioia]

Let's brainstorm here on how to define SLO and SLA in DPDS.
The ideas collected in this thread could become the foundation for a new RFC proposal.

What is a SLO/SLA anyway?
An SLO is a service level objective: a target value or range of values for a service level that is measured by a service level indicator (SLI). An SLI is a service level indicator—a carefully defined quantitative measure of some aspect of the level of service that is provided.
Finally, SLAs are service level agreements: an explicit or implicit contract with your users that includes consequences of meeting (or missing) the SLOs they contain. The consequences are most easily recognized when they are financial—a rebate or a penalty—but they can take other forms.

Note: A SLI measures a service in terms of its operational efficiency (output). A Key Performance Indicator (KPI) instead measures a service in terms of its business efficacy (outcome). Both are service measures. We should evaluate whether to manage them in the same way or not.

Why it matters?
TODO

Some useful resources

1. Site Reliability Engineering (Chapter 4 - SLO)
2. SLA vs. SLO vs. SLI: What’s the difference?
3. OpneSLO
4. Implementing Service Level Objectives (Section II - SLO Implementation)4.