Add definitions for SLI, SLO, SLA, error budget and toil (bregman-arie#9077)

* add definitions for SLI, SLO, SLA, error budget and toil

* add credit

* Add credits section

* add google sre book under questions

Author: ronan-s1

topics/sre/README.md

@@ -3,9 +3,71 @@
 ## SRE Questions
 
 <details>
-<summary>What is SLO (service-level objective)?</summary><br><b>
-</b></details>
+<summary>What is an SLI (Service-Level Indicator)?</summary>
+<b>
+An SLI is a measurement used to assess the actual performance or reliability of a service. It serves as the basis for defining SLOs.
+
+Examples:
+- Request latency
+- Processing throughput
+- Request failures per unit of time
+
+Read more: [Google SRE Handbook](https://sre.google/sre-book/table-of-contents/)
+</b>
+</details></br>
+
+<details>
+<summary>What is an SLO (Service-Level Objective)?</summary>
+<b>
+
+An SLO is a target value or range of values for a service level that is measured by an SLI
+
+Example: 99% across 30 days for a specific collection of SLIs.
+
+It's also worthy to note that the SLO also serves as a lower bound, indicating that there is no requirement to be more reliable than necessary because doing so can delay the rollout of new features.
+
+Read more: [Google SRE Handbook](https://sre.google/sre-book/table-of-contents/)
+</b>
+</details><br>
+
+<details>
+<summary>What is an SLA (Service-Level Agreement)?</summary>
+<b>
+
+AN SLA is a formal agreement between a service provider and customers, specifying the expected service quality and consequences for not meeting it.
+
+SRE doesn't typically get involved in constructing SLAs, because SLAs are closely tied to business and product decisions
+
+Read more: [Google SRE Handbook](https://sre.google/sre-book/table-of-contents/)
+</b>
+</details><br>
+
+<details>
+<summary>What is an Error Budget?</summary>
+<b>
+
+An Error Budget represents the acceptable amount of downtime or errors a service can experience while still meeting its SLO.
+
+An error budget is 1 minus the SLO of the service. A 99.9% SLO service has a 0.1% error budget.
+
+If our service receives 1,000,000 requests in four weeks, a 99.9% availability SLO gives us a budget of 1,000 errors over that period.
+
+The error budget is a mechanism for balancing innovation and stability. If the SRE cannot enforce the error budget, the whole system breaks down.
+
+Read more: [Google SRE Handbook](https://sre.google/sre-book/table-of-contents/)
+</b>
+</details></br>
 
 <details>
-<summary>What is SLA (service-level agreement)?</summary><br><b>
-</b></details>
+<summary>What is Toil?</summary>
+<b>
+
+Toil is the kind of work that tends to be manual, repetitive, automatable, tactical, devoid of enduring value, and that scales linearly as a service grows.
+
+If you can be automate a task, you should probably automate the task.
+
+Automation significantly reduces Toil. Investing in automation results in valuable work with lasting impact, offering scalability potential with minimal adjustments as your system expands.
+
+Read more: [Google SRE Handbook](https://sre.google/sre-book/table-of-contents/)
+</b>
+</details>