Sla, Slo , Sli

📜1️⃣ SLA – Service Level Agreement

🔍 Definition

SLA is a formal contract between a service provider and the customer.
It defines what level of service is promised and what happens if that level isn’t met.

It often includes penalties or compensation for not meeting the target.
⚙️ Real-world Example

If AWS promises 99.99% uptime per month, that’s an SLA.

If uptime drops below 99.99%, AWS might:

• Credit you part of your monthly bill.

• Publish an incident RCA.

🎯 2️⃣ SLO – Service Level Objective

🔍 Definition

SLO is the target goal or threshold for your SLI — the performance level you want to maintain.

It’s what you aim for internally as a reliability objective.

💡 Example

• SLI: 99.95% availability

• SLO: “Our goal is to maintain 99.9% or higher availability for the payment API per month.”

If your SLI drops below 99.9%, it’s a signal that reliability is degrading and you need to take corrective actions.

⚙️ Real-world Example

A video streaming service may define:

• SLI: Percentage of video plays without buffering.

• SLO: 99.5% of all videos should play without buffering.

If buffering increases due to CDN issues, you’re breaching your SLO.

⚙️ 3️⃣ SLI – Service Level Indicator

🔍 Definition

SLI is a measurable metric that shows how a system is performing — basically, “How are we doing?”

It’s a quantitative measurement of a specific aspect of reliability, such as:

• Availability (uptime)

• Latency (response time)

• Error rate (failed requests)

• Throughput (requests handled per second)

📈 Formula Example

Availability SLI = (Number of successful requests) / (Total requests)

If you had:

• 999,500 successful requests out of 1,000,000 total
  → SLI = 99.95% availability

🧩 Real-world Example

In a payment gateway API, an SLI could be:

“Percentage of successful transactions in the last 30 days.”

If 0.1% of transactions failed, SLI = 99.9%.