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SLA, SLO, and SLI Metrics

Understanding SLA, SLO, and SLI​

Service Level Agreement (SLA)​

  • Definition: A contract between a service provider and a customer specifying the expected level of service.
  • Purpose: Defines obligations and consequences if the agreed*upon reliability is not met.
  • Example: "The service will be available 99.9% of the time per month. If this is not met, a refund of 10% will be issued."

Service Level Objective (SLO)​

  • Definition: A specific, measurable target for the level of service reliability.
  • Purpose: Serves as a benchmark to ensure the SLA is met.
  • Example: "99.95% of HTTP requests will return a response within 200ms."
  • Relation to SLA: Typically stricter than the SLA, giving room to address failures before breaching the SLA.

Service Level Indicator (SLI)​

  • Definition: A metric that quantifies system performance to track compliance with SLOs.
  • Purpose: Acts as the raw data used to measure whether an SLO is met.
  • Example: "The percentage of successful HTTP requests over the past 30 days."

Example Relationships Between SLA, SLO, and SLI​

  • SLI: Measured uptime = 99.93%
  • SLO: Target uptime = 99.95%
  • SLA: Guaranteed uptime = 99.90%

Calculating Availability and Reliability​

Availability is often expressed as a percentage of uptime over a given period:

Availability (%)=(UptimeTotal Time)×100\text{Availability (\%)} = \left( \frac{\text{Uptime}}{\text{Total Time}} \right) \times 100

Example:

  • Total Time: 30 days (43,200 minutes)
  • Downtime: 30 minutes
Availability=(43,200∗3043,200)×100=99.93%\text{Availability} = \left( \frac{43,200 * 30}{43,200} \right) \times 100 = 99.93\%

Reliability measures the likelihood of a system performing without failure over a specific time:

Reliability (%)=e∗(Total FailuresTotal Time)\text{Reliability (\%)} = e^{*\left(\frac{\text{Total Failures}}{\text{Total Time}}\right)}

Example:

  • Failures: 2
  • Time: 100 hours
Reliability=e∗(2100)=e∗0.02≈98.02%\text{Reliability} = e^{*\left(\frac{2}{100}\right)} = e^{*0.02} \approx 98.02\%

Examples of Defining Metrics for a Service​

Let’s define SLAs, SLOs, and SLIs for a simple web API​

1. SLI Examples:

  • Latency: 95% of requests complete within 200ms.
  • Uptime: Percentage of time the service is reachable.
  • Error Rate: Percentage of failed requests.

2. SLO Examples:

  • Uptime SLO: "The service uptime will be at least 99.95% per month."
  • Latency SLO: "95% of requests will complete within 200ms over the past week."
  • Error Rate SLO: "The error rate will not exceed 0.1% of total requests over the past month."

3. SLA Example:

  • SLA: "The service will maintain 99.9% uptime per month. For every 0.1% below this threshold, a 5% refund of the monthly fee will be issued."

Monitoring and Implementation​

Monitoring SLIs:

  • Latency: Use tools like Prometheus to track response time.
  • Uptime: Use uptime monitoring tools like Pingdom or a custom Prometheus exporter.
  • Error Rate: Count HTTP 4xx and 5xx responses using metrics.

Visualizing in Grafana:

  • Create panels for each metric to display SLI performance over time.
  • Set alerts when an SLO is violated (e.g., error rate exceeds 0.1%).