1) Meaning

A Service Level Indicator (SLI) is a specific, measurable metric that reflects the actual performance of a service in relation to what has been agreed or expected.

Think of it as the “thermometer” that tells you how well a service is running.
In practice, it’s the raw measurement used to evaluate whether a service is meeting its Service Level Objective (SLO).

2) Relationship with SLA / SLO

  • SLI (Indicator): The metric being measured.
    • Example: “Percentage of requests completed within 300 ms.”
  • SLO (Objective): The target/goal for the SLI.
    • Example: “99% of requests must complete within 300 ms.”
  • SLA (Agreement): The formal contract that may include penalties if SLOs are not met.
    • Example: “If availability drops below 99%, provider must credit the customer.”

3) Examples of SLIs

(a) Reliability & Availability

  • Service uptime % (e.g., 99.9% availability)
  • Mean time between failures (MTBF)

(b) Performance

  • Response time (average latency in ms)
  • Throughput (requests per second handled successfully)

(c) Quality

  • Error rate (% of failed requests)
  • Data accuracy (e.g., % of transactions processed correctly)

(d) Supply Chain Context

  • On-time delivery %
  • Fill rate %
  • Stockout frequency %

4) Example Scenario

E-commerce Website:

  • SLI: 95% of checkout requests completed in < 2 seconds.
  • SLO: Target = 99% of requests in < 2 seconds.
  • SLA: Contract says provider must compensate if uptime falls below 98%.

5) Why It Matters

  • Transparency: Gives objective evidence of performance.
  • Decision-making: Helps prioritize improvements.
  • Accountability: Forms the foundation of SLOs and SLAs.
  • Customer trust: Customers know what metrics are monitored and guaranteed.

Bottom line:
SLI (Service Level Indicator) = the measurable metric that shows how well a service is performing. It feeds into SLOs (objectives) and SLAs (agreements), making it the most granular layer of service-level management.