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System Design

Service Level Agreements (SLAs)

A guide to understanding the key components of service level management: SLAs, SLOs, and SLIs.

Table of Contents

The Hierarchy of Service Levels

SLAs, SLOs, and SLIs form a hierarchy for defining and measuring the reliability and performance of a service:

  1. SLA (The Promise): The overall agreement with the user.
  2. SLO (The Goal): The internal targets required to meet the SLA.
  3. SLI (The Measurement): The real-time data that tracks performance against the SLOs.

Service Level Agreement (SLA)

An SLA is a formal agreement between a service provider and a client or user. It defines the level of service expected, including metrics for uptime, performance, and responsibilities.

  • What it is: A contract that specifies the promises made to users.
  • Example: “We guarantee 99.9% uptime for our service.”
  • Consequences: SLAs often include penalties for the provider if the agreed-upon terms are not met (e.g., service credits, refunds).

Service Level Objective (SLO)

An SLO is a specific, measurable objective that a team must meet to comply with the SLA. It is an internal goal that is more aggressive than the SLA to provide a buffer.

  • What it is: A target for a specific metric, such as uptime or response time.
  • Example: To meet a 99.9% uptime SLA, the internal SLO might be 99.95% uptime. This buffer allows for maintenance or unexpected issues without violating the SLA.

Service Level Indicator (SLI)

An SLI is a quantitative measure of a specific aspect of the service’s performance. It is the actual, real-time measurement of how the system is performing.

  • What it is: The raw data or metric being measured.
  • Examples:
    • The percentage of successful HTTP requests.
    • The latency of the 95th percentile of requests.
    • The proportion of a month that the service was available.

Note: An SLI provides the data to determine whether you are meeting your SLO.

Summary

ConceptDescription
SLAThe formal agreement with the user (the promise).
SLOThe internal goal to meet the SLA (the target).
SLIThe real measurement of performance (the metric).

Why It Matters

Understanding and implementing SLAs, SLOs, and SLIs is crucial for:

  • Aligning Expectations: Clearly defines what users can expect from a service.
  • Data-Driven Decisions: Provides a quantitative basis for engineering and operational decisions.
  • Prioritization: Helps teams focus on what matters most to users.
  • Accountability: Creates a clear framework for accountability and continuous improvement.