November 5, 2021

8 min read

SLA vs. SLO vs. SLI: The Full Breakdown for Reliable Systems

Explore the roles of SLIs, SLOs, and SLAs in site reliability engineering and how they empower your team to plan, prioritize, and perform with confidence.

Written by

JJ Tang

SLA vs. SLO vs. SLI: The Full Breakdown for Reliable Systems

Table of contents

TL;DR: Last updated: July 17th, 2025

This guide has been updated to provide the clearest, most current explanation of SLAs, SLOs, and SLIs. Here’s the gist:

SLI (Indicator): The raw metric you measure (e.g., latency, uptime percentage).
SLO (Objective): The internal goal or target for that metric (e.g., 99.9% uptime).
SLA (Agreement): The formal promise to a customer, often with financial penalties if you miss the goal.

Key Takeaways

SLA (Service Level Agreement): A formal, contractual promise between provider and customer, outlining acceptable service levels and consequences if unmet.
SLO (Service Level Objective): The measurable reliability goal teams commit to internally. Think of it as the target.
SLI (Service Level Indicator): The metric you monitor to see if you’re hitting that target—availability, latency, throughput, etc.
How They Work Together: SLIs inform SLOs, which guide SLAs—creating a reliable chain of measurement, goal-setting, and accountability.

SLAs, SLOs, and SLIs form the foundation of modern site reliability engineering (SRE). They influence how incidents are tracked, how engineering teams prioritize efforts, and how businesses maintain customer trust. Yet, too often, these terms are lumped together without clarity or used interchangeably. This guide aims to clean up the confusion.

We’ll walk through each term—starting with SLIs as the building blocks, SLOs as the internal north stars, and SLAs as the external commitments. Along the way, we’ll touch on common challenges, real-world examples, and strategies for getting these right.

What Is an SLI (Service Level Indicator)?

Definition

An SLI is a data-driven measurement of system behavior. It quantifies how your service is performing from the user’s point of view—things like availability, latency, error rates, and system throughput.

Challenges

The hardest part of working with SLIs is not the math—it’s the relevance. Choosing an SLI that doesn’t reflect the customer experience can lead teams to optimize the wrong things. Worse, if the data pipeline is unreliable or poorly defined, decisions made from those SLIs can derail service improvement.

Who Needs It

SLIs are used by SREs, DevOps engineers, QA teams, and anyone responsible for uptime and reliability. They feed alerting systems, support capacity planning, and inform incident reviews.

Examples

99.95% of HTTP requests returned a 2xx status code
95% of database queries completed within 100ms
Less than 0.01% of API responses failed over 24 hours

What Is an SLO (Service Level Objective)?

Definition

An SLO is a clearly defined performance target based on SLIs. It’s a statement of intent: "We aim to achieve 99.9% availability of our login service over the past 30 days."

Challenges

Teams often struggle to set achievable SLOs. Set them too low, and they’re meaningless. Set them too high, and they set you up for alert fatigue or frequent failure. There’s also the challenge of making sure product and engineering agree on what "good enough" means.

Who Needs It

Product managers, SREs, and engineering leaders rely on SLOs to prioritize reliability without slowing down progress. They become the baseline for error budgets—how much unreliability is acceptable within a given period.

Examples

99.9% of requests to the homepage respond in under 300ms
No more than 1% error rate in transaction processing per week

SLOs and Error Budgets

SLOs create accountability, but error budgets allow flexibility. An error budget lets your team innovate and deploy changes as long as the budget isn't burned. Once it is, it's a signal to pause and focus on stability.

Setting Good SLOs

Start with historical data—what’s your system currently capable of? Then, bring product and engineering together to define what reliability means. Revisit regularly as your system and customer expectations evolve.

What Is an SLA (Service Level Agreement)?

Definition

An SLA is a legal document or contract between a service provider and a customer. It defines what level of service is guaranteed, and what penalties apply if those promises aren’t met.

Challenges

The stakes are higher here. Overpromising in an SLA can cost your company—financially, reputationally, or both. And if the metrics aren’t grounded in reliable data (SLIs) and reasonable targets (SLOs), you’re flying blind.

Who Needs It

SaaS vendors, cloud infrastructure providers, managed service providers—anyone delivering digital services under contract. Clients rely on SLAs to ensure accountability and performance.

Examples

99.99% monthly uptime guarantee with 10% service credit if violated
24/7 customer support with 1-hour response time for high-severity tickets

SLAs vs. SLOs

Think of SLAs as promises to the outside world. SLOs are promises to yourself. SLAs carry consequences. SLOs drive alignment. They must inform one another, but they are not the same.

Writing Effective SLAs

Start with what your system can realistically deliver. Include exceptions (e.g., scheduled maintenance), remedies (credits or refunds), and response timelines. Most importantly, don’t treat SLAs as static—review them as your service evolves.

Comparison Table: SLA vs. SLO vs. SLI

Understanding how SLAs, SLOs, and SLIs differ isn’t just helpful—it’s essential for building resilient systems. The table below simplifies their distinctions, so you can make confident, data-driven decisions in your reliability strategy.

Aspect	SLI	SLO	SLA
Type	Metric	Target goal	Legal contract
Purpose	Track system behavior	Guide internal reliability	Define external accountability
Audience	Engineers, SREs	Product & engineering teams	Clients, legal, customer success
Scope	Specific system metric	Broader performance threshold	Comprehensive service definition
Example	99.95% success rate	99.9% uptime last 30 days	99.9% uptime with penalty clause
Penalty for Breach	None	Internal alerts or SRE pause	Service credits or refunds
Update Frequency	Frequently	Occasionally	Rarely

While the distinctions in the table are clear-cut, what truly matters is how your team interprets and applies them. SLAs, SLOs, and SLIs aren’t just policy terms—they’re living agreements between your system, your teams, and your users.

When these three align, you not only gain technical clarity but also empower your team to prioritize the work that matters most. Reliability becomes a shared responsibility, not just an SRE concern.

Why Are SLAs, SLOs, and SLIs Important?

Align Technical and Business Goals

Reliability doesn’t exist in a vacuum. SLAs, SLOs, and SLIs give everyone—from engineers to executives—a shared language to measure success. This alignment ensures that technical metrics translate into real business impact.

Drive Accountability

Whether you're a platform team managing microservices or a SaaS company supporting customers, these frameworks create transparency. They help define who owns what, when action is required, and what success looks like. As a result, teams can operate with greater autonomy and clarity.

Reduce Alert Fatigue

SLOs define what good looks like. They help filter out unnecessary noise from alerting systems and keep engineers focused on meaningful incidents. This focus ultimately reduces burnout and supports sustainable on-call practices.

Build Trust

SLAs aren’t just paperwork—they’re promises. When honored, they build long-term loyalty and reinforce credibility. And when breached, they offer a structured path to make things right and maintain the customer relationship.