Last week, I ordered a pizza on a food delivery app. And they promised the delivery in 30 minutes.
Similarly, all digital services: Apps, websites, cloud platforms, etc, make promises about speed, uptime, and reliability. The difference is how they track and measure those promises.
That’s where SLA, SLO, and SLI come in. These three metrics define what “reliable” actually means. They turn a vague claim like “99.9% uptime” into something you can measure, track, and act on.
For DevOps and SRE teams, these aren’t just technical terms. They’re the framework that helps you build trust with customers and keep systems running smoothly.
Let’s break down what each one means and how they work together.
Table of Contents
SLA vs. SLO vs. SLI
| Term | Who Uses It | What It Tracks | What Happens If You Miss It |
|---|---|---|---|
| SLA (Service Level Agreement) | Customers and service providers | The formal promise about uptime, response time, and availability | Breach triggers penalties, service credits, or legal consequences |
| SLO (Service Level Objective) | Internal teams (product, engineering, SRE) | Internal performance target (e.g., 99.95% uptime) | Signals you’re close to breaking the SLA, but no direct penalty yet |
| SLI (Service Level Indicator) | Engineering and monitoring teams | Actual measured performance (latency, error rate, uptime %) | When it drops below target, you’ve missed your SLO |
What is SLA (Service Level Agreement)?
An SLA is a formal, external agreement between a service provider and a customer (or consumer of that service) that defines what level of service will be delivered and what happens if the provider fails to meet that level.
Example of an SLA
Let’s say AWS promises 99.95% monthly uptime for EC2 instances. And if uptime drops below that, customers receive 10% service credits on their next bill. That’s SLA in action. It is measurable, enforceable, and customer-facing.
Why SLA Matters
If your customers don’t know what you guarantee, you risk misunderstandings, loss of trust, or legal liability. SLAs help you formalize the promise. Additionally, SLAs are also crucial in competitive markets. When two providers offer similar features, the one with a stronger SLA often wins the deal.
Key Components of SLA
An SLA typically includes:
- Scope of service (what is covered, what is excluded)
- Metrics and targets (e.g., uptime, response time)
- Measurement and reporting method
- Penalties or remedies for breach
- Review and revision clauses
Best Practices of SLA
- When creating an SLA, define clear and measurable metrics like uptime, latency, and error rate so customers can easily understand what reliability means in practice.
- Keep your commitments realistic and regularly reviewed to reflect your system’s true performance. Overpromising can harm credibility and trust.
- Finally, include response timelines and exclusions such as scheduled maintenance to set fair, transparent expectations for both parties.
Who Needs SLA?
External-facing platforms, cloud providers, SaaS tools, managed IT services, and telecom companies; essentially, anyone selling reliability as part of their offering.
Even internal IT teams use “operational SLAs” to manage expectations between departments.
What is SLO (Service Level Objective)?
An SLO is an internal, measurable target that defines what “acceptable reliability” looks like for your team. It sits between the SLA (the promise) and the SLI (the data).
In short, if the SLA is the contract, the SLO is the goal that makes sure you’ll never break it.
Example of SLO
Netflix engineers might define SLOs for streaming availability or video start latency, such as:
“99.98% of video play requests start within 2 seconds.”
If the actual measured uptime (SLI) dips below that, they know they’re burning error budget and must investigate before the SLA is threatened.
Why SLO Matters
SLOs give your team a clear target to aim for. Without them, you’re flying blind between raw metrics and customer promises.
They create breathing room between what you measure and what you guarantee, so one bad day doesn’t break your SLA.
SLOs also guide decisions. When you’re deciding whether to push a release or fix a bug, your SLO tells you how much reliability margin you have left to spend.
Key Components of SLO
- Target metric (the aspect of reliability you’re measuring, e.g., uptime, latency)
- Threshold value (sets the acceptable performance level, like 99.9% availability)
- Time window (specifies the period over which performance is measured: weekly, monthly, quarterly)
- Error budget (Indicates how much failure is tolerable before action is needed)
Best Practices of SLO
- Always tie SLOs to real user experience metrics like availability or latency so they reflect how customers experience your service.
- Keep them visible and realistic, maintaining a healthy buffer between SLOs and SLAs to handle unexpected incidents.
- Use SLO breaches as feedback, not failure. They are opportunities to review, refine, and improve reliability goals.
Who Needs SLO?
Every DevOps and SRE team. Whether you run an API, database, or microservice, SLOs help you quantify “good enough” reliability.
They’re also vital in post-incident reviews, helping you learn from misses and adjust goals.
How SLO Relates to SLA
Most teams define stricter SLOs than SLAs to maintain a cushion called the error budget.
If your SLA guarantees 99.9% uptime, you might set an SLO of 99.95%. That 0.05% difference allows some room for internal experimentation or feature releases without risking the customer contract.
What is SLI (Service Level Indicator)?
An SLI is the concrete measurement of service performance. It answers the question: “How are we doing right now?”
SLIs are typically numbers your monitoring tools track, such as uptime percentage, latency, or error rate, and they form the foundation for SLOs and SLAs.
Examples of SLI
- Availability (percentage of time service is up)
- Latency (time taken to respond)
- Error rate (fraction of requests that fail)
- Throughput (number of requests processed per unit time)
Why SLI Matters
SLIs are your source of truth. They show what actually happens right now, not what you hope happens. Without accurate SLIs, your SLOs become guesses and your SLAs turn into promises you can’t verify.
SLIs also drive action. They feed dashboards, alerts, and incident response. When something breaks, your SLI gives the first signal about where to look.
Key Components of SLI
- Metric definition (outlines what’s being measured, availability, error rate, and latency)
- Data source (identifies where the measurement comes from, logs, monitoring tools, or observability platforms)
- Calculation method (how the metric is computed, e.g., successful requests ÷ total requests)
- Measurement frequency (how often the system collects and evaluates the data)
Best Practices of SLI
- Choose SLIs that directly reflect user experience, not just internal system metrics, so your measurements stay meaningful.
- Keep them accurate, consistent, and easy to track over time, using visual dashboards and alerts to spot reliability risks early.
- Focus on a few key SLIs that matter most, and use error budgets to understand when you’re nearing your reliability limits.
Who Needs SLI?
Anyone responsible for monitoring service health, like, SREs, DevOps engineers, and infrastructure teams.
If you’re setting up dashboards, alert rules, or on-call rotations, SLIs are your source of truth.
How SLI Relates to SLA and SLO
The relationship is simple:
SLI → SLO → SLA
Metric → Target → Promise
You define an SLI (like request success rate), set a target threshold (SLO), and then use that to support your external commitment (SLA).
How SLIs Feed into SLOs? If your SLI shows that 99.92% of requests succeeded last month, and your SLO target is 99.95%, you’ve missed the objective, even if users didn’t notice. That’s your early warning to improve before breaking an SLA.
How SLA, SLO, and SLI Work Together
SLA, SLO, and SLI form a layered reliability system. Let’s put it all together with a scenario for a DevOps/SRE team like yours.
Your SaaS product tells customers: “We guarantee 99.9% uptime each month.” That is your SLA. Internally, you might set: “We aim for 99.95% uptime each month.” That’s your SLO. Your monitoring system tracks the actual uptime percentage, which is the SLI.
Suppose you ran the system and measured uptime = 99.92%. You missed the internal SLO (99.95%), but you still met the SLA (99.9%). This signals to your team: reliability margin is narrowing, you need to fix underlying issues, or you risk breaking SLA.
Here are some Boundary cases for clarity:
You might have internal SLOs for services that don’t have a customer-facing SLA (for internal teams) — that’s fine.
You could breach an SLO but still not breach the SLA (as above).
Once you breach the SLA, you trigger the penalty/credit and risk customer dissatisfaction.
What Happens When You Miss an SLO vs When You Break an SLA
When you missed SLO but SLA is still met, there’s no penalty for the customer, but take it as an internal flag: “We are losing reliability margin, hence, take action”.
But when the SLA is broken, you need to understand that here the customer is impacted. You may owe credits or face reputation/legal consequences while the contract kicks in.
Turning Metrics into Meaning
For DevOps engineers and SREs, SLA, SLO, and SLI aren’t paperwork. They’re the backbone of observability, accountability, and trust.
These work together to form a reliability loop:
- Measure performance (SLI).
- Set internal goals (SLO).
- Deliver external commitments (SLA).
As digital services scale, these three abbreviations become your compass, guiding decisions that keep systems trustworthy, customers happy, and teams aligned.
When done right, they prevent burnout, reduce firefighting, and turn reliability into a shared responsibility, not an afterthought.
As Margaret Hamilton once said, “The difference between theory and practice is greater in practice than in theory.”
SLAs, SLOs, and SLIs bridge that gap, transforming good intentions into measurable reliability.
FAQs
1. What happens if you break an SLA?
Breaking an SLA means failing to meet the promised service level. It can lead to penalties, service credits, or loss of customer trust depending on the contract terms.
2. How to calculate SLA, SLO, and SLI?
- SLI: Actual measured performance (e.g., uptime % = available time / total time).
- SLO: Target value for that metric (e.g., 99.9% uptime).
- SLA: The external commitment based on SLOs, often with legal or financial consequences.
3. What does 99.9 SLA mean?
It means the service promises 99.9% uptime, allowing for roughly 43 minutes of downtime per month before breaching the agreement.