These three terms are commonly mixed up in reliability, observability, and customer contracts. This page gives you clear definitions, a side by side comparison table, and practical examples that you can copy into docs.
Quick List: SLI vs SLO vs SLA in Plain English
Use this list at the top of a listicle. It is designed for skimming and quick understanding.
1) SLI (Service Level Indicator)
The metric you measure. It describes the service level you are actually delivering.
- ✅What it is: A clearly defined quantitative measure (latency, error rate, availability, throughput).
- 🧠Think: "What number tells us the user experience for this service?"
- 🧪Example: "Percentage of successful HTTP requests."
2) SLO (Service Level Objective)
The target you want your SLI to hit across a defined period.
- 🎯What it is: A target value or range for an SLI (including the measurement window).
- 🧠Think: "How good is good enough, and over what time window?"
- 🧪Example: "Request success rate at or above X% over 30 days."
3) SLA (Service Level Agreement)
The customer agreement. It usually includes one or more SLOs, plus consequences or remedies if the commitment is missed.
- 🧾What it is: A contract or published commitment with defined outcomes if targets are not met.
- 🧠Simple test: If there is no consequence, it is likely an SLO, not an SLA.
- 🧪Example: "If availability is below X% in a month, customer receives service credits."
Fast rule to avoid confusion
SLI is the measurement, SLO is the internal reliability target, SLA is the external promise plus consequences.
SLO vs SLA vs SLI Comparison Table
Side by side view of what each term means, who owns it, and what happens when it is missed.
| Term | Meaning | Core purpose | Typical owner | Audience | If missed | Example statement |
|---|---|---|---|---|---|---|
| SLI | A quantitative indicator of service level. | Measure what users experience. | Engineering, SRE, platform teams. | Mainly internal. | No direct consequence by itself (it is a metric). | "Successful requests divided by total requests." |
| SLO | A target for an SLI over a defined window. | Define reliability expectations and tradeoffs. | Engineering plus product, with stakeholder input. | Internal, sometimes published. | Triggers internal action (priorities, releases, paging policies). | "Success rate at or above X% over 30 days." |
| SLA | A customer facing agreement that may include SLOs and remedies. | Set contractual expectations with accountability. | Product, legal, leadership, with SRE input. | External customers and internal teams. | Defined consequence or remedy (service credits, penalties, support escalation). | "If availability is below X% in a month, credits apply per contract." |
Important nuance
Many teams use "SLA" informally to mean response time targets inside a support desk. In reliability engineering, SLA usually means a customer commitment with consequences. If you mean an internal target, label it SLO.
What is an SLI (Service Level Indicator)
An SLI is the number you track. It should be clearly defined so that different people measuring it get the same result.
Definition
SLI stands for Service Level Indicator. It is a carefully defined quantitative measure of some aspect of the service level you provide.
What makes an SLI usable
- It measures something users actually experience (not only internal system health).
- It has clear numerator and denominator where relevant (for example, good events vs total events).
- It is defined with scope (which endpoints, which regions, which customer tier).
- It is defined with a window and sampling method (for example, rolling 30 days, per minute).
Common SLI types
- Availability or success rate
- Latency (for example, 95th percentile request latency)
- Error rate
- Throughput or freshness (for data pipelines)
What an SLI is not
- Not a goal by itself (that is an SLO).
- Not a contract (that is an SLA).
- Not a vanity metric with unclear user impact.
SLI example formula (success rate)
This is a common pattern for SLIs in web services.
What is an SLO (Service Level Objective)
An SLO turns measurement into a target. It defines what reliability you aim to deliver for a service over a time window.
Definition
SLO stands for Service Level Objective. It is a target value or range of values for a service level that is measured by an SLI.
What a strong SLO includes
- The SLI it uses (exactly defined).
- A target threshold (or range).
- A measurement window (for example, rolling 28 or 30 days).
- Scope (what traffic is included and excluded).
What an SLO is used for
- Balancing reliability work vs feature work.
- Creating alerting that matches user impact.
- Setting release guardrails (slow down when reliability is suffering).
- Creating shared expectations across teams.
What an SLO is not
- Not a guarantee to customers unless it is part of an SLA.
- Not necessarily "as high as possible" (too strict can slow delivery without improving user outcomes).
SLO example (availability)
This is a template. Replace X and the window with your actual target.
SLO accuracy checklist
- 1Window defined: rolling 30 days, calendar month, or another clear period.
- 2Scope defined: endpoints, regions, paid tiers, and exclusions.
- 3SLI defined: good event definition is explicit.
- 4Target defined: threshold or range is specific and measurable.
What is an SLA (Service Level Agreement)
An SLA is where reliability becomes a customer commitment. The key difference is consequences or remedies if the commitment is missed.
Definition
SLA stands for Service Level Agreement. It is an agreement with users or customers that describes a level of service and what happens if the service level is not met.
What SLAs typically include
- One or more service commitments (often stated like an SLO).
- How the service level is measured (definitions, exclusions, windows).
- Remedies or consequences (service credits, penalties, termination rights, support escalation).
- Claims process and timelines (how customers request remedies).
Why teams should be careful with SLAs
- Legal and financial risk increases as commitments become stricter.
- Ambiguous measurement language leads to disputes.
- Teams often need internal SLOs that are stricter than the external SLA to create buffer.
What an SLA is not
- Not the same as an internal "target" unless you actually publish remedies.
- Not a single metric by default (it is an agreement, it can include multiple metrics).
SLA example template (copy and edit)
This is an example pattern. Actual SLAs should be reviewed by legal and product owners.
Remedy: If availability is below X%, service credits apply according to the credit schedule and claim process.
How SLIs, SLOs, and SLAs Fit Together
This is the clean mental model you can reuse in your article, docs, or onboarding material.
Step 1: Define the SLI
Pick the metric that represents user experience for the service.
- 📈Choose user visible signals (success, latency, freshness).
- 🧩Define scope and exclusions clearly.
Step 2: Set the SLO
Choose a target and window that matches the business and user expectations.
- 🎯Pick a window (rolling or calendar) and a threshold.
- ⚖️Balance reliability and shipping velocity.
Step 3: Decide if it becomes an SLA
Only commit externally when you can define measurement and remedies clearly.
- 🧾Define remedies and a claim process.
- 🛡️Use internal buffer (often stricter SLO than SLA).
The consequence test
If missing the target triggers a defined remedy or penalty, you are in SLA territory. If it triggers internal action only, it is an SLO.
Examples You Can Reuse: SLIs, SLOs, and SLAs
The goal is clarity. These are written as templates so you can plug in your values and service details.
Example 1: API request success
Good for web apps, APIs, and microservices.
- SLISuccess rate = good requests divided by total requests.
- SLOSuccess rate at or above X% over a rolling 30 days.
- SLAIf success rate is below X% in a calendar month, remedies apply.
Example 2: Latency (percentile based)
Good for user experience sensitive endpoints.
- SLIp95 request latency for a defined endpoint set.
- SLOp95 latency below Y ms over a rolling 28 days.
- SLAIf p95 latency exceeds Y ms in a month, remedies apply (if you publish it).
Common mistakes to avoid
- ⚠️Vague SLIs: "Uptime" without defining what counts as downtime or which users are included.
- ⚠️SLO without a window: Targets must include a measurement period to be meaningful.
- ⚠️SLA without measurement language: If you cannot measure it consistently, you cannot contract it safely.
- ⚠️Internal goals called SLA: If there is no remedy, call it an SLO to reduce confusion.
FAQ: SLI, SLO, and SLA Questions People Ask
These answers are short on purpose and written to work well for featured snippets.
Is an SLO the same as an SLA
No. An SLO is a target for an SLI. An SLA is a customer facing agreement that may include SLOs plus remedies or consequences.
Can you have SLOs without SLAs
Yes. Many teams use SLOs purely for internal reliability management without publishing an SLA to customers.
Can one SLA include multiple SLOs
Yes. An SLA can include multiple commitments (for example, availability and support response commitments) as long as measurement and remedies are defined.
What should come first, SLI or SLO
Define the SLI first. If you cannot measure the indicator clearly, you cannot set a meaningful objective for it.
One sentence summary for the intro
SLI is what you measure, SLO is the target you aim for, SLA is the promise you make to customers with consequences if you miss it.
SLO Error Budgets Explained
Where to add it:
Place this immediately after the “What is an SLO (Service Level Objective)” section and before “What is an SLA.”
Content:
An SLO by itself defines your reliability target. An error budget defines how much unreliability you are allowed.
If your SLO is 99.9% success rate over 30 days, your error budget is 0.1% over that same window.
Error Budget = 100% − SLO
If you burn through the entire error budget early in the window, you have exceeded your acceptable reliability risk. That should trigger internal action.
Error budgets are used to:
- Decide when to slow down feature releases
- Shift focus from shipping to reliability work
- Align product and engineering priorities
- Prevent over-engineering when reliability is already good enough
Without error budgets, SLOs become passive numbers. With error budgets, SLOs become decision frameworks.
This concept is central in modern reliability practices and helps teams balance speed and stability.
How to Choose the Right SLI and SLO
Where to add it:
Place this right after the “How SLIs, SLOs, and SLAs Fit Together” section and before “Examples You Can Reuse.”
Content:
Choosing the wrong SLI makes every SLO and SLA built on top of it meaningless.
Start by asking: what does the user actually experience?
Good SLIs usually measure:
- Successful requests from the user’s perspective
- Latency that users actually feel
- Data freshness where delays impact decisions
- End-to-end behavior, not just internal service health
Avoid internal system metrics that do not map to user impact. CPU usage is rarely a good SLI. Request success rate often is.
When choosing an SLO:
- Make it realistic but meaningful
- Avoid chasing perfection if users cannot tell the difference
- Include a clear time window
- Make sure measurement is automated and consistent
If you cannot explain why the SLO matters to a customer or stakeholder, it is likely the wrong target.
Sources and References
These links back up the definitions and help keep terminology aligned with widely used SRE guidance.
Primary references
- 1 Google SRE Book: Service Level Objectives (SLOs) and how they relate to SLIs and SLAs: sre.google/sre-book/service-level-objectives
- 2 Google SRE Workbook: Implementing SLOs (practical guidance and examples): sre.google/workbook/implementing-slos
- 3 AWS CloudWatch docs: Service Level Objectives and tracking SLI performance against goals: docs.aws.amazon.com (CloudWatch SLOs)
- 4 Microsoft Well-Architected Reliability: reliability targets and SLOs: learn.microsoft.com (Reliability metrics)
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