What is Service Level Agreement (SLA)?

A Service Level Agreement (SLA) is a formal, often contractual commitment to customers about the level of service they will receive, such as 99.9% uptime, with defined consequences (credits or penalties) if the commitment is not met.

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IN DEPTH

In depth.

An SLA is the externally-facing, business-level promise. It is usually negotiated, written into contracts, and backed by penalties, service credits if uptime drops below a threshold, for instance. Because the consequences are real (money and trust), SLAs are typically set looser than the internal targets a team actually aims for, leaving a safety margin.

That is the key relationship interviewers probe. A Service Level Indicator (SLI) is the actual measurement (observed uptime or latency). A Service Level Objective (SLO) is the internal target the team aims for. The SLA is the external promise, deliberately less strict than the SLO so the team can miss its internal goal occasionally without breaching a customer contract. For example: SLO 99.95% internally, SLA 99.9% to customers.

For QA and SDET work, SLAs shape priorities: the journeys and metrics tied to SLAs get the most testing and monitoring, because breaching them has contractual cost. Performance tests assert against SLA-relevant thresholds, and synthetic monitoring watches the SLIs that feed them.

WHY IT MATTERS

Why interviewers ask about this.

SLAs come up in interviews for SDET, performance, platform, and lead roles, and the SLA/SLO/SLI distinction trips people up. Knowing that the SLA is the external contractual promise (looser than the internal SLO) signals reliability and business awareness.

EXAMPLE

Example scenario.

A SaaS vendor commits to a 99.9% uptime SLA with service credits for breaches. Internally the team targets a stricter 99.95% SLO, so a bad week that drops them to 99.92% misses the internal goal but still honors the customer SLA, exactly the safety margin the gap is designed to provide.

TIP

Interview tip.

Separate the three cleanly: SLI is the measurement, SLO is the internal target, SLA is the external contractual promise with penalties, set looser than the SLO. Then connect SLAs to what gets prioritized for testing and monitoring.

FAQ

Frequently asked questions.

What is the difference between an SLA and an SLO?

An SLA is the external, contractual commitment to customers, usually with penalties for breach. An SLO is the internal target the team aims for, typically stricter than the SLA so missing the internal goal does not immediately breach a customer contract.

What happens if an SLA is breached?

SLAs usually define consequences, commonly service credits or refunds to the customer, and sometimes the right to terminate the contract. That financial and relationship cost is why SLA-related journeys get heavy testing and monitoring.