In modern manufacturing IT, SLI (Service Level Indicator) measures actual system performance as a concrete metric, SLO (Service Level Objective) defines the internal target for that metric, and SLA (Service Level Agreement) is the contractual commitment to a customer or user that a specific performance level will be met. These three terms describe a clear hierarchy: without measurement, there is no goal; without a goal, there is no meaningful contract.
The most common mistake in industrial IT: companies negotiate SLAs with software providers without ever defining which SLIs they actually measure or which SLOs apply internally. The result is a contract whose compliance no one can validly verify.
The correct logic flows from the bottom up:
An SLA not anchored in a defined SLO is a document without an operational foundation.
An SLI is a precise, quantifiable metric that describes the state or performance of a system at a specific time. It is the raw material for all further service-level evaluations.
| SLI | Metric | Relevance in Manufacturing |
| Availability | % of time the system is reachable | Risk of production standstill |
| Latency | Response time in milliseconds | Delayed machine data, control errors |
| Error Rate | % of failed transactions | Data loss in quality protocols |
| Throughput | Data points processed per second | Bottleneck for high-frequency machine data |
| Data Freshness | Age of the last written record | Critical for real-time OEE and Traceability |
An SLO is an internally defined performance goal for one or more SLIs, typically over a fixed measurement period. A classic example: "The MES shall be available 99.9% of the time on a monthly average."
SLOs align operations, IT, and business departments toward a common, measurable goal. They are not a promise to the outside world—they are the internal steering mechanism.
Practical Warning: The SLO must always be stricter than the SLA. If the SLA guarantees 99.5% availability, the internal SLO should be 99.7% or higher. This acts as a buffer to identify issues before a breach of contract occurs.
Originating from the Google SRE model and now entering industrial IT, the Error Budget makes risk management operational. If your SLO defines 99.9% availability, you have an Error Budget of 0.1% (approx. 43 minutes of tolerated downtime per month). Once this budget is exhausted, risky deployments are halted until it regenerates.
An SLA is a legally binding agreement between a provider and a client. It defines which SLOs are guaranteed, how compliance is measured, the consequences of falling short (penalties), and escalation paths.
The "Single-Incident" Trap: Most SLAs define availability as a yearly or monthly average. 99.9% availability sounds solid but allows up to 8.7 hours of downtime per year. If these 8.7 hours occur in a single event, but your production requires an RTO (Recovery Time Objective) of 2 hours, you are contractually under-protected. Always negotiate the Maximum Single-Incident Downtime.
| Level | Term | Purpose | Audience |
| Measure | SLI | Quantify actual state | IT Ops, Monitoring |
| Steer | SLO | Internal performance target | Ops, Production IT, Mgmt |
| Guarantee | SLA | Contractual minimum | Customers, Auditors |
A comprehensive SLI/SLO/SLA model makes the operational quality of digital production systems objective and manageable. As manufacturers become increasingly dependent on cloud-MES and digital platforms, they must manage these dependencies with the same rigor they use for machine capacity and quality KPIs: through measurable targets and clear accountability.