What is an SLA?
A Service Level Agreement (SLA) is Microsoft’s uptime guarantee for a service.
It’s expressed as a percentage of availability per month.
Common SLA Numbers
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99% → ~7.3 hours downtime/month
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99.9% (three nines) → ~43 minutes downtime/month
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99.95% → ~22 minutes downtime/month
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99.99% (four nines) → ~4.4 minutes downtime/month
SLAs for Azure Services
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Single VM (no redundancy) → 99.9%
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VMs in an Availability Set → 99.95%
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VMs in Availability Zones → 99.99%
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Azure Storage (RA-GRS) → 99.99% read access
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SQL Database (Premium tier) → 99.99%
Combining Services for Higher SLA
When services are combined, the effective SLA decreases (multiply them).
Example:
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Azure App Service SLA = 99.95%
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Azure SQL Database SLA = 99.99%
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Combined SLA = 99.95% × 99.99% ≈ 99.94%
Designing for High Availability
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Use Availability Zones to survive datacenter outages.
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Use Load Balancers to distribute traffic across instances.
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Enable Auto-Scaling with VM Scale Sets.
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Use Geo-Redundant Storage (GRS/RA-GRS) for durability.
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Consider Disaster Recovery with Azure Site Recovery (ASR).
Confusion Buster 🚨
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SLA vs SLO vs SLI
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SLA = official Microsoft guarantee.
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SLO = your internal target.
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SLI = actual measured uptime.
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Availability Set vs Zone for SLA
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Set = 99.95% (within datacenter).
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Zone = 99.99% (across datacenters).
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Exam trap: If question says “survive datacenter failure” → Zone, not Set.
Simple Example
A healthcare app:
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Deploys VMs across Availability Zones for 99.99% uptime.
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Uses Load Balancer to ensure traffic fails over to healthy VMs.
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Stores medical records in RA-GRS Storage for cross-region durability.
Exam Tip
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“Single VM SLA” → 99.9%.
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“Availability Set” → 99.95%.
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“Availability Zone” → 99.99%.
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“Combined SLA of multiple services” → multiply them.
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“Survive datacenter outage” → Availability Zone.
What to Expect in the Exam
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Direct Q: “What is the SLA for a VM deployed in Availability Zones?” → 99.99%.
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Scenario: “Company requires <5 min downtime/month.” → Must use Availability Zones.
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Scenario: “What’s the combined SLA of App Service (99.95%) and SQL DB (99.99%)?” → ~99.94%.
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Trick Q: “Azure guarantees 100% uptime with Availability Zones.” (False).