"This is where the series comes together. The Well-Architected Framework, HA patterns, architecture design patterns, and cost optimisation are the lenses every AZ-305 scenario question is written through. Learn to see through those lenses and every exam question becomes a design conversation."
The Urban Planner Analogy
The Well-Architected Framework — Your Design Lens
Every AZ-305 scenario question is written through one or more of the five WAF pillars. Learning to identify which pillar a question is testing tells you what the answer should optimise for.
High Availability — The SLA Spectrum
Every HA design has a cost. The architect's job is to match the design to the SLA requirement — not exceed it unnecessarily (wastes money) and not fall short (breaks the SLA agreement).
HIGH AVAILABILITY SLA SPECTRUM
99.9% SLA (3 nines) -- single region, single zone
Downtime allowed: ~8.7 hours per year
Design: App Service (single region) + Azure SQL (standard)
Cost: baseline
99.95% SLA -- single region, redundant instances
Downtime allowed: ~4.4 hours per year
Design: App Service (2+ instances) + availability set for VMs
Cost: +slight (second instance)
99.99% SLA (4 nines) -- multi-zone within region
Downtime allowed: ~52 minutes per year
Design: App Service (Zone Redundant) + Azure SQL (Business Critical)
Resources spread across Availability Zones 1, 2, 3
Cost: ++significant (Zone Redundant tier is premium)
99.999% SLA (5 nines) -- multi-region active-active
Downtime allowed: ~5 minutes per year
Design: Front Door -> App Service (Zone Redundant) in 2 regions
Azure SQL (Business Critical + Geo-Replication auto-failover)
Cost: +++expensive (duplicate infrastructure in 2 regions)
AVAILABILITY ZONES:
Physical separation within one Azure region
3 zones per region -- each in a different building, power, cooling
VMs in different AZs survive datacenter-level failure
Active-Active vs Active-Passive — The Multi-Region Decision
When a workload requires multi-region deployment, the choice between Active-Active and Active-Passive determines both cost and recovery speed.
Architecture Patterns — The AZ-305 Vocabulary
AZ-305 scenario questions map to recognised architecture patterns. Knowing these patterns by name — and when to apply them — is the architect's vocabulary.
Strangler Fig — The AZ-305 Migration Pattern
The Strangler Fig is named after a tropical tree that grows around a host tree, gradually replacing it. Route traffic through a new facade, add new cloud capabilities, and gradually transfer functionality — eventually decommissioning the legacy system without a big-bang cutover.
STRANGLER FIG MIGRATION PATTERN
PHASE 1: Add routing layer (no functionality change)
Users -> Front Door / API Management -> Legacy system (100% traffic)
Legacy continues to serve everything. No change visible to users.
PHASE 2: Add new Azure service for ONE feature
Users -> Front Door -> Route /new-feature -> Azure App Service (new)
-> Route /everything-else -> Legacy (still running)
PHASE 3: Gradually expand routing
Users -> Front Door -> Route /checkout -> Azure (new)
-> Route /catalog -> Azure (new)
-> Route /account -> Azure (new)
-> Route /legacy-reports -> Legacy (shrinking)
PHASE 4: Legacy decommissioned
Users -> Front Door -> All routes -> Azure services
Legacy system removed. Migration complete.
Zero downtime. Fully reversible at each phase.
Cost Optimisation — The FinOps Toolkit
Cost optimisation scenarios are common in AZ-305. The exam tests whether you know the specific tools and their appropriate use cases.
ENTERPRISE REFERENCE ARCHITECTURE -- 3-Tier HA Web Application
Internet Users (Global)
|
+-------------+-------------+
| Azure Front Door |
| WAF + CDN + Anycast |
+-------------|-------------+
| (routes to nearest region)
+----------------------------+
| Region A | Region B |
| App Service App Service |
| Zone Redundant (auto-scale 2-10) |
+-----------+----------------+
|
+-----------+----------+
| Azure SQL Business |
| Critical (Primary) |---> SQL (Secondary) Region B
| Auto-failover group |
+----------------------+
|
+-----------------+-----------------+
| | |
Azure Key Vault App Insights Entra ID
(secrets) (APM + logs) (identity)
Architect Scenario — Global E-Commerce
A global e-commerce company has users in Europe and Asia. Their requirement: RTO less than 5 minutes and RPO less than 1 minute during a complete regional Azure outage. Their current architecture is a single App Service + Azure SQL in West Europe only. What is the correct redesign?
Show Answer + Reasoning
Active-Active architecture: Azure Front Door + App Service Zone Redundant in 2 regions + Azure SQL Business Critical with Geo-Replication auto-failover group.
Why Active-Active, not Active-Passive? RPO < 1 minute means we cannot afford replication lag. Azure Site Recovery (Active-Passive) has RPO of minutes — not guaranteed sub-1-minute. RTO < 5 minutes means we cannot wait for DNS TTL to propagate (Traffic Manager failover typically takes 2-6 minutes including DNS caching). Front Door detects failure in seconds and reroutes — meeting the 5-minute RTO.
Why the current single-region architecture fails: A regional outage takes the entire application offline. RTO = hours to days (Azure region restoration or manual recreation). RPO = all data since last backup. Neither meets the requirement.
Complete redesign:
- Azure Front Door (WAF + performance routing): routes European users to West Europe, Asian users to Southeast Asia. On failure: reroutes within seconds.
- App Service (Zone Redundant) in West Europe + Southeast Asia: both serving live traffic at all times.
- Azure SQL Business Critical in West Europe (primary) with Geo-Replication auto-failover group to Southeast Asia: auto-failover triggers when primary is unreachable — application continues writing to secondary, which becomes new primary.
- Key Vault in each region: each App Service accesses its regional Key Vault.
🎯 Final Series Quiz — Module 6
Q1: A company plans to auto-scale their App Service based on CPU usage. Which Well-Architected Framework pillar does this primarily address?
Show Answer
Performance Efficiency. Auto-scaling ensures the application has enough compute capacity to handle load efficiently — matching resource supply to demand. It also touches Cost Optimisation (scale down when load is low = pay less) and Reliability (more instances = fewer individual failures causing downtime), but the PRIMARY pillar is Performance Efficiency — the application must perform adequately under variable load. When you see "handle peak traffic" or "respond quickly under load" in a scenario, that is a Performance Efficiency signal.
Q2: What is the Strangler Fig pattern and when does AZ-305 expect you to recommend it?
Show Answer
The Strangler Fig pattern migrates a legacy monolith to cloud services incrementally — without a big-bang rewrite. A routing layer (Front Door or API Management) is placed in front of the legacy system. New cloud services are built for specific features and traffic is gradually redirected. The legacy system shrinks as each feature migrates until it is decommissioned entirely. Recommend it when: a scenario describes a legacy application needing modernisation but the company cannot afford downtime or a full rewrite; OR when the scenario mentions "incremental migration", "zero-downtime modernisation", "move to cloud without disrupting users". It is the safe migration pattern — each step is independently reversible.
Q3: A company runs a stable, predictable data analytics workload on 20 Azure VMs, 24 hours a day, 7 days a week, with no expected changes for the next 3 years. What is the most cost-effective pricing option?
Show Answer
3-Year Reserved Instances — up to 72% discount versus pay-as-you-go. The signals are: stable (no need for flexibility), predictable (known size and region), 24/7 (VMs running continuously — full discount benefit), and 3-year horizon (long enough to commit). Spot VMs (90% discount) are not appropriate — they can be evicted with 30 seconds notice, unacceptable for a continuous analytics workload that cannot tolerate interruption. 1-year Reserved Instances give less discount (40%) for the same commitment risk when the 3-year horizon is already confirmed.
Series Complete — The Complete AZ-305 Foundation
- WAF pillars are the AZ-305 design lenses: identify the primary pillar in each scenario, optimise for that pillar without violating the others
- HA spectrum: 99.9% (single region) to 99.99% (Availability Zones) to 99.999% (Active-Active multi-region) — design to the stated SLA, not beyond it
- Active-Active (Front Door) for near-zero RTO/RPO; Active-Passive (ASR/Traffic Manager) for moderate RTO/RPO at lower cost
- Strangler Fig = the safe incremental migration pattern for legacy modernisation — never big-bang rewrite when Strangler Fig is an option
- Cost optimisation: Reserved Instances for stable 24/7 workloads, Spot VMs for interruptible batch, auto-shutdown for dev/test, Lifecycle Policies for aging data
- You have now covered the complete AZ-104 to AZ-305 gap: Identity, Services, Networking, IaC, Monitoring, Design Thinking