Progressive Deployment

Overview

Every production change follows a staged rollout that expands blast radius gradually: One-Box → One-AZ → Regional → Global. Each stage has mandatory bake time during which alarms are monitored. Any alarm breach triggers automatic rollback to the last known-good state. The goal is to detect problems when they affect the fewest customers possible.

When to Use

• Every code deployment to production
• Configuration changes that affect runtime behavior
• Infrastructure changes (new instance types, scaling policies)
• Database schema migrations with application-level changes
• Any change that could impact customer-facing behavior

Amazon Context

At Amazon, no service deploys to all hosts simultaneously. The pipeline enforces progressive deployment because the cost of a bad deployment to 100% of hosts is measured in customer trust and revenue. A 1-box deployment that catches a bug costs almost nothing. A full-fleet deployment that causes a critical incident costs millions and erodes customer confidence. Teams that skip stages eventually cause high-severity events—this is not theoretical, it is observed repeatedly.

The Process

Stage 1: One-Box (Canary)

1. Deploy to a single host in the lowest-traffic availability zone
2. Route a small percentage of traffic (typically <1%) to this host
3. Monitor for 30-60 minutes minimum (bake time)
4. Compare metrics against the rest of the fleet:
   • Latency p50, p99, p99.9
   • Error rate (5xx, 4xx anomalies)
   • CPU, memory, GC pauses
   • Business metrics (conversion, success rate)
5. Automated gate: proceed only if all metrics are within threshold

Stage 2: One-AZ

1. Deploy to all hosts in one Availability Zone (~33% in a 3-AZ setup)
2. Bake time: 60-120 minutes
3. Monitor the same metrics at AZ level
4. Watch for cross-AZ dependency issues
5. Verify the AZ can handle full load if others need to absorb traffic during rollback

Stage 3: Regional

1. Deploy to all hosts in one region
2. Bake time: 2-4 hours (longer for low-traffic regions: 4-8 hours)
3. Verify region-level metrics and cross-region replication
4. Monitor for subtle issues that only appear at higher traffic volumes
5. Choose initial region carefully: not the largest, not the smallest

Stage 4: Global

1. Deploy to remaining regions in waves (not all at once)
2. Stagger by region size: small regions first, largest last
3. Bake time between region waves: 30-60 minutes
4. Final bake after all regions: 2-4 hours before marking deployment complete

Rollback Triggers

Automatic rollback fires when any of these conditions are met:

• Error rate exceeds baseline + 0.1% for 3 consecutive minutes
• p99 latency exceeds baseline + 20% for 5 consecutive minutes
• Any P1 or P2 alarm fires
• Dependency health check failures exceed threshold
• Deployment health monitor (synthetic canary) fails

Rollback Execution

1. Stop forward deployment immediately
2. Revert to previous code/config on affected hosts
3. Verify rollback success with synthetic tests
4. Page on-call if rollback fails or metrics don't recover within 5 minutes
5. Create ticket for investigation regardless of auto-recovery

Mechanisms Over Good Intentions

Common Rationalizations

Red Flags

• Skipping one-box because "it's a small change"
• Bake time set to 5 minutes ("just to get through the gate")
• Manual rollback as the only rollback mechanism
• No synthetic canary validating the deployment
• Same bake time for all regions regardless of traffic volume
• Deploying to the largest region first
• No alarm covering the specific functionality being changed
• "We'll add the alarm after we deploy this"

Verification

Before marking a deployment complete, confirm:

• One-box stage completed with full bake time
• One-AZ stage completed with full bake time
• Regional deployment completed with full bake time
• All automated rollback triggers are active and tested
• Metrics dashboard shows no degradation at any percentile
• Synthetic canary passed at every stage
• Business metrics (if applicable) show no negative movement
• Rollback was tested (either actually triggered or dry-run verified within the last 30 days)

Tenets

1. Blast radius minimization is not optional. Every change starts small and grows. No exceptions for "simple" changes.
2. Bake time is not idle time. It is active verification that the change behaves correctly under real traffic.
3. Automatic rollback is faster than any human. By the time you notice, diagnose, and decide to rollback, thousands of customers have been impacted.
4. The pipeline is the authority, not the developer. No individual should be able to bypass safety stages without VP-level approval and a documented reason.
5. Speed comes from confidence, not from skipping steps. Teams that invest in deployment safety deploy more frequently because they trust their pipeline.