Auto Scaling

Also Known As

TL;DR

Explanation

Horizontal scaling adds/removes instances; vertical scaling changes instance size. Auto scaling groups (AWS ASG, GCE managed instance groups) scale horizontally based on CloudWatch metrics (CPU, request rate, queue depth). Scale-out policies add capacity when metrics exceed thresholds; scale-in policies remove capacity when load drops. Cooldown periods prevent oscillation. For PHP-FPM, scaling means adding more servers — each request is stateless, so any server can handle any request.

Diagram

flowchart TD
    MON[CloudWatch<br/>Metrics] -->|CPU > 70%| SCALE_OUT[Scale Out<br/>Add instances]
    MON -->|CPU < 20%| SCALE_IN[Scale In<br/>Remove instances]
    subgraph Auto Scaling Group
        S1[Instance 1]
        S2[Instance 2]
        S3[Instance 3 - new]
    end
    LB[Load Balancer] --> S1 & S2
    SCALE_OUT --> S3
    LB --> S3
    SESS[(Redis Sessions<br/>Shared)] -.->|stateless app| S1 & S2 & S3
style S3 fill:#238636,color:#fff
style SESS fill:#1f6feb,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Scaling on CPU alone — request rate or queue depth is often a better signal for PHP-FPM workloads.
• No scale-in protection for instances processing long-running jobs — scale-in terminates them mid-job.
• Sessions stored on individual instances — scale-in removes the instance and all its sessions; use Redis for session storage.
• No warm-up period — new instances need time to compile opcache before handling production traffic.

Code Examples

# Sticky sessions on application servers — blocks scale-in:
# nginx upstream:
upstream app {
    ip_hash;  # Sticky sessions — user X always goes to server 1
    server app1:80;
    server app2:80;
}
# Scale-in removes app1 — all of app1's sessions lost

# Stateless app servers + Redis sessions = scale freely:
# php.ini / app config:
session.save_handler = redis
session.save_path = 'tcp://redis.internal:6379'

# nginx — no sticky sessions needed:
upstream app {
    least_conn;   # Route to least busy server
    server app1:80;
    server app2:80;
}

References

• ↗ https://aws.amazon.com/autoscaling/

Tags

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