Introduction

Fastly custom logging endpoint fails when log format is invalid or destination is unreachable. This guide provides step-by-step diagnosis and resolution with specific commands and configuration examples.

Symptoms

Typical symptoms and error messages when this issue occurs:

bash
CDN error: content not delivered
Check CDN configuration and origin server
Verify cache rules and SSL settings

Observable indicators: - CDN returns errors to end users - Content not being cached as expected - SSL or security configuration issues

Common Causes

  1. 1.Fastly issues are typically caused by:
  2. 2.VCL syntax errors or incorrect logic
  3. 3.Backend health check configuration issues
  4. 4.Cache key or stale content configuration
  5. 5.TLS certificate pending validation

Step-by-Step Fix

Step 1: Check Current State

bash
curl -I -H "Fastly-Debug: 1" https://example.com/path

Step 2: Identify Root Cause

bash
fastly log-tail --service-id <service-id>

Step 3: Apply Primary Fix

bash
# Primary configuration fix
Cache-Control: public, max-age=86400
CDN-Cache-Control: max-age=31536000

Apply this configuration in the CDN dashboard or via API.

Step 4: Apply Alternative Fix (If Needed)

bash
# Alternative fix: adjust TTL
Edge-Cache-TTL: 3600
Stale-While-Revalidate: 86400

Step 5: Verify the Fix

After applying the fix, verify with:

bash
curl -I -H "Fastly-Debug: 1" https://example.com/test | grep -E "X-Served-By|X-Cache"

Expected output should show proper caching headers and successful content delivery.

Common Pitfalls

  • VCL subroutine not defined before use
  • Backend health check path returning wrong status
  • Cache key including too many query parameters
  • Shielding not configured for high traffic

Best Practices

  • Use VCL snippets for reusable code
  • Configure shielding for high-traffic origins
  • Use edge dictionaries for configuration
  • Test VCL changes locally before deployment
  • Fastly VCL Syntax Error
  • Fastly Backend Health Check Failing
  • Fastly Cache Stale Serving
  • Fastly Rate Limiter Blocking Legitimate