Fastly VCL Syntax Error

Introduction

Fastly VCL has syntax error when VCL code has incorrect syntax or undefined subroutine. 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

VCL syntax error at line 42:
Expected ";" but found "if"
Undefined subroutine: custom_deliver

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

Common Causes

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

``` # Fastly VCL configuration sub vcl_recv { # Normalize host set req.http.Host = "www.example.com";

# Cache static content if (req.url ~ "^/static/") { return (pass); }

# Custom logic if (req.http.X-Custom-Header) { set req.http.X-Processed = "true"; } }

sub vcl_deliver { # Add debug header set resp.http.X-Served-By = server.identity; } ```

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