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fastly-multi-service-routing-debug

Testing & Quality
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Debug Fastly Compute services where code changes don't affect behavior due to multi-service routing architecture. Use when: (1) deployed changes have no effect, (2) unexpected redirects persist after code updates, (3) DNS resolves to Fastly but responses don't match deployed code, (4) `fastly service list` shows multiple related services. The root cause is often a routing/gateway service intercepting requests before they reach your content service.

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Fastly Multi-Service Routing Debug

Problem

You deploy changes to a Fastly Compute service but the behavior doesn't change. Requests still return old responses, redirects, or errors even after:

  • Deploying new code with fastly compute publish
  • Purging the cache with fastly purge --all
  • Verifying the correct service version is active

Context / Trigger Conditions

  • DNS resolves to Fastly IPs (151.101.x.x)
  • Response headers show x-served-by: cache-xxx (Fastly cache)
  • Deployed code should return different status/headers but doesn't
  • Multiple Fastly services exist for the same project (e.g., divine-web, divine-router)
  • Wildcard subdomain routing is involved (*.example.com)

Solution

Step 1: Identify Which Service Handles the Domain

# List all services
fastly service list

# Check which service handles your domain using domain-v1
fastly domain-v1 list | grep your-domain.com

The output shows the service ID for each domain:

*.divine.video      glh3AfBEmZKzmAmByvGyAg  76fTayX6mBKa8faLeZ1fet  2025-12-28...
divine.video        1JttvGPc6AlOVg4koUGhPg  76fTayX6mBKa8faLeZ1fet  2025-11-23...

The third column is the service ID that handles that domain.

Step 2: Verify Service Architecture

Common patterns:

  • Router + Content: A routing service (often Rust) sits in front, handling subdomain logic, then passes through to a content service
  • Gateway Pattern: Edge gateway handles auth/rate-limiting, then proxies to origin
  • Wildcard Routing: *.domain.com may be handled by a different service than domain.com

Step 3: Debug the Correct Service

Once you identify the routing service:

# Check its active version
fastly service-version list --service-id <router-service-id>

# Check its domains
fastly domain list --service-id <router-service-id> --version active

# Find and read its source code (usually in a different repo)

Step 4: Trace Request Flow

Add debug logging to understand the flow:

  1. In the routing service: log the Host header and routing decision
  2. In the content service: log if requests are even reaching it
  3. Check Fastly logs: fastly log-tail --service-id <id>

Step 5: Fix the Correct Service

If the routing service is doing unwanted redirects:

  • Update its routing logic to pass through instead of redirect
  • Or update it to return the desired response directly
  • Deploy to the routing service, not just the content service

Verification

# Test with cache bypass
curl -sI -H "Cache-Control: no-cache" "https://subdomain.your-domain.com/?t=$(date +%s)"

# Verify new behavior appears
# Check response headers match what your code should return

Example

Symptom: rabble.divine.video returns 301 redirect despite divine-web having code to serve HTML directly.

Discovery:

$ fastly domain-v1 list | grep divine.video
*.divine.video      ...  76fTayX6mBKa8faLeZ1fet  ...  # This is divine-router!
divine.video        ...  76fTayX6mBKa8faLeZ1fet  ...  # Same - router handles both

Root Cause: divine-router (Rust service) was returning 301 redirects in its serve_profile() function, never letting requests reach divine-web.

Fix: Updated divine-router to return the desired response directly instead of redirecting.

Notes

  • The fastly domain list command shows domains on a specific service/version
  • The fastly domain-v1 list command shows ALL domains across ALL services
  • Wildcard domains (*.domain.com) often route to a different service than apex
  • When using Fastly Compute backends, the request may go: Router → Content → Origin
  • Cache purging only affects the service that's caching; if a router caches, purge there
  • Response headers like x-publisher-server-collection indicate which service responded

Architecture Patterns

Pattern 1: Router → Content

User → DNS → Fastly Router Service → Fastly Content Service → Origin/KV Store

The router handles subdomain logic, auth, or routing rules.

Pattern 2: Direct

User → DNS → Fastly Content Service → Origin/KV Store

Single service handles everything.

Debugging Tip

If you're unsure which pattern applies, check the [setup.backends] section in each service's fastly.toml to see if one service proxies to another's edgecompute.app URL.