Great question! According to Google's App Engine Flexible documentation, you should be able to route directly to a specific service using the pattern http://SERVICE_ID.MY_CUSTOM_DOMAIN without needing a dispatch.yaml file. But if you're seeing requests still land on the default service despite trying this, here are some common pitfalls to check:

• Custom domain isn't mapped to the target service: It's easy to only map your custom domain to the default service. Make sure you've added the subdomain SERVICE_ID.MY_CUSTOM_DOMAIN to your GAE project's domain mappings and linked it explicitly to the service you want to access.

• Service ID mismatch: Double-check that the SERVICE_ID in your URL exactly matches the ID defined in your service's app.yaml (case matters!). A tiny typo here will send your request straight to the default service.

• DNS propagation lag: If you just set up the subdomain, DNS changes can take time to roll out across the internet—anywhere from a few minutes to a full day. Give it some time, or use a DNS lookup tool to confirm your subdomain is pointing to GAE's servers.

• Leftover dispatch rules: Even if you don't have a dispatch.yaml deployed now, old rules might still be active in your project. Head to the GAE console's "Dispatch rules" section to make sure there's nothing overriding the default subdomain routing.

If you still can't get the subdomain pattern working, using dispatch.yaml is a rock-solid fallback (as you've already experienced). A simple rule would look like this:

dispatch:
  - url: "service-id.my-custom-domain.com/*"
    service: service-id

This explicitly tells GAE to route all traffic for that subdomain to your target service, bypassing any default routing ambiguities.

内容的提问来源于stack exchange，提问作者RamyaKarri