Step 1 — Prepare Two Target Groups
- Create (or reuse) group A for the website targets.
- Create group B for the API targets.
- Confirm both groups show healthy targets.
Path-Based Routing on ALB
Start here
What Is Path-Based Routing?
On an Application Load Balancer, path-based routing sends different URL paths to different target groups.
Example:
/api/*→ API servers/*→ website servers
Beginner tip.
One DNS name, one ALB, multiple backends. A Network Load Balancer cannot do this.
Why it matters
In Simple Words
What problem does it solve?
Microservices often share www.example.com but live on different instances. Path rules keep one certificate and one hostname while splitting traffic.
Path vs host
Path = after the domain (/api). Host = the domain name itself (api.example.com). Related redirects: www to non-www.
Official docs.
Listener rules for Application Load Balancers.
Dictionary
Words You Will See
| Word | Plain meaning |
|---|---|
Listener | ALB port/protocol that receives client traffic (80/443) |
Rule | Conditions (path/host/…) + action (forward/redirect) |
Priority | Order rules are checked; lower number wins first |
Path pattern | Match like /api/* or /images/* |
Default action | Catch-all when no other rule matches |
Target group | Backends that receive the forwarded requests |
Checklist
What You Need First
- An internet-facing ALB with an HTTP or HTTPS listener.
- Two target groups (or follow Target groups and ALB).
- Security groups that allow client→ALB and ALB→targets.
Follow along
Step-by-Step
Step 2 — Add a Listener Rule for /api/*
- Open the ALB → Listeners → view rules for port 80 or 443.
- Choose Add rule.
- Condition: Path is
/apior/api/*(match your app). - Action: Forward to the API target group.
- Set a priority number lower (earlier) than the catch-all.
Step 3 — Default Action for Everything Else
- Keep the listener default action forwarding to the website target group.
- Or add an explicit
/*rule with a higher priority number than/api/*.
How It Works
- Specific paths first, broad paths last.
- Only one rule “wins” per request.
Step 4 — Test Both Paths
- Open
http://ALB-DNS/— should hit the website group. - Open
http://ALB-DNS/api/...— should hit the API group. - If both look the same, check priorities, path spelling, and target health.
Avoid these
Common Beginner Mistakes
Catch-all before /api
A high-priority /* steals API traffic. Put /api/* first.
Unhealthy API targets
Rules forward only to healthy targets—fix health checks.
Expecting NLB path rules
Use ALB (Layer 7) for HTTP path matching.
Forgetting trailing wildcards
/api and /api/* match differently—match what your clients request.
Remember
Key Takeaways
One line: two target groups, a specific /api/* rule, then a default forward for the rest.
Frequently Asked Questions
Path-based routing uses Application Load Balancer listener rules to forward requests to different target groups based on the URL path, such as /api/* versus /*.
No. A single default forward rule is enough for one app. Add path rules when two services share one hostname.
Host-based rules match the Host header (api.example.com vs www.example.com). Path-based rules match the path after the domain. You can combine both.
No. NLB works at Layer 4 and does not read HTTP paths. Use an ALB for path-based routing.
ALB evaluates listener rules in priority order (lower number first). Put more specific paths (like /api/*) ahead of a catch-all /* rule.
Yes for two different backends. Create one target group per service, register its targets, then point each path rule at the matching group.
Next: Target groups and ALB
Register instances and wire groups before adding more listener rules.
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