## TL;DR
No server available means every backend server is marked down by health checks (or none are configured). The servers might genuinely be down, or the health checks might be wrong: checking the wrong port, path, or protocol, or marked down by a too-strict rise/fall threshold. Check the stats page first, then verify what the health check actually tests.

## The query
```text
haproxy "backend has no server available": health check debugging
```

## Use this when
- HAProxy logs or stats show no server available
- All backend servers marked down simultaneously
- Health checks seem misconfigured
- After backend or HAProxy changes

## Not for when
- Frontend binding or ACL problems
- SSL termination issues
- Request routing logic

## Steps

### Step 1: Check the stats page for server states
Open the HAProxy stats page to see each server's state and the check results. All-down simultaneously suggests a check or network problem; one-down suggests a server problem.
Expected output: per-server states with check details.

### Step 2: Manually run the health check
Reproduce the health check by hand from the HAProxy host: same protocol, port, path, and expected response. If your manual check fails, the servers are really unhealthy; if it passes, the check configuration is wrong.
Expected output: the check's verdict validated independently.

### Step 3: Verify check port, path, and protocol
Compare the configured check against what the servers actually serve. Common misses: checking HTTP on an HTTPS port, a health path that was renamed, or expecting 200 from an endpoint that returns 204.
Expected output: the check aligned with the servers' real health endpoints.

### Step 4: Review rise/fall thresholds
Overly strict thresholds (fall 1, or aggressive intervals) mark servers down on single blips. Tune so transient failures do not drain the whole backend: a few consecutive failures before marking down is the sane default.
Expected output: thresholds that tolerate blips but catch real outages.

### Step 5: Check the network path from HAProxy to backends
If manual checks from your laptop pass but HAProxy's fail, the HAProxy-to-backend network path is the problem: firewalls, security groups, or routing. Test from the HAProxy host itself, not from yours.
Expected output: the network path proven or identified as broken.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_EPMq1oaXw8K7Doqb4_0GUg