Overview

The message “no available server” is a generic connectivity failure, not a single product-specific error. It indicates that a client (application, driver, or service) attempted to connect to a backend but was unable to find any endpoint that reported itself as healthy and ready to accept traffic.

This condition can originate at multiple layers of your stack, including:

• Network routing and firewalls
• DNS and hostname resolution
• Load balancers and reverse proxies
• Application or API processes
• Database servers and connection pools
• Container orchestration platforms such as Kubernetes
• TLS/SSL termination and certificate validation
• Client configuration and connection strings

This reference-style guide is structured to help you quickly isolate where the failure occurs, then drill into targeted checks for each layer.

Architecture & Typical Failure Points

In a typical modern deployment, a client request flows through several components before it reaches the actual service process:

1. Client (browser, mobile app, service, database driver)
2. DNS resolution for the service hostname
3. Network path (VPCs, security groups, firewalls, routing)
4. Load balancer or proxy (NGINX, HAProxy, AWS ALB/NLB, GCP LB, Azure LB)
5. Service endpoint (VM, container, pod, or serverless function)
6. Optional backend (database, cache, message broker)

A “no available server” error typically means that at some point in this chain, the component that should forward or handle the request has zero healthy targets or cannot establish a TCP/TLS connection at all.

Common Root Causes

While the exact wording varies by platform, most “no available server” errors map to one or more of the following conditions:

• Network connectivity failures or blocked ports (firewalls, security groups, routing rules)
• DNS lookup failures, stale records, or misconfigured hostnames
• Load balancer or proxy with no healthy backends in its target pool
• Application process crashed, out-of-memory, or stuck during startup
• Database server unreachable or connection pool fully exhausted
• Kubernetes pods not ready, failing probes, or CrashLoopBackOff states
• Expired or invalid TLS/SSL certificates, SNI issues, or protocol mismatches
• Incorrect endpoint URLs, ports, or connection string parameters

Rapid Triage Checklist (First 5 Minutes)

Use this fast path to determine whether the problem is client-side, network-related, or backend-related before you dive into detailed diagnostics:

1. Capture the exact error and timestamp from client logs or monitoring tools.
2. Check provider status pages for any known outages on cloud or managed services.
3. Test reachability from a host in the same network:

   curl -v https://service.example.com/health

4. Verify DNS resolution:

   dig service.example.com
   nslookup service.example.com

5. Inspect load balancer or proxy health and backend pool status in your console or logs.

If you confirm that DNS works and the endpoint is reachable from another host, the issue is likely higher up the stack (load balancer configuration, application process, or database).

Layer-by-Layer Troubleshooting

1. Client & Application Logs

Start at the edge where the error is observed:

• Inspect client-side logs for:
  • Connection timeouts
  • Socket errors or refused connections
  • Authentication or TLS handshake failures
  • High retry counts or exponential backoff behavior
• Correlate error timestamps with:
  • Recent deployments or configuration changes
  • Service restarts or failovers
  • Traffic spikes or incidents reported by monitoring

This correlation often tells you whether the issue is transient, deployment-related, or systemic.

2. Network Connectivity & DNS Resolution

From the host where the client or application runs, verify connectivity to the target hostname and port:

ping -c 4 service.example.com
dig +short service.example.com
curl -v http://service.example.com:PORT/health

# Direct TCP connectivity test
nc -vz service.example.com 5432

Interpretation guidelines:

• Ping fails, dig/nslookup fail:
  • DNS record missing, misconfigured, or not propagated
  • Split-horizon DNS inconsistencies between environments
• DNS resolves but nc/curl fail:
  • Firewall, security group, or network ACL blocking the port
  • Route misconfiguration or wrong target IP

If DNS returns unexpected IP addresses, verify your DNS zone configuration, TTLs, and any conditional forwarding or private zones that might override public records.

3. Load Balancer or Proxy Health

When a load balancer or proxy reports “no available server”, it usually means there are zero healthy targets in its backend pool.

Check the following:

• Backend health status:
  • For managed LBs (AWS ALB/NLB, GCP, Azure), inspect the console for target health.
  • For NGINX/HAProxy, review status pages or metrics endpoints if enabled.
• Health check configuration:
  • Verify the health check path or command (for example, /health or /ready).
  • Confirm port, protocol (HTTP/HTTPS/TCP), and expected status codes.
  • Ensure the health endpoint responds quickly and does not require authentication.
• Recent changes:
  • Did you deploy a new version that removed or changed the health endpoint?
  • Did TLS termination move from the LB to the app or vice versa?

4. Service Process & Resource Limits

On the server or instance that should handle the request, verify that the service is actually running and healthy:

systemctl status my-service
journalctl -u my-service -n 200
ps aux | grep myapp
free -m  # check available memory

Key checks:

• Process not running:
  • Investigate recent deploys, configuration changes, or crashes.
  • Look for OOM kill messages or segmentation faults in logs.
• High resource usage:
  • CPU or memory saturation can slow responses and cause health checks to fail.
  • Consider horizontal scaling or performance optimizations.

5. Database-specific Diagnostics

Many client libraries for databases raise messages similar to “no available servers” when they cannot select a suitable node or when connection pools are exhausted.

MongoDB

Typical indicators:

• Errors mentioning server selection or timeout, such as MongoTimeoutError.
• Replica set hostnames or ports misconfigured in the connection string.

Checks:

• Validate the connection string, including replica set name and node list.
• Confirm that each host and port is reachable from the client.
• Review MongoDB logs for stepdowns, elections, or network partitions.
• Use the Mongo shell or client with verbose logging:

  mongo --host your-mongo-host:27017

PostgreSQL

Postgres errors can surface if the server is unreachable or if max_connections is reached.

From a connected session or admin tool, inspect active connections:

SELECT count(*) FROM pg_stat_activity;

If the count approaches or equals max_connections:

• Review application connection pool sizes.
• Shorten idle timeouts and close unused connections.
• Increase max_connections cautiously if resources allow.

Redis

To confirm Redis availability:

redis-cli -h host -p port ping

A successful response should be PONG. If you cannot connect:

• Check firewall rules and security groups.
• Verify the Redis instance is running and bound to the expected interface.

For all databases, if connection pools are exhausted, adjust pool sizes carefully and optimize query duration and connection reuse to avoid resource starvation.

6. Kubernetes Environments

In Kubernetes, “no available server” often maps to services with no ready pods behind them, or to pods that fail readiness checks.

Useful commands:

kubectl get pods -n your-namespace
kubectl describe pod <pod-name> -n your-namespace
kubectl get svc -n your-namespace
kubectl get endpoints -n your-namespace
kubectl logs <pod-name> -n your-namespace

Focus on:

• Pod status:
  • CrashLoopBackOff, ImagePullBackOff, or similar indicates pods are not healthy.
• Readiness and liveness probes:
  • If readiness probes fail, pods will not be added to service endpoints, which looks like “no available server” to clients.
  • Check probe paths, ports, and timing parameters such as initialDelaySeconds and timeoutSeconds.
• Endpoints objects:
  • If kubectl get endpoints shows zero addresses, the service has no ready pods behind it.
• NetworkPolicy:
  • Overly restrictive policies can block traffic between pods or from ingress controllers.

7. TLS / SSL & Certificate Validation

Some clients report generic connectivity errors when the TLS handshake fails due to certificate problems or SNI mismatches.

Inspect the certificate chain and handshake using:

openssl s_client -connect host:443 -servername host

Review:

• Certificate expiration dates
• Common Name (CN) and Subject Alternative Name (SAN) entries
• Whether the presented certificate matches the hostname you are connecting to

Expired or mismatched certificates can cause clients to abort connections, which may surface as “no available server” from the application perspective.

8. Authentication & Authorization

Even if the network path is open, invalid credentials or revoked permissions can prevent establishing a usable session.

• Verify client credentials, API keys, or service account tokens.
• Check IAM roles, database roles, and any relevant policies for recent changes.
• Review service provider logs for authentication failures or access denials.

9. Rate Limiting, DDoS Protection & Provider Outages

High traffic levels or security protections can also result in connection failures:

• Inspect metrics from WAFs, API gateways, or DDoS protection services.
• Look for throttling, rate limiting, or blocking rules that might apply to your clients.
• Confirm cloud provider and managed service status pages for ongoing incidents.

Scenario-Based Examples & Fixes

Scenario 1: MongoDB Driver Reports “No Available Servers”

Likely cause: Incorrect replica set hostnames or a network partition.

Targeted checks and remediation:

• Validate the MongoDB connection string:
  • Replica set name matches the server configuration.
  • All hosts and ports are correct and reachable.
• Test connectivity to each member from the application host.
• Review MongoDB logs for elections, primary stepdowns, or connectivity issues.

Scenario 2: Load Balancer Shows No Healthy Backends

Likely cause: Misconfigured health check path or unhealthy backend instances.

Targeted checks and remediation:

• Ensure the health check path and port map to a lightweight endpoint that returns HTTP 200 when the app is ready.
• Avoid heavy authentication or long-running logic in health endpoints.
• Restart or redeploy backend instances that have crashed or are stuck.

Scenario 3: Kubernetes Readiness Probe Failing

Likely cause: Application startup time longer than probe configuration, or incorrect probe endpoint.

Targeted checks and remediation:

• Increase initialDelaySeconds and timeoutSeconds to accommodate actual startup time.
• Ensure the readiness endpoint only returns success when the application is fully initialized.
• Monitor pod startup logs to identify slow initialization tasks or blocking operations.

Preventive Practices

To reduce the likelihood and impact of “no available server” incidents, adopt the following practices:

• Robust health checks and graceful shutdowns:
  • Implement separate readiness and liveness endpoints.
  • Ensure services signal unavailability before shutdown so load balancers stop routing traffic.
• Autoscaling and capacity planning:
  • Use autoscaling to handle traffic spikes and avoid saturating connection pools.
  • Track utilization metrics to plan capacity ahead of peak loads.
• Monitoring and alerting:
  • Instrument metrics for error rates, latency, connection pool usage, and backend health.
  • Use tools like Prometheus, Grafana, or CloudWatch to set actionable alerts.
• Resilient client behavior:
  • Implement retries with exponential backoff and jitter.
  • Use circuit breakers to avoid overwhelming unhealthy services.