Troubleshooting · Applications
Application unavailable
Users report an application is down or unusably slow.
Work through these in order
- 01
Establish scope and change
All users or some? Since when? What changed, deployment, certificate, patch, network change?
- 02
Check the front door
DNS resolution, certificate validity, load balancer health probe status and backend pool members.
- 03
Check dependencies
Identity provider, database, message queue, file storage, external API, in that order of frequency.
- 04
Check resource saturation
CPU, memory, connection pools, thread pools and disk latency on application and database tiers.
- 05
Read the application log
Correlate errors with the user reported timeline; look for authentication and timeout patterns.
- 06
Confirm with a synthetic test
Reproduce from outside and inside the network to isolate access path issues.
Background reading
Load Balancing
Distributing client requests across multiple backend servers to provide scale, resilience and maintainability.
Databases
Systems that store, protect and serve structured data with transactional guarantees, indexing and controlled concurrent access.
Monitoring
Collecting and evaluating signals about availability, performance and capacity so problems are detected before users report them.
TLS
Transport Layer Security provides confidentiality, integrity and server authentication for application traffic, most visibly as HTTPS.
APIs
Programmatic interfaces that let systems exchange data and invoke functions across boundaries, and the main integration surface of modern IT.