Too many open files

The Too many open files error indicates that a process has exceeded the maximum number of file descriptors allowed by the operating system. File descriptors are used for files, sockets, pipes, and network connections. This root error commonly occurs on Linux and macOS systems, Java and Spring Boot applications, Docker containers, databases, and high-traffic servers that open many files or connections simultaneously.

When does this error occur?

  • Applications opening many files or network sockets without closing them
  • High-concurrency servers handling large numbers of connections
  • Long-running processes with file descriptor leaks
  • Database or message brokers under heavy load
  • Containers running with low default file descriptor limits

Root cause of Too many open files

This error occurs at the OS level when a process reaches its configured file descriptor limit. Operating systems enforce limits to prevent resource exhaustion. When applications fail to close files or sockets properly, or when limits are set too low for the workload, new open operations are denied and the system returns this error.

How to fix the error (step-by-step)

Linux / macOS

Check the current file descriptor limit for the shell or process. 

ulimit -n

Temporarily increase the limit for the current session. 

ulimit -n 65535

For permanent changes, update system configuration files. 

/etc/security/limits.conf

Windows

Windows manages handles differently, but similar limits can be hit by applications. Review application handle usage and ensure resources are released properly.

Java / Spring Boot

Ensure files, streams, and sockets are closed after use. 

try (FileInputStream fis = new FileInputStream(file)) {
    // use stream
}

Monitor open file descriptors for the Java process. 

lsof -p <pid>

Docker / containers

Check container file descriptor limits. 

docker inspect <container-name>

Run containers with higher limits if required. 

docker run --ulimit nofile=65535:65535 <image>

Database / network services

Databases and servers often open many connections. Review service-specific limits and connection pooling settings to reduce descriptor usage.

Verify the fix

After applying changes, restart the affected service or application. Monitor open file descriptor counts and confirm that new files or connections can be opened without triggering the error.

Common mistakes to avoid

  • Increasing limits without fixing file descriptor leaks
  • Ignoring connection pooling best practices
  • Setting limits too high without considering system capacity
  • Forgetting to restart services after configuration changes
  • Monitoring only application logs and not OS resource usage

Quick tip

Regularly monitor open file descriptors on production systems to detect leaks before limits are reached.

FAQ

Q: Does this error only apply to files?

A: No. It applies to files, network sockets, pipes, and other resources using file descriptors.

Q: Is increasing ulimit always safe?

A: Only if the system has enough resources and the application manages descriptors correctly.

Conclusion

The Too many open files error signals exhaustion of file descriptor limits. Closing resources properly and configuring appropriate limits resolves the issue. Check related root error references on ErrorFixHub for deeper system troubleshooting.

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