Graceful Restarts for High-Availability Services with ecdysis

The Challenge of Zero-Downtime Service Upgrades

Upgrading network services handling millions of requests per second without disrupting user connections is a fundamental challenge in system design. The naive approach of stopping an old process and starting a new one inevitably leads to a service gap, causing dropped connections (ECONNREFUSED for new connections) and abrupt termination of established connections. This can result in significant performance degradation and business impact, especially for critical infrastructure components like traffic routers or firewalls.

Limitations of SO_REUSEPORT for Graceful Restarts

While the SO_REUSEPORT socket option allows multiple processes to bind to the same address:port, enabling kernel-level load balancing of new connections, it introduces issues during transitions. If a process exits after the kernel assigns it a new connection but before it calls accept(), the connection becomes orphaned and is terminated. This makes SO_REUSEPORT unsuitable for truly graceful restarts where no connections should be dropped.

ecdysis: A NGINX-Inspired Approach to Graceful Restarts

Cloudflare's ecdysis library implements a robust graceful restart mechanism, inspired by NGINX, to address these issues. The core idea is to leverage Unix fork-exec model combined with socket inheritance. This ensures that the listening socket remains open and active throughout the upgrade process, preventing any service interruption.

1. The parent process forks a new child process.
2. The child process uses execve() to replace its image with the new version of the service code.
3. Crucially, the child inherits the listening socket file descriptors from the parent via a named pipe, allowing both processes to share the same underlying kernel data structure.
4. The parent continues to accept and process connections while the child initializes.
5. Once the child is ready, it signals the parent, which then closes its copy of the listening socket and begins draining its existing connections.
6. The child then takes over accepting new connections, ensuring no gap in service.

Security Considerations and Integration

The fork-then-exec model ensures the child starts with a clean address space, and ecdysis explicitly manages inherited file descriptors to prevent sensitive data leakage. It supports asynchronous Rust services via Tokio and integrates with systemd for lifecycle management and socket activation, making it suitable for modern Rust-based microservices at scale.