Graceful startup and shutdown for Phoenix applications

Phoenix applications benefit from the built-in capabilities of the BEAM for long-lived processes, but a Graceful Shutdown is essential for cleaning up when those processes end, especially when additional processes are involved. In the context of a Phoenix application, a Graceful Shutdown involves stopping incoming work, completing ongoing work, and safely bringing down the system without disrupting the user experience, particularly during frequent code deployments. This process is supported by Phoenix's default configuration, including components like Plug.Cowboy.Drainer, which manages HTTP connection closures. However, challenges can arise with clustered Phoenix applications, such as maintaining accurate presence information. Implementing strategies such as Phoenix.Tracker.graceful_permdown helps ensure the system accurately reflects node status, preventing outdated presence data. Properly managing the application lifecycle, including the order of process termination and utilizing health checks, ensures a smooth transition during deployments, allowing for zero-downtime and a seamless user experience.