How do large companies manage legacy code and ensure apps stay online during updates?

Java and PHP are still widely used and not considered obsolete. They run a significant number of tech services today. When companies are looking to transition, they often don’t rewrite the entire system at once. Instead, they break it down into smaller parts and gradually update each endpoint, sometimes moving to microservices architecture. This way, they can test and deploy updates without causing widespread downtime.

For big businesses, a lot goes into planning updates to ensure zero downtime. Usually, they architect and build the replacement systems first. Once that’s done, they start testing in a safe environment, then carefully implement side-by-side operations of the new and old systems while monitoring everything. It's like having an extra safety net before fully switching.

Yes, companies like Reddit do document their transitions, and typically they don’t go for large-scale rewrites all at once. It’s much more common to update services little by little. They build new functionalities parallel to existing systems, ensuring users don’t notice any disruptions.

For zero downtime deployments, feature flags can be a game-changer. You can introduce new code behind these flags and roll it out progressively to users. Once everything is stable, you can clean up the old code without any hiccups.

From my experience managing high-stakes projects where downtime costs a fortune, we often utilize rolling updates and extensive testing. Critical transitions, like moving from one database engine to another, can take a ton of work to ensure continuity, but it's entirely manageable with great teamwork and careful planning.