removed_simple_executor.md

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title: Removed `SimpleExecutor`
pull_requests: [21176]

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Bevy has removed the previously deprecated SimpleExecutor, one of the SystemExecutors in Bevy alongside SingleThreadedExecutor and MultiThreadedExecutor (which aren't going anywhere any time soon). The MultiThreadedExecutor is great at large schedules and async heavy work, and the SingleThreadedExecutor is good at smaller schedules or schedules that have fewer parallelizable systems. So what was SimpleExecutor good at? Not much. That's why it was removed. Removing it reduced some maintenance and consistency burdens on maintainers, allowing them to focus on more exciting features!

If you were using SimpleExecutor, consider upgrading to SingleThreadedExecutor instead, or try MultiThreadedExecutor if if fits the schedule. It's worth mentioning that SimpleExecutor ran deferred commands inbetween each system, regardless of it it was needed. The other executors are more efficient about this, but that means they need extra information about when to run those commands. In most schedules, that information comes from the contents and ordering of systems, via before, after, chain, etc. If a schedule that was previously using SimpleExecutor still needs commands from one system to be applied before another system runs, make sure that ordering is enforced explicitly by these methods, rather than implicitly by the order of add_systems. If you are looking for a quick fix, chain is the easiest way to do this.