Flink removed `-n` option after FLIP-6, change to ResourceManager start a new worker when required. But I think maintain a certain amount of slots is necessary. These workers will start immediately when ResourceManager starts and would not release even if all slots are free.
Here are some resons:
- Users actually know how many resources are needed when run a single job, initialize all workers when cluster starts can speed up startup process.
- Job schedule in topology order, next operator won't schedule until prior execution slot allocated. The TaskExecutors will start in several batchs in some cases, it might slow down the startup speed.
- Flink support FLINK-12122 [Spread out tasks evenly across all available registered TaskManagers], but it will only effect if all TMs are registered. Start all TMs at begining can slove this problem.
- Add config "taskmanager.minimum.numberOfTotalSlots" and "taskmanager.maximum.numberOfTotalSlots", default behavior is still like before.
- Start plenty number of workers to satisfy minimum slots when ResourceManager accept leadership(subtract recovered workers).
- Don't comlete slot request until minimum number of slots are registered, and throw exeception when exceed maximum.