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Index components in asterixdb can be in one of the following states:

• INACTIVE: The component is inactive (Unreadable and Unwritable).
• READABLE_WRITABLE: The component can be read from and can be written to
• READABLE_UNWRITABLE: Immutable component that can be read from but not written

...

• to (Note: Unless it is a force operation)
• READABLE_UNWRITABLE_FLUSHING: A component that is being flushed. Can be read from but not written to
• UNREADABLE_UNWRITABLE: A component that has completed flushing but still has some readers inside. This is equivalent to a DEACTIVATING state
• READABLE_MERGING: A disk component that is being merged

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• A memory component start as an empty component with its state being either READABLE_WRITABLE (first component) or INACTIVE (other components).
• A A READABLE_WRITABLE component becomes READABLE_UNWRITABLE prior  prior to flushing it to allow for current writers inside the components to exit before flushing.
  • This operation is done through the primary index operation tracker and it affects all indexes/partitions of the dataset.
  • Hence, an empty component can become become READABLE_UNWRITABLE and schedule flush will get called.
• If schedule flush is called on an empty component, its state is switched back from from READABLE_UNWRITABLE to READABLE_WRITABLE.
• On scheduling a flush, the memory component state is transitioned to READABLE_UNWRITABLE_FLUSHING. The previous state can either be
  READABLE_UNWRITABLE or (READABLE_WRITABLE: not clear when).
• When a flush is scheduled, the activation flag is set for the next component.