Rollover should not swap aliases when `is_write_index` is set to `true`.
Instead, both the new and old indices should have the rollover alias,
with the newly created index as the new write index
Updates Rollover to leverage the ability to preserve aliases and swap which is the write index.
Historically, Rollover would swap which index had the designated alias for writing documents against. This required users to keep a separate read-alias that enabled reading against both rolled over and newly created indices, whiles the write-alias was being re-assigned at every rollover.
With the ability for aliases to designate a write index, Rollover can be a bit more flexible with its use of aliases.
Updates include:
- Rollover validates that the target alias has a write index (the index that is being rolled over). This means that the restriction that aliases only point to one index is no longer necessary.
- Rollover explicitly (and atomically) swaps which index is the write-index by explicitly assigning the existing index to have `is_write_index: false` and have the newly created index have its rollover alias as `is_write_index: true`. This is only done when `is_write_index: true` on the write index. Default behavior of removing the alias from the rolled over index stays when `is_write_index` is not explicitly set
Relevant things that are staying the same:
- Rollover is rejected if there exist any templates that match the newly-created index and configure the rollover-alias
- I think this existed to prevent the situation where an alias pointed to two indices for a short while. Although this can technically be relaxed, the specific cases that are safe are really particular and difficult to reason, so leaving the broad restriction sounds good
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