ctlog: write pending tiles to protect against concurrent sequencers #2
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If two sequencers (somehow) end up running at the same time, they could scribble over each other's uploads to the backend storage. Protect against that special case by writing the tiles in two steps. First, all tiles are staged in a unique pending directory (keyed by the hash of the new tree), and only copying them over to their final location after a successful lock update. In the normal case, this will incur an extra copy and delete for every uploaded tile, doubling the number of write operations. The copy code is simple but does introduce new requirements on the backend: - The backend must implement copy, list, and delete. - The copy must maintain the original headers. - The list must be read-after-write consistent. These properties should be true for S3. To make sure that the (rare) recovery code path is well-tested, use the same code path in both the recovery and normal case. A special IAM policy could allow deletes only of pending files.
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If two sequencers (somehow) end up running at the same time, they could scribble over each other's uploads to the backend storage.
Protect against that special case by writing the tiles in two steps. First, all tiles are staged in a unique pending directory (keyed by the hash of the new tree), and only copying them over to their final location after a successful lock update.
In the normal case, this will incur an extra copy and delete for every uploaded tile, doubling the number of write operations.
The copy code is simple but does introduce new requirements on the backend:
To make sure that the (rare) recovery code path is well-tested, use the same code path in both the recovery and normal case.
A special IAM policy could allow deletes only of pending files.