15bea069a9
this is a follow-up change of 1519904fb9, to incorporate the comment
from Anna Stuchlik.
Signed-off-by: Anna Stuchlik <anna.stuchlik@scylladb.com>
Closes scylladb/scylladb#17671
38 lines
2.3 KiB
Markdown
38 lines
2.3 KiB
Markdown
# Timestamp conflict resolution
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The fundamental rule for ordering cells that insert, update, or delete data in a given row and column
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is that the cell with the highest timestamp wins.
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However, it is possible that multiple such cells will carry the same `TIMESTAMP`.
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In this case, conflicts must be resolved in a consistent way by all nodes.
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Otherwise, if nodes would have picked an arbitrary cell in case of a conflict and they would
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reach different results, reading from different replicas would detect the inconsistency and trigger
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read-repair that will generate yet another cell that would still conflict with the existing cells,
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with no guarantee for convergence.
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The first tie-breaking rule when two cells have the same write timestamp is that
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dead cells win over live cells; and if both cells are deleted, the one with the later deletion time prevails.
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If both cells are alive, their expiration time is examined.
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Cells that are written with a non-zero TTL (either implicit, as determined by
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the table's default TTL, or explicit, `USING TTL`) are due to expire
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TTL seconds after the time they were written (as determined by the coordinator,
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and rounded to 1 second resolution). That time is the cell's expiration time.
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When cells expire, they become tombstones, shadowing any data written with a write timestamp
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less than or equal to the timestamp of the expiring cell.
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Therefore, cells that have an expiration time win over cells with no expiration time.
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If both cells have an expiration time, the one with the latest expiration time wins;
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and if they have the same expiration time (in whole second resolution),
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their write time is derived from the expiration time less the original time-to-live value
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and the one that was written at a later time prevails.
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Finally, if both cells are live and have no expiration, or have the same expiration time and time-to-live,
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the cell with the lexicographically bigger value prevails.
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Note that when multiple columns are inserted (`INSERT`) or updated (`UPDATE`) using the same timestamp,
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selecting (`SELECT`) those columns might return a result that mixes cells from either upsert.
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This may happen when both upserts have no expiration time, or both their expiration time and TTL are the
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same, respectively (in whole second resolution). In such a case, cell selection would be based on the cell values
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in each column, independently of each other.
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