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Cassandra High Availability by Robbie Strickland

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Deleting data

We have established that Cassandra employs a log-structured storage engine, where all writes are immutable appends to the log. The implication is that data cannot actually be deleted at the time a DELETE statement is issued. Cassandra solves this by writing a marker, called a tombstone, with a timestamp greater than the previous value. This has the effect of overwriting the previous value with an empty one, which will then be compiled in subsequent queries for that column in the same manner as any other update. The actual value of the tombstone is set to the time of deletion, which is used to determine when the tombstone can be removed.

Garbage collection

Eventually these tombstones are reconciled with earlier values as part of the ...

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