Understanding SSTable immutiability

Question

I'm trying to understand better the immutability of sstables in Cassandra. It's very clear what happens both in an insert operation, or in update/delete operation when the data exists in the memtable. But it's not clear what happens when I want to modify data that has already been flushed out.

So I understand the simple senario: I execute an insert opertaion and the data is written to a memtable. When the memtable is full then it's flushed to an sstable.

Now, how does modification of data occur? What happens when I execute a delete or update command (when the data has been flushed out)? If the sstable is immutable, so how will the data get deleted/updated? And how does the memtable work in delete and update commands (of data that does not exist in it because it has been flushed out)? What will the memtable contain?

Answer 1

In Cassandra / Scylla you ALWAYS append. Meaning any operation, whether it's insert / update / delete will create a new entry for that partition containing the new data and new timestamp. In case of a delete operation the new entry will actually be a tombstone with the new timestamp (indicating that the previous data was deleted). This applies whether the data is still in memory (memtable) or already flushed to disk -> sstable created.

Several "versions" of the same partition with different data and different timestamps can reside in multiple sstables (and even in memory) at the same time. SStables will be merged duration compaction and there are several compaction strategies that can be applied.

When the gc_grace_period (default: 10 days, tunable) has expired, on the next compaction that tombstone will be removed, meaning the data that was deleted and the tombstone indicating the latest action (delete), will not get merged into the new sstable.

The internal implementation of the memtables might be slightly different between Scylla and Cassandra but for the sake of simplicity let's assume it is the same.

You are welcomed to read more about the architecture in the following documentation:

• SStables

• Compaction strategies