CODEWITHSUNDEEP
cassandra
Programmer2030
Programmer2030

Reputation: 39

Cassandra read path

I'm learning Cassandra's read path.

According to some sources: "When Cassandra receives the read request, data will be searched first in the Memtable, then data will be searched in SSTables and if data exists it is returned"

Also, I know that Memtables are periodically flushed to SSTables on disk.

My questions:

  1. are memtables fully deleted from RAM after flushing to SSTables?

  2. Suppose, we have a read request on a node. Node contains both memtables and SSTables. Is it possible for Cassandra to get required data only from Memtables without accessing SSTables? If yes, when it is possible and how can Cassandra determine that required data stored only in Memtables and there are no other related data stored on disk (SSTables)?

Upvotes: 1

Views: 302

Answers (1)

Alex Ott
Alex Ott

Reputation: 87369

Short answer to 2nd question - No. Cassandra will always check SSTables even if the data is in the memtable. The reason for that is the data in the memtable could be older than data in the SSTable. For example, if you're explicitly set write timestamp for records, or data is replayed from the hints on other node. When memtable is flushed, data is removed from memory. But in some cases you can use row cache if you have data that is often accessed.

You can read more about read path in the DSE arch guide.

Upvotes: 2

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