Define Mem-table in Cassandra.
It is a memory-resident data structure. After commit log, the data will be written to the mem-table. Mem-table is in-memory/write-back cache space consisting of content in key and column format. The data in mem- table is sorted by key, and each column family consists of a distinct mem-table that retrieves column data via key. It stores the writes until it is full, and then flushed out.
