Glossary - Cassandra: The Definitive Guide - page 311

Cassandra generate one for you. The representation of the token is dependent on the kind of
partitioner used.
With a Random Partitioner , the token is an integer in the range 0-2 127 , generated by
applying an MD5 hash on keys. This is represented by the

org.apache.cassandra.dht.BigIntegerToken class.
With an Order-Preserving Partitioner , the token is a UTF-8 string, based on the key. This is
represented by the org.apache.cassandra.dht.StringToken class.
Tokens are represented in Cassandra by the org.apache.cassandra.dht.Token class.

Tombsttone
Cassandra does not immediately delete data following a delete operation. Instead, it marks
the data with a “tombstone,” an indicator that the column has been deleted but not removed
entirely yet. The tombstone can then be propagated to other replicas.
Tombstones are discarded on major Compaction .

Vecttor C r Cllock

Vector clocks allow for partial, causal ordering of events in a distributed system. A vector
clock maintains an array of logical clocks, one for each process, and each process contains a
local copy of the clock.

In order to keep the entire set of processes in a consistent logical state, one process will send
its clock to another process, which is then updated. In order to ensure consistency, some ver-
sion of the following steps are typically followed.
All clocks start at 0. Each time a process experiences an event, its clock is incremented by 1.
Each time a process prepares to send a message, this too counts as an event, so it increments
its clock by 1 and then sends its entire vector to the external process along with the message.
Each time a process receives a message, this too counts as an event, so it updates its own
clock by 1 and then compares its vector to the vector wrapped in the incoming message from
the external process. It updates its own clock with the maximum value from the comparison.
A vector clock event synchronization strategy will likely be introduced in a future version of
Cassandra.

Weak C k Consiisttency

For reads, weak consistency improves performance by first returning results, and afterward
performing any necessary Read Repair .