A counter is a special kind of column used to store a number that incrementally counts the occurrences of a particular event or process. For example, you might use a counter column to count the number of times a page is viewed.
Counter column tables must use Counter data type. Counters may only be stored in dedicated tables.
After a counter is defined, the client application then updates the counter column value by incrementing (or decrementing) it. A client update to a counter column passes the name of the counter and the increment (or decrement) value; no timestamp is required.
Internally, the structure of a counter column is a bit more complex. Cassandra tracks the distributed state of the counter as well as a server-generated timestamp upon deletion of a counter column. For this reason, it is important that all nodes in your cluster have their clocks synchronized using a source such as network time protocol (NTP).
Unlike normal columns, a write to a counter requires a read in the background to ensure that distributed counter values remain consistent across replicas. Typically, you use a consistency level of ONE with counters because during a write operation, the implicit read does not impact write latency.