In the preceding chapters, you learned how to use Cassandra as a primary data store for the MyStatus application, with a focus on modeling the data that drives the main user experience on the site. In this chapter, we'll shift focus to another popular use of Cassandra: aggregating data that we observe over time. In particular, we'll build a small analytics component into our schema, which will allow us to keep track of how many times a given status update was viewed on a given day.
In order to do this, we'll introduce a new type of column, the counter column, which is a special numeric column type that can be discretely incremented or decremented. Counter columns have a lot in common with collection columns, which we explored in the previous chapter: you can make discrete changes to them without reading their current value, and they're good for scenarios in which many threads or processes might need to update the same piece of data at the same time...