User based trace enable in Cassandra

If you foresee that moving to Cassandra is definitely in your future, it will be easier to do while your dataset is still small and manageable. Also, as you learn and get a feel for Cassandra, a small dataset is a better one to make mistakes on (and thus, easier to correct them). That way your data model is solid by the time your dataset gets big, and that's when it really matters. And IMHO, there is no such thing as "too small" for Cassandra.

One of our applications uses Cassandra and ElasticSearch in prod. Based on those experiences, I would offer some caution about using ElasticSearch as a persistent datastore. We've seen it lose writes fairly often. Have a read through this discussion on Quora, appropriately titled: Why should I NOT use ElasticSearch as my primary datastore? That being said, it works great as a search engine.

There are a couple of concepts which need to be distinguished. One is about structure and the other about schema.

Structured data is one where the application knows in advance the meaning of each byte it receives. A good example is measurements from a sensor. In contrast a Twitter stream is unstructured. Schema is about how much of the structure is communicated to the DBMS as how it is asked to enforce this. It controls how much the DBMS parses the data it stores. A schema-required DBMS such as SQL Server can store unparsed data (varbinary) or optionally-parsed data (xml) and fully parsed data (columns).

NoSQL DBMSs lie on a spectrum from no parsing (key-value stores) upwards. Cassandra offers reatively rich functionality in this respect. Where they differ markedly to relational stores is in the uniformity of the data. Once a table is defined only data which matches that definition may be held there. In Cassandra, however, even if columns and families are defined there is no requirement for any two rows in the same table to look anything like each other. It falls to the application designer to decide how much goes in a single row (also referred to as a document) and what is held separately, linked by pointers. In effect, how much denormalisation do you want.

The advantage is you can retrieve a full set of data with a single sequential read. This is fast. One downside is that you, the application programer, are now solely responsible for all data integrity and backward compatibility concerns, for ever, for every bit of code that ever touches this data store. That can be difficult to get right. Also, you are locked into one point of view on the data. If you key your rows by order number, how do you report on the sale on one particular product, or region, or customer?