Michael Noll wraps up a series on Apache Kafka. First up is the fundamentals of Kafka Streams:
A table is a, well, table in the ordinary technical sense of the word, and we have already talked a bit about tables before (a table in Kafka is today more like an RDBMS materialized view than an RDBMS table, because it relies on a change being made elsewhere rather than being directly updatable itself). Seen through the lens of event streaming however, a table is also an aggregated stream. This is a reference to the stream-table duality we discussed in part 1.
Streams and tables are always fault tolerant because their data is stored reliably and durably in Kafka. This should be relatively easy to understand for streams by now as they map to Kafka topics in a straightforward manner. If something breaks while processing a stream, then we just need to re-read the underlying topic again.
For tables, it is more complex because they must maintain additional information—their state—to allow for stateful processing such as joins and aggregations like
SUM(). To achieve this while also ensuring high processing performance, tables (through their state stores) are materialized on local disk within a Kafka Streams application instance or a ksqlDB server. But machines and containers can be lost, along with any locally stored data. How can we make tables fault tolerant, too?
This was a nice series.