What is an event?¶
ksqlDB is a database that's purpose-built for stream processing applications. The main focus of stream processing is modeling computation over unbounded streams of events.
An event is anything that occurred and was recorded. It could be something high-level that happened in a business, like the sale of an item or the submission of an invoice. Or it could be something low-level, like a log line emitted by a web server when a request is received. Anything that happens at a point in time is an event.
Because events are so fundamental to stream processing, they are ksqlDB's core unit of data. All of ksqlDB's features are oriented around making it easy to solve problems using events. Although it's easy to think about individual events, figuring out how to store related events together is a bit more challenging. Fortunately, the idea of storing related events is well-explored territory. Apache Kafka® leads the way, which is why ksqlDB is built directly on top of it.
Kafka is a distributed streaming platform for working with events. It’s horizontally scalable, fault-tolerant, and extremely fast. Although working with it directly can be low-level, it has a strong and opinionated approach for modeling both individual events and stored events. For this reason, ksqlDB borrows heavily from some of Kafka's abstractions. It doesn’t aim to make you learn all of Kafka, but it also doesn't reinvent the wheel where there's already something really good to use.
ksqlDB represents events by using a simple key/value model, which is very similar to Kafka's notion of a record. The key represents some form of identity about the event. The value represents information about the event that occurred. This combination of key and value makes it easy to model stored events, since multiple events with the same key represent the same identity, irrespective of their values.
But events in ksqlDB carry more information than just a key and value. Similar to Kafka, they also describe the time at which the event was true.
ksqlDB aims to raise the abstraction from working with a lower-level stream
processor. Usually, an event is called a "row", as if it were a row in a
relational database. Each row is composed of a series of columns. Columns
are either read from the event's key or value. ksqlDB also supports 3
pseudo columns, available on every row:
ROWTIME, which represents the
time of the event, as well as
ROWOFFSET, which represent
the partition and offset of the source record, respectively.
In addition, windowed sources have
WINDOWEND system columns.
Starting in ksqlDB 0.24, you can create columns that are populated by a record's header field. Kafka headers are a list of zero or more key-value pairs, where the keys are (not necessarily unique) strings and the values are byte arrays. Headers typically contain metadata about records, which can then be used for routing or processing For example, the header could store information about how to deserialize strings in the value.