Content Filter

Events in an Event Processing Application can often be very large. We tend to capture data exactly as it arrives, and then process it, rather than processing it first and only storing the results. So the event that we want to consume often contains much more information than we actually need for the task in hand.

For example, we might pull in a product feed from a third-party API and store that data exactly as it was received. Later, we might ask the question, "How many products are in each product category?" and find that every event contains 100 fields, when we're really only interested in counting one. At the very least, this is inefficient; the network, memory, and serialization costs are 100x higher than they need to be. But manually inspecting the data actually becomes painful -- hunting through 100 fields to find and check the one that we care about.

Equally important, we may have security and data privacy concerns to address. Imagine that we have a stream of data representing users' personal details and site preferences. If the marketing department wants to get more information about our global customer base, we might be able to share the users' timezone and currency settings, but only those fields.

We need a method of storing complete events while only giving consumers a subset of the event fields.


How can I simply consume only a few data items from a large event?


content filter

Create an Event Processor that inspects each event, pulls out the fields of interest, and passes new, smaller events downstream for further processing.


As an example, in the streaming database ksqlDB, we can use a SELECT statement to easily transform a rich event stream into a stream of simpler events.

Assume that we have an event stream called products, where each event contains a huge number of fields. We are only interested in four fields: producer_id, category, sku, and price. We can prune down the events to just those fields with the following query:

  FROM products;

Or we can perform an equivalent transformation using the Apache Kafka® client library Kafka Streams, perhaps as part of a larger processing pipeline:"products", Consumed.with(Serdes.Long(), productSerde))
        (product) -> {
          ProductSummary summary = new ProductSummary();


          return summary;
    .to("product_summaries", Produced.with(Serdes.Long(), productSummarySerde));


Since filtering the content creates a new stream, it's worth considering how the new stream will be partitioned, as discussed in the Partitioned Placement pattern. By default, the new stream will inherit the same partitioning key as its source, but we can repartition the data to suit our new use case (for example, by specifying a PARTITION BY clause in ksqlDB).

In the example above, our third-party product feed might be partitioned by the vendor's unique product_id, but for this use case, it might make more sense to partition the filtered events by their category.

See the ksqlDB documentation for details.


  • This pattern is derived from Content Filter in Enterprise Integration Patterns, by Gregor Hohpe and Bobby Woolf.
  • For filtering out entire events from a stream, consider the Event Filter pattern.