Understanding Confluent KSQL

Confluent KSQL is a powerful stream processing engine built on top of Apache Kafka. It allows you to perform real-time data processing and analysis using a SQL-like language. In this guide, we’ll explore the key concepts and features of Confluent KSQL and how it can be used for stream processing in Kafka.

Key Concepts

  1. Streams: In KSQL, a stream represents an unbounded sequence of data records. Each record in a stream consists of a key, value, and timestamp. Streams are created from Kafka topics and can be continuously updated as new data arrives.

  2. Tables: A table in KSQL represents a view of a stream or another table at a specific point in time. Tables are derived from streams and are automatically updated as new data arrives in the underlying stream. Tables can be used for aggregations, joins, and other stateful operations.

  3. Queries: KSQL supports two types of queries: persistent queries and transient queries. Persistent queries continuously run in the background and update their results as new data arrives. Transient queries are ad-hoc queries that return a result based on the current state of the data and terminate immediately.

  4. User-Defined Functions (UDFs): KSQL allows you to define custom functions using Java to extend its functionality. UDFs can be used to perform complex transformations, aggregations, or custom business logic on the streaming data.

Creating Streams and Tables

To create a stream in KSQL, you use the CREATE STREAM statement. Here’s an example:

  order_id BIGINT,
  product_id BIGINT,
  quantity INTEGER,
  price DOUBLE
) WITH (
  KAFKA_TOPIC = 'orders',

This statement creates a stream named orders with the specified columns and maps it to the Kafka topic orders. The VALUE_FORMAT property specifies the format of the data in the topic, in this case, JSON.

To create a table, you use the CREATE TABLE statement. Here’s an example:

CREATE TABLE order_summary AS
SELECT product_id, SUM(quantity) AS total_quantity, SUM(price) AS total_price
FROM orders
GROUP BY product_id;

This statement creates a table named order_summary that aggregates the data from the orders stream. It calculates the total quantity and total price for each product_id.

Stream Processing with KSQL

KSQL provides a rich set of stream processing capabilities through its SQL-like language. Some common operations include:

  1. Filtering: You can filter streams based on specific conditions using the WHERE clause. For example:

    SELECT * FROM orders WHERE quantity > 10;
  2. Transformations: KSQL allows you to transform the data in streams using SELECT statements. You can select specific columns, perform calculations, or apply functions to the data. For example:

    SELECT order_id, quantity * price AS total_amount FROM orders;
  3. Aggregations: You can perform aggregations on streams using the GROUP BY clause and aggregate functions like SUM, COUNT, AVG, etc. For example:

    SELECT product_id, SUM(quantity) AS total_quantity FROM orders GROUP BY product_id;
  4. Joins: KSQL supports joining streams and tables based on a common key. You can perform inner joins, left joins, and outer joins. For example:

    SELECT o.order_id, o.quantity, p.product_name
    FROM orders o
    JOIN products p ON o.product_id = p.product_id;
  5. Windowing: KSQL allows you to perform windowed operations on streams, such as tumbling windows, hopping windows, and session windows. Windowing enables you to analyze data over a specific time period. For example:

    SELECT product_id, SUM(quantity) AS total_quantity
    FROM orders
    GROUP BY product_id;

Best Practices

  1. Partition Data Correctly: Ensure that your Kafka topics are partitioned based on the keys that will be used for joining and aggregating data in KSQL. This enables efficient and scalable stream processing.

  2. Use Appropriate Data Formats: Choose the appropriate data formats for your Kafka topics, such as Avro, JSON, or Protobuf. KSQL supports various data formats, and using a schema-based format like Avro can provide better compatibility and schema evolution.

  3. Monitor and Scale KSQL Clusters: Monitor the performance and resource utilization of your KSQL clusters. Scale the clusters horizontally by adding more nodes to handle increasing data volumes and processing requirements.

  4. Manage State Stores: KSQL uses state stores for stateful operations like aggregations and joins. Ensure that you have sufficient disk space and memory for the state stores. Configure the state store retention period based on your data retention requirements.

  5. Test and Validate Queries: Test and validate your KSQL queries in a non-production environment before deploying them to production. Ensure that the queries produce the expected results and perform efficiently.