Frequently Asked Questions

What are the benefits of KSQL?

KSQL allows you to query, read, write, and process data in Apache Kafka in real-time and at scale using intuitive SQL-like syntax. KSQL does not require proficiency with a programming language such as Java or Scala, and you don’t have to install a separate processing cluster technology.

What are the technical requirements of KSQL?

KSQL only requires:

  1. A Java runtime environment
  2. Access to an Apache Kafka cluster for reading and writing data in real-time. The cluster can be on-premises or in the cloud. KSQL works with clusters running vanilla Apache Kafka as well as with clusters running the Kafka versions included in Confluent Platform.

We recommend the use of Confluent Platform or Confluent Cloud for running Apache Kafka.

Is KSQL owned by the Apache Software Foundation?

No, KSQL is owned and maintained by Confluent Inc. as part of its free Confluent Open Source product. However, KSQL is licensed under the Apache 2.0 license.

How does KSQL compare to Apache Kafka’s Streams API?

KSQL is complementary to the Kafka Streams API, and indeed executes queries through Kafka Streams applications. They share some similarities such as having very flexible deployment models so you can integrate them easily into your existing technical and organizational processes and tooling, regardless of whether you have opted for containers, VMs, bare-metal machines, cloud services, or on-premise environments.

One of the key benefits of KSQL is that it does not require the user to develop any code in Java or Scala. This enables users to leverage a SQL-like interface alone to construct streaming ETL pipelines, to respond to real-time, continuous business requests, to spot anomalies, and more. KSQL is a great fit when your processing logic can be naturally expressed through SQL.

For full-fledged stream processing applications Kafka Streams remains a more appropriate choice. For example, implementing a finite state machine that is driven by streams of data is easier to achieve in a programming language such as Java or Scala than in SQL. In Kafka Streams you can also choose between the DSL (a functional programming API) and the Processor API (an imperative programming API), and even combine the two.

As with many technologies, each has its sweet-spot based on technical requirements, mission-criticality, and user skillset.

Does KSQL work with vanilla Apache Kafka clusters, or does it require the Kafka version included in Confluent Platform?

KSQL works with both vanilla Apache Kafka clusters as well as with the Kafka versions included in Confluent Platform.

Does KSQL support Kafka’s exactly-once processing semantics?

Yes, KSQL supports exactly-once processing, which means it will compute correct results even in the face of failures such as machine crashes.

Can I use KSQL with my favorite data format (e.g. JSON, Avro)?

KSQL currently supports formats:

  • DELIMITED (e.g. comma-separated value)
  • JSON
  • Avro message values are supported. Avro keys are not yet supported. Requires Schema Registry and ksql.schema.registry.url in the KSQL server configuration file. For more information, see Avro and Schema Registry.

Is KSQL fully compliant to ANSI SQL?

KSQL is a dialect inspired by ANSI SQL. It has some differences because it is geared at processing streaming data. For example, ANSI SQL has no notion of “windowing” for use cases such as performing aggregations on data grouped into 5-minute windows, which is a commonly required functionality in the streaming world.

How do I shutdown a KSQL environment?

  • To stop DataGen tasks that were started with the -daemon flag (cf. ksql_clickstream-local).

    $ jps | grep DataGen
    25379 DataGen
    $ kill 25379
    
  • Exit KSQL.

    ksql> exit
    
  • Stop Confluent Platform by shutting down all services including Kafka.

    $ confluent stop
    
  • To remove all data, topics, and streams:

    $ confluent destroy
    

How do I add KSQL servers to an existing KSQL cluster?

You can add or remove KSQL servers during live operations. KSQL servers that have been configured to use the same Kafka cluster (bootstrap.servers) and the same KSQL service ID (ksql.service.id) form a given KSQL cluster.

To add a KSQL server to an existing KSQL cluster the server must be configured with the same bootstrap.servers and ksql.service.id settings as the KSQL cluster it should join. For more information, see Configuring KSQL Server.

How can I lock-down KSQL servers for production and prevent interactive client access?

You can configure your servers to run a set of predefined queries by using ksql.queries.file or the --queries-file command line flag. For more information, see Configuring KSQL Server.

How do I use Avro data and integrate with Confluent Schema Registry?

Configure the ksql.schema.registry.url property in the KSQL server configuration to point to Schema Registry (see Avro and Schema Registry).

Important

  • To use Avro data with KSQL you must have Schema Registry installed. This is included by default with Confluent Platform.
  • Avro message values are supported. Avro keys are not yet supported.

How can I scale out KSQL?

The maximum parallelism depends on the number of partitions.

  • To scale out: start additional KSQL servers with same config. This can be done during live operations. See How do I add KSQL servers to an existing KSQL cluster?.
  • To scale in: stop the desired running KSQL servers, but keep at least one server running. This can be done during live operations. The remaining servers should have sufficient capacity to take over work from stopped servers.

Tip

Idle servers will consume a small amount of resource. For example, if you have 10 KSQL servers and run a query against a two-partition input topic, only two servers perform the actual work, but the other eight will run an “idle” query.

Can KSQL connect to an Apache Kafka cluster over SSL?

Yes. Internally, KSQL uses standard Kafka consumers and producers. The procedure to securely connect KSQL to Kafka is the same as connecting any app to Kafka. For more information, see Configuring Kafka Encrypted Communication.

Can KSQL connect to an Apache Kafka cluster over SSL and authenticate using SASL?

Yes. Internally, KSQL uses standard Kafka consumers and producers. The procedure to securely connect KSQL to Kafka is the same as connecting any app to Kafka.

For more information, see Configuring Kafka Authentication.

Will KSQL work with Confluent Cloud?

Yes. Running KSQL against an Apache Kafka cluster running in the cloud is pretty straight forward. For more information, see Connecting KSQL to Confluent Cloud.

Will KSQL work with a HTTPS Confluent Schema Registry?

Yes. KSQL can be configured to communicate with the Confluent Schema Registry over HTTPS. For more information, see Configuring KSQL for Secured Confluent Schema Registry.