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Frequently Asked Questions

What are the benefits of ksqlDB?

ksqlDB allows you to query, read, write, and process data in Apache Kafka® in real-time and at scale using intuitive SQL-like syntax. ksqlDB 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 ksqlDB?

ksqlDB 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. ksqlDB 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 ksqlDB owned by the Apache Software Foundation?

No, ksqlDB is owned and maintained by Confluent Inc. as part of its Confluent Platform product. However, ksqlDB is licensed under the Confluent Community License.

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

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

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

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

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

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

ksqlDB currently supports the following 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 ksqlDB server configuration file. For more information, see Configure ksqlDB for Avro.
• KAFKA (for example, a BIGINT that’s serialized using Kafka’s standard LongSerializer).

See Serialization Formats for more details.

Is ksqlDB fully compliant to ANSI SQL?

ksqlDB 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 shut down a ksqlDB environment?

Exit ksqlDB CLI:

ksql> exit

If you’re running with Confluent CLI, use the confluent stop command:

confluent stop ksql

If you’re running ksqlDB in Docker containers, stop the cp-ksqldb-server container:

docker stop <cp-ksqldb-server-container-name>

If you’re running ksqlDB as a system service, use the systemctl stop command:

sudo systemctl stop confluent-ksql

For more information on shutting down Confluent Platform, see Install and Upgrade Confluent Platform.

How do I configure the target Kafka cluster?

Define bootstrap.servers in the Configure ksqlDB Server.

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

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

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

How can I lock-down ksqlDB 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 Configure ksqlDB Server.

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

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

Important

• To use Avro data with ksqlDB 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 ksqlDB?

The maximum parallelism depends on the number of partitions.

• To scale out: start additional ksqlDB servers with same config. This can be done during live operations. See How do I add ksqlDB servers to an existing ksqlDB cluster?
• To scale in: stop the desired running ksqlDB 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 ksqlDB 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 ksqlDB connect to an Apache Kafka cluster over SSL?

Yes. Internally, ksqlDB uses standard Kafka consumers and producers. The procedure to securely connect ksqlDB to Kafka is the same as connecting any app to Kafka. For more information, see Configure ksqlDB for Secured Apache Kafka clusters.

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

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

For more information, see Configure Kafka Authentication.

Will ksqlDB work with Confluent Cloud?

Yes. Running ksqlDB against an Kafka cluster running in the cloud is pretty straightforward. For more information, see Confluent Cloud ksqlDB Quick Start.

Also, you can run fully managed KSQL in Confluent Cloud. For more information, see Confluent Cloud ksqlDB Quick Start.

Will ksqlDB work with a Apache Kafka cluster secured using Kafka ACLs?

Yes. For more information, see Configure Authorization of ksqlDB with Kafka ACLs.

Will ksqlDB work with a HTTPS Confluent Schema Registry?

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

Where are ksqlDB-related data and metadata stored?

In interactive mode, ksqlDB stores metatada in and builds metadata from the ksqlDB command topic. To secure the metadata, you must secure the command topic.

The ksqlDB command topic stores all data definition language (DDL) statements: CREATE STREAM, CREATE TABLE, DROP STREAM, and DROP TABLE. Also, the ksqlDB command topic stores TERMINATE statements, which stop persistent queries based on CREATE STREAM AS SELECT (CSAS) and CREATE TABLE AS SELECT (CTAS).

Currently, data manipulation language (DML) statements, like UPDATE and DELETE aren’t available.

In headless mode, ksqlDB stores metadata in the config topic. The config topic stores the ksqlDB properties provided to ksqlDB when the application was first started. ksqlDB uses these configs to ensure that your ksqlDB queries are built compatibly on every restart of the server.

Which ksqlDB queries read or write data to Kafka?

SHOW STREAMS and EXPLAIN <query> statements run against the ksqlDB server that the ksqlDB client is connected to. They don’t communicate directly with Kafka.

CREATE STREAM WITH <topic> and CREATE TABLE WITH <topic> write metadata to the ksqlDB command topic.

Persistent queries based on CREATE STREAM AS SELECT and CREATE TABLE AS SELECT read and write to Kafka topics.

Non-persistent queries based on SELECT that are stateless only read from Kafka topics, for example SELECT … FROM foo WHERE ….

Non-persistent queries that are stateful read and write to Kafka, for example, COUNT and JOIN. The data in Kafka is deleted automatically when you terminate the query with CTRL-C.

How do I check the health of a ksqlDB server?

Use the ps command to check whether the ksqlDB server process is running, for example:

ps -aux | grep ksql

Your output should resemble:

jim       2540  5.2  2.3 8923244 387388 tty2   Sl   07:48   0:33 /usr/lib/jvm/java-8-oracle/bin/java -cp /home/jim/confluent-5.0.0/share/java/monitoring-interceptors/* ...

If the process status of the JVM isn’t Sl or Ssl, the ksqlDB server may be down.

If you’re running ksqlDB server in a Docker container, run the docker ps or docker-compose ps command, and check that the status of the ksql-server container is Up. Check the health of the process in the container by running docker logs <ksql-server-container-id>.

Check runtime stats for the ksqlDB server that you’re connected to.

• Run SHOW STREAMS or SHOW TABLES, then run DESCRIBE EXTENDED <stream|table>.
• Run SHOW QUERIES, then run EXPLAIN <query>.

The ksqlDB REST API supports a “server info” request (for example, http://<ksql-server-url>/info), which returns info such as the ksqlDB version. For more info, see REST API Index.

What if automatic topic creation is turned off?

If automatic topic creation is disabled, ksqlDB and Kafka Streams applications continue to work. ksqlDB and Kafka Streams applications use the Admin Client, so topics are still created.