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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 Confluent Platform product. However, KSQL is licensed under the Confluent Community 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 Configuring Avro and Schema Registry for KSQL.
• KAFKA (for example, a BIGINT that’s serialized using Kafka’s standard LongSerializer).

See Formats for more details.

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 shut down a KSQL environment?

Exit KSQL CLI:

ksql> exit

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

confluent stop KSQL

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

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

If you’re running KSQL 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.

How do I configure the target Kafka cluster?

Define bootstrap.servers in the KSQL server configuration.

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 and Scaling KSQL.

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 Configuring Avro and Schema Registry for KSQL).

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 ksqlDB to Confluent Cloud.

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

Yes. For more information, see Configuring Authorization of KSQL with Kafka ACLs.

Will KSQL work with a HTTPS Confluent Schema Registry?

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

Where are KSQL-related data and metadata stored?

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

The KSQL command topic stores all data definition language (DDL) statements: CREATE STREAM, CREATE TABLE, DROP STREAM, and DROP TABLE. Also, the KSQL 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, KSQL stores metadata in the config topic. The config topic stores the KSQL properties provided to KSQL when the application was first started. KSQL uses these configs to ensure that your KSQL queries are built compatibly on every restart of the server.

Which KSQL queries read or write data to Kafka?

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

CREATE STREAM WITH <topic> and CREATE TABLE WITH <topic> write metadata to the KSQL 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 KSQL server?

Use the ps command to check whether the KSQL 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 KSQL server may be down.

If you’re running KSQL 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 KSQL server that you’re connected to.

• Run ksql-print-metrics on a server host. The tool connects to a KSQL server that’s running on localhost and collects JMX metrics from the server process. Metrics include the number of messages, the total throughput, the throughput distribution, and the error rate.
• Run SHOW STREAMS or SHOW TABLES, then run DESCRIBE EXTENDED <stream|table>.
• Run SHOW QUERIES, then run EXPLAIN <query>.

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

What if automatic topic creation is turned off?

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