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KSQL Operations

Watch the screencast of Taking KSQL to Production on YouTube.

Local Development and Testing with Confluent CLI

For development and testing purposes, you can use Confluent CLI to spin up services on a single host. For more information, see the Confluent Platform Quick Start.


The Confluent CLI is meant for development purposes only and is not suitable for a production environment. The data that are produced are transient and are intended to be temporary. For production-ready workflows, see Install and Upgrade.

Installing and Configuring KSQL

You have a number of options when you set up KSQL Server. For more information on installing and configuring KSQL, see the following topics.

Starting and Stopping KSQL Clusters

KSQL provides start and stop scripts.

This script starts the KSQL server. It requires a server configuration file as an argument and is located in the /bin directory of your Confluent Platform installation. For more information, see Starting the KSQL Server.
This script stops the KSQL server. It is located in the /bin directory of your Confluent Platform installation.


  • The KSQL REST API supports a “server info” request at http://<server>:8088/info.
  • Check runtime stats for the KSQL server that you are connected to via DESCRIBE EXTENDED <stream or table> and EXPLAIN <name of query>.
  • Run ksql-print-metrics on a KSQL server. For example, see this blog post.

Monitoring and Metrics

KSQL includes JMX (Java Management Extensions) metrics which give insights into what is happening inside your KSQL servers. These metrics include the number of messages, the total throughput, throughput distribution, error rate, and more.

To enable JMX metrics, set JMX_PORT before starting the KSQL server:

export JMX_PORT=1099 && \
<path-to-confluent>/bin/ksql-server-start <path-to-confluent>/etc/ksql/

The ksql-print-metrics command line utility collects these metrics and prints them to the console. You can invoke this utility from your terminal:

$ <path-to-confluent>/bin/ksql-print-metrics

Your output should resemble:

messages-consumed-avg: 96416.96196183885
messages-consumed-min: 88900.3329377909
error-rate: 0.0
num-persistent-queries: 2.0
messages-consumed-per-sec: 193024.78294586178
messages-produced-per-sec: 193025.4730374501
num-active-queries: 2.0
num-idle-queries: 0.0
messages-consumed-max: 103397.81191436431

For more information about Kafka Streams metrics, see Monitoring Streams Applications.

Capacity Planning

The Capacity Planning guide describes how to size your KSQL clusters.


SELECT query hangs and doesn’t stop?

Queries in KSQL, including non-persistent queries such as SELECT * FROM myTable, are continuous streaming queries. Streaming queries will not stop unless explicitly terminated. To terminate a non-persistent query in the KSQL CLI you must type Ctrl + C.

No results from SELECT * FROM table or stream?

This is typically caused by the query being configured to process only newly arriving data instead, and no new input records are being received. To fix, do one of the following:

Can’t create a stream from the output of windowed aggregate?

The output of a windowed aggregate is a record per grouping key and per window, and is not a single record. This is not currently supported in KSQL.

KSQL doesn’t clean up its internal topics?

Make sure that your Apache Kafka® cluster is configured with delete.topic.enable=true. For more information, see deleteTopics.

KSQL CLI doesn’t connect to KSQL server?

The following warning may occur when you start the KSQL CLI.

**************** WARNING ******************
Remote server address may not be valid:
Error issuing GET to KSQL server

Caused by: Connection reset
Caused by: Connection reset

Also, you may see a similar error when you create a KSQL query by using the CLI.

Error issuing POST to KSQL server
Caused by: Connection reset
Caused by: Connection reset

In both cases, the CLI can’t connect to the KSQL server, which may be caused by one of the following conditions.

  • KSQL CLI isn’t connected to the correct KSQL server port.
  • KSQL server isn’t running.
  • KSQL server is running but listening on a different port.

Check the port that KSQL CLI is using

Ensure that the KSQL CLI is configured with the correct KSQL server port. By default, the server listens on port 8088. For more info, see Starting the KSQL CLI.

Check the KSQL server configuration

In the KSQL server configuration file, check that the list of listeners has the host address and port configured correctly. Look for the listeners setting:


For more info, see Starting KSQL Server.

Check for a port conflict

There may be another process running on the port that the KSQL server listens on. Use the following command to check the process that’s running on the port assigned to the KSQL server. This example checks the default port, which is 8088.

netstat -anv | egrep -w .*8088.*LISTEN

Your output should resemble:

tcp4  0 0  *.8088       *.*    LISTEN      131072 131072    46314      0

In this example, 46314 is the PID of the process that’s listening on port 8088. Run the following command to get info on the process.

ps -wwwp <pid>

Your output should resemble: ./config/

If the KsqlServerMain process isn’t shown, a different process has taken the port that KsqlServerMain would normally use. Check the assigned listeners in the KSQL server configuration, and restart the KSQL CLI with the correct port.

Replicated topic with Avro schema causes errors?

Confluent Replicator renames topics during replication, and if there are associated Avro schemas, they aren’t automatically matched with the renamed topics.

In the KSQL CLI, the PRINT statement for a replicated topic works, which shows that the Avro schema ID exists in Schema Registry, and KSQL can deserialize the Avro message. But CREATE STREAM fails with a deserialization error:

CREATE STREAM pageviews_original (viewtime bigint, userid varchar, pageid varchar) WITH (kafka_topic='pageviews.replica', value_format='AVRO');

[2018-06-21 19:12:08,135] WARN task [1_6] Skipping record due to deserialization error. topic=[pageviews.replica] partition=[6] offset=[1663] (org.apache.kafka.streams.processor.internals.RecordDeserializer:86)
org.apache.kafka.connect.errors.DataException: pageviews.replica
        at io.confluent.connect.avro.AvroConverter.toConnectData(
        at io.confluent.ksql.serde.connect.KsqlConnectDeserializer.deserialize(
        at io.confluent.ksql.serde.connect.KsqlConnectDeserializer.deserialize(

The solution is to register schemas manually against the replicated subject name for the topic:

# Original topic name = pageviews
# Replicated topic name = pageviews.replica
curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" --data "{\"schema\": $(curl -s http://localhost:8081/subjects/pageviews-value/versions/latest | jq '.schema')}" http://localhost:8081/subjects/pageviews.replica-value/versions

Check KSQL server logs

If you’re still having trouble, check the KSQL server logs for errors.

confluent log ksql-server

Look for logs in the default directory at /usr/local/logs or in the LOG_DIR that you assign when you start the KSQL CLI. For more info, see Starting the KSQL CLI.