<a id="integrate-ksql-with-confluent-control-center"></a>

# Connect ksqlDB Server Instances to Confluent Control Center on Confluent Platform

ksqlDB Server integrates with Control Center by default when you install Confluent Platform,
and you can configure Control Center to interact with other ksqlDB Server
instances that run on separate hosts. In Control Center, you use the ksqlDB
user interface to create Apache Kafka® topics and develop persistent queries in the
query editor.

![Screenshot of the ksqlDB Create Stream interface in Confluent Control Center.](ksqldb/images/c3-ksql-flow-2.png)

## Configuration settings for ksqlDB and Control Center

Set up the integration between ksqlDB and Control Center by assigning
properties in the ksqlDB Server and Control Center configuration files.

* By default, the ksqlDB Server configuration file is installed at
  CONFLUENT_HOME/etc/ksqldb/ksql-server.properties\`.
* By default, the Control Center configuration file is installed at
  `CONFLUENT_HOME/etc/confluent-control-center/control-center.properties`.

You can set up ksqlDB Server to communicate securely with other components in
Confluent Platform. For more information, see
[Configure Security for ksqlDB](operate-and-deploy/installation/security.md#ksqldb-installation-security).

#### NOTE
To run ksqlDB queries from Control Center, both ksqlDB and Control Center
must be running on either HTTP or HTTPS.

### Network configuration for ksqlDB and Control Center

Use these configuration settings to set up network
connectivity between ksqlDB and Control Center.

* In the ksqlDB Server configuration file, set the
  [listeners](operate-and-deploy/installation/server-config.md#ksqldb-install-configure-server-configuring-listeners) property
  to the IP address of the REST API endpoint for ksqlDB Server. Typical values
  are `http://0.0.0.0:8088` and `http://localhost:8088`.
* In the Control Center configuration file, set the
  `confluent.controlcenter.ksql.<ksql-cluster-name>.url`
  property to the hostnames and listener ports for the ksqlDB cluster specified
  by `<ksql-cluster-name>`.
  This setting specifies how Control Center communicates with ksqlDB Server for
  regular HTTP
  requests. For more information, see [ksqlDB settings](https://docs.confluent.io/control-center/current/installation/configuration.html#controlcenter-ksql-settings).
* If ksqlDB Server communicates over an internal DNS that is not externally
  resolvable or routable, set the
  `confluent.controlcenter.ksql.<ksql-cluster-name>.advertised.url`
  property in the Control Center configuration file. This setting specifies how the
  browser communicates with ksqlDB Server for websocket requests. For more
  information,
  see [ksqlDB settings](https://docs.confluent.io/control-center/current/installation/configuration.html#controlcenter-ksql-settings).

When ksqlDB Server and Confluent Control Center run on the same host, you can use the default
configuration defined by Confluent Platform setup.

## When ksqlDB and Control Center run on different hosts

If ksqlDB Server and Control Center run on different hosts, you must specify a
configuration that ensures ksqlDB Server and Control Center can communicate.
This is necessary when ksqlDB Server and Control Center:

* Run in separate containers.
* Run in separate virtual machines.
* Communicate over a virtual private network (VPN).
* Use different URLs, for example when the ksqlDB Server host publishes a
  public URL that’s different from the private URL for ksqlDB Server.

#### NOTE
When ksqlDB and Control Center communicate over a virtual private network (VPN),
Control Center proxies your queries, but query results stream directly
from ksqlDB Server back to your browser without going through
Control Center. Over a VPN, the advertised URL isn’t `localhost`. Instead,
it’s the hostname of the remote server.

Assign the following configuration properties to integrate ksqlDB Server with
Control Center when they run on separate hosts.

### ksqlDB server configuration

In the ksqlDB Server configuration file, set `listeners` to bind to all
interfaces:

```properties
listeners=http://0.0.0.0:8088
```

Configure ksqlDB to accept cross-origin requests from the Confluent Control Center UI by adding the following CORS settings to the ksqlDB Server configuration file.

```properties
access.control.allow.origin=*
access.control.allow.methods=GET,POST,HEAD
access.control.allow.headers=X-Requested-With,Content-Type,Accept,Origin,Authorization
```

### Control Center configuration

In the Control Center configuration file, set
`confluent.controlcenter.ksql.<ksql-cluster-name>.url`
to a list of URLs for the ksqlDB Server hosts, which must be reachable from the host
that Control Center is installed on. Replace `<ksql-cluster-name>` with the name
that Control Center uses to identify the ksqlDB cluster.

```none
confluent.controlcenter.ksql.<ksql-cluster-name>.url=<internally-resolvable-hostname1>, <internally-resolvable-hostname2>, ...
```

Also, set `confluent.controlcenter.ksql.<ksql-cluster-name>.advertised.url`
to the public IP addresses published by the ksqlDB Server hosts, which must be a
list of URLs that the browser can resolve through externally available DNS.

```none
confluent.controlcenter.ksql.<ksql-cluster-name>.advertised.url=<externally-resolvable-hostname1>, <externally-resolvable-hostname2>, ...
```

The Control Center configuration must match the ksqlDB Server `listeners` values.

Use the `curl` command to check whether these URLs are reachable. Depending
on your deployment, you may need to check from two different hosts:

* Check from the host where Control Center is running, which is relevant
  for the `confluent.controlcenter.ksql.<ksql-cluster-name>.url` setting.
* Check from the host where the browser is running, which is relevant for the
  `confluent.controlcenter.ksql.<ksql-cluster-name>.advertised.url` setting.

On both hosts, run the following command to confirm that the ksqlDB Server
cluster is reachable. The `hostname` value is one of the hosts in the
listed in the `confluent.controlcenter.ksql.<ksql-cluster-name>.url` and
`confluent.controlcenter.ksql.<ksql-cluster-name>.advertised.url`
configuration settings.

```bash
curl http://<hostname>:8088/info \
{"KsqlServerInfo":{"version":"8.3.1","kafkaClusterId":"<ksql-cluster-name>","ksqlServiceId":"default_"}}%
```

#### NOTE
You must specify the ports in the ksqlDB URL settings. For example, if the
public URL is `http://ksql-server-677739697.us-east-1.elb.amazonaws.com:80`,
be sure to include port `80`, or the Control Center connection to ksqlDB Server
will fail.

### Check network connectivity between ksqlDB and Control Center

Use a web browser to check the configuration of an advertised URL. Make sure
that your browser can reach the `info` endpoint at `http://<ksql.advertised.url>/info`.
If the configuration is wrong, and the browser can’t resolve the URL of the
ksqlDB Server host, you’ll receive an error:
`Websocket error when communicating with <ksql.advertised.url>`.

### Check ksqlDB server network binding

If Control Center doesn’t connect with your ksqlDB Server instance, check the network
binding on the ksqlDB Server host:

```bash
sudo netstat -plnt|grep $(ps -ef|grep KsqlServerMain|grep -v grep|awk '')
```

If your ksqlDB server is bound to `localhost` only, your output should
resemble:

```bash
tcp6 0 0 127.0.0.1:8088 :::* LISTEN 64383/java
tcp6 0 0 :::34791 :::* LISTEN 64383/java
```

If `0.0.0.0` isn’t listed, ksqlDB Server isn’t accepting external
connections. In the `ksql-server.properties` file, set
`listeners=http://0.0.0.0:8088` and restart ksqlDB Server.

## Next steps

* [Connecting ksqlDB to Confluent Cloud](/cloud/current/cp-component/ksql-cloud-config.html)
* [Configure Security for ksqlDB](operate-and-deploy/installation/security.md#ksqldb-installation-security)