To troubleshoot YCQL API connection issues, you should perform a number of checks.

Are YugabyteDB processes running?

You should ensure that the expected YugabyteDB processes are on the current node. At a minimum, the YB-TServer process needs to be running to be able to connect to this node with a YCQL client or application.

Additionally, depending on the setup, you might expect a YB-Master process to also be running on this node.

For detailed instructions, see Check YugabyteDB servers.

Is the Cassandra-compatible YCQL API running?

If the YB-TServer process is running, execute the following command to verify that the YCQL API is enabled and listening on the YCQL port (default 9042):

lsof -i :9042
COMMAND     PID   USER   FD   TYPE     DEVICE SIZE/OFF NODE NAME
yb-tserve 81590 centos  131u  IPv4 0xdeadbeef      0t0  TCP localhost:9042 (LISTEN)

You may need to install lsof first.

When running a local cluster with yb-ctl, you should see all the nodes with different IP addresses, as per the following example:

lsof -i :9042
COMMAND     PID   USER   FD   TYPE     DEVICE SIZE/OFF NODE NAME
yb-tserve 81590 centos  131u  IPv4 0xdeadbeef      0t0  TCP localhost:9042 (LISTEN)
yb-tserve 81593 centos  131u  IPv4 0xdeadbeef      0t0  TCP 127.0.0.2:9042 (LISTEN)
yb-tserve 81596 centos  131u  IPv4 0xdeadbeef      0t0  TCP 127.0.0.3:9042 (LISTEN)

If another process is using this port, you might need to stop that and restart the YB-TServer process. Otherwise, if no process is listening but the YB-TServer is running, check the value of the --cql_proxy_bind_address flag passed to the YB-TServer process.

Can ycqlsh connect locally?

Once on the machine where YugabyteDB is running, use ycqlsh to connect to the local YugabyteDB instance, as follows:

ycqlsh <yb-local-address>

Depending on your configuration, you may need to install ycqlsh; otherwise, it is available in the YugabyteDB bin directory.

In the preceding command, <yb-local-address> is the address where the YugabyteDB YCQL server is listening (for example, as returned by lsof). For instance, it maps to localhost in the examples presented in this document (or, additionally, 127.0.0.2 and 127.0.0.3 for the yb-ctl case).

If ycqlsh can connect, the issue is likely a network issue with the original client not being able to access the node where YugabyteDB is running.

See also Are YB-Master and YB-TServer endpoints accessible?