"Communications link failure" in the MySQL agent (4228229)

A MySQLAgent cannot connect to the MySQL instance. The full error message usually looks like this:

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server. com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

This is one of the most common errors and indicates that the DB isn't reachable with the connection properties provided.

This can have one or more of the following causes:

1. IP address or hostname in JDBC URL is wrong.
2. Hostname in JDBC URL is not recognized by local DNS server.
3. Port number is missing or wrong in JDBC URL.
4. DB server is down.
5. DB server doesn't accept TCP/IP connections.
6. DB server has run out of connections.
7. Something in between Java and DB is blocking connections, e.g. a firewall or proxy.

Each one can be tested with the corresponding number solution:

1. Verify and test them with ping.
2. Refresh DNS or use IP address in JDBC URL instead.
3. Verify it based on my.cnf of MySQL DB.
4. Start the DB.
5. Verify if mysqld is started without the --skip-networking option.
6. Close some connections or increase max_connections server variable.
7. Disable firewall and/or configure firewall/proxy to allow/forward the port