MyCATApache/Mycat-Server · critical · ConnectionException

err.errno

err.errno

Error message

errMsg

What it means

During the MySQL handshake/authentication phase, MySQLConnectionAuthenticator.handle received an ErrorPacket from the server and rethrows it as ConnectionException(err.errno, errMsg). The message 'can't connect to mysql server, errmsg:...' is logged first. This surfaces the MySQL server's own error (e.g. access denied, unknown database, max connections) at the mycat backend-connection layer.

Solutions

1. Read errMsg/errno in the exception to see the MySQL server's reason (1045=access denied, 1049=unknown database, 1040=too many connections)
2. Fix the datasource credentials in mycat config to match a MySQL account with access from the mycat host
3. Grant privileges: CREATE USER ...@'<mycat-host>'; GRANT ... and FLUSH PRIVILEGES on the MySQL server
4. Verify network reachability and that the MySQL server is accepting connections (mysql -h host -u user -p)

Example fix

// before (schema.xml datasource)
<dataSource ... user="app" password="wrongpass" />
// after
<dataSource ... user="app" password="correctpass" /> <!-- matches MySQL grant for mycat host -->

try (Connection c = DriverManager.getConnection(url, user, password)) { /* validate credentials before registering datasource with mycat */ } catch (SQLException e) { /* fail fast */ }

try { ... } catch (ConnectionException e) { if (e.getErrorCode() == 1045) { fix credentials } else if (e.getErrorCode() == 1040) { retry later / raise max_connections } else { alert ops with errno + errMsg } }

When it happens

Trigger: The backend MySQL server rejects the connection during authentication: wrong user/password in mycat's datasource config, host not allowed, too many connections, or server shutting down.

Common situations: Misconfigured user/password in schema.xml datasources; MySQL grants missing for the mycat host; server-side limits (max_connections, blocked host after failed logins).

Related errors

• can't connect to mysql server ,errmsg
• Unknown Packet!
• global seq and share join and special…
• Unexpected exception
• txIsolation

AI-assisted analysis of MyCATApache/Mycat-Server@65f8d8beb7 (2026-09-11). Data as JSON: /api/errors/88ca7e0f6ea1baff. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/io/mycat/backend/mysql/nio/MySQLConnectionAuthenticator.java:93

				source.setHandler(new MySQLConnectionHandler(source));
				source.setAuthenticated(true);
				boolean clientCompress = Capabilities.CLIENT_COMPRESS==(Capabilities.CLIENT_COMPRESS & packet.serverCapabilities);
				boolean usingCompress= MycatServer.getInstance().getConfig().getSystem().getUseCompression()==1 ;
				if(clientCompress&&usingCompress)
				{
					source.setSupportCompress(true);
				}
				if (listener != null) {
					listener.connectionAcquired(source);
				}
				break;
			case ErrorPacket.FIELD_COUNT:
				ErrorPacket err = new ErrorPacket();
				err.read(data);
				String errMsg = new String(err.message);
				LOGGER.warn("can't connect to mysql server ,errmsg:"+errMsg+" "+source);
				//source.close(errMsg);
				throw new ConnectionException(err.errno, errMsg);

			case EOFPacket.FIELD_COUNT:
				auth323(data[3]);
				break;
			default:
				packet = source.getHandshake();
				if (packet == null) {
					processHandShakePacket(data);
					// 发送认证数据包
					source.authenticate();
					break;
				} else {
					throw new RuntimeException("Unknown Packet!");
				}

			}

		} catch (RuntimeException e) {

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