MariaDB gcomm Backend Connection Failed 110 Explained: Troubleshooting Galera Cluster Issues

• MariaDB: A relational database management system similar to MySQL.
• Galera: A clustering technology for MariaDB that enables data replication across multiple servers, creating a highly available database.
• gcomm: The communication layer used by Galera for cluster communication between nodes.

Error Breakdown:

The error indicates an issue with establishing a connection between Galera nodes using the gcomm communication layer. The error code "110" typically signifies a connection timeout.

Possible Causes:

• Network Issues: Firewalls blocking communication ports, network congestion, or incorrect network configuration on any Galera node can prevent connections.
• Node Status: A Galera node might be down or unreachable by other nodes in the cluster.
• Galera Configuration: Incorrect configuration of the wsrep_cluster_address variable in the MariaDB configuration file (my.cnf) on any node can lead to connection failures.

Troubleshooting Steps:

1. Check Network Connectivity: Ensure firewalls allow communication on ports used by Galera (typically 4504 and 4504 UDP). Verify network connectivity between nodes using tools like ping.
2. Verify Node Status: Check if all Galera nodes are running using a service manager like systemctl.
3. Review Galera Configuration: Ensure the wsrep_cluster_address variable in each node's my.cnf file lists the correct IP addresses or hostnames of all Galera nodes in the cluster.

systemctl status mariadb

This command (assuming systemd is your init system) checks the status of the MariaDB service and indicates if it's running or not.

Reviewing Galera Configuration (my.cnf):

Here's an example snippet from the my.cnf file showing the wsrep_cluster_address variable:

[galera]
# Mandatory settings
wsrep_on=ON
wsrep_provider=/usr/lib/galera/libgalera_smm.so
wsrep_cluster_address="gcomm://192.168.1.10,192.168.1.20"
binlog_format=row
default_storage_engine=InnoDB
innodb_autoinc_lock_mode=2

This configuration defines:

• wsrep_on=ON: Enables Galera functionality.
• wsrep_provider: Path to the Galera library.
• wsrep_cluster_address: Lists IP addresses of all Galera nodes separated by commas.

Initializing a New Cluster (if applicable):

This command (only needed for the first node) initializes a new Galera cluster on systems using systemd:

galera_new_cluster

Instead of manually pinging each node, you can leverage system monitoring tools. These tools can provide a more comprehensive view of network connectivity and potential bottlenecks. Some options include:

• nmap: A free and open-source network scanner that can identify open ports and potential firewall issues.
• iftop: Monitors network traffic in real-time, helping to identify congestion or unusual activity.

Analyzing MariaDB logs:

The MariaDB error logs might provide more specific details about the connection failure. These logs are typically located in /var/log/mysql/error.log. You can use tools like grep to search for relevant entries containing keywords like "gcomm" or "connection timeout."

Checking Firewall Rules:

Firewalls can sometimes block communication on ports used by Galera. You can use temporary firewall rule adjustments to test if this is the cause. Remember to re-enable the original rules after troubleshooting.

Forced Initialization (Risky):

As a last resort, on some systems (not recommended for production environments), you might attempt a forced initialization of a Galera node. This can overwrite existing configurations, so proceed with caution. Consult your MariaDB documentation for specific steps on forced initialization for your version.