Galera Load Balancer (GLB) is a third-party tool produced by FromDual. Downloads and official documentation is available on their site: http://www.fromdual.com/. Similar to Galera, GLB only runs on Linux.
It consists of a daemon called glbd.
There is no client to manage GLB. To send administrative commands to GLB, for example to add or drop nodes, the generic nc tool can be used. The nc tool can communicate with a TCP daemon and print the reply on the screen, which is basically all we need to manage GLB. Here's the general syntax to send a command to glbd from the command line:
echo "<command>" | nc -q 1 <host_address> <port>
The host_address variable is the hostname or IP address, which glbd is running on, probably 127.0.0.1. The port variable is the port which glbd is listening to. There is no standard port; we have to specify it when we start the daemon.
The syntax to start glbd is as follows:
glbd [OPTIONS] <port> <node_list>
The port variable can be a complete address if the machine has multiple network interfaces: address:port.
The
node_list variable is a space-separated list of Galera nodes. Each node can be specified as address:port:weight. The weight is an important concept for glbd, but it is only used if the daemon was started with the --top or --single option, or if the used policy (described next) takes the node's weight into account. With the
--top option, the nodes with a higher weight will always be used if at least one of them is running. This option has no effect if all the running nodes have the same weight. With the
--single option, only one node with the highest weight is used until it crashes. The node's weight is a useful feature if some servers run on machines with low resources, and should only be used for replication, unless the other nodes crash.
The standard informative options are supported in glbd. The --help and --version options can be used, respectively, to print a help message or the version number and exit the program. The --verbose option can be used to print more information on the screen.
Some of the most important options are:
- The
--daemonoption runsglbdas a daemon. - The
--control <port>option specifies which port will be used to accept administrative commands vianc. - The
--discoveryoption enables the autodiscovery of new nodes when they are added to the cluster. The list of nodes is obtained from running known nodes. - The
--topand--singleoptions instructglbdto take the node's weight into account, as explained previously. - The
--max_con <number>option sets the maximum number of accepted connections to avoid overloading the cluster. Even if this option is not specified, a limit is imposed by the operating system. - The
--threads <number>option specifies the number of threads to be used. By default, only one thread is used. - Normally,
glbdmerges small packets of data into bigger packets to optimize the network usage. The--nodelayoption disables this mechanism.
Unless the --single option is used, we usually want to determine the policy that glbd will use to choose the destination of each SQL statement. The policy can be chosen by specifying the corresponding option. The following are the supported policies:
- The least connected is the default policy, which is used when no other policy is specified. It redirects each connection to the node that has received few connections until now. The node's weight is also kept into account, so heavy nodes will receive more connections than the lighter nodes.
- In the
--roundoption,glbduses a circular list of nodes. When a connection request is received, it is redirected to the current node, and the cursor advances or goes back to the first node in the list. - In the
--randomoption, each connection is redirected to a random node. - In the
--sourceoption, each client is assigned to a different server. All connection requests from the same client will always be redirected to the same node, unless the node crashes.
Here are a couple of examples of the glbd invocations:
glbd --daemon --control 8765 --threads 4 3306 host1:4567:1 host2:4567:1 host3:4567:1
In this example, the daemon will listen to the 3306 port (the standard MariaDB and MySQL port) for client connections, and the 8765 port for administrative commands. We have three hosts with the same weight and the standard policy is used. The glbd variable will use four concurrent threads:
glbd --daemon --single 3306 host1:4567:3 host2:4567:2 host3:4567:1
In this example, glbd runs on the standard MariaDB port but does not listen to any port for the administrative commands, so it will not be possible to modify the list of nodes at runtime. There are three nodes with different heights, but only host1 will be used. If host1 crashes, host2 will be used, and if that node crashes too, glbd will use host3.
In the following example, we will use nc to add a new host:
echo "host4:4567:1" | nc -q 1 127.0.0.1 8765
The daemon is supposed to run on the local machine and listen to the 8765 port for administrative commands. The added node is node4. Its weight is 1.
The following example shows how to eliminate a host from the list:
echo "host2:4567:-1" | nc -q 1 127.0.0.1 8765
In the preceding example, we are setting host2 with a negative weight (-1). Negative weights are used to drop servers from the list, so host2 will not be used anymore.
We can also use nc to get some usage statistics from the daemon, as shown in the following example:
echo "getinfo" | nc -q 1 127.0.0.1 8765 Router: ---------------------------------------------------- Address : weight usage conns 191.52.7.1:4567 : 1.000 0.000 0 191.52.7.2:4567 : 1.000 0.000 0 191.52.7.3:4567 : 1.000 0.000 0 ---------------------------------------------------- Destinations: 3, total connections: 0