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Galera Load Balancer Documentation


glbd is a simple TCP connection balancer made with scalability and performance in mind. It was inspired by pen, but unlike pen its functionality is limited only to balancing generic TCP connections.


  • list of backend servers is configurable in runtime.
  • supports server draining, i.e. does not allocate new connections to server, but does not kill existing ones, waiting for them to end gracefully.
  • can use epoll API provided by Linux version 2.6 and higher for ultimate routing performance.
  • glb is multithreaded, so it can utilize multiple CPU cores. Also, if your OS does not support epoll API, consider using several threads even on a single core machine as it will lessen poll() overhead proportionally and can improve overall performance by a factor of 2 or more.
  • optional watchdog module can monitor destinations and adjust routing table automatically.

libglb is a shared library that provides 0-effort connection balancing to any Linux application that uses standard libc connect() call by overloading that function in runtime. No other program functionality is affected and no program modification or recompilation is needed. See below for details.

Balancing Policies

GLB supports five balancing "policies":

  1. "least connected" - new connection will be directed to the server with least connections (corrected for server "weight"). This policy is default.
  2. "round-robin" - each new connection is routed to the next destination in the list in circular order.
  3. "single" - all connections are routed to a single server with the highest weight available. All routing will stick to that server until it fails or a server with a strictly higher weight is introduced.
  4. "random" - connections are distributed randomly between the servers.
  5. "source tracking" - connections originating from the same address are directed to the same server. For details about this policy see below.

-T|--top option was introduced in GLB 0.9.2. It restricts all balancing policies to a subset of destinations with top weight. E.g. if there are servers configured with weight 1 and 2, all balancing will happen only between servers with weight 2 as long as at least one of them is available.

Maximum concurrent connections

Maximum connections that can be opened via glbd simultaneously depends on the system open files limit and is 493 for a standard limit of 1024. If needed glbd will attempt to increase open files limit as much as allowed by the effective user privileges. For unprivileged user it is normally 4096 which results in max 2029 connections.
On Linux open files limit may be checked with 'ulimit -n' and if necessary increased in /etc/security/limits.conf.

Command line options

See output of the --help option.

Runtime management

Runtime management can be done either through FIFO file, or network socket. By default network socket is not opened; address and port to listen at must be explicitly specified with -c option.

To add/modify/delete backend server: send server specification in the form :[:weight] where weight is an integer to the daemon. Connections are distributed proportionally to the weight. Default weight is 1. Weight of 0 means drain the server.
Negative weight means delete the server completely (all connections to that server are closed immediately). This works both on socket connection and on FIFO file.

To see the stats: send "getstat" command to the daemon. This works only on socket connection since it implies response.

Example (here glbd is listening at

$ echo "" | nc -q 1 4444
$ echo "" | nc -q 1 4444
$ echo "getinfo" | nc -q 1 4444
   Address       : weight   usage  conns :  5.000   0.000    0 :  5.000   0.000    0
Destinations: 2, total connections: 0 

'usage' here is some dimensionless metric of how much destination is staffed with connections (relative to weight). Ranges from 0 (totally unused) to 1.0 (very busy). Router tries to keep 'usage' equal on all destinations.

Address conventions

All network addresses are specified in the form IP|HOSTNAME:PORT:WEIGHT. Depending on the context some parts can be optional, in that case they can be omitted. For example address to listen for client connections can be specified either as HOSTNAME:PORT or just PORT. In the latter case glbd will listen for client connections on all interfaces. Backend servers can be specified as HOSTNAME1,HOSTNAME2,HOSTNAME3. In that case incoming port number will be used for PORT value and 1 will be used for WEIGHT value.

Performance statistics

GLB allows to query raw performance statistics through control socket using 'getstat' command. The client can use these data to obtain useful information, e.g. average number of reads per poll() call.

$ echo "getstat" | nc -q 1 4444
in: 6930 out: 102728 recv: 109658 / 45 send: 109658 / 45 conns: 0 / 4 poll: 45 / 0 / 45 elapsed: 1.03428

Statistics line consists of fields separated by spaces for ease of parsing in scripts. A few description fields are added to assist in human reading. Value fields are all even and go as follows:

  2  - number of bytes received on incoming interface (client requests)
  4  - number of bytes sent from incoming interface (server responses)
  6  - number of bytes passed through recv() call
  8  - number of recv() calls
  10 - number of bytes passed through send() call (should be equal to p.6)
  12 - number of send() calls
  14 - number of created connections
  16 - number of concurrent connections
  18 - number of read-ready file descriptors returned by poll()/epoll_wait()
  20 - number of write-ready file descriptors returned by poll()/epoll_wait()
  22 - number of times poll()/epoll_wait() triggered
  24 - time elapsed since last statistics report (seconds)

All values except for 16 and 24 are totals accumulated since the last report. In order to obtain some variable rate it must be divided by the elapsed time. On 32-bit architectures the values are stored in 4-byte integers and can overflow if too much time elapsed, so the first statistics report in the series may need to be discarded.

Source tracking capability

GLB features simple source tracking capability where connections originating from one address can be routed to the same destination, chosen randomly among available destinations according to their weights. One limitation of such scheme is that when the destination list changes, destination choice for new connections changes as well while established connections naturally stay unchanged. Also when a destination is marked unavailable, all connections that would be routed to it will failover to another, randomly chosen destination. When the original target becomes available, all new connections will be routed back to it.

In other words source tracking should be considered a best effort feature and will work best for short-lived connections.


(NOTE: this is a work in progress and neither functionality nor terminology is final.)

Without the watchdog GLB can check destination availability only via the ability to establish TCP connection to destination. For most use cases TCP connectivity is an insufficient check. E.g. for HTTP server it is important to be able to serve webpages and for DBMS server - to be able to execute queries. Watchdog module implements asynchronus monitoring of destinations via backends made to check particular service availability.

Watchdog is enabled with a -w|--watchdog option. Its parameter is a string containing the backend ID string, optionally followed by a colon and backend configuration options. Example:

$ glbd -w exec:" -utest -ptestpass" -t 2 3306

This initializes "exec" backend that executes external programs. In this case it will execute script (can be found in the files/ directory, must be placed in PATH) with provided parameters to check MySQL servers at192.168.0.1 and (in parallel threads). To pass server address to a script, 'exec' backend will insert host:port string as the first argument after a script/command name, so the actual command that would be executed in this case looks like:

" -utest -ptestpass"

Check interval is set with -i|--interval parameter (fractional seconds, 1.0).

Destination discovery

If destinations can supply information about other members of the cluster it can be used to automatically populate watchdog destination list if -D|--discover option is supplied. Currently only MySQL/Galera nodes are known to do this, so it is a Galera-only option.

Running glbd as a "service"

See README in the files/ directory.

Using libglb

Using libglb requres 2 environment variables to be set:

GLB_OPTIONS='options string'

Allows to specify GLB options to libglb the same way as command line parameters for glbd. It is limited however in that ptions and their values cannot contain whitespaces and commas and short options cannot be concatenated (i.e. '-qri10' should go as '-q -r -i 10'). Parsing errors and options unsupported by libglb (like -d) will be silently ignored. In particular, watchdog option cannot be specified this way.
(See GLB_WATCHDOG environment variable below)


$ LD_PRELOAD=src/.libs/ \
GLB_OPTIONS="--random 3306" \
mysql -uroot -prootpass -h127.0.0.1 -P3306

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 76
Server version: 5.5.28 Source distribution, wsrep_24dev.7.r3830

Additional libglb parameters:

In case GLB_OPTIONS is not sufficient (e.g. watchdog option needs to be specified) below there is a list of additional environment variables (which take precedence over GLB_OPTIONS). Note however, that all these options except GLB_WATCHDOG are considered deprecated at the moment.


Interpreted the same way as --watchdog parameter.


Is a comma-separated list of target servers among which the client connections must be distributed.


Where is interpreted the same way as LISTEN_ADDR parameter for glbd. Whenever application attempts to initiate connection to this address, the request will be intercepted and connection established to one of the servers from GLB_TARGETS list according to balancing rules.


Default libglb balancing policy is "round-robin", "single", "random" and"source tracking" policies can be specified with GLB_POLICY variable.

(The meaning of GLB_POLICY=source in this case is that all connections from this client will be routed to the same random destination, and failover to another if the primary destination fails. Thus client-server affinity is achieved, however load from many clients will be spread over all available destinations.)


Interpreted the same way as --control parameter of glbd. Application will open a socket at a specified address to listen to control commands. Due to library functionality limited only to connect() call, no traffic statistics or connection count is maintained, so "getstat" command is a noop and "getinfo" only prints out a routing table.

Source: glb-1.0.0/README, Mailing list, Copyright (C) 2007-2013 Codership Oy