gistfile1.txt

      source      The source IP address is hashed and divided by the total
                  weight of the running servers to designate which server will
                  receive the request. This ensures that the same client IP
                  address will always reach the same server as long as no
                  server goes down or up. If the hash result changes due to the
                  number of running servers changing, many clients will be
                  directed to a different server. This algorithm is generally
                  used in TCP mode where no cookie may be inserted. It may also
                  be used on the Internet to provide a best-effort stickiness
                  to clients which refuse session cookies. This algorithm is
                  static by default, which means that changing a server's
                  weight on the fly will have no effect, but this can be
                  changed using "hash-type".

      uri         This algorithm hashes either the left part of the URI (before
                  the question mark) or the whole URI (if the "whole" parameter
                  is present) and divides the hash value by the total weight of
                  the running servers. The result designates which server will
                  receive the request. This ensures that the same URI will
                  always be directed to the same server as long as no server
                  goes up or down. This is used with proxy caches and
                  anti-virus proxies in order to maximize the cache hit rate.
                  Note that this algorithm may only be used in an HTTP backend.
                  This algorithm is static by default, which means that
                  changing a server's weight on the fly will have no effect,
                  but this can be changed using "hash-type".

                  This algorithm supports two optional parameters "len" and
                  "depth", both followed by a positive integer number. These
                  options may be helpful when it is needed to balance servers
                  based on the beginning of the URI only. The "len" parameter
                  indicates that the algorithm should only consider that many
                  characters at the beginning of the URI to compute the hash.
                  Note that having "len" set to 1 rarely makes sense since most
                  URIs start with a leading "/".

                  The "depth" parameter indicates the maximum directory depth
                  to be used to compute the hash. One level is counted for each
                  slash in the request. If both parameters are specified, the
                  evaluation stops when either is reached.

      url_param   The URL parameter specified in argument will be looked up in
                  the query string of each HTTP GET request.

                  If the modifier "check_post" is used, then an HTTP POST
                  request entity will be searched for the parameter argument,
                  when it is not found in a query string after a question mark
                  ('?') in the URL. Optionally, specify a number of octets to
                  wait for before attempting to search the message body. If the
                  entity can not be searched, then round robin is used for each
                  request. For instance, if your clients always send the LB
                  parameter in the first 128 bytes, then specify that. The
                  default is 48. The entity data will not be scanned until the
                  required number of octets have arrived at the gateway, this
                  is the minimum of: (default/max_wait, Content-Length or first
                  chunk length). If Content-Length is missing or zero, it does
                  not need to wait for more data than the client promised to
                  send. When Content-Length is present and larger than
                  <max_wait>, then waiting is limited to <max_wait> and it is
                  assumed that this will be enough data to search for the
                  presence of the parameter. In the unlikely event that
                  Transfer-Encoding: chunked is used, only the first chunk is
                  scanned. Parameter values separated by a chunk boundary, may
                  be randomly balanced if at all.

                  If the parameter is found followed by an equal sign ('=') and
                  a value, then the value is hashed and divided by the total
                  weight of the running servers. The result designates which
                  server will receive the request.

                  This is used to track user identifiers in requests and ensure
                  that a same user ID will always be sent to the same server as
                  long as no server goes up or down. If no value is found or if
                  the parameter is not found, then a round robin algorithm is
                  applied. Note that this algorithm may only be used in an HTTP
                  backend. This algorithm is static by default, which means
                  that changing a server's weight on the fly will have no
                  effect, but this can be changed using "hash-type".

      hdr(<name>) The HTTP header <name> will be looked up in each HTTP request.
                  Just as with the equivalent ACL 'hdr()' function, the header
                  name in parenthesis is not case sensitive. If the header is
                  absent or if it does not contain any value, the roundrobin
                  algorithm is applied instead.

                  An optional 'use_domain_only' parameter is available, for
                  reducing the hash algorithm to the main domain part with some
                  specific headers such as 'Host'. For instance, in the Host
                  value "haproxy.1wt.eu", only "1wt" will be considered.

                  This algorithm is static by default, which means that
                  changing a server's weight on the fly will have no effect,
                  but this can be changed using "hash-type".