Table of Contents

Prototype: selectservers(hostlist, port, query, regex, maxbytes, array)

Return type: int

Description: Returns the number of tcp servers from hostlist which respond with a reply matching regex to a query send to port, and populates array with their names.

The regular expression is anchored. If query is empty, then no reply checking is performed (any server reply is deemed to be satisfactory), otherwise at most maxbytes bytes are read from the server and matched.

This function allows discovery of all the TCP ports that are active and functioning from an ordered list, and builds an array of their names. This allows maintaining a list of pretested failover alternatives.


  • hostlist: string, in the range: @[(][a-zA-Z0-9_$(){}\[\].:]+[)]
  • port: string, in the range: .*
  • query: string, in the range: .*
  • regex: regular expression, in the range: .*
  • maxbyes: int, in the range: 0,99999999999
  • array: string, in the range: [a-zA-Z0-9_$(){}\[\].:]+


    bundle agent example

     "hosts" slist => { "", "", "" };
     "fhosts" slist => { "", "" };

     "up_servers" int =>  selectservers("@(hosts)","80","","","100","alive_servers");
     "has_favicon" int =>
                "@(hosts)", "80",
            "GET /favicon.ico HTTP/1.0$(const.n)Host:$(const.n)$(const.n)",
            "200", "favicon_servers");


      "someone_alive" expression => isgreaterthan("$(up_servers)","0");

      "has_favicon" expression => isgreaterthan("$(has_favicon)","0");

        "Number of active servers $(up_servers)";

        "First server $(alive_servers[0]) fails over to $(alive_servers[1])";

        "At least $(favicon_servers[0]) has a favicon.ico";


If there is a multi-line response from the server, special care must be taken to ensure that newlines are matched, too. Note the use of (?s) in the example, which allows . to also match newlines in the multi-line HTTP response.