Version Tokens requirements. By default, the client token list is empty, which matches any server token list. When a client sets its token list to a nonempty value, matching may succeed or fail, depending on the server version token list. To define the version token list for a server, the management application calls the version_tokens_set()  function. (There are also functions for modifying and displaying the token list, described later.) For example, the application might send these statements to a group of three servers: Server 1: mysql>  SELECT version_tokens_set(’emp=read;prod=read’); +------------------------------------------+ | version_tokens_set(’emp=read;prod=read’) | +------------------------------------------+ | 2 version tokens set. | +------------------------------------------+ Server 2: mysql>  SELECT version_tokens_set(’emp=write;prod=read’); +-------------------------------------------+ | version_tokens_set(’emp=write;prod=read’) | +-------------------------------------------+ | 2 version tokens set. | +-------------------------------------------+ Server 3: mysql>  SELECT version_tokens_set(’emp=read;prod=write’); +-------------------------------------------+ | version_tokens_set(’emp=read;prod=write’) | +-------------------------------------------+ | 2 version tokens set. | +-------------------------------------------+ The token list in each case is specified as a semicolon-separated list of  name = value  pairs. The resulting token list values result in these server assingments: •Any server accepts reads for either database. •Only server 2 accepts updates for the  emp  database. •Only server 3 accepts updates for the  prod  database. In addition to assigning each server a version token list, the management application also maintains a cache that reflects the server assignments. Before communicating with the servers, a client application contacts the management application and retrieves information about server assignments. Then the client selects a server based on those assignments. Suppose that a client wants to perform both reads and writes on the  emp  database. Based on the preceding assignments, only server 2 qualifies. The client connects to server 2 and registers its server requirements there by setting its  version_tokens_session  system variable: mysql>  SET @@SESSION.version_tokens_session = ’emp=write’; For subsequent statements sent by the client to server 2, the server compares its own version token list to the client list to check whether they match. If so, statements execute normally: mysql>  UPDATE emp.employee SET salary = salary * 1.1 WHERE id = 4981; Query OK, 1 row affected (0.07 sec) Rows matched: 1 Changed: 1 Warnings: 0 1101