protected long getUserId(Connection connection) {
      // single user mode
      if (UserAttributes.userAttributes.get() == null) {
        return connection.getServer().getUserId();
      }
      // multi user mode
      Long id = UserAttributes.userAttributes.get().getServerToId().get(connection.getServer());
      if (id == null) {
        return NOT_A_USER_ID;
      }
      return id;
    }
Sends the auth credentials to the server for a particular user. Used in multiple user mode of
authentication.

@param location The ServerLocation instance whose connection instance will be used to perform
       the operation.
@param pool The connection pool to use for this operation.
@return Object unique user-id.
geode-core/src/main/java/org/apache/geode/cache/client/internal/AuthenticateUserOp.java