code: private Set<String> getToleratedAuthenticationProviders() {
  
          // Parse list of auth providers whose internal failures should be
          // tolerated
          try {
              return environment.getProperty(SKIP_IF_UNAVAILABLE, Collections.<String>emptySet());
          }
  
          // Use empty set by default if property cannot be parsed
          catch (GuacamoleException e) {
              logger.warn("The list of authentication providers specified via the \"{}\" property could not be parsed: {}", SKIP_IF_UNAVAILABLE.getName(), e.getMessage());
              logger.debug("Unable to parse \"{}\" property.", SKIP_IF_UNAVAILABLE.getName(), e);
              return Collections.<String>emptySet();
          }
  
      }
docs: Returns the set of identifiers of all authentication providers whose
  internal failures should be tolerated during the authentication process.
  If the identifier of an authentication provider is within this set,
  errors during authentication will result in the authentication provider
  being ignored for that authentication attempt, with the authentication
  process proceeding as if that authentication provider were not present.
  By default, errors during authentication halt the authentication process
  entirely.
  
  @return
  The set of identifiers of all authentication providers whose
  internal failures should be tolerated during the authentication
  process.