currently only redis cache store is supported
function cache(app) {
  if (app.locals.config.cacheDb && 'redis' === app.locals.config.cacheDb.connector) {
    // use redis for cache
    let redis = require("redis").createClient(app.locals.config.cacheDb);
    redis.on("error", function (err) {
      app.error(err);
    });
    // clear existing cache on startup
    redis.del('zlSites', 'themeTrees', 'themesConfig');

    return {
      // for getting data
      get: function () {
        if (3 == arguments.length) {
          // hash request
          redis.hget(arguments[0], arguments[1], arguments[2]);
        }
        else if (2 == arguments.length) {
          // key request
          redis.get(arguments[0], arguments[1]);
        }
      },
      // for setting data
      set: function () {
        if (4 == arguments.length) {
          // hash request
          redis.hset(arguments[0], arguments[1], arguments[2], arguments[3]);
        }
        else if (3 == arguments.length) {
          // key request
          redis.set(arguments[0], arguments[1], arguments[2]);
        }
      },
      // for setting volatile data with an expiration date
      setvol: function () {
        if (4 == arguments.length) {
          // key request
          redis.set(arguments[0], arguments[1], () => {
            redis.expire(arguments[0], arguments[2], arguments[3]);
          });
        }
      },
      // for deleting data
      del: function () {
        if (3 == arguments.length) {
          // hash request
          redis.hdel(arguments[0], arguments[1], arguments[2]);
        }
        else if (2 == arguments.length) {
          // key request
          redis.del(arguments[0], arguments[1]);
        }
      }
    }
  }
  else {
    // not using cache
    app.error('No config.cacheDb provided. This may affect performance.');
    let e = function() { arguments[arguments.length-1](); }
    return { get: e, set: e, setvol: e, del: e }
  }
}