数据缓存、如何保证实时性又能减轻数据库的压力 [ 2.0 版本 ]

为了保证数据的实时性，可以做缓存依赖。

这个可以列个专题讨论

缓存依赖 $dependency

    /**
     * Stores a value identified by a key into cache.
     * If the cache already contains such a key, the existing value and
     * expiration time will be replaced with the new ones, respectively.
     *
     * @param mixed $key a key identifying the value to be cached. This can be a simple string or
     * a complex data structure consisting of factors representing the key.
     * @param mixed $value the value to be cached
     * @param integer $duration the number of seconds in which the cached value will expire. 0 means never expire.
     * @param Dependency $dependency dependency of the cached item. If the dependency changes,
     * the corresponding value in the cache will be invalidated when it is fetched via [[get()]].
     * This parameter is ignored if [[serializer]] is false.
     * @return boolean whether the value is successfully stored into cache
     */
    public function set($key, $value, $duration = 0, $dependency = null)
    {
        if ($dependency !== null && $this->serializer !== false) {
            $dependency->evaluateDependency($this);
        }
        if ($this->serializer === null) {
            $value = serialize([$value, $dependency]);
        } elseif ($this->serializer !== false) {
            $value = call_user_func($this->serializer[0], [$value, $dependency]);
        }
        $key = $this->buildKey($key);

        return $this->setValue($key, $value, $duration);
    }