1. 缓存是什么，各级缓存

2. 本地缓存原理和实现

3. 缓存应用和缓存更新

缓存：为了快速获取结果，在第一次获取数据后存起来，下次直接使用

缓存一般用在哪里？

1. 会重复的请求

2. 数据相对稳定

3. 耗时/耗资源

4. 体积不大

配置文件；菜单-权限；省市区；类别数据；

热搜；公告；技能/属性；数据字典；

分页(只要数据不是经常变)

如果一个数据缓存一次，能够被有效查询4次，那这个缓存就是值得的(大型系统的时候，为了性能，为了压力，需要更多的缓存)

缓存本身是共享的，所以每个缓存应该是唯一的

本地缓存空间小，不能跨进程共享

小项目一般随便缓存

中大型项目不够用的，一般就会使用分布式缓存

如：

Memcached:内存管理

Redis: REmote DIctionary Server

下面附上我学习使用的自建简单Cahce缓存代码：

public class CustomeCache{/// <summary>/// 线程安全的Dictionary集合/// </summary>public static Dictionary<string, KeyValuePair<object, DateTime>> CustomCacheDictionary;/// <summary>/// 缓存需要自动清理:防止过多无用缓存/// </summary>static CustomeCache(){CustomCacheDictionary = new Dictionary<string, KeyValuePair<object, DateTime>>();Console.WriteLine($"{DateTime.Now.ToString("MM-dd HH:mm:ss fff")}初始化缓存");Task.Run(() =>{while (true){LockAction(new Action(() => {List<string> list = new List<string>();foreach (var key in CustomCacheDictionary.Keys){var valueTime = CustomCacheDictionary[key];if (valueTime.Value > DateTime.Now){//没有过期就不用管}else {//过期就统一将key放入list中list.Add(key);}}list.ForEach(key => CustomCacheDictionary.Remove(key));}));Thread.Sleep(1000 * 60 * 15);//每15分钟清理一次}});}private static readonly object CustomCache_Lock = new object();public static void LockAction(Action action){lock (CustomCache_Lock){action.Invoke();}}/// <summary>/// 添加缓存数据/// </summary>/// <param name="key"></param>/// <param name="value"></param>/// <param name="second"></param>public static void Add(string key, object value, int second = 1800) {LockAction(new Action(() =>{CustomCacheDictionary.Add(key, new KeyValuePair<object, DateTime>(value, DateTime.Now.AddSeconds(second)));}));}/// <summary>/// 保存数据  -存在则更新 不存在则新增/// </summary>/// <param name="key"></param>/// <param name="value"></param>/// <param name="second"></param>public static void SaveOrUpdate(string key, object value, int second = 1800) {LockAction(new Action(() =>{CustomCacheDictionary[key] = new KeyValuePair<object, DateTime>(value, DateTime.Now.AddSeconds(second));}));}public static T Find<T>(string key, Func<T> func, int second = 1800) {T t = default(T);if (!Exsit(key)){t = func.Invoke();CustomeCache.Add(key, t, second);}else {t = Get<T>(key);}return t;}/// <summary>/// 获取数据 没有则异常/// </summary>/// <typeparam name="T"></typeparam>/// <param name="key"></param>/// <returns></returns>public static T Get<T>(string key) {return (T)CustomCacheDictionary[key].Key;}/// <summary>/// 检查是否存在/// </summary>/// <param name="key"></param>/// <returns></returns>public static bool Exsit(string key){if (CustomCacheDictionary.ContainsKey(key)){var valueTime = CustomCacheDictionary[key];if (valueTime.Value > DateTime.Now){   //时候未到return true;}else {//时晨已到  上台LockAction(new Action(() =>{CustomCacheDictionary.Remove(key);}));return false;}}else {return false;}}public static void Remove(string key) {LockAction(new Action(() =>{CustomCacheDictionary.Remove(key);}));}public static void RemoveAll() {LockAction(new Action(() =>{CustomCacheDictionary.Clear();}));}public static void RemoveCondition(Func<string, bool> func) {LockAction(new Action(() =>{List<string> list = new List<string>();foreach (var key in CustomCacheDictionary.Keys){if (func.Invoke(key)){list.Add(key);}}list.ForEach(key => CustomCacheDictionary.Remove(key));}));}}