關於 ASP.NET Core IMemoryCache RegisterPostEvictionCallback 的觸發時機

同事提到想用 ASP.NET Core 的 IMemoryCache 來處理 application 本身的 cache，無奈小弟學藝不精沒有太多想法可以參與討論，所以趕緊惡補，藉這個機會學習也順便做些 POC，大致上與過去使用 System.Runtime.Caching 的 MemoryCache 用法概念接近，但實際用法則不全然相同

基本比較

基本環境說明

1. macOS Mojave 10.14.4
2. .NET Core SDK 2.2.107
3. NuGet package
   • NLog 4.6.3
   • NLog.Web.AspNetCore 4.8.2
4. ASP.NET Core MVC 預設專案範本

前置準備

1. 註冊 IMemoryCache

   • Startup.cs 中的 ConfigureServices 方法中加入

     services.AddMemoryCache();
     

   • 準備 RegisterPostEvictionCallback

     private void CacheExpireHandler(object key, object value, EvictionReason reason, object state)
     {
         _logger.LogDebug($"Cache Expire ; key :{key};valu:{value};reason:{reason};state:{state}");
         setCache();
     }
     

2. 註冊 NLog

   • 加入 nlog.config

     <?xml version="1.0" encoding="utf-8" ?>
     <nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         autoReload="true"
         internalLogLevel="Info"
         internalLogFile="internal-nlog.txt">
     
         <!-- enable asp.net core layout renderers -->
         <extensions>
             <add assembly="NLog.Web.AspNetCore"/>
         </extensions>
     
         <!-- the targets to write to -->
         <targets>
             <!-- write logs to file  -->
             <target xsi:type="File" name="allfile" fileName="./logs/nlog-all-${shortdate}.log"
                     layout="${longdate}|${event-properties:item=EventId_Id}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}" />
     
             <!-- another file log, only own logs. Uses some ASP.NET core renderers -->
             <target xsi:type="File" name="ownFile-web" fileName="./logs/nlog-own-${shortdate}.log"
                     layout="${longdate}|${event-properties:item=EventId_Id}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}|url: ${aspnet-request-url}|action: ${aspnet-mvc-action}" />
         </targets>
     
         <!-- rules to map from logger name to target -->
         <rules>
             <!--All logs, including from Microsoft-->
             <logger name="*" minlevel="Trace" writeTo="allfile" />
     
             <!--Skip non-critical Microsoft logs and so log only own logs-->
             <logger name="Microsoft.*" maxlevel="Info" final="true" /> <!-- BlackHole without writeTo -->
             <logger name="*" minlevel="Trace" writeTo="ownFile-web" />
         </rules>
     </nlog>
     

   • Program.cs 的 CreateWebHostBuilder 加入 .UseNLog();

     public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
         WebHost.CreateDefaultBuilder(args)
                 .UseStartup<Startup>()
                 .UseNLog();
     }
     

3. 使用 IMemoryCache 與 NLog

   private static IMemoryCache _cache;
   private ILogger<HomeController> _logger;
   
   public HomeController(IMemoryCache cache,ILogger<HomeController> logger)
   {
       _cache = cache;
       _logger = logger;
   }
   

觸發時機

理想狀況就是在 cache expire 當下就觸發 RegisterPostEvictionCallback，即可立馬重新 recache，但實際情況不如預期

1. 確認觸發時機

   public class HomeController : Controller
   {
       private static IMemoryCache _cache;
       private ILogger<HomeController> _logger;
   
       public HomeController(IMemoryCache cache,ILogger<HomeController> logger)
       {
           _cache = cache;
           _logger = logger;
       }
   
   
       public void SetCache()
       {
           _logger.LogDebug($"before SetCache:{JsonConvert.SerializeObject(_cache)}");
           _logger.LogDebug("SetCache Action");
           setCache();
           _logger.LogDebug($"after SetCache:{JsonConvert.SerializeObject(_cache)}");
   
       }
   
       public IActionResult Index()
       {
           _logger.LogDebug($"GetCache @ {DateTime.Now}");
           _logger.LogDebug($"before getCache:{JsonConvert.SerializeObject(_cache)}");
   
   
           var cacheData = _cache.Get<string>("dateTimeNow");
           _logger.LogDebug($"after getCache:{JsonConvert.SerializeObject(_cache)}");
   
           return View((object) cacheData);
       }
   
       public IActionResult Privacy()
       {
           return View();
       }
           
       private void setCache()
       {
           MemoryCacheEntryOptions cacheExpirationOptions = new MemoryCacheEntryOptions();
           var datTimeNow = DateTime.Now;
           _logger.LogDebug($"SetCache @ {datTimeNow}");
           cacheExpirationOptions.AbsoluteExpiration = datTimeNow.AddSeconds(10);
           cacheExpirationOptions.Priority = CacheItemPriority.High;
           cacheExpirationOptions.RegisterPostEvictionCallback(CacheExpireHandler, this);
           _cache.Set<string>("dateTimeNow", datTimeNow.ToString(), cacheExpirationOptions);
       }
   
       private void CacheExpireHandler(object key, object value, EvictionReason reason, object state)
       {
           _logger.LogDebug($"Cache Expire ; key :{key};valu:{value};reason:{reason};state:{state}");
           setCache();
       }
   
       [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
       public IActionResult Error()
       {
           return View(new ErrorViewModel {RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier});
       }
   }
   

2. 實際結果

   

心得

以測試結果來看， cache 的 expire 機制是被動式的：cache 在無人存取時不會主動刪除，在存取時當下檢查 expire 與否，確定 expire 會連帶刪除 cache 內容接著觸發 RegisterPostEvictionCallback

而觸發 RegisterPostEvictionCallback 的 cache 當次存取除了會造成刪除 cache 內容還會直接回傳 cache miss 無法取得預期中的 cache 內容，因此光憑 RegisterPostEvictionCallback 是沒辦法達成絕對的定期 recache

參考資訊

1. MemoryCache Class
2. MemoryCache Class
3. 快取在記憶體中的 ASP.NET Core
4. Getting started with ASP.NET Core 2