问题描述
我正在开发ASP.NET Core(ASP.NET 5)Web API应用程序，并且必须借助实体标签来实现HTTP缓存.早些时候，我使用CacheCow来实现相同功能，但是到目前为止，它似乎还不支持ASP.NET Core.我也没有找到其他相关的库或框架支持详细信息.
我可以为它编写自定义代码，但是在此之前，我想看看是否有可用的代码.请分享是否已有可用的东西，以及实现它的更好方法是什么.
尝试使用中间件一段时间后，我发现 MVC操作过滤器实际上更适合此功能.
public class ETagFilter : Attribute, IActionFilter
{
private readonly int[] _statusCodes;
public ETagFilter(params int[] statusCodes)
{
_statusCodes = statusCodes;
if (statusCodes.Length == 0) _statusCodes = new[] { 200 };
}
public void OnActionExecuting(ActionExecutingContext context)
{
}
public void OnActionExecuted(ActionExecutedContext context)
{
if (context.HttpContext.Request.Method == "GET")
{
if (_statusCodes.Contains(context.HttpContext.Response.StatusCode))
{
//I just serialize the result to JSON, could do something less costly
var content = JsonConvert.SerializeObject(context.Result);
var etag = ETagGenerator.GetETag(context.HttpContext.Request.Path.ToString(), Encoding.UTF8.GetBytes(content));
if (context.HttpContext.Request.Headers.Keys.Contains("If-None-Match") && context.HttpContext.Request.Headers["If-None-Match"].ToString() == etag)
{
context.Result = new StatusCodeResult(304);
}
context.HttpContext.Response.Headers.Add("ETag", new[] { etag });
}
}
}
}
// Helper class that generates the etag from a key (route) and content (response)
public static class ETagGenerator
{
public static string GetETag(string key, byte[] contentBytes)
{
var keyBytes = Encoding.UTF8.GetBytes(key);
var combinedBytes = Combine(keyBytes, contentBytes);
return GenerateETag(combinedBytes);
}
private static string GenerateETag(byte[] data)
{
using (var md5 = MD5.Create())
{
var hash = md5.ComputeHash(data);
string hex = BitConverter.ToString(hash);
return hex.Replace("-", "");
}
}
private static byte[] Combine(byte[] a, byte[] b)
{
byte[] c = new byte[a.Length + b.Length];
Buffer.BlockCopy(a, 0, c, 0, a.Length);
Buffer.BlockCopy(b, 0, c, a.Length, b.Length);
return c;
}
}
然后将其用作属性所需的操作或控制器:
[HttpGet("data")]
[ETagFilter(200)]
public async Task<IActionResult> GetDataFromApi()
{
}
中间件和过滤器之间的重要区别是，中间件可以在MVC中间件之前和之后运行，并且只能与HttpContext一起使用.而且，一旦MVC开始将响应发送回客户端，就为时已晚，无法对其进行任何更改.
另一方面，过滤器是MVC中间件的一部分.他们可以访问MVC上下文，在这种情况下，可以更轻松地实现此功能. 有关过滤器的更多信息及其在MVC中的管道.I am working on ASP.NET Core (ASP.NET 5) Web API application and have to implement HTTP Caching with the help of Entity Tags. Earlier I used CacheCow for the same but it seems it does not support ASP.NET Core as of now. I also didn't find any other relevant library or framework support details for the same.
I can write custom code for the same but before that I want to see if anything is already available. Kindly share if something is already available and what is the better way to implement that.
After a while trying to make it work with middleware I figured out that MVC action filters are actually better suited for this functionality.
public class ETagFilter : Attribute, IActionFilter
{
private readonly int[] _statusCodes;
public ETagFilter(params int[] statusCodes)
{
_statusCodes = statusCodes;
if (statusCodes.Length == 0) _statusCodes = new[] { 200 };
}
public void OnActionExecuting(ActionExecutingContext context)
{
}
public void OnActionExecuted(ActionExecutedContext context)
{
if (context.HttpContext.Request.Method == "GET")
{
if (_statusCodes.Contains(context.HttpContext.Response.StatusCode))
{
//I just serialize the result to JSON, could do something less costly
var content = JsonConvert.SerializeObject(context.Result);
var etag = ETagGenerator.GetETag(context.HttpContext.Request.Path.ToString(), Encoding.UTF8.GetBytes(content));
if (context.HttpContext.Request.Headers.Keys.Contains("If-None-Match") && context.HttpContext.Request.Headers["If-None-Match"].ToString() == etag)
{
context.Result = new StatusCodeResult(304);
}
context.HttpContext.Response.Headers.Add("ETag", new[] { etag });
}
}
}
}
// Helper class that generates the etag from a key (route) and content (response)
public static class ETagGenerator
{
public static string GetETag(string key, byte[] contentBytes)
{
var keyBytes = Encoding.UTF8.GetBytes(key);
var combinedBytes = Combine(keyBytes, contentBytes);
return GenerateETag(combinedBytes);
}
private static string GenerateETag(byte[] data)
{
using (var md5 = MD5.Create())
{
var hash = md5.ComputeHash(data);
string hex = BitConverter.ToString(hash);
return hex.Replace("-", "");
}
}
private static byte[] Combine(byte[] a, byte[] b)
{
byte[] c = new byte[a.Length + b.Length];
Buffer.BlockCopy(a, 0, c, 0, a.Length);
Buffer.BlockCopy(b, 0, c, a.Length, b.Length);
return c;
}
}
And then use it on the actions or controllers you want as an attribute:
[HttpGet("data")]
[ETagFilter(200)]
public async Task<IActionResult> GetDataFromApi()
{
}
The important distinction between Middleware and Filters is that your middleware can run before and after MVC middlware and can only work with HttpContext. Also once MVC starts sending the response back to the client it's too late to make any changes to it.
Filters on the other hand are a part of MVC middleware. They have access to the MVC context, with which in this case it's simpler to implement this functionality. More on Filters and their pipeline in MVC.
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