在Ocelot中使用自定义的中间件（二）

李明成

发布于 2020-03-19 09:44:53

8690

发布于 2020-03-19 09:44:53

在上文中《在Ocelot中使用自定义的中间件（一）》，我介绍了如何在Ocelot中使用自定义的中间件来修改下游服务的response body。今天，我们再扩展一下设计，让我们自己设计的中间件变得更为通用，使其能够应用在不同的Route上。比如，我们可以设计一个通用的替换response body的中间件，然后将其应用在多个Route上。

Ocelot的配置文件

我们可以将Ocelot的配置信息写在appsettings.json中，当然也可以将其放在单独的json文件里，然后通过ConfigureAppConfiguration的调用，将单独的json文件添加到配置系统中。无论如何，基于JSON文件的Ocelot配置都是可以加入我们自定义的内容的，基于数据库的或者其它存储的配置文件信息或许扩展起来并不方便，因此，使用JSON文件作为配置源还是一个不错的选择。比如，我们可以在ReRoute的某个配置中添加以下内容：

{
  "DownstreamPathTemplate": "/api/themes",
  "DownstreamScheme": "http",
  "DownstreamHostAndPorts": [
    {
      "Host": "localhost",
      "Port": 5010
    }
  ],
  "UpstreamPathTemplate": "/themes-api/themes",
  "UpstreamHttpMethod": [ "Get" ],
  "CustomMiddlewares": [
    {
      "Name": "themeCssMinUrlReplacer",
      "Enabled": true,
      "Config": {
        "replacementTemplate": "/themes-api/theme-css/{name}"
      }
    }
  ]
}

然后就需要有一个方法能够解析这部分配置内容。为了方便处理，可以增加以下配置Model，专门存放CustomMiddlewares下的配置信息：

public class CustomMiddlewareConfiguration
{
    public string DownstreamPathTemplate { get; set; }
    public string UpstreamPathTemplate { get; set; }
    public int ReRouteConfigurationIndex { get; set; }
    public string Name { get; set; }
    public bool Enabled { get; set; }
    public Dictionary<string, object> Config { get; set; }
}

然后定义下面的扩展方法，用以从IConfiguration对象中解析出所有的CustomMiddleware的配置信息：


public static IEnumerable<CustomMiddlewareConfiguration> GetCustomMiddlewareConfigurations(this IConfiguration config)

{

    var reRoutesConfigSection = config.GetSection("ReRoutes");

    if (reRoutesConfigSection.Exists())

    {

        var reRoutesConfigList = reRoutesConfigSection.GetChildren();

        for (var idx = 0; idx < reRoutesConfigList.Count(); idx++)

        {

            var reRouteConfigSection = reRoutesConfigList.ElementAt(idx);

            var upstreamPathTemplate = reRouteConfigSection.GetSection("UpstreamPathTemplate").Value;

            var downstreamPathTemplate = reRouteConfigSection.GetSection("DownstreamPathTemplate").Value;

            var customMidwareConfigSection = reRouteConfigSection.GetSection("CustomMiddlewares");

            if (customMidwareConfigSection.Exists())

            {

                var customMidwareConfigList = customMidwareConfigSection.GetChildren();

                foreach (var customMidwareConfig in customMidwareConfigList)

                {

                    var customMiddlewareConfiguration = customMidwareConfig.Get<CustomMiddlewareConfiguration>();

                    customMiddlewareConfiguration.UpstreamPathTemplate = upstreamPathTemplate;

                    customMiddlewareConfiguration.DownstreamPathTemplate = downstreamPathTemplate;

                    customMiddlewareConfiguration.ReRouteConfigurationIndex = idx;

                    yield return customMiddlewareConfiguration;

                }

            }

        }

    }

 

    yield break;

}

CustomMiddleware基类

为了提高程序员的开发体验，我们引入CustomMiddleware基类，在Invoke方法中，CustomMiddleware对象会读取所有的CustomMiddleware配置信息，并找到属于当前ReRoute的CustomMiddleware配置信息，从而决定当前的CustomMiddleware是否应该被执行。相关代码如下：


public abstract class CustomMiddleware : OcelotMiddleware

{

    #region Private Fields

 

    private readonly ICustomMiddlewareConfigurationManager customMiddlewareConfigurationManager;

    private readonly OcelotRequestDelegate next;

 

    #endregion Private Fields

 

    #region Protected Constructors

 

    protected CustomMiddleware(OcelotRequestDelegate next,

        ICustomMiddlewareConfigurationManager customMiddlewareConfigurationManager,

        IOcelotLogger logger) : base(logger)

    {

        this.next = next;

        this.customMiddlewareConfigurationManager = customMiddlewareConfigurationManager;

    }

 

    #endregion Protected Constructors

 

    #region Public Methods

 

    public async Task Invoke(DownstreamContext context)

    {

        var customMiddlewareConfigurations = from cmc in this

                                                .customMiddlewareConfigurationManager

                                                .GetCustomMiddlewareConfigurations()

                                             where cmc.DownstreamPathTemplate == context

                                                    .DownstreamReRoute

                                                    .DownstreamPathTemplate

                                                    .Value &&

                                                   cmc.UpstreamPathTemplate == context

                                                    .DownstreamReRoute

                                                    .UpstreamPathTemplate

                                                    .OriginalValue

                                             select cmc;

 

        var thisMiddlewareName = this.GetType().GetCustomAttribute<CustomMiddlewareAttribute>(false)?.Name;

        var customMiddlewareConfiguration = customMiddlewareConfigurations.FirstOrDefault(x => x.Name == thisMiddlewareName);

        if (customMiddlewareConfiguration?.Enabled ?? false)

        {

            await this.DoInvoke(context, customMiddlewareConfiguration);

        }

 

        await this.next(context);

    }

 

    #endregion Public Methods

 

    #region Protected Methods

 

    protected abstract Task DoInvoke(DownstreamContext context, CustomMiddlewareConfiguration configuration);

 

    #endregion Protected Methods

}

接下来就简单了，只需要让自定义的Ocelot中间件继承于CustomMiddleware基类就行了，当然，为了解耦类型名称与中间件名称，使用一个自定义的CustomMiddlewareAttribute：


[CustomMiddleware("themeCssMinUrlReplacer")]

public class ThemeCssMinUrlReplacer : CustomMiddleware

{

    private readonly Regex regex = new Regex(@"\w+://[a-zA-Z0-9]+(\:\d+)?/themes/(?<theme_name>[a-zA-Z0-9_]+)/bootstrap.min.css");

    public ThemeCssMinUrlReplacer(OcelotRequestDelegate next,

        ICustomMiddlewareConfigurationManager customMiddlewareConfigurationManager,

        IOcelotLoggerFactory loggerFactory)

        : base(next, customMiddlewareConfigurationManager, loggerFactory.CreateLogger<ThemeCssMinUrlReplacer>())

    {

    }

 

    protected override async Task DoInvoke(DownstreamContext context, CustomMiddlewareConfiguration configuration)

    {

        var downstreamResponseString = await context.DownstreamResponse.Content.ReadAsStringAsync();

        var downstreamResponseJson = JObject.Parse(downstreamResponseString);

        var themesArray = (JArray)downstreamResponseJson["themes"];

        foreach(var token in themesArray)

        {

            var cssMinToken = token["cssMin"];

            var cssMinValue = cssMinToken.Value<string>();

            if (regex.IsMatch(cssMinValue))

            {

                var themeName = regex.Match(cssMinValue).Groups["theme_name"].Value;

                var replacementTemplate = configuration.Config["replacementTemplate"].ToString();

                var replacement = $"{context.HttpContext.Request.Scheme}://{context.HttpContext.Request.Host}{replacementTemplate}"

                    .Replace("{name}", themeName);

                cssMinToken.Replace(replacement);

            }

        }

 

        context.DownstreamResponse = new DownstreamResponse(

            new StringContent(downstreamResponseJson.ToString(Formatting.None), Encoding.UTF8, "application/json"),

            context.DownstreamResponse.StatusCode, context.DownstreamResponse.Headers, context.DownstreamResponse.ReasonPhrase);

    }

}

自定义中间件的注册

在上文介绍的BuildCustomOcelotPipeline扩展方法中，加入以下几行，就完成所有自定义中间件的注册：

var customMiddlewareTypes = from type in typeof(Startup).Assembly.GetTypes()
                            where type.BaseType == typeof(CustomMiddleware) &&
                                  type.IsDefined(typeof(CustomMiddlewareAttribute), false)
                            select type;
foreach (var customMiddlewareType in customMiddlewareTypes)
{
    builder.UseMiddleware(customMiddlewareType);
}

当然，app.UseOcelot的调用要调整为：

app.UseOcelot((b, c) => b.BuildCustomOcelotPipeline(c).Build()).Wait();

运行

重新运行API网关，得到结果跟之前的一样。所不同的是，我们可以将ThemeCssMinUrlReplacer在其它的ReRoute配置上重用了。

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原始发表：2020-03-18，如有侵权请联系 cloudcommunity@tencent.com 删除

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