在Ocelot中使用自定义的中间件（一）

Ocelot是ASP.NET Core下的API网关的一种实现，在微服务架构领域发挥了非常重要的作用。本文不会从整个微服务架构的角度来介绍Ocelot，而是介绍一下最近在学习过程中遇到的一个问题，以及如何使用中间件（Middleware）来解决这样的问题。

问题描述

在上文中，我介绍了一种在Angular站点里基于Bootstrap切换主题的方法。之后，我将多个主题的boostrap.min.css文件放到一个ASP.NET Core Web API的站点上，并用静态文件的方式进行分发，在完成这部分工作之后，调用这个Web API，就可以从服务端获得主题信息以及所对应的样式文件。例如：

// GET http://localhost:5010/api/themes
{
    "version": "1.0.0",
    "themes": [
        {
            "name": "蔚蓝 (Cerulean)",
            "description": "Cerulean",
            "category": "light",
            "cssMin": "http://localhost:5010/themes/cerulean/bootstrap.min.css",
            "navbarClass": "navbar-dark",
            "navbarBackgroundClass": "bg-primary",
            "footerTextClass": "text-light",
            "footerLinkClass": "text-light",
            "footerBackgroundClass": "bg-primary"
        },
        {
            "name": "机械 (Cyborg)",
            "description": "Cyborg",
            "category": "dark",
            "cssMin": "http://localhost:5010/themes/cyborg/bootstrap.min.css",
            "navbarClass": "navbar-dark",
            "navbarBackgroundClass": "bg-dark",
            "footerTextClass": "text-dark",
            "footerLinkClass": "text-dark",
            "footerBackgroundClass": "bg-light"
        }
    ]
}

当然，整个项目中不仅仅是有这个themes API，还有另外2-3个服务在后台运行，项目是基于微服务架构的。为了能够让前端有统一的API接口，我使用Ocelot作为服务端的API网关，以便为Angular站点提供API服务。于是，我定义了如下ReRoute规则：

{
    "ReRoutes": [
        {
            "DownstreamPathTemplate": "/api/themes",
            "DownstreamScheme": "http",
            "DownstreamHostAndPorts": [
                {
                    "Host": "localhost",
                    "Port": 5010
                }
            ],
            "UpstreamPathTemplate": "/themes-api/themes",
            "UpstreamHttpMethod": [ "Get" ]
        }
    ]
}

假设API网关运行在http://localhost:9023，那么基于上面的ReRoute规则，通过访问http://localhost:9023/themes-api/themes，即可转发到后台的http://localhost:5010/api/themes，完成API的调用。运行一下，调用结果如下：

// GET http://localhost:9023/themes-api/themes
{
    "version": "1.0.0",
    "themes": [
        {
            "name": "蔚蓝 (Cerulean)",
            "description": "Cerulean",
            "category": "light",
            "cssMin": "http://localhost:5010/themes/cerulean/bootstrap.min.css",
            "navbarClass": "navbar-dark",
            "navbarBackgroundClass": "bg-primary",
            "footerTextClass": "text-light",
            "footerLinkClass": "text-light",
            "footerBackgroundClass": "bg-primary"
        },
        {
            "name": "机械 (Cyborg)",
            "description": "Cyborg",
            "category": "dark",
            "cssMin": "http://localhost:5010/themes/cyborg/bootstrap.min.css",
            "navbarClass": "navbar-dark",
            "navbarBackgroundClass": "bg-dark",
            "footerTextClass": "text-dark",
            "footerLinkClass": "text-dark",
            "footerBackgroundClass": "bg-light"
        }
    ]
}

看上去一切正常，但是，每个主题设置的css文件地址仍然还是指向下游服务的URL地址，比如上面的cssMin中，还是使用的http://localhost:5010。从部署的角度，外部是无法访问除了API网关以外的其它服务的，于是，这就造成了css文件无法被访问的问题。

解决这个问题的思路很简单，就是API网关在返回response的时候，将cssMin的地址替换掉。如果在Ocelot的配置中加入以下ReRoute设置：

{
  "DownstreamPathTemplate": "/themes/{name}/bootstrap.min.css",
  "DownstreamScheme": "http",
  "DownstreamHostAndPorts": [
    {
      "Host": "localhost",
      "Port": 5010
    }
  ],
  "UpstreamPathTemplate": "/themes-api/theme-css/{name}",
  "UpstreamHttpMethod": [ "Get" ]
}

那么只需要将下游response中cssMin的值（比如http://localhost:5010/themes/cyborg/bootstrap.min.css）替换为Ocelot网关中设置的上游URL（比如http://localhost:9023/themes-api/theme-css/cyborg），然后将替换后的response返回给API调用方即可。这个过程，可以使用Ocelot中间件完成。

使用Ocelot中间件

Ocelot中间件是继承于OcelotMiddleware类的子类，并且可以在Startup.Configure方法中，通过app.UseOcelot方法将中间件注入到Ocelot管道中，然而，简单地调用IOcelotPipelineBuilder的UseMiddleware方法是不行的，它会导致整个Ocelot网关不可用。比如下面的方法是不行的：

app.UseOcelot((builder, config) =>
{
    builder.UseMiddleware<ThemeCssMinUrlReplacer>();
});

这是因为没有将Ocelot的其它Middleware加入到管道中，Ocelot管道中只有ThemeCssMinUrlReplacer中间件。要解决这个问题，我目前的方法就是通过使用扩展方法，将所有Ocelot中间全部注册好，然后再注册自定义的中间件，比如：

public static IOcelotPipelineBuilder BuildCustomOcelotPipeline(this IOcelotPipelineBuilder builder,
    OcelotPipelineConfiguration pipelineConfiguration)
{
    builder.UseExceptionHandlerMiddleware();
    builder.MapWhen(context => context.HttpContext.WebSockets.IsWebSocketRequest,
        app =>
        {
            app.UseDownstreamRouteFinderMiddleware();
            app.UseDownstreamRequestInitialiser();
            app.UseLoadBalancingMiddleware();
            app.UseDownstreamUrlCreatorMiddleware();
            app.UseWebSocketsProxyMiddleware();
        });
    builder.UseIfNotNull(pipelineConfiguration.PreErrorResponderMiddleware);
    builder.UseResponderMiddleware();
    builder.UseDownstreamRouteFinderMiddleware();
    builder.UseSecurityMiddleware();
    if (pipelineConfiguration.MapWhenOcelotPipeline != null)
    {
        foreach (var pipeline in pipelineConfiguration.MapWhenOcelotPipeline)
        {
            builder.MapWhen(pipeline);
        }
    }
    builder.UseHttpHeadersTransformationMiddleware();
    builder.UseDownstreamRequestInitialiser();
    builder.UseRateLimiting();

    builder.UseRequestIdMiddleware();
    builder.UseIfNotNull(pipelineConfiguration.PreAuthenticationMiddleware);
    if (pipelineConfiguration.AuthenticationMiddleware == null)
    {
        builder.UseAuthenticationMiddleware();
    }
    else
    {
        builder.Use(pipelineConfiguration.AuthenticationMiddleware);
    }
    builder.UseClaimsToClaimsMiddleware();
    builder.UseIfNotNull(pipelineConfiguration.PreAuthorisationMiddleware);
    if (pipelineConfiguration.AuthorisationMiddleware == null)
    {
        builder.UseAuthorisationMiddleware();
    }
    else
    {
        builder.Use(pipelineConfiguration.AuthorisationMiddleware);
    }
    builder.UseClaimsToHeadersMiddleware();
    builder.UseIfNotNull(pipelineConfiguration.PreQueryStringBuilderMiddleware);
    builder.UseClaimsToQueryStringMiddleware();
    builder.UseLoadBalancingMiddleware();
    builder.UseDownstreamUrlCreatorMiddleware();
    builder.UseOutputCacheMiddleware();
    builder.UseHttpRequesterMiddleware();
    
    return builder;
}

然后再调用app.UseOcelot即可：

app.UseOcelot((builder, config) =>
{
    builder.BuildCustomOcelotPipeline(config)
    .UseMiddleware<ThemeCssMinUrlReplacer>()
    .Build();
});

这种做法其实听起来不是特别的优雅，但是目前也没找到更合适的方式来解决Ocelot中间件注册的问题。

以下便是ThemeCssMinUrlReplacer中间件的代码，可以看到，我们使用正则表达式替换了cssMin的URL部分，使得css文件的地址可以正确被返回：

public class ThemeCssMinUrlReplacer : OcelotMiddleware
{
    private readonly Regex regex = new Regex(@"w+://[a-zA-Z0-9]+(:d+)?/themes/(?<theme_name>[a-zA-Z0-9_]+)/bootstrap.min.css");
    private const string ReplacementTemplate = "/themes-api/theme-css/{name}";
    private readonly OcelotRequestDelegate next;

    public ThemeCssMinUrlReplacer(OcelotRequestDelegate next,
        IOcelotLoggerFactory loggerFactory) : base(loggerFactory.CreateLogger<ThemeCssMinUrlReplacer2>())
        => this.next = next;

    public async Task Invoke(DownstreamContext context)
    {
        if (!string.Equals(context.DownstreamReRoute.DownstreamPathTemplate.Value, "/api/themes"))
        {
            await next(context);
        }

        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 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);
    }

}

执行结果如下：

// GET http://localhost:9023/themes-api/themes
{
  "version": "1.0.0",
  "themes": [
    {
      "name": "蔚蓝 (Cerulean)",
      "description": "Cerulean",
      "category": "light",
      "cssMin": "http://localhost:9023/themes-api/theme-css/cerulean",
      "navbarClass": "navbar-dark",
      "navbarBackgroundClass": "bg-primary",
      "footerTextClass": "text-light",
      "footerLinkClass": "text-light",
      "footerBackgroundClass": "bg-primary"
    },
    {
      "name": "机械 (Cyborg)",
      "description": "Cyborg",
      "category": "dark",
      "cssMin": "http://localhost:9023/themes-api/theme-css/cyborg",
      "navbarClass": "navbar-dark",
      "navbarBackgroundClass": "bg-dark",
      "footerTextClass": "text-dark",
      "footerLinkClass": "text-dark",
      "footerBackgroundClass": "bg-light"
    }
  ]
}

总结

本文介绍了使用Ocelot中间件实现下游服务response body的替换任务，在ThemeCssMinUrlReplacer的实现代码中，我们使用了context.DownstreamReRoute.DownstreamPathTemplate.Value来判断当前执行的URL是否需要由该中间件进行处理，以避免不必要的中间件逻辑执行。这个设计可以再优化一下，使用一个简单的框架让程序员可以通过Ocelot的配置文件来更为灵活地使用Ocelot中间件，下文介绍这部分内容。