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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 <a href="http://localhost:5010/api/themes">http://localhost:5010/api/themes</a> { "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 ThemeCssMinUrlReplacer2(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。
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