目錄：

• 面向.NET開發(fā)人員的Dapr——前言

• 面向.NET開發(fā)人員的Dapr——分布式世界

• 面向.NET開發(fā)人員的Dapr——俯瞰Dapr

• 面向.NET開發(fā)人員的Dapr——入門

• 面向.NET開發(fā)人員的Dapr——參考應(yīng)用程序

• 面向.NET開發(fā)人員的Dapr——狀態(tài)管理

• 面向.NET開發(fā)人員的Dapr——服務(wù)調(diào)用

The Dapr publish & subscribe building block

Dapr 發(fā)布 & 訂閱構(gòu)建塊

The?Publish-Subscribe pattern?(often referred to as "pub/sub") is a well-known and widely used messaging pattern. Architects commonly embrace it in distributed applications. However, the plumbing to implement it can be complex. There are often subtle feature differences across different messaging products. Dapr offers a building block that significantly simplifies implementing pub/sub functionality.

發(fā)布-訂閱模式?(通常稱為 "發(fā)布/訂閱" ) 是眾所周知且廣泛使用的消息模式。架構(gòu)師通常在分布式應(yīng)用程序中采用它。但是，實現(xiàn)可能會很復(fù)雜。在不同的消息隊列產(chǎn)品中，通常會有細微的功能差異。Dapr 提供了一個構(gòu)建基塊，可顯著簡化實現(xiàn)發(fā)布/訂閱功能。

What it solves

解決方法

The primary advantage of the Publish-Subscribe pattern is?loose coupling, sometimes referred to as?temporal decoupling. The pattern decouples services that send messages (the?publishers) from services that consume messages (the?subscribers). Both publishers and subscribers are unaware of each other - both are dependent on a centralized?message broker?that distributes the messages.

發(fā)布/訂閱 模式的主要優(yōu)點是松耦合，有時稱為時間上解耦。此模式分離服務(wù)為發(fā)送消息的服務(wù)（稱為發(fā)布者）和消費消息的服務(wù)（稱為訂閱者）。發(fā)布者和訂閱者都不知道對方的存在，兩者都依賴于分發(fā)消息的集中式消息代理。

Figure 7-1 shows the high-level architecture of the pub/sub pattern.

圖7-1 顯示了發(fā)布/訂閱模式的高層架構(gòu)。

Figure 7-1. The pub/sub pattern.

圖 7-1。發(fā)布/訂閱模式。

From the previous figure, note the steps of the pattern:

Publishers send messages to the message broker.

Subscribers bind to a subscription on the message broker.

The message broker forwards a copy of the message to interested subscriptions.

Subscribers consume messages from their subscriptions.

從上圖中，注意模式的步驟：

發(fā)布者將消息發(fā)送到消息代理。

訂閱者綁定到消息代理上的訂閱。

消息代理將消息的副本轉(zhuǎn)發(fā)給感興趣的訂閱。

訂閱者從其訂閱消費消息。

Most message brokers encapsulate a queueing mechanism that can persist messages once received. With it, the message broker guarantees?durability?by storing the message. Subscribers don't need to be immediately available or even online when a publisher sends a message. Once available, the subscriber receives and processes the message. Dapr guarantees?At-Least-Once?semantics for message delivery. Once a message is published, it will be delivered at least once to any interested subscriber.

大多數(shù)消息代理封裝了一個排隊機制，這種機制可以保證在收到消息后可以持久保存消息的。利用它，消息代理通過存儲消息來保證?持久性?。當發(fā)布者發(fā)送消息時，訂閱者無需立即就緒，甚至不需要在線。一旦訂閱者就緒，訂閱者將接收并處理消息。Dapr 為消息傳遞 提供至少一次的語義保證。發(fā)布消息后，該消息將至少傳遞一次到任何相關(guān)訂閱者。

If your service can only process a message once, you'll need to provide an?idempotency check?to ensure that the same message is not processed multiple times. While such logic can be coded, some message brokers, such as Azure Service Bus, provide built-in?messaging capabilities.

如果要求服務(wù)只能處理一次消息，則需要提供?冪等性檢查?，確保不會多次處理同一消息。雖然可以編碼這種邏輯，但某些消息代理（如 Azure Service Bus）提供內(nèi)置的消息傳遞?重復(fù)檢測?功能。

There are several message broker products available - both commercially and open-source. Each has advantages and drawbacks. Your job is to match your system requirements to the appropriate broker. Once selected, it's a best practice to decouple your application from message broker plumbing. You achieve this functionality by wrapping the broker inside an?abstraction. The abstraction encapsulates the message plumbing and exposes generic pub/sub operations to your code. Your code communicates with the abstraction, not the actual message broker. While a wise decision, you'll have to write and maintain the abstraction and its underlying implementation. This approach requires custom code that can be complex, repetitive, and error-prone.

有多種可用的消息代理產(chǎn)品-商業(yè)和開源。各有優(yōu)缺點。你需要按系統(tǒng)要求選擇相應(yīng)的代理。選擇后，最佳做法是分離應(yīng)用程序與消息代理系統(tǒng)?？梢酝ㄟ^抽象?代理來實現(xiàn)此功能。抽象封裝消息代理并向代碼公開通用的發(fā)布/訂閱操作。你的代碼與抽象交互，而不是實際的消息代理。明智的決定是，你必須編寫和維護抽象及其底層實現(xiàn)。此方法需要自定義代碼，這些代碼可能會很復(fù)雜、重復(fù)并且容易出錯。

The Dapr publish & subscribe building block provides the messaging abstraction and implementation out-of-the-box. The custom code you would have had to write is prebuilt and encapsulated inside the Dapr building block. You bind to it and consume it. Instead of writing messaging plumbing code, you and your team focus on creating business functionality that adds value to your customers.

Dapr 發(fā)布 & 訂閱構(gòu)建塊提供開箱即用的消息傳遞抽象和實現(xiàn)。以前您必須編寫的自定義代碼已在 Dapr 構(gòu)建基塊內(nèi)預(yù)構(gòu)建和封裝。綁定到并使用這些封裝。你和你的團隊只需聚焦于創(chuàng)建能為客戶帶來價值的業(yè)務(wù)功能上，而無需再編寫消息管道代碼。

How it works

工作原理

The Dapr publish & subscribe building block provides a platform-agnostic API framework to send and receive messages. Your services publish messages to a named?topic. Your services subscribe to a topic to consume messages.

Dapr 發(fā)布 & 訂閱構(gòu)建塊提供了平臺無關(guān)的 API 框架來發(fā)送和接收消息。你的服務(wù)將消息發(fā)布到一個命名?主題。你的服務(wù)訂閱主題來消費消息。

The service calls the pub/sub API on the Dapr sidecar. The sidecar then makes calls into a pre-defined Dapr pub/sub component that encapsulates a specific message broker product. Figure 7-2 shows the Dapr pub/sub messaging stack.

服務(wù)在 Dapr 邊車上調(diào)用 pub/sub API。然后，邊車將調(diào)用一個預(yù)定義的 Dapr pub/sub 組件（封裝了特定的消息代理產(chǎn)品）。圖7-2 顯示了 Dapr pub/sub 消息傳遞棧。

?

Figure 7-2. The Dapr pub/sub stack.

圖 7-2。Dapr pub/sub 棧。

The Dapr publish & subscribe building block can be invoked in many ways.

可以通過多種方式調(diào)用 Dapr 發(fā)布 & 訂閱構(gòu)建塊。

At the lowest level, any programming platform can invoke the building block over HTTP or gRPC using the?Dapr native API. To publish a message, you make the following API call:

在最低級別，任何編程平臺均可通過 HTTP 或 gRPC 使用 Dapr 本機 API 來調(diào)用構(gòu)建塊。若要發(fā)布消息，請執(zhí)行以下 API 調(diào)用：

http://localhost:<dapr-port>/v1.0/publish/<pub-sub-name>/<topic>

There are several Dapr specific URL segments in the above call:

• <dapr-port>?provides the port number upon which the Dapr sidecar is listening.

• <pub-sub-name>?provides the name of the selected Dapr pub/sub component.

• <topic>?provides the name of the topic to which the message is published.

以上調(diào)用中有幾個 Dapr 特定的 URL 段：

• <dapr-port>?提供 Dapr 邊車正在偵聽的端口號。

• <pub-sub-name>?提供選定的 Dapr pub/sub 組件的名稱。

• <topic>?提供消息要發(fā)布到的主題的名稱。

Using the?curl?command-line tool to publish a message, you can try it out:

可以嘗試一下使用 curl 的命令行工具發(fā)布消息：

curl -X POST http://localhost:3500/v1.0/publish/pubsub/newOrder \-H "Content-Type: application/json" \-d '{ "orderId": "1234", "productId": "5678", "amount": 2 }'

You receive messages by subscribing to a topic. At startup, the Dapr runtime will call the application on a well-known endpoint to identify and create the required subscriptions:

通過訂閱主題來接收消息。在啟動時，Dapr 運行時將調(diào)用應(yīng)用程序上的已知終結(jié)點來識別和創(chuàng)建所需的訂閱：

http://localhost:<appPort>/dapr/subscribe

• <appPort>?informs the Dapr sidecar of the port upon which the application is listening.

• <appPort>?通知 Dapr 邊車應(yīng)用程序偵聽的端口。

You can implement this endpoint yourself. But Dapr provides more intuitive ways of implementing it. We'll address this functionality later in this chapter.

可以自行實現(xiàn)此終結(jié)點。但 Dapr 提供了更直觀的實現(xiàn)方法。本章稍后將介紹此功能。

The response from the call contains a list of topics to which the applications will subscribe. Each includes an endpoint to call when the topic receives a message. Here's an example of a response:

對此終結(jié)點調(diào)用的響應(yīng)包含應(yīng)用程序?qū)⒂嗛喌闹黝}的列表。每個都包括在主題收到消息時要調(diào)用的終結(jié)點。下面是響應(yīng)的示例：

[{"pubsubname": "pubsub","topic": "newOrder","route": "/orders"},{"pubsubname": "pubsub","topic": "newProduct","route": "/productCatalog/products"} ]

In the JSON response, you can see the application wants to subscribe to topics?newOrder?and?newProduct. It registers the endpoints?/orders?and?/productCatalog/products?for each, respectively. For both subscriptions, the application is binding to the Dapr component named?pubsub.

在 JSON 響應(yīng)中，可以看到應(yīng)用程序要訂閱主題?newOrder?和?newProduct?。分別為每個終結(jié)點注冊終結(jié)點?/orders和?/productCatalog/products?。對于這兩個訂閱，應(yīng)用程序?qū)⒔壎ǖ矫麨榈?Dapr 組件?pubsub?。

Figure 7-3 presents the flow of the example.

圖7-3 顯示了該示例的消息流。

Figure 7-3. pub/sub flow with Dapr.

圖 7-3。Dapr 的發(fā)布/訂閱流。

From the previous figure, note the flow:

The Dapr sidecar for Service B calls the?/dapr/subscribe?endpoint from Service B (the consumer). The service responds with the subscriptions it wants to create.

The Dapr sidecar for Service B creates the requested subscriptions on the message broker.

Service A publishes a message at the?/v1.0/publish/<pub-sub-name>/<topic>?endpoint on the Dapr Service A sidecar.

The Service A sidecar publishes the message to the message broker.

The message broker sends a copy of the message to the Service B sidecar.

The Service B sidecar calls the endpoint corresponding to the subscription (in this case?/orders) on Service B. The service responds with an HTTP status-code?200 OK?so the sidecar will consider the message as being handled successfully.

在上圖中，請注意以下 消息流：

服務(wù)B的Dapr邊車調(diào)用服務(wù)B（訂閱者）的終結(jié)點/dapr/subscribe?。服務(wù)會返回它要創(chuàng)建的訂閱作為響應(yīng)。

服務(wù) B 的 Dapr 邊車在消息代理上創(chuàng)建要求的訂閱。

服務(wù) A 在其Dapr邊車的終結(jié)點/v1.0/publish/<pub-sub-name>/<topic>?上發(fā)布一條消息。

服務(wù) A 的Dapr邊車將消息發(fā)布到消息代理。

消息代理發(fā)送消息副本到服務(wù) B 的Dapr邊車。

服務(wù) B 的Dapr邊車調(diào)用服務(wù) b 上與訂閱對應(yīng)的終結(jié)點（此處為 /orders）。服務(wù)以 HTTP 狀態(tài)碼?200 OK?進行響應(yīng)，邊車將認為消息已被成功處理。

In the example, the message is handled successfully. But if something goes wrong while Service B is handling the request, it can use the response to specify what needs to happen with the message. When it returns an HTTP status-code?404, an error is logged and the message is dropped. With any other status-code than?200?or?404, a warning is logged and the message is retried. Alternatively, Service B can explicitly specify what needs to happen with the message by including a JSON payload in the body of the response:

示例中，消息已被成功處理。但是，如果服務(wù) B 處理請求時出現(xiàn)問題，則可以使用響應(yīng)指定需要對消息執(zhí)行的操作。當它返回 HTTP 狀態(tài)碼404時?，將記錄錯誤并丟棄消息。對于任何其他狀態(tài)碼（非200?404），將記錄警告，并重試消息?；蛘?#xff0c;服務(wù) B 可以通過在響應(yīng)正文中包含 JSON 負荷，顯式指定需要對消息執(zhí)行的操作。

{"status": "<status>" }

The following table shows the available?status?values:

TABLE 1StatusAction

SUCCESS	The message is considered as processed successfully and dropped.
RETRY	The message is retried.
DROP	A warning is logged and the message is dropped.
Any other status	The message is retried.

下表顯示了?status的可用?值：

表 1狀態(tài)操作

SUCCESS	消息被視為已成功處理和丟棄。
RETRY	重試消息。
DROP	將記錄警告，并丟棄消息。
任何其他狀態(tài)	重試消息。

Competing consumers

消費者競爭

When scaling out an application that subscribes to a topic, you have to deal with competing consumers. Only one application instance should handle a message sent to the topic. Luckily, Dapr handles that problem. When multiple instances of a service with the same application-id subscribe to a topic, Dapr delivers each message to only one of them.

橫向擴展訂閱某個主題的應(yīng)用程序時，必須處理消費者競爭。只有一個應(yīng)用程序?qū)嵗龖?yīng)處理發(fā)送到主題的消息。幸運的是，Dapr 處理這一問題。當具有相同應(yīng)用程序 id 的服務(wù)的多個實例訂閱主題時，Dapr 僅將每條消息傳遞給其中的一個。

SDKs

Making HTTP calls to the native Dapr APIs is time-consuming and abstract. Your calls are crafted at the HTTP level, and you'll need to handle plumbing concerns such as serialization and HTTP response codes. Fortunately, there's a more intuitive way. Dapr provides several language-specific SDKs for popular development platforms. At the time of this writing, Go, Node.js, Python, .NET, Java, and JavaScript are available.

對本機 Dapr Api 進行 HTTP 調(diào)用非常耗時且更抽象。你的調(diào)用是HTTP 級別的，你將需要處理諸如序列化和 HTTP 響應(yīng)代碼這樣的管道相關(guān)問題。幸運的是，有一種更直觀的方式。Dapr 為常用開發(fā)平臺提供多種語言特定的 Sdk。撰寫本文時，可以使用 Node.js、Python、.NET、Java 和 JavaScript。

Use the Dapr .NET SDK

使用 Dapr .NET SDK

For .NET Developers, the?Dapr .NET SDK?provides a more productive way of working with Dapr. The SDK exposes a?DaprClient?class through which you can directly invoke Dapr functionality. It's intuitive and easy to use.

對于 .NET 開發(fā)人員而言，?Dapr .NET SDK?提供了更高效的 Dapr 處理方式。SDK 公開了一個?DaprClient?類，通過該類可以直接調(diào)用 Dapr 功能，直觀且易于使用。

To publish a message, the?DaprClient?exposes a?PublishEventAsync?method.

DaprClient?公開了一個?PublishEventAsync?方法來發(fā)布消息 。

var data = new OrderData {orderId = "123456",productId = "67890",amount = 2 };var daprClient = new DaprClientBuilder().Build();await daprClient.PublishEventAsync<OrderData>("pubsub", "newOrder", data);

• The first argument?pubsub?is the name of the Dapr component that provides the message broker implementation. We'll address components later in this chapter.

• The second argument?neworder?provides the name of the topic to send the message to.

• The third argument is the payload of the message.

• You can specify the .NET type of the message using the generic type parameter of the method.

• 第一個參數(shù)?pubsub?是提供消息代理實現(xiàn)的 Dapr 組件的名稱。本章稍后將介紹這些組件。

• 第二個參數(shù)?neworder?提供要向其發(fā)送消息的主題的名稱。

• 第三個參數(shù)是消息的載體。

• 您可以使用方法的泛型類型參數(shù)來指定消息的 .NET 類型。

To receive messages, you bind an endpoint to a subscription for a registered topic. The AspNetCore library for Dapr makes this trivial. Assume, for example, that you have an existing ASP.NET WebAPI action method entitled?CreateOrder:

若要接收消息，請將終結(jié)點綁定到訂閱。用于 Dapr 的 AspNetCore 庫使此變得簡單。例如，假設(shè)你有一個名為?CreateOrder?的 ASP.NET WebAPI 操作方法?：

[HttpPost("/orders")] public async Task<ActionResult> CreateOrder(Order order)

You must add a reference to the?Dapr.AspNetCore?NuGet package in your project to consume the Dapr ASP.NET Core integration.

必須在項目中添加對?Dapr.AspNetCore?NuGet 包的引用，才能使用 Dapr ASP.NET Core 集成。

To bind this action method to a topic, you decorate it with the?Topic?attribute:

若要將此操作方法綁定到主題，請使用Topic特性?對其進行修飾?：

[Topic("pubsub", "newOrder")] [HttpPost("/orders")] public async Task<ActionResult> CreateOrder(Order order)

You specify two key elements with this attribute:

• The Dapr pub/sub component to target (in this case?pubsub).

• The topic to subscribe to (in this case?newOrder).

指定此特性的兩個關(guān)鍵元素：

• Dapr的發(fā)布/訂閱組件（此處為pubsub）

• 訂閱的主題 (此處為?newOrder) 。

Dapr then invokes that action method as it receives messages for that topic.

Dapr 將調(diào)用該操作方法，以接收該主題的消息。

You'll also need to enable ASP.NET Core to use Dapr. The Dapr .NET SDK provides several extension methods that can be invoked in the?Startup?class.

還需要啟用 ASP.NET Core 來使用 Dapr。Dapr .NET SDK 提供了可在Startup?類中調(diào)用的多個擴展方法?。

In the?ConfigureServices?method, you must add the following extension method:

在?ConfigureServices?方法中，需要添加以下擴展方法：

public void ConfigureServices(IServiceCollection services) {// ...services.AddControllers().AddDapr(); }

Appending the?AddDapr?extension-method to the?AddControllers?extension-method registers the necessary services to integrate Dapr into the MVC pipeline. It also registers a?DaprClient?instance into the dependency injection container, which then can be injected anywhere into your service.

將?AddDapr?擴展方法追加到?AddControllers?擴展方法會注冊必要的服務(wù)，以將 Dapr 集成到 MVC 管道中。它還將?DaprClient?實例注冊到依賴關(guān)系注入容器，將來可以在任何需要DaprClient的服務(wù)中注入它。

In the?Configure?method, you must add the following middleware components to enable Dapr:

在?Configure?方法中，必須添加以下中間件組件來啟用 Dapr：

public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {// ...app.UseCloudEvents();app.UseEndpoints(endpoints =>{endpoints.MapSubscribeHandler();// ...}); }

The call to?UseCloudEvents?adds?CloudEvents?middleware into to the ASP.NET Core middleware pipeline. This middleware will unwrap requests that use the CloudEvents structured format, so the receiving method can read the event payload directly.

?對UseCloudEvents?的調(diào)用用于將 CloudEvents 中間件添加到 ASP.NET Core 中間件管道。此中間件將解包使用 CloudEvents 結(jié)構(gòu)化格式的請求，因此接收方法可以直接讀取事件載體。

CloudEvents?is a standardized messaging format, providing a common way to describe event information across platforms. Dapr embraces CloudEvents. For more information about CloudEvents, see the?cloudevents specification.

CloudEvents?是一種標準化的消息傳遞格式，提供跨平臺描述事件信息的通用方式。Dapr 采用 CloudEvents。有關(guān) CloudEvents 的詳細信息，請參閱?CloudEvents 規(guī)范。

The call to?MapSubscribeHandler?in the endpoint routing configuration will add a Dapr subscribe endpoint to the application. This endpoint will respond to requests on?/dapr/subscribe. When this endpoint is called, it will automatically find all WebAPI action methods decorated with the?Topic?attribute and instruct Dapr to create subscriptions for them.

終結(jié)點路由配置中對MapSubscribeHandler的調(diào)用會為應(yīng)用程序添加Dapr 訂閱終結(jié)點。此終結(jié)點將響應(yīng)到/dapr/subscribe?上的請求?。調(diào)用此終結(jié)點時，它將自動查找所有用Topic?特性修飾的WebAPI 操作方法，?并指示 Dapr 為它們創(chuàng)建訂閱。

Pub/sub components

Pub/sub 組件

Dapr?pub/sub components?handle the actual transport of the messages. Several are available. Each encapsulates a specific message broker product to implement the pub/sub functionality. At the time of writing, the following pub/sub components were available:

Dapr?pub/sub 組件?處理消息的實際傳輸。有多個底層實現(xiàn)可用。每個都封裝特定消息代理產(chǎn)品以實現(xiàn)發(fā)布/訂閱功能。撰寫本文時，可以使用以下發(fā)布/訂閱組件：

• Apache Kafka

• Azure Event Hubs

• Azure Service Bus

• AWS SNS/SQS

• GCP Pub/Sub

• Hazelcast

• MQTT

• NATS

• Pulsar

• RabbitMQ

• Redis Streams

Note

The Azure cloud stack has both messaging functionality (Azure Service Bus) and event streaming (Azure Event Hub) availability.

備注

Azure cloud stack 具有消息傳遞（Azure Service Bus）和事件流(Azure Event Hub) 功能。

These components are created by the community in a?component-contrib repository on GitHub. You're encouraged to write your own Dapr component for a message broker that isn't yet supported.

這些組件由?GitHub 上的?component-contrib存儲庫中的社區(qū)創(chuàng)建。建議為尚不受支持的消息代理編寫自己的 Dapr 組件。

Configure pub/sub components

配置發(fā)布/訂閱組件

Using a Dapr configuration file, you can specify the pub/sub component(s) to use. This configuration contains several fields. The?name?field specifies the pub/sub component that you want to use. When sending or receiving a message, you need to specify this name (as you saw earlier in the?PublishEventAsync?method signature).

使用 Dapr 配置文件，您可以指定要使用的發(fā)布/訂閱組件。此配置包含多個字段。?name字段指定要使用的發(fā)布/訂閱組件。發(fā)送或接收消息時，需要指定此名稱， (如之前在PublishEventAsync?方法簽名) 中看到的那樣?。

Below you see an example of a Dapr configuration file for configuring a RabbitMQ message broker component:

下面你將看到一個用于配置 RabbitMQ 消息代理組件的 Dapr 配置文件示例：

apiVersion: dapr.io/v1alpha1 kind: Component metadata:name: pubsub-rq spec:type: pubsub.rabbitmqversion: v1metadata:- name: hostvalue: "amqp://localhost:5672"- name: durablevalue: true

In this example, you can see that you can specify any message broker-specific configuration in the?metadata?block. In this case, RabbitMQ is configured to create durable queues. But the RabbitMQ component has more configuration options. Each of the components' configuration will have its own set of possible fields. You can read which fields are available in the documentation of each?pub/sub component.

在此示例中，可以看到，你可以在metadata?塊中指定任何特定于消息代理的配置?。此處，RabbitMQ 被配置為創(chuàng)建持久隊列。但 RabbitMQ 組件具有更多的配置選項。每個組件的配置將有自己的一組字段。您可以閱讀每個發(fā)布?/訂閱組件的文檔來了解其可用的字段。

Next to the programmatic way of subscribing to a topic from code, Dapr pub/sub also provides a declarative way of subscribing to a topic. This approach removes the Dapr dependency from the application code. Therefore, it also enables an existing application to subscribe to topics without any changes to the code. The following example shows a Dapr configuration file for configuring a subscription:

除了以編程方式訂閱主題外，Dapr pub/sub 還提供了一種聲明方式來訂閱主題。此方法會從應(yīng)用程序代碼中刪除 Dapr 依賴項。然而，它還允許現(xiàn)有應(yīng)用程序無需更改代碼，就能夠訂閱主題?。以下示例演示了用于配置訂閱的 Dapr 配置文件：

apiVersion: dapr.io/v1alpha1 kind: Subscription metadata:name: newOrder-subscription spec:pubsubname: pubsubtopic: newOrderroute: /orders scopes: - ServiceB - ServiceC

You have to specify several elements with every subscription:

• The name of the Dapr pub/sub component you want to use (in this case?pubsub).

• The name of the topic to subscribe to (in this case?newOrder).

• The API operation that needs to be called for this topic (in this case?/orders).

• The?scope?can specify which services can publish and subscribe to a topic.

必須為每個訂閱指定多個元素：

• Dapr pub/sub 組件的名稱（此處為pubsub）?。

• 訂閱的主題名稱 (此處為?newOrder) 。

• 需要為本主題調(diào)用 的API 操作 (此處為?/orders) 。

• 作用域可以指定哪些服務(wù)可以發(fā)布和訂閱主題。

Reference application: eShopOnDapr

參考應(yīng)用程序：eShopOnDapr

The accompanying?eShopOnDapr?app provides an end-to-end reference architecture for constructing a microservices application implementing Dapr. eShopOnDapr is an evolution of the widely popular?eShopOnContainers?app, created several years ago. Both versions use the pub/sub pattern for communicating?integration events?across microservices. Integration events include:

• When a user checks-out a shopping basket.

• When a payment for an order has succeeded.

• When the grace-period of a purchase has expired.

隨附的?eShopOnDapr?應(yīng)用提供端到端參考架構(gòu)，用于構(gòu)造實現(xiàn) Dapr 的微服務(wù)應(yīng)用程序。eShopOnDapr 是在幾年前創(chuàng)建的廣泛流行的?eShopOnContainers?應(yīng)用程序的演變。這兩個版本都使用發(fā)布/訂閱模式來跨微服務(wù)傳遞?集成事件?。集成事件包括：

• 當用戶結(jié)賬時。

• 訂單的付款成功時。

• 購買的寬限期已過期時。

Eventing in eShopOnContainers is based on the following?IEventBus?interface:

EShopOnContainers 中的事件基于以下?IEventBus?接口：

public interface IEventBus {void Publish(IntegrationEvent integrationEvent);void Subscribe<T, THandler>()where TEvent : IntegrationEventwhere THandler : IIntegrationEventHandler<T>; }

Concrete implementations of this interface exist in eShopOnContainers for both RabbitMQ and Azure Service Bus. Each implementation included a great deal of custom plumbing code that was complex to understand and difficult to maintain.

eShopOnContainers 中存在此接口的具體實現(xiàn)（RabbitMQ 和Azure Service Bus） 。每個實現(xiàn)都包含大量自定義的管道代碼，這些代碼非常復(fù)雜，難于理解和維護。

The newer eShopOnDapr significantly simplifies pub/sub behavior by using Dapr. For example, the?IEventBus?interface was reduced to a single method:

較新的 eShopOnDapr 通過使用 Dapr 大大簡化了發(fā)布/訂閱行為。例如，?IEventBus?接口被縮減到單一方法：

public interface IEventBus {Task PublishAsync(IntegrationEvent integrationEvent); }

Publish events

發(fā)布事件

In the updated eShopOnDapr, a single?DaprEventBus?implementation can support any Dapr-supported message broker. The following code block shows the simplified Publish method. Note how the?PublishAsync?method uses the Dapr client to publish an event:

在更新了的 eShopOnDapr 中，一個?DaprEventBus?實現(xiàn)可以支持任何 Dapr 支持的消息代理。下面的代碼塊顯示了簡化的發(fā)布方法。請注意該?PublishAsync?方法如何使用 DaprClient來發(fā)布事件：

public class DaprEventBus : IEventBus {private const string PubSubName = "pubsub";private readonly DaprClient _daprClient;private readonly ILogger<DaprEventBus> _logger;public DaprEventBus(DaprClient daprClient, ILogger<DaprEventBus> logger){_daprClient = daprClient ?? throw new ArgumentNullException(nameof(daprClient));_logger = logger ?? throw new ArgumentNullException(nameof(logger));}public async Task PublishAsync(IntegrationEvent integrationEvent){var topicName = integrationEvent.GetType().Name;// Dapr uses System.Text.Json which does not support serialization of a// polymorphic type hierarchy by default. Using object as the type// parameter causes all properties to be serialized.await _daprClient.PublishEventAsync<object>(PubSubName, topicName, integrationEvent);} }

As you can see in the code snippet, the topic name is derived from event type's name. Because all eShop services use the?IEventBus?abstraction, retrofitting Dapr required?absolutely no change?to the mainline application code.

如代碼片段中所示，主題名稱從事件類型的名稱獲得。因為所有 eShop services 都使用?IEventBus?抽象，替換底層組件完全不更改?主線應(yīng)用程序代碼。

Important

The Dapr SDK uses?System.Text.Json?to serialize/deserialize messages. However,?System.Text.Json?doesn't serialize properties of derived classes by default. In the eShop code, an event is sometimes explicitly declared as an?IntegrationEvent, the base class for integration events. This is done because the concrete event type is determined dynamically at run time based on business logic. As a result, the event is serialized using the type information of the base class and not the derived class. To force?System.Text.Json?to serialize all properties of the derived class in this case, the code uses?object?as the generic type parameter. For more information, see the?.NET documentation.

重要

Dapr SDK 使用?System.Text.Json?來序列化/反序列化消息。但是，?System.Text.Json?默認情況下不會序列化派生類的屬性。在 eShop 代碼中，事件有時顯式聲明為?IntegrationEvent?（集成事件的基類）。這是因為具體事件類型是在運行時基于業(yè)務(wù)邏輯動態(tài)確定的。因此，使用基類而不是派生類的類型信息對事件進行序列化。若要?System.Text.Json?在這種情況下強制序列化派生類的所有屬性，代碼需使用?object?作為泛型類型參數(shù)。有關(guān)詳細信息，請參閱?.net 文檔。

With Dapr, the infrastructure code is?dramatically simplified. It doesn't need to distinguish between the different message brokers. Dapr provides this abstraction for you. And if needed, you can easily swap out message brokers or configure multiple message broker components.

借助 Dapr，可大大簡化基礎(chǔ)結(jié)構(gòu)代碼。不需要區(qū)分不同的消息代理。Dapr 為你提供此抽象。如果需要，可以輕松地替換消息代理或配置多個消息代理組件。

Subscribe to events

訂閱事件

The earlier eShopOnContainers app contains?SubscriptionManagers?to handle the subscription implementation for each message broker. Each manager contains complex message broker-specific code for handling subscription events. To receive events, each service has to explicitly register a handler for each event-type.

早前的 eShopOnContainers 應(yīng)用程序包含?訂閱管理器?來處理每個消息代理的訂閱實現(xiàn)。每個管理器都包含用于處理訂閱事件的消息代理特定的復(fù)雜代碼。若要接收事件，每個服務(wù)必須為每個事件類型顯式注冊一個處理程序。

eShopOnDapr streamlines the plumbing for event subscriptions by using Dapr ASP.NET Core libraries. Each event is handled by an action method in the controller. A?Topic?attribute decorates the action method with the name of the corresponding topic to subscribe to. Here's a code snippet taken from the?PaymentService:

eShopOnDapr 使用 Dapr ASP.NET Core 庫優(yōu)化了事件訂閱的管道。每個事件都由控制器中的操作方法處理。?Topic特性使用主題的名稱修飾操作方法。下面是從PaymentService?中獲取的代碼片段?：

[Route("api/v1/[controller]")] [ApiController] public class IntegrationEventController : ControllerBase {private const string DAPR_PUBSUB_NAME = "pubsub";private readonly IServiceProvider _serviceProvider;public IntegrationEventController(IServiceProvider serviceProvider){_serviceProvider = serviceProvider;}[HttpPost("OrderStatusChangedToValidated")][Topic(DAPR_PUBSUB_NAME, "OrderStatusChangedToValidatedIntegrationEvent")]public async Task OrderStarted(OrderStatusChangedToValidatedIntegrationEvent integrationEvent){var handler = _serviceProvider.GetRequiredService<OrderStatusChangedToValidatedIntegrationEventHandler>();await handler.Handle(integrationEvent);} }

In the?Topic?attribute, the name of the .NET type of the event is used as the topic name. For handling the event, an event handler that already existed in the earlier eShopOnContainers code base is invoked. In the previous example, messages received from the?OrderStatusChangedToValidatedIntegrationEvent?topic invoke the existing?OrderStatusChangedToValidatedIntegrationEventHandler?event-handler. Because Dapr implements the underlying plumbing for subscriptions and message brokers, a large amount of original code became obsolete and was removed from the code-base. Much of this code was complex to understand and challenging to maintain.

在?Topic?特性中，事件的 .net 類型的名稱將用作主題名稱。在處理事件時，將調(diào)用以前的 eShopOnContainers 代碼中已有的事件處理程序。在上面的示例中，從OrderStatusChangedToValidatedIntegrationEvent?主題接收消息后調(diào)用了現(xiàn)有的?OrderStatusChangedToValidatedIntegrationEventHandler?事件處理程序。由于 Dapr 實現(xiàn)了訂閱和消息代理的底層管道，因此大量原始代碼已過時，并已從代碼中刪除。這些代碼對于理解和維護非常復(fù)雜。

Use pub/sub components

使用發(fā)布/訂閱組件

?Within the eShopOnDapr repository, a?deployment?folder contains files for deploying the application using different deployment modes:?Docker Compose?and?Kubernetes. A?dapr?folder exists within each of these folders that holds a?components?folder. This folder holds a file?eshop-pubsub.yaml?containing the configuration of the Dapr pub/sub component that the application will use for pub/sub behavior. As you saw in the earlier code snippets, the name of the pub/sub component used is?pubsub. Here's the content of the?eshop-pubsub.yaml?file in the?deployment/compose/dapr/components?folder:

在 eShopOnDapr 存儲庫中，?deployment?文件夾包含使用不同部署模式（Docker Compose?和?Kubernetes）?部署應(yīng)用程序的文件。compose文件夾或kubernetes文件夾中的dapr文件夾中都有一個components文件夾（此句不會翻譯）。此文件夾包含一個eshop-pubsub.yaml文件，該文件包含應(yīng)用程序?qū)⑹褂玫?Dapr pub/sub 組件的配置。正如您在前面的代碼片段中所看到的那樣，所使用的 pub/sub 組件的名稱為?pubsub?。下面是deployment/compose/dapr/components?文件夾中?eshop-pubsub.yaml?文件的內(nèi)容?：

apiVersion: dapr.io/v1alpha1 kind: Component metadata:name: pubsubnamespace: default spec:type: pubsub.natsversion: v1metadata:- name: natsURLvalue: nats://demo.nats.io:4222

The preceding configuration specifies the desired?NATS message broker?for this example. To change message brokers, you need only to configure a different message broker, such as RabbitMQ or Azure Service Bus and update the yaml file. With Dapr, there are no changes to your mainline service code when switching message brokers.

前面的配置指定此示例使用的?NATS 消息代理?。若要更改消息代理，只需更新 yaml 文件來配置一個不同的消息代理，如 RabbitMQ 或 Azure Service Bus。使用Dapr，切換消息代理時，無需對主線服務(wù)代碼進行任何更改。

Finally, you might ask, "Why would I need multiple message brokers in an application?". Many times a system will handle workloads with different characteristics. One event may occur 10 times a day, but another event occurs 5,000 times per second. You may benefit by partitioning messaging traffic to different message brokers. With Dapr, you can add multiple pub/sub component configurations, each with a different name.

最后，您可能會問："為什么需要在一個應(yīng)用程序中使用多個消息代理？"。很多時候系統(tǒng)將處理具有不同特征的工作負荷。一個事件可能一天發(fā)生10次，但另一個事件每秒發(fā)生5000次?？梢酝ㄟ^將消息傳送流量分區(qū)給不同消息代理來受益。使用 Dapr，可以添加多個 pub/sub 組件配置，每個配置使用不同的名稱。

Summary

總結(jié)

The pub/sub pattern helps you decouple services in a distributed application. The Dapr publish & subscribe building block simplifies implementing this behavior in your application.

Pub/sub 模式可幫助你解耦分布式應(yīng)用程序中的服務(wù)。在應(yīng)用程序中使用Dapr 發(fā)布 & 訂閱構(gòu)建塊可簡化此工作。

Through Dapr pub/sub, you can publish messages to a specific?topic. As well, the building block will query your service to determine which topic(s) to subscribe to.

通過 Dapr pub/sub，你可以將消息發(fā)布到特定?主題。同時，構(gòu)建塊將查詢你的服務(wù)，以確定訂閱的主題。

You can use Dapr pub/sub natively over HTTP or by using one of the language-specific SDKs, such as the .NET SDK for Dapr. The .NET SDK tightly integrates with the ASP.NET core platform.

你可以通過 HTTP 或使用特定于語言的 Sdk（如 .NET SDK for Dapr）使用 Dapr pub/sub。.NET SDK 與 ASP.NET core 平臺緊密集成。

With Dapr, you can plug a supported message broker product into your application. You can then swap message brokers without requiring code changes to your application.

使用 Dapr，可以將受支持的消息代理插入應(yīng)用程序。然后，你可以在無需對應(yīng)用程序進行代碼更改的情況下替換消息代理。

目錄：

• 面向.NET開發(fā)人員的Dapr——前言

• 面向.NET開發(fā)人員的Dapr——分布式世界

• 面向.NET開發(fā)人員的Dapr——俯瞰Dapr

• 面向.NET開發(fā)人員的Dapr——入門

• 面向.NET開發(fā)人員的Dapr——參考應(yīng)用程序

• 面向.NET開發(fā)人員的Dapr——狀態(tài)管理

• 面向.NET開發(fā)人員的Dapr——服務(wù)調(diào)用

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