Teach you to learn dapr - 6 Publish subscription

Introduce

Release / Subscription mode allows microservices to communicate with each other using messages . Producers or publishers send messages to topics without knowing which application will receive them . This involves writing them to the input channel . Again , Consumers or subscribers subscribe to the topic and receive its messages , I don't know what service generated these messages . This involves receiving messages from the output channel . The intermediate message broker is responsible for copying each message from the input channel to the output channel of all subscribers interested in the message . When you need to separate microservices from each other , This model is particularly useful .

Dapr Release in / subscribe API Provide at least one （at-least-once） Guarantee , It integrates with various message brokers and queue systems . The specific implementation used by your service is pluggable , And configured to run at runtime Dapr Pub/Sub Components . This approach eliminates the dependency of your services , So that your service can be more portable , More flexibility to adapt to changes .

Dapr Release / The subscription building block provides a platform independent API To send and receive messages . Your service publishes messages to named topics , And subscribe to topics to use these messages .

The figure below shows a “shipping” Service and a “email” Examples of services , They're all subscribed to “cart” The topic of service publishing . Each service will be loaded to point to the same publication / Subscribe to the publication of the message bus component / Subscribe to the component configuration file , for example Redis Streams、NATS Streaming、Azure Service Bus or GCP Pub/Sub.

The following figure has the same service , But this time it shows Dapr Release API, It sends “ Order ” Order endpoints on topic and subscription services , These subject messages are provided by Dapr Publish to .

characteristic

Cloud Events The message format

To enable message routing and provide additional context for each message ,Dapr Use CloudEvents 1.0 Specification as its message format . Application usage Dapr Any message sent to the subject is automatically “ packing ” stay Cloud Events In the envelope , Use datacontenttype Attribute Content-Type Header value .

Dapr The following is achieved Cloud Events Field ：

• id
• source
• specversion
• type
• datacontenttype (Optional)

The following example shows CloudEvent v1.0 Serialized as JSON Of XML Content ：

{    "specversion" : "1.0",    "type" : "xml.message",    "source" : "https://example.com/message",    "subject" : "Test XML Message",    "id" : "id-1234-5678-9101",    "time" : "2020-09-23T06:23:21Z",    "datacontenttype" : "text/xml",    "data" : "<note><to>User1</to><from>user2</from><message>hi</message></note>"}

News subscription

Dapr Applications can subscribe to published topics . Dapr There are two ways to allow your application to subscribe to topics ：

• declarative , Where the subscription is defined in an external file
• Procedural , Define subscriptions in user code

The messaging

In principle, , When the subscriber responds with a non error response after processing the message ,Dapr Consider that the message has been successfully delivered . For finer control ,Dapr Release / subscribe API An explicit state defined in the response payload is also provided , Subscribers can use these states to send messages to Dapr Indicates a specific processing instruction （ for example RETRY or DROP）. If two different applications （ Different app-ID） Subscribed to the same topic ,Dapr Pass each message to only one instance of each application .

Subject area

By default , All support Dapr Release / Subscription component （ for example Kafka、Redis Stream、RabbitMQ） The topics of are available for each application that configures the component . To limit which applications can publish or subscribe to topics ,Dapr Provides a range of topics . This enables you to allow the application to publish and subscribe to what topics .

Message lifetime (TTL)

Dapr Timeout messages can be set on a per message basis , This means that if not from pub/sub Component reads the message , The message will be discarded . This is to prevent the accumulation of unread messages . In the queue than configured TTL Long news is called dead news .

notes ： You can also set messages for a given queue when the component is created TTL.

And no use Dapr and CloudEvents Application communication for

For an application, use Dapr A scenario that is not used by another application , You can disable... For publishers or subscribers CloudEvent packing . This allows the use of... Not all at once Dapr Some of our applications use Dapr Publish subscribe .

Use .Net call Dapr Publish subscription

The following example creates an application to publish and subscribe to an application named deathStarStatus The theme of

precondition

function dapr init when ,Redis Streams It is installed on the local machine by default . If it is dapr init --slim You need to do something yourself , I'm not going to do that here .

Verify by opening your component file %UserProfile%\.dapr\components\pubsub.yaml

apiVersion: dapr.io/v1alpha1kind: Componentmetadata:  name: pubsubspec:  type: pubsub.redis  version: v1  metadata:  - name: redisHost    value: localhost:6379  - name: redisPassword    value: ""

notes ： here redisHost Of value It can be set according to the actual situation , For example, a cloud redis Examples, etc . Someone asked if you can switch the default db, Certainly. ,name Set up redisDB,value Set to the... You want to use db that will do

If you want more details yaml Configuration parameters , For example, concurrency settings 、 The maximum number of retries and so on can be seen here https://docs.dapr.io/reference/components-reference/supported-pubsub/setup-redis-pubsub/

Subscribe to topics

There are two ways to subscribe to a topic ： declarative 、 Procedural

Declarative subscriptions

You can use the following custom resource definitions (CRD) Subscribe to topics . Create a file called subscription.yaml And paste the following ：

apiVersion: dapr.io/v1alpha1kind: Subscriptionmetadata:  name: myevent-subscriptionspec:  topic: deathStarStatus  route: /dsstatus  pubsubname: pubsubscopes:- app1- app2

The above example shows a pair of pubsub Components pubsub The theme deathStarStatus Event subscription for .

take CRD Put the file in the component directory , I won't go on here .

Programmed subscription

Dapr The instance calls your application at startup and looks forward to the topic subscription JSON Respond to ：

• pubsubname：Dapr Which should I use pub/sub Components
• topic： Which topic to subscribe to
• route： When the message relates to the subject ,Dapr Call which endpoint

notes ： You might think , Isn't it troublesome ？ Yes , So we use dapr dotnet-sdk To help us automate these things

.Net subscribe

The above is to let dapr sidecar Know who the subscription to this message will eventually go to . But how do we write it in our program ？

If you choose a declarative subscription , You make one route that will do , If it is a programmed subscription, you don't need to write one more yaml file , And it can support , So let's see .Net SDK How to do it .

establish Assignment.Server(Sub)

establish ASP.NET Core empty project , At the same time, add... According to the content of the previous article Dapr.AspNetCoreNuGet Package and modifier ports are 5000

modify program.cs Code

using Microsoft.AspNetCore.Mvc;var builder = WebApplication.CreateBuilder(args);var app = builder.Build();app.UseRouting();app.UseCloudEvents();app.UseEndpoints(endpoints =>{    endpoints.MapSubscribeHandler();});app.MapPost("/dsstatus", ([FromBody] string word) => Console.WriteLine($"Hello {word}!")).WithTopic("pubsub", "deathStarStatus");app.Run();

notes ： To tell Dapr The message was processed successfully , Please return 200 OK Respond to . If Dapr Received except 200 Any other return status code other than , Or if your application crashes ,Dapr Will try to follow At-Least-Once Semantic retransmission of messages .

function Assignment.Server

Use Dapr CLI To start up , First use the command line tool to jump to the directory dapr-study-room\Assignment06\Assignment.Server, Then execute the following command

dapr run --app-id testpubsub --app-port 5000 --dapr-http-port 3500 --dapr-grpc-port 50001 dotnet run

establish Assignment.Client(Publish)

establish Console project , And modify it program.cs

var client = new Dapr.Client.DaprClientBuilder().Build();await client.PublishEventAsync<string>("pubsub", "deathStarStatus", "World");

function Assignment.Client You can see Assignment.Server Will print Hello World!

Route messages to different event handlers

Content based routing is a way to use DSL Instead of the messaging pattern of imperative application code .PubSub Routing is an implementation of this pattern , It allows developers to use expressions based on CloudEvents The content of the is routed to different nodes in the application URI/ Paths and event handlers .

notes ： This is a preview function , If you are interested, you can try it yourself , It is worth mentioning that Common Expression Language (CEL) Very interesting , Here is just a piece of code to see .

        [Topic("pubsub", "inventory", "event.type ==\"widget\"", 1)]        [HttpPost("widgets")]        public async Task<ActionResult<Stock>> HandleWidget(Widget widget, [FromServices] DaprClient daprClient)        {            // Logic            return stock;        }        [Topic("pubsub", "inventory", "event.type ==\"gadget\"", 2)]        [HttpPost("gadgets")]        public async Task<ActionResult<Stock>> HandleGadget(Gadget gadget, [FromServices] DaprClient daprClient)        {            // Logic            return stock;        }        [Topic("pubsub", "inventory")]        [HttpPost("products")]        public async Task<ActionResult<Stock>> HandleProduct(Product product, [FromServices] DaprClient daprClient)        {            // Logic            return stock;        }

Release / Subscribe to topic access

A namespace or component scope can be used to restrict component access to a particular application . The scope of these applications added to the component is limited only to those with specific characteristics ID Your application can use this component .

Beyond the scope of this common component , Release / The subscription component can also limit the following ：

• What topics can be used （ Published or subscribed ）
• Which applications are allowed to publish to specific topics
• Which applications are allowed to subscribe to specific topics

Subject access

apiVersion: dapr.io/v1alpha1kind: Componentmetadata:  name: pubsub  namespace: defaultspec:  type: pubsub.redis  version: v1  metadata:  - name: redisHost    value: "localhost:6379"  - name: redisPassword    value: ""  - name: publishingScopes    value: "app1=topic1;app2=topic2,topic3;app3="  - name: subscriptionScopes    value: "app2=;app3=topic1"

The following table shows which applications are allowed to publish to topics ：

	topic1	topic2	topic3
app1	X
app2		X	X
app3

The following table shows which applications are allowed to subscribe to topics ：

	topic1	topic2	topic3
app1	X	X	X
app2
app3	X

notes ： If the application is not listed （ for example subscriptionScopes Medium app1）, You are allowed to subscribe to all topics . Because not used allowedTopics And app1 No subscription scope , So it can also use other topics not listed above .

Limit allowed topics

If Dapr The application sends a message to it , A theme will be created . In some cases , You should control the creation of this topic . for example ：

• stay Dapr In the application , Errors in generating topic names can cause an unlimited number of topics to be created
• Compact topic name and total number , Prevent the theme from growing indefinitely

apiVersion: dapr.io/v1alpha1kind: Componentmetadata:  name: pubsub  namespace: defaultspec:  type: pubsub.redis  version: v1  metadata:  - name: redisHost    value: "localhost:6379"  - name: redisPassword    value: ""  - name: allowedTopics    value: "topic1,topic2,topic3"

combination allowedTopics and scopes

Sometimes you want to combine these two scopes , Therefore, only a fixed set of topics is allowed , And assign the scope to a specific application .

apiVersion: dapr.io/v1alpha1kind: Componentmetadata:  name: pubsub  namespace: defaultspec:  type: pubsub.redis  version: v1  metadata:  - name: redisHost    value: "localhost:6379"  - name: redisPassword    value: ""  - name: allowedTopics    value: "A,B"  - name: publishingScopes    value: "app1=A"  - name: subscriptionScopes    value: "app1=;app2=A"

notes ： The third application is not listed , Because if an application does not specify in scope , It is allowed to use all topics .

The following table shows the applications that are allowed to be published to topics ：

	A	B	C
app1	X
app2	X	X
app3	X	X

The following table shows which applications can subscribe to topics :

	A	B	C
app1
app2	X
app3	X	X

news TTL

Same status management , Use metadata Of ttlInSeconds

Source code of this chapter

Assignment06

https://github.com/doddgu/dapr-study-room

We are acting , New framework 、 New ecology

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After several months of production project practice , Completed POC, At present, the previous accumulation is being refactored into new open source projects

At present, the source code has been synchronized to Github（ The document site is under planning , Will gradually improve ）：

MASA.BuildingBlocks

MASA.Contrib

MASA.Utils

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BlazorComponent

MASA.Blazor

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