EIP

Ref:https://camel.apache.org/manual/latest/enterprise-integration-patterns.html

1. 消息系统 MESSAGING SYSTEMS

• 信道
• 消息
• 管道
• 转换器
• 终结点

消息信道 Message Channel.

How does one application communicate with another using messaging?

Connect the applications using a Message Channel, where one application writes information to the channel and the other one reads that information from the channel.

可以理解为队列。

Eg: 标识一个JMS信道：

jms:queue:foo

消息 Message.

How can two applications connected by a message channel exchange a piece of information?

信息的载体

• id
• header
• body
• attachment

管道过滤 PIPELINE and Filters

How can we perform complex processing on a message while maintaining independence and flexibility?

消息路由 Message Router

How can you decouple individual processing steps so that messages can be passed to different filters depending on a set of conditions?

在inQueue和outQueue之间。

Eg: 是一个choice路由。

RouteBuilder builder = new RouteBuilder() {
    public void configure() {
        from("direct:a")
            .choice()
                .when(simple("${header.foo} == 'bar'"))
                    .to("direct:b")
                .when(simple("${header.foo} == 'cheese'"))
                    .to("direct:c")
                .otherwise()
                    .to("direct:d");
    }
};

消息转换 Message Translator.

How can systems using different data formats communicate with each other using messaging?

在消息与消息之间进行转换。即使用Processor。

Eg:使用Bean处理器

from("activemq:SomeQueue")
  .bean("myTransformerBean", "myMethodName")
  .to("mqseries:AnotherQueue");

Eg：使用Velocity转换

from("activemq:My.Queue")
    .to("velocity:com/acme/MyResponse.vm")
    .to("activemq:Another.Queue");

消息终结点 Message EndPoint.

How does an application connect to a messaging channel to send and receive messages?

2. 消息信道 MESSAGING CHANNELS

• 点对点
• 发布订阅
• 死信
• 担保发送

点对点信道 Point To Point Channel.

How can the caller be sure that exactly one receiver will receive the document or perform the call?

from("direct:start")
    .to("jms:queue:foo");

发布订阅信道 Publish Subscribe Channel.

How can the sender broadcast an event to all interested receivers?

from("direct:a")
    .multicast()
      .to("direct:b")
      .to("direct:c")
      .to("direct:d")
    .end() // end multicast

死信信道 Dead Letter Channel

What will the messaging system do with a message it cannot deliver?

errorHandler(deadLetterChannel("jms:queue:dead")
    .maximumRedeliveries(3).redeliveryDelay(5000));

担保发送 Guaranteed Delivery

How can the sender make sure that a message will be delivered, even if the messaging system fails?

• File for using file systems as a persistent store of messages
• JMS when using persistent delivery (the default) for working with JMS Queues and Topics for high performance, clustering and load balancing
• JPA for using a database as a persistence layer, or use any of the many other database component such as SQL, JDBC, MyBatis

信道适配器 CHANNEL ADAPTER

How can you connect an application to the messaging system so that it can send and receive messages?

from("kafka:cheese?brokers={{kafka.host}}:{{kafka.port}}"
    .to("bean:cheeseBean");

//or
public class CheeseBean {
  @Consume("kafka:cheese?brokers={{kafka.host}}:{{kafka.port}}")
  public void receiveCheeseData(byte[] data) {
    // do something
  }
}

MESSAGING BRIDGE

How can multiple messaging systems be connected so that messages available on one are also available on the others??

from("mq:queue:foo")
  .to("activemq:queue:foo")

MESSAGE BUS

from("direct:start")
    .pollEnrich("file:inbox?fileName=data.txt")
    .to("jms:queue:foo");

CHANGE DATA CAPTURE

3. 消息构造 MESSAGE CONSTRUCTION

• 事件消息
• 请求响应
• 返回地址
• 关联标志

事件消息 Event Message.

How can messaging be used to transmit events from one application to another?

使用InOnly和InOut区分。

指定为单向：

foo:bar?exchangePattern=InOnly

from("mq:someQueue").
  setExchangePattern(ExchangePattern.InOnly).
  bean(Foo.class);

from("mq:someQueue").
  inOnly("mq:anotherQueue");

请求响应 Request Reply.

When an application sends a message, how can it get a response from the receiver?

For example when using JMS with InOut the component will by default perform these actions

• 默认创建一个临时inbound队列 create by default a temporary inbound queue
• set the JMSReplyTo destination on the request message
• set the JMSCorrelationID on the request message
• send the request message
• consume the response and associate the inbound message to the request using the JMSCorrelationID (as you may be performing many concurrent request/responses).

强制：

jms:MyQueue?exchangePattern=InOut

返回地址 Return Address

How does a replier know where to send the reply?

关联标识 Correlation Identifier.

How does a requestor that has received a reply know which request this is the reply for?

Exchange.CORRELATION_ID

from("direct:start")
    .to(ExchangePattern.InOut, "jms:queue:foo?useMessageIDAsCorrelationID=true")
    .to("mock:result");

消息过期 Message Expiration.

How can a sender indicate when a message should be considered stale and thus shouldn’t be processed?

  from("direct:cheese")
    .to("jms:queueu:cheese?timeToLive=5000");

4. 消息路由 MESSAGE ROUTING

基于内容路由 Content Based Router.

How do we handle a situation where the implementation of a single logical function (e.g., inventory check) is spread across multiple physical systems?

RouteBuilder builder = new RouteBuilder() {
    public void configure() {
        errorHandler(deadLetterChannel("mock:error"));

        from("direct:a")
            .choice()
                .when(header("foo").isEqualTo("bar"))
                    .to("direct:b")
                .when(header("foo").isEqualTo("cheese"))
                    .to("direct:c")
                .otherwise()
                    .to("direct:d");
    }
};

过滤器 Message Filter.

How can a component avoid receiving uninteresting messages?

Predicate

from("direct:a")
    .filter(simple("${header.foo} == 'bar'"))
        .to("direct:b");

from("direct:start").
        filter().xpath("/person[@name='James']").
        to("mock:result");

from("direct:start")
    .filter().method(MyBean.class, "isGoldCustomer")
      .to("mock:gold")
    .end()
    .to("mock:all");

public static class MyBean {
    public boolean isGoldCustomer(@Header("level") String level) {
        return level.equals("gold");
    }
}

Exchange.FILTER_MATCHED

动态路由

How can you avoid the dependency of the router on all possible destinations while maintaining its efficiency?

from("direct:start")
    // use a bean as the dynamic router
    .dynamicRouter(method(DynamicRouterTest.class, "slip"));

public String slip(String body) {
    bodies.add(body);
    invoked++;

    if (invoked == 1) {
        return "mock:a";
    } else if (invoked == 2) {
        return "mock:b,mock:c";
    } else if (invoked == 3) {
        return "direct:foo";
    } else if (invoked == 4) {
        return "mock:result";
    }

    // no more so return null
    return null;
}

@DynamicRouter

• 收件人列表 Recipient List

How do we route a message to a list of (static or dynamically) specified recipients?

• 分割器 Splitter

How can we process a message if it contains multiple elements, each of which may have to be processed in a different way?

• 聚合器 Aggregator

How do we combine the results of individual, but related messages so that they can be processed as a whole?

• 限流 Throttler

How can I throttle messages to ensure that a specific endpoint does not get overloaded, or we don’t exceed an agreed SLA with some external service?

5. MESSAGE TRANSFORMATION

内容丰富 Content Enricher.

How do we communicate with another system if the message originator does not have all the required data items available?

• 内容过滤 Content Filter.

How do you simplify dealing with a large message, when you are interested only in a few data items?

• 权限检查 Claim Check. How can we reduce the data volume of message sent across the system without sacrificing information content?

• 归一化 Normalizer How do you process messages that are semantically equivalent, but arrive in a different format?

• 排序 Sort. How can I sort the body of a message?

• 脚本 Script. How do I execute a script which may not change the message?

• 验证 Validate.

How can I validate a message?

6. MESSAGING ENDPOINTS

• 消息是配置 Message Adapter.

How do you move data between domain objects and the messaging infrastructure while keeping the two independent of each other?

• 事件驱动消费者 Event Driven Consumer.

How can an application automatically consume messages as they become available?

• 轮训消费者 Polling Consumer.

How can an application consume a message when the application is ready?

7. SYSTEM MANAGEMENT