Flume NG源码分析（三）使用Event接口表示数据流

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Flume NG有4个主要的组件：

Event表示在Flume各个Agent之间传递的数据流

Source表示从外部源接收Event数据流，然后传递给Channel

Channel表示对从Source传递的Event数据流的临时存储

Sink表示从Channel中接收存储的Event数据流，并传递给下游的Source或者终点仓库

这篇看一下Event接口表示的数据流。Source, Channel, Sink操作的数据流都是基于Event接口的封装。

public interface Event {  /**   * Returns a map of name-value pairs describing the data stored in the body.   */  public Map<String, String> getHeaders();  /**   * Set the event headers   * @param headers Map of headers to replace the current headers.   */  public void setHeaders(Map<String, String> headers);  /**   * Returns the raw byte array of the data contained in this event.   */  public byte[] getBody();  /**   * Sets the raw byte array of the data contained in this event.   * @param body The data.   */  public void setBody(byte[] body);}

Event接口非常简单，数据流分为两个部分：消息头和消息体。消息头是一个Key-Value的，存储字符串的结构。消息体是普通的字节数组。

Event的类层次结构如下

来看一下常用的SimpleEvent的具体实现，ExecSource等组件都是使用它来封装本地日志数据。它的实现非常简单，就是设置了header和body两部分数据。

public class SimpleEvent implements Event {  private Map<String, String> headers;  private byte[] body;  public SimpleEvent() {    headers = new HashMap<String, String>();    body = new byte[0];  }  @Override  public Map<String, String> getHeaders() {    return headers;  }  @Override  public void setHeaders(Map<String, String> headers) {    this.headers = headers;  }  @Override  public byte[] getBody() {    return body;  }  @Override  public void setBody(byte[] body) {    if(body == null){      body = new byte[0];    }    this.body = body;  }  @Override  public String toString() {    Integer bodyLen = null;    if (body != null) bodyLen = body.length;    return "[Event headers = " + headers + ", body.length = " + bodyLen + " ]";  }}

看一下如何创建Event对象实例

public class EventBuilder {  /**   * Instantiate an Event instance based on the provided body and headers.   * If <code>headers</code> is <code>null</code>, then it is ignored.   * @param body   * @param headers   * @return   */  public static Event withBody(byte[] body, Map<String, String> headers) {    Event event = new SimpleEvent();    if(body == null) {      body = new byte[0];    }    event.setBody(body);    if (headers != null) {      event.setHeaders(new HashMap<String, String>(headers));    }    return event;  }}

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