esper（4-1）-简单context

1、创建context语法

create context context_name 
partition [by] event_property [and event_property [and ...]] 
from stream_def [, event_property [...] from stream_def] [, ...]

context_name为context的名字，并且唯一。如果重复，会说明已存在。

event_property为事件的属性名，多个属性名之间用and连接，也可以用逗号连接。

stream_def就是view。

例如：

//一个流：id和name是User的属性
create context NewUser partition by id and name from User

//多个流：sid是Student的属性，tid是Teacher的属性
create context Person partition by sid from Student, tid from Teacher

多个流一定要注意，每个流的中用于context的属性的数量要一样，数据类型也要一致。比如下面这几个就是错误的：

// 错误：sid是int，tname是String，数据类型不一致
create context Person partition by sid from Student, tname from Teacher

// 错误：Student有一个属性，Teacher有两个属性，属性数量不一致
create context Person partition by sid from Student, tid,tname from Teacher

// 错误：sid对应tname，sname对应tid，并且sname和tname是String，sid和tid是int，属性数量一样，但是对应的数据类型不一致
create context Person partition by sid,sname from Student, tname,tid from Teacher

可以对进入context的事件增加过滤条件，不符合条件的就被过滤掉，就像下面这样：

// age大于20的Student事件才能建立或者进入context
create context Person partition by sid from Student(age > 20)

例子：

import cn.hutool.core.lang.Console;
import com.espertech.esper.client.*;
import com.espertech.esper.client.context.ContextPartitionSelectorSegmented;
import lombok.*;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * @author yaoyuan2
 * @date 2019/3/26
 */
public class SimpleContext {
    static EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider();
    static EPAdministrator admin = epService.getEPAdministrator();
    static String view = User.class.getName();
    public static void main(String[] args) {
        String epl1 = "create context esbtest partition by id from "+view;//必须首先创建，否则报错：没有declare context
        String epl2 = "context esbtest select avg(amount) as avgAmount,id from " + view;
        Console.log(epl1);
        Console.log(epl2);
        EPStatement context = admin.createEPL(epl1);
        EPStatement state = admin.createEPL(epl2);
        state.addListener(new UpdateListener() {
            @Override
            public void update(EventBean[] newEvents, EventBean[] oldEvents) {
                if (newEvents != null) {
                    Double avgAmount = (Double) (newEvents[0].get("avgAmount"));
                    Integer id = (Integer) (newEvents[0].get("id"));
                    Console.log("id={},avgAmount={}",id,avgAmount);
                }
            }
        });
        EPRuntime runtime = epService.getEPRuntime();
        //查看id为1的平均金额
        ContextPartitionSelectorSegmented selectCtx = new ContextPartitionSelectorSegmented() {

            // 该方法的实现方式与context定义的properties有关，如果有两个property id和time，则Object数组长度为2，obj[0]为id值，
            // obj[1]为time值，然后再添加到list中并返回
            @Override
            public List<Object[]> getPartitionKeys() {
                Object[] o = new Object[1];
                o[0] = 1;
                List<Object[]> list = new ArrayList<Object[]>();
                list.add(o);
                return list;
            }
        };
        User u1 = new User(1,20);
        Console.log("sendEvent: id={},amount={}",u1.getId(),u1.getAmount());
        runtime.sendEvent(u1);
        User u2 = new User(2,30);
        Console.log("sendEvent: id={},amount={}",u2.getId(),u2.getAmount());
        runtime.sendEvent(u2);
        User u3 = new User(1,30);
        Console.log("sendEvent: id={},amount={}",u3.getId(),u3.getAmount());
        runtime.sendEvent(u3);

        Iterator<EventBean> it = state.iterator(selectCtx);
        EventBean event = it.hasNext() ? it.next() : null;
        Console.log("Iterator context:id=1,avgAmount={}",event.get("avgAmount"));

        User u4 = new User(2,40);
        Console.log("sendEvent: id={},amount={}",u4.getId(),u4.getAmount());
        runtime.sendEvent(u4);

    }
}
@Setter
@Getter
@ToString
@NoArgsConstructor
@AllArgsConstructor
class User implements Serializable {
    private int id;
    private int amount;
}

输出

create context esbtest partition by id from com.ebc.User
context esbtest select avg(amount) as avgAmount,id from com.ebc.User

sendEvent: id=1,amount=20
id=1,avgAmount=20.0
sendEvent: id=2,amount=30
id=2,avgAmount=30.0
sendEvent: id=1,amount=30
id=1,avgAmount=25.0
Iterator context:id=1,avgAmount=25.0
sendEvent: id=2,amount=40
id=2,avgAmount=35.0

通过例子发现，partition by id会根据id分组，每组进入各自的context。上例中，会建2个context，一个id=1的，另外1个id=2的。

如果partition by后边有同一个流的多个属性值，如：partition by id,name。A事件id=1,amount=20，B事件id=1,amount=21，C事件id=1,amount=20，那么A和C在一个context，B在另外1个context。