KeySpaceNotification键空间通知
1、Redis键淘汰机制简介
在Redis中，内存的大小是有限的，所以为了防止内存饱和，需要实现某种键淘汰策略。主要有两种方法，一种是当Redis内存不足时所采用的内存释放策略。第二种是对过期键进行删除的策略，也可以在某种程度上释放内存。
1.1Redis键过期淘汰的策略
当需要进行内存释放的时候，需要用某种策略对保存的的对象进行删除。Redis有六种策略：
volatile-lru：从已设置过期时间的数据集（server.db[i].expires）中挑选最近最少使用的数据淘汰
volatile-ttl：从已设置过期时间的数据集中挑选将要过期的数据淘汰
volatile-random：从已设置过期时间的数据集中任意选择数据淘汰
allkeys-lru：从数据集（server.db[i].dict）中挑选最近最少使用的数据淘汰
allkeys-random：从数据集（server.db[i].dict）中任意选择数据淘汰
no-enviction（驱逐）：禁止驱逐数据
2、KeySpaceNotification功能
2.1开启KeySpaceNotification功能
默认情况下，该功能是关闭的，因为该功能消耗部分CPU。开启该功能需要修改redis的配置文件。
当前需要修改的配置的机器有两台：Redis1、Redis2
配置文件的路径均为：/etc/redis
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图1Redis1中需要修改的配置文件
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图2Redis2中需要修改的配置文件
2.2修改配置文件
1.使用vi6379.conf进入文件；
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图3使用vi指令进入配置文件
2.输入/keyspace，回车后定位到该功能开启部分；
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图4定位keyspace
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图4查看通知功能介绍
3.修改配置文件为下图所示KEA表示所有的操作都会向相应的频道中发布通知信息；
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图5修改后的配置文件
4.按下Esc，输入:wq!回车保存文件；
5.按照上述过程将所有的配置文件。
6.重启Redis1、Redis2，使键空间通知功能开启。
3、使用键空间通知功能
3.1实现频道的订阅
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publicclassjedisSubscribe{
publicstaticvoidmain(String[]args){
SetjedisClusterNodes=newHashSet();
JedisClustercluster;
cluster=newJedisCluster(RedisPool.loadServers());
Stringhost1="192.168.1.34";
JedisPubSubjedisPubSub=null;
jedisPubSub=newJedisPubSub(){
/
常规模式：关闭订阅时触发arg0key值arg1订阅数量
/
publicvoidonUnsubscribe(Stringarg0,intarg1){
}
/
常规模式：启动订阅时触发arg0key值arg1订阅数量
/
publicvoidonSubscribe(Stringarg0,intarg1){
System.out.println("SuccessonSubscribe"+arg0);
}
/
常规模式：收到匹配key值的消息时触发arg0key值arg1收到的消息值
/
publicvoidonMessage(Stringarg0,Stringarg1){
System.out.println("GetMessage"+arg1);
System.out.println("Get"+arg0);
Stringkey=cluster.get(arg0);
System.out.println("Success"+key);
}
/
正则模式：关闭正则类型订阅时触发
arg0key的正则表达式arg1订阅数量
/
publicvoidonPUnsubscribe(Stringarg0,intarg1){
}
/
正则模式：启动正则类型订阅时触发
arg0key的正则表达式
arg1订阅数量
/
publicvoidonPSubscribe(Stringarg0,intarg1){
}
/
正则模式：收到匹配key值的消息时触发
arg0订阅的key正则表达式
arg1匹配上该正则key值
arg2收到的消息值
/
publicvoidonPMessage(Stringarg0,Stringarg1,Stringarg2){
}
};
Stringchannel4="__keyevent@0__:expire";//设置订阅的频道channels
JedisPooljedisPool1=newJedisPool(host1);
jedisPool1.getResource().subscribe(jedisPubSub,channel4);
}
}
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3.2对Redis中的键进行操作
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publicstaticvoidmain(String[]args)throwsInterruptedException{
SetjedisClusterNodes=newHashSet();
JedisClustercluster;
cluster=newJedisCluster(RedisPool.loadServers());
Stringkey="name";//设置一个key
Stringvalue="zpf";//设置该key的value
cluster.set(key,value);//向redis中写入该(key,value)
cluster.expire(key,2);//设置key过期时间为2秒
cluster.del(key);
}
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3.3运行结果
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4.车辆信息的实时性维护
4.1利用键空间通知的方法
Redis会在设置过期时间的键集合中随机抽选进行键的判断，如果过期则进行删除操作，同时会向频道__keyevent@0__:expire中过期消息，并且监听这个频道的程序会得到过期的键是哪一个键。也会向频道__keyspace@0__:key中发布该键过期的消息，其中这里的频道根据过期的键key不同发布的频道也不同。进行删除操作时，Redis也会向频道__keyevent@0__:del和__keyspace@0__:key中分别发布消息。
为了维护车辆信息的实时性，需要对过期的车辆数据进行相应的删除操作，但是Redis中的删除操作只能将对应的key删除掉，这里的需求是进行网格以及其他相应的维护操作，删除操作是不相同的。因此需要再删除之前得到过期的key的值value。
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根据上面的描述，无法再订阅频道__keyevent@0__:expire接收到通知的时候得到过期键的值value。为了实现借助Redis的删除功能需要在该频道接收到信息之前得到要过期的key的值value。但是这样的话又成了对redis的判断操作。为了实现得到过期键的value，可以通过在得到该数据并进行设置过期时间的时候提前构造一个map其中key为车牌，value为车牌+原始信息的value。这样在频道__keyevent@0__:expire接收到通知时，就可以使用map得到相应的value值。
4.2使用该方法造成的问题
构造map时，需要对每一辆车进行判断，如果车牌key不在此map中时，需要将该信息放入map中；如果车牌key存在此map中，需要更新map。那么存储该map需要占用额外的存储空间。
参考资料：
RedisKeyspaceNotifications
IMPORTANTKeyspacenotificationsisafeatureavailablesince2.8.0
Featureoverview
KeyspacenotificationsallowsclientstosubscribetoPub/SubchannelsinordertoreceiveeventsaffectingtheRedisdatasetinsomeway.
Examplesoftheeventsthatispossibletoreceivearethefollowing:
Allthecommandsaffectingagivenkey.
AllthekeysreceivinganLPUSHoperation.
Allthekeysexpiringinthedatabase0.
EventsaredeliveredusingthenormalPub/SublayerofRedis,soclientsimplementingPub/Subareabletousethisfeaturewithoutmodifications.
BecauseRedisPub/Subisfireandforgetcurrentlythereisnowaytousethisfeatureifyouapplicationdemandsreliablenotificationofevents,thatis,ifyourPub/Subclientdisconnects,andreconnectslater,alltheeventsdeliveredduringthetimetheclientwasdisconnectedarelost.
Inthefuturethereareplanstoallowformorereliabledeliveringofevents,butprobablythiswillbeaddressedatamoregeneralleveleitherbringingreliabilitytoPub/Subitself,orallowingLuascriptstointerceptPub/Submessagestoperformoperationslikepushingtheeventsintoalist.
Typeofevents
KeyspacenotificationsareimplementedsendingtwodistincttypeofeventsforeveryoperationaffectingtheRedisdataspace.ForinstanceaDELoperationtargetingthekeynamedmykeyindatabase0willtriggerthedeliveringoftwomessages,exactlyequivalenttothefollowingtwoPUBLISHcommands:
PUBLISH__keyspace@0__:mykeydel
PUBLISH__keyevent@0__:delmykey
Itiseasytoseehowonechannelallowstolistentoalltheeventstargetingthekeymykeyandtheotherchannelallowstoobtaininformationaboutallthekeysthataretargetofadeloperation.
Thefirstkindofevent,withkeyspaceprefixinthechanneliscalledaKey-spacenotification,whilethesecond,withthekeyeventprefix,iscalledaKey-eventnotification.
Intheaboveexampleadeleventwasgeneratedforthekeymykey.Whathappensisthat:
TheKey-spacechannelreceivesasmessagethenameoftheevent.
TheKey-eventchannelreceivesasmessagethenameofthekey.
Itispossibletoenableonlyonekindofnotificationinordertodeliverjustthesubsetofeventsweareinterestedin.
Configuration
BydefaultkeyspaceeventsnotificationsaredisabledbecausewhilenotverysensiblethefeatureusessomeCPUpower.Notificationsareenabledusingthenotify-keyspace-eventsofredis.conforviatheCONFIGSET.
Settingtheparametertotheemptystringdisablesnotifications.Inordertoenablethefeatureanon-emptystringisused,composedofmultiplecharacters,whereeverycharacterhasaspecialmeaningaccordingtothefollowww.wang027.comwingtable:
KKeyspaceevents,publishedwith__keyspace@__prefix.
EKeyeventevents,publishedwith__keyevent@__prefix.
gGenericcommands(non-typespecific)likeDEL,EXPIRE,RENAME,...
$Stringcommands
lListcommands
sSetcommands
hHashcommands
zSortedsetcommands
xExpiredevents(eventsgeneratedeverytimeakeyexpires)
eEvictedevents(eventsgeneratedwhenakeyisevictedformaxmemory)
AAliasforg$lshzxe,sothatthe"AKE"stringmeansalltheevents.
AtleastKorEshouldbepresentinthestring,otherwisenoeventwillbedeliveredregardlessoftherestofthestring.
ForinstancetoenablejustKey-spaceeventsforlists,theconfigurationparametermustbesettoKl,andsoforth.
ThestringKEAcanbeusedtoenableeverypossibleevent.
Eventsgeneratedbydifferentcommands
Differentcommandsgeneratedifferentkindofeventsaccordingtothefollowinglist.
DELgeneratesadeleventforeverydeletedkey.
RENAMEgeneratestwoevents,arename_fromeventforthesourcekey,andarename_toeventforthedestinationkey.
EXPIREgeneratesanexpireeventwhenanexpireissettothekey,oraexpiredeventeverytimesettinganexpireresultsintothekeybeingdeleted(seeEXPIREdocumentationformoreinfo).
SORTgeneratesasortstoreeventwhenSTOREisusedtosetanewkey.Iftheresultinglistisempty,andtheSTOREoptionisused,andtherewasalreadyanexistingkeywiththatname,theresultisthatthekeyisdeleted,soadeleventisgeneratedinthiscondition.
SETandallitsvariants(SETEX,SETNX,GETSET)generatesetevents.HoweverSETEXwillalsogenerateanexpireevents.
MSETgeneratesaseparatedseteventforeverykey.
SETRANGEgeneratesasetrangeevent.
INCR,DECR,INCRBY,DECRBYcommandsallgenerateincrbyevents.
INCRBYFLOATgeneratesanincrbyfloatevents.
APPENDgeneratesanappendevent.
LPUSHandLPUSHXgeneratesasinglelpushevent,eveninthevariadiccase.
RPUSHandRPUSHXgeneratesasinglerpushevent,eveninthevariadiccase.
RPOPgeneratesanrpopevent.Additionallyadeleventisgeneratedifthekeyisremovedbecausethelastelementfromthelistwaswww.baiyuewang.netpopped.
LPOPgeneratesanlpopevent.Additionallyadeleventisgeneratedifthekeyisremovedbecausethelastelementfromthelistwaspopped.
LINSERTgeneratesanlinsertevent.
LSETgeneratesanlsetevent.
LREMgeneratesanlremevent,andadditionallyadeleventiftheresultinglistisemptyandthekeyisremoved.
LTRIMgeneratesanltrimevent,andadditionallyadeleventiftheresultinglistisemptyandthekeyisremoved.
RPOPLPUSHandBRPOPLPUSHgenerateanrpopeventandanlpushevent.Inbothcasestheorderisguaranteed(thelpusheventwillalwaysbedeliveredaftertherpopevent).Additionallyadeleventwillbegeneratediftheresultinglistiszerolengthandthekeyisremoved.
HSET,HSETNXandHMSETallgenerateasinglehsetevent.
HINCRBYgeneratesanhincrbyevent.
HINCRBYFLOATgeneratesanhincrbyfloatevent.
HDELgeneratesasinglehdelevent,andanadditionaldeleventiftheresultinghashisemptyandthekeyisremoved.
SADDgeneratesasinglesaddevent,eveninthevariadiccase.
SREMgeneratesasinglesremevent,andanadditionaldeleventiftheresultingsetisemptyandthekeyisremoved.
SMOVEgeneratesansremeventforthesourcekey,andansaddeventforthedestinationkey.
SPOPgeneratesanspopevent,andanadditionaldeleventiftheresultingsetisemptyandthekeyisremoved.
SINTERSTORE,SUNIONSTORE,SDIFFSTOREgeneratesinterstore,sunionostore,sdiffstoreeventsrespectively.Inthespecialcasetheresultingsetisempty,andthekeywheretheresultisstoredalreadyexists,adeleventisgeneratedsincethekeyisremoved.
ZINCRgeneratesazincrevent.
ZADDgeneratesasinglezaddeventevenwhenmultipleelementsareadded.
ZREMgeneratesasinglezremeventevenwhenmultipleelementsaredeleted.Whentheresultingsortedsetisemptyandthekeyisgenerated,anadditionaldeleventisgenerated.
ZREMBYSCOREgeneratesasinglezrembyscoreevent.Whentheresultingsortedsetisemptyandthekeyisgenerated,anadditionaldeleventisgenerated.
ZREMBYRANKgeneratesasinglezrembyrankevent.Whentheresultingsortedsetisemptyandthekeyisgenerated,anadditionaldeleventisgenerated.
ZINTERSTOREandZUNIONSTORErespectivelygeneratezinterstoreandzunionstoreevents.Inthespecialcasetheresultingsortedsetisempty,andthekeywheretheresultisstoredalreadyexists,adeleventisgeneratedsincethekeyisremoved.
Everytimeakeywithatimetoliveassociatedisremovedfromthedatasetbecauseitexpired,anexpiredeventisgenerated.
Everytimeakeyisevictedfromthedatasetinordertofreememoryasaresultofthemaxmemorypolicy,anevictedeventisgenerated.
IMPORTANTallthecommandsgenerateeventsonlyifthetargetkeyisreallymodified.ForinstanceanSREMdeletinganon-existingelementfromaSetwillnotactuallychangethevalueofthekey,sonoeventwillbegenerated.
Ifindoubtabouthoweventsaregeneratedforagivencommand,thesimplestthingtodoistowatchyourself:
$redis-cliconfigsetnotify-keyspace-eventsKEA
$redis-cli--csvpsubscribe''__key__:''
Readingmessages...(pressCtrl-Ctoquit)
"psubscribe","__key__:",1
Atthispointuseredis-cliinanotherterminaltosendcommandstotheRedisserverandwatchtheeventsgenerated:
"pmessage","__key__:","__keyspace@0__:foo","set"
"pmessage","__key__:","__keyevent@0__:set","foo"
...
Timingofexpiredevents
KeyswithatimetoliveassociatedareexpiredbyRedisintwoways:
Whenthekeyisaccessedbyacommandandisfoundtobeexpired.
Viaabackgroundsystemthatlooksforexpiredkeysinbackground,incrementally,inordertobeabletoalsocollectkeysthatareneveraccessed.
Theexpiredeventsaregeneratedwhenakeyisaccessedandisfoundtobeexpiredbyoneoftheabovesystems,asaresulttherearenoguaranteesthattheRedisserverwillbeabletogeneratetheexpiredeventatthetimethekeytimetolivereachesthevalueofzero.
Ifnocommandtargetsthekeyconstantly,andtherearemanykeyswithaTTLassociated,therecanbeasignificantdelaybetweenthetimethekeytimetolivedropstozero,andthetimetheexpiredeventisgenerated.
BasicallyexpiredeventsaregeneratedwhentheRedisserverdeletesthekeyandnotwhenthetimetolivetheoreticallyreachesthevalueofzero.
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