SCAN的作用是簡(jiǎn)化客戶端連接數(shù)據(jù)庫的配置，無論集群增加還是刪除節(jié)點(diǎn)SCAN名稱會(huì)一直保持不變，客戶端不需要做任何的修改。SCAN是一個(gè)名稱，通過DNS被解析成最多3個(gè)IP地址（SCAN VIP）

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SCAN VIP的作用是接收客戶端連接，SCAN VIP必須與集群的公網(wǎng)資源處于相同的子網(wǎng)，每一個(gè)SCAN VIP都會(huì)有對(duì)應(yīng)的SCAN LISTENER，并且SCAN LISTENER只會(huì)監(jiān)聽對(duì)應(yīng)的SCAN VIP，而且它們必須運(yùn)行在同一個(gè)節(jié)點(diǎn)。SCAN VIP會(huì)均勻地分布在集群的各個(gè)節(jié)點(diǎn)，例如：一個(gè)包含了8個(gè)節(jié)點(diǎn)的集群，會(huì)選擇其中3個(gè)節(jié)點(diǎn)運(yùn)行SCAN VIP，當(dāng)運(yùn)行SCAN VIP的節(jié)點(diǎn)出現(xiàn)問題時(shí)，SCAN VIP會(huì)漂移到其他正常節(jié)點(diǎn)，對(duì)于一個(gè)兩節(jié)點(diǎn)的集群，如果配置了3個(gè)SCAN VIP，一定會(huì)有一個(gè)節(jié)點(diǎn)存在兩個(gè)SCAN VIP ，另一個(gè)節(jié)點(diǎn)存在一個(gè)SCAN VIP。

SCAN VIP和SCAN LISTENER以集群資源的形式存在，由集群的代理進(jìn)程進(jìn)行統(tǒng)一管理，SCAN VIP依賴于集群的公網(wǎng)資源，而SCAN LISTENER依賴于對(duì)應(yīng)的SCAN VIP資源，當(dāng)本地節(jié)點(diǎn)出現(xiàn)問題SCAN VIP漂移到其他節(jié)點(diǎn)后，對(duì)應(yīng)的SCAN LISTENER也需要隨著SCAN VIP一起漂移。

通過SCAN連接到數(shù)據(jù)庫

階段1：每個(gè)節(jié)點(diǎn)的PMON進(jìn)程通過訪問數(shù)據(jù)庫信息將本地實(shí)例提供的服務(wù)注冊(cè)到本地節(jié)點(diǎn)的監(jiān)聽程序和每一個(gè)SCAN監(jiān)聽程序。其中l(wèi)ocal_listener指定了PMON需要注冊(cè)的本地監(jiān)聽程序位置，而remote_listener指定了需要注冊(cè)的SCAN監(jiān)聽程序位置，之后，無論是本地節(jié)點(diǎn)的監(jiān)聽程序，還是SCAN監(jiān)聽程序都已經(jīng)為接收客戶端的連接做好準(zhǔn)備。

階段2：客戶端通過tnsnames.ora中的別名向SCAN監(jiān)聽程序發(fā)起連接，由于SCAN監(jiān)聽程序分布在集群的不同節(jié)點(diǎn)，連接到哪個(gè)節(jié)點(diǎn)取決于DNS將SCAN名解析成哪一個(gè)節(jié)點(diǎn)的SCAN VIP（DNS使用輪詢的方式返回SCAN對(duì)應(yīng)的IP地址）。

階段3：SCAN監(jiān)聽程序在收到了客戶端發(fā)送的請(qǐng)求后，根據(jù)每個(gè)節(jié)點(diǎn)的負(fù)責(zé)情況，將客戶端連接轉(zhuǎn)發(fā)給負(fù)載最低的節(jié)點(diǎn)本地監(jiān)聽程序，之后對(duì)應(yīng)節(jié)點(diǎn)的本地監(jiān)聽程序完成與客戶端的連接。

服務(wù)器端負(fù)載均衡

數(shù)據(jù)庫實(shí)例的PMON進(jìn)程會(huì)將本地實(shí)例的服務(wù)信息和工作負(fù)載信息注冊(cè)到SCAN LISTENER上，每個(gè)服務(wù)在每個(gè)節(jié)點(diǎn)的工作負(fù)載信息在SCAN LISTENER層面就存在了，SCAN LISTENER已經(jīng)能夠完成服務(wù)器端負(fù)載均衡的工作，將收到的連接直接發(fā)送給負(fù)載最低的節(jié)點(diǎn)的本地監(jiān)聽程序。

階段1：客戶端通過指定SCAN的tnsnames.ora別名連接到某一個(gè)SCAN監(jiān)聽程序。

階段2：SCAN監(jiān)聽程序分析包含申請(qǐng)服務(wù)的每個(gè)節(jié)點(diǎn)的最新工作負(fù)載信息，選擇負(fù)載最低的節(jié)點(diǎn)，并將客戶端連接分發(fā)給對(duì)應(yīng)節(jié)點(diǎn)的監(jiān)聽程序。

階段3：本地節(jié)點(diǎn)的監(jiān)聽程序接收到了連接后，檢查是否有對(duì)應(yīng)的服務(wù)注冊(cè)，并通知客戶端重新連接本地節(jié)點(diǎn)監(jiān)聽程序。

階段4：客戶端接收到服務(wù)器對(duì)應(yīng)節(jié)點(diǎn)的監(jiān)聽程序發(fā)送的信息后，重新連接新的位置。

階段5：本地節(jié)點(diǎn)監(jiān)聽程序在接收到連接信息之后，完成對(duì)數(shù)據(jù)庫的連接。

查詢SCAN的配置 [root@wallet01?~]#?su?-?grid [grid@wallet01?~]$?srvctl?config?scan SCAN?name:?wallet.gscaifu.com,?Network:?1/192.168.40.0/255.255.255.0/eth0 SCAN?VIP?name:?scan1,?IP:?/wallet.gscaifu.com/192.168.40.103 SCAN?VIP?name:?scan2,?IP:?/wallet.gscaifu.com/192.168.40.101 SCAN?VIP?name:?scan3,?IP:?/wallet.gscaifu.com/192.168.40.102 查詢SCAN的狀態(tài) [grid@wallet01?~]$?srvctl?status?scan SCAN?VIP?scan1?is?enabled SCAN?VIP?scan1?is?running?on?node?wallet01 SCAN?VIP?scan2?is?enabled SCAN?VIP?scan2?is?running?on?node?wallet03 SCAN?VIP?scan3?is?enabled SCAN?VIP?scan3?is?running?on?node?wallet02 查詢SCAN?LISTENER的配置 [grid@wallet01?~]$?srvctl?config?scan_listener SCAN?Listener?LISTENER_SCAN1?exists.?Port:?TCP:1521 SCAN?Listener?LISTENER_SCAN2?exists.?Port:?TCP:1521 SCAN?Listener?LISTENER_SCAN3?exists.?Port:?TCP:1521 查詢SCAN?LISTENER的狀態(tài) [grid@wallet01?~]$?srvctl?status?scan_listener SCAN?Listener?LISTENER_SCAN1?is?enabled SCAN?listener?LISTENER_SCAN1?is?running?on?node?wallet01 SCAN?Listener?LISTENER_SCAN2?is?enabled SCAN?listener?LISTENER_SCAN2?is?running?on?node?wallet03 SCAN?Listener?LISTENER_SCAN3?is?enabled SCAN?listener?LISTENER_SCAN3?is?running?on?node?wallet02 查詢本地LISTENER的狀態(tài) [grid@wallet01?~]$?lsnrctl?status?LISTENER LSNRCTL?for?Linux:?Version?11.2.0.4.0?-?Production?on?07-MAY-2019?15:36:20 Copyright?(c)?1991,?2013,?Oracle.??All?rights?reserved. Connecting?to?(DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=LISTENER))) STATUS?of?the?LISTENER ------------------------ Alias?????????????????????LISTENER Version???????????????????TNSLSNR?for?Linux:?Version?11.2.0.4.0?-?Production Start?Date????????????????07-MAY-2019?13:20:31 Uptime????????????????????0?days?2?hr.?15?min.?48?sec Trace?Level???????????????off Security??????????????????ON:?Local?OS?Authentication SNMP??????????????????????OFF Listener?Parameter?File???/u01/app/11.2.0.4/grid/network/admin/listener.ora Listener?Log?File?????????/u01/app/grid/diag/tnslsnr/wallet01/listener/alert/log.xml Listening?Endpoints?Summary... ??(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=LISTENER))) ??(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=192.168.40.34)(PORT=1521))) ??(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=192.168.40.134)(PORT=1521))) Services?Summary... Service?"+ASM"?has?1?instance(s). ??Instance?"+ASM1",?status?READY,?has?1?handler(s)?for?this?service... Service?"gspublic"?has?1?instance(s). ??Instance?"wallet1",?status?READY,?has?1?handler(s)?for?this?service... Service?"wallet"?has?1?instance(s). ??Instance?"wallet1",?status?READY,?has?1?handler(s)?for?this?service... Service?"walletXDB"?has?1?instance(s). ??Instance?"wallet1",?status?READY,?has?1?handler(s)?for?this?service... The?command?completed?successfully 查詢LISTENER_SCAN1的狀態(tài) [grid@wallet01?~]$?lsnrctl?status?LISTENER_SCAN1 LSNRCTL?for?Linux:?Version?11.2.0.4.0?-?Production?on?07-MAY-2019?15:36:57 Copyright?(c)?1991,?2013,?Oracle.??All?rights?reserved. Connecting?to?(DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=LISTENER_SCAN1))) STATUS?of?the?LISTENER ------------------------ Alias?????????????????????LISTENER_SCAN1 Version???????????????????TNSLSNR?for?Linux:?Version?11.2.0.4.0?-?Production Start?Date????????????????07-MAY-2019?13:20:31 Uptime????????????????????0?days?2?hr.?16?min.?26?sec Trace?Level???????????????off Security??????????????????ON:?Local?OS?Authentication SNMP??????????????????????OFF Listener?Parameter?File???/u01/app/11.2.0.4/grid/network/admin/listener.ora Listener?Log?File?????????/u01/app/11.2.0.4/grid/log/diag/tnslsnr/wallet01/listener_scan1/alert/log.xml Listening?Endpoints?Summary... ??(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=LISTENER_SCAN1))) ??(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=192.168.40.103)(PORT=1521))) Services?Summary... Service?"SYS$SYS.SCHEDULER$_EVENT_QUEUE.WALLET"?has?1?instance(s). ??Instance?"wallet2",?status?READY,?has?1?handler(s)?for?this?service... Service?"gspublic"?has?1?instance(s). ??Instance?"wallet1",?status?READY,?has?1?handler(s)?for?this?service... Service?"wallet"?has?3?instance(s). ??Instance?"wallet1",?status?READY,?has?1?handler(s)?for?this?service... ??Instance?"wallet2",?status?READY,?has?1?handler(s)?for?this?service... ??Instance?"wallet3",?status?READY,?has?1?handler(s)?for?this?service... Service?"walletXDB"?has?3?instance(s). ??Instance?"wallet1",?status?READY,?has?1?handler(s)?for?this?service... ??Instance?"wallet2",?status?READY,?has?1?handler(s)?for?this?service... ??Instance?"wallet3",?status?READY,?has?1?handler(s)?for?this?service... The?command?completed?successfully

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