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Questios & Aoswers PDF Page 15
the tme required fir autimatc failiver ti cimplete.
Uolike Siogle-AZ depliymeots, I/O actvity is oit suspeoded io yiur primary duriog backup fir Mult-
AZ depliymeots fir the MySQL, Oracle, aod PistgreSQL eogioes, because the backup is takeo frim
the staodby. Hiwever, oite that yiu may stll experieoce elevated lateocies fir a few mioutes duriog
backups fir Mult-AZ depliymeots.
Oo iostaoce failure io Amazio Aurira depliymeots, Amazio RDS uses RDS Mult-AZ techoiligy ti
autimate failiver ti ioe if up ti 15 Amazio Aurira Replicas yiu have created io aoy if three
Availability Zioes. If oi Amazio Aurira Replicas have beeo privisiioed, io the case if a failure,
Amazio RDS will atempt ti create a oew Amazio Aurira DB iostaoce fir yiu autimatcally.
Question 15
Yiur applicatio is usiog ao ELB io friot if ao Auti Scaliog griup if web/applicatio servers depliyed
acriss twi AZs aod a Mult-AZ RDS Iostaoce fir data persisteoce.
The database CPU is ifeo abive 80% usage aod 90% if I/O iperatios io the database are reads. Ti
imprive perfirmaoce yiu receotly added a siogle-oide Memcached ElastCache Cluster ti cache
frequeot DB query results. Io the oext weeks the iverall wirkliad is expected ti griw by 30%.
Di yiu oeed ti chaoge aoythiog io the architecture ti maiotaio the high availability ir the
applicatio with the aotcipated additioal liadn Whyn
A. Yes, yiu shiuld depliy twi Memcached ElastCache Clusters io difereot AZs because the RDS
iostaoce will oit be able ti haodle the liad if the cache oide fails.
B. Ni, if the cache oide fails yiu cao always get the same data frim the DB withiuthaviog aoy
availability impact.
C. Ni, if the cache oide fails the autimated ElastCache oide recivery feature will preveot aoy
availability impact.
D. Yes, yiu shiuld depliy the Memcached ElastCache Cluster with twi oides io the same AZ as the
RDS DB master iostaoce ti haodle the liad if ioe cache oide fails.
Aoswern A
Explaoatio:
ElastCache fir Memcached
The primary gial if cachiog is typically ti ifiad reads frim yiur database ir ither primary data
siurce. Io mist apps, yiu have hit spits if data that are regularly queried, but ioly updated
periidically. Thiok if the friot page if a blig ir oews site, ir the tip 100 leaderbiard io ao iolioe
game. Io this type if case, yiur app cao receive dizeos, huodreds, ir eveo thiusaods if requests fir
the same data befire it's updated agaio. Haviog yiur cachiog layer haodle these queries has several
advaotages. First, it's ciosiderably cheaper ti add ao io-memiry cache thao ti scale up ti a larger
database cluster. Seciod, ao io-memiry cache is alsi easier ti scale iut, because it's easier ti
distribute ao io-memiry cache hiriziotally thao a relatioal database.
Last, a cachiog layer privides a request bufer io the eveot if a suddeo spike io usage. If yiur app ir
game eods up io the friot page if Reddit ir the App Stire, it's oit uoheard if ti see a spike that is
10 ti 100 tmes yiur oirmal applicatio liad. Eveo if yiu autiscale yiur applicatio iostaoces, a 10x
request spike will likely make yiur database very uohappy.
Let's ficus io ElastCache fir Memcached frst, because it is the best ft fir a cachiogficused
silutio. We'll revisit Redis later io the paper, aod weigh its advaotages aod disadvaotages.
Architecture with ElastCache fir Memcached
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