输入验证(Validating Input)
updatingthestoredmetadataintheprocess,ortoreplacethestoredresponse(s)withanewresponse.Thisprocessisknownas"validating"or"revalidating"thestoredresponse
itfirsthastocheckwiththeoriginserver(orpossiblyanintermediatecachewithafreshresponse)toseeifitscachedentryisstillusable.Wecallthis"validating"thecacheentry.Sincewedonotwanttohavetopaytheoverheadofretransmittingthefullresponseifthecachedentryisgood