[MongoDB] Read Preference 의미와 종류

Intro

mongodb의 read preference에 대해 알아보겠습니다. 

 

Read Preference?

read preference란 mongo db에서 replica set 설정 시 primary, secondary에 대한 설정입니다. 이를 위해 mongo db는 5가지 방법을 지원하고 있습니다. 

 

Read Preference ModeDescription

primary	Default mode. All operations read from the current replica set primary. Multi-document transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.
primaryPreferred	In most situations, operations read from the primary but if it is unavailable, operations read from secondary members. Starting in version 4.4, primaryPreferred supports hedged reads on sharded clusters.
secondary	All operations read from the secondary members of the replica set. Starting in version 4.4, secondary supports hedged reads on sharded clusters.
secondaryPreferred	In most situations, operations read from secondary members but if no secondary members are available, operations read from the primary on sharded clusters. Starting in version 4.4, secondaryPreferred supports hedged reads on sharded clusters.
nearest	Operations read from a random eligible replica set member, irrespective of whether that member is a primary or secondary, based on a specified latency threshold. The operation considers the following when calculating latency: The localThresholdMS connection string option The maxStalenessSeconds read preference option Any specified tag set lists Starting in version 4.4, nearest supports hedged reads on sharded clusters and enables the hedged read option by default.

• primary : 모든 명령이 primary에서만 동작합니다. 
• primaryPreferred : primary가 unavilable 할 경우, read operation을 secondary로 전달합니다. 
• secondary : 모든 read 명령을 secondary로 보내 처리합니다. 
• secondaryPreferred : secondary members 모두 unavilable 한 경우 read 명령을 primary로 넘겨 처리합니다. 
• nearest : primary, secondary 상관없이 latency threshold가 가작 적은 인스턴스로 보내 처리합니다.

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https://www.mongodb.com/docs/manual/core/read-preference/