Amazon DocumentDB Read Preferences Guide

Tags: mongodb documentdb aws

Written by: Tushar Sharma

Amazon DocumentDB is a fully managed, MongoDB-compatible database service provided by AWS. It replicates data six times across three Availability Zones (two copies per zone) for high durability and availability.

Connection String Example with Read Preferences

Specify read preferences directly in the connection string:

mongodb://host:port/database?readPreference=secondaryPreferred

Consistency Tradeoffs

NoSQL databases like DocumentDB often use eventual consistency by default. This means reads immediately after writes might return stale data, as updates propagate asynchronously to replicas. This design prioritizes availability and scalability over strict consistency. You can override this behavior using read preferences, though this may impact scalability.

Read Preference Mode	Consistency	Behavior	Use Case	Consideration
primaryPreferred	Read-after-write consistency	Routes reads to primary by default - Fails over to replicas if unavailable	Critical reads requiring fresh data	Creates primary bottleneck (not ideal for read-heavy workloads)
secondaryPreferred	Eventual consistency	Prioritizes replica reads - Uses primary only if replicas unavailable	Read scaling & high availability	Distributes read traffic across replicas
nearest	Variable (depends on replication lag)	Selects lowest-latency nodes - Uses both primary and secondary nodes	Latency-sensitive applications	Fast responses but potentially stale data

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