Configuring Read Preferences

1. Setting Read Preference

Mode	Behavior
primary (default)	Always primary
primaryPreferred	Primary, else secondary
secondary	Only secondary
secondaryPreferred	Secondary, else primary
nearest	Lowest latency member

2. Using primaryPreferred Mode

Use Case	Detail
Read availability	Falls back to secondary if primary down
Consistency	Reads usually fresh; stale during failover

3. Using secondaryPreferred Mode

Use Case	Detail
Offload reads	Reduce primary load
Caveat	Secondary lag → stale reads

4. Using nearest Mode

Aspect	Detail
Selection	Lowest ping within latency window (15ms default)
Geo-distributed apps	Reads from same region

5. Using Read Preference Tags

Aspect	Detail
Form	readPreference=secondary&readPreferenceTags=region:us-east
Multiple sets	Tried in order
Setup	Tags configured per member in rs.conf()

6. Setting maxStalenessSeconds

Aspect	Detail
Purpose	Limit acceptable lag (90s minimum)
Effect	Skip members lagging more than threshold
Not with primary	Only secondary modes

7. Understanding Read Preference Behavior

Stage	Detail
Server selection	Topology check + mode filter + tags + staleness
Latency window	fastest + localThresholdMS (default 15ms)
Random pick	Among eligible servers

8. Using Read Preference with Transactions

Rule	Detail
All reads	Must target primary (or "primary" RP)
Driver enforces	Override RP in txn options if needed

9. Configuring Read Preference in Connection String

Option	Example
readPreference	secondaryPreferred
readPreferenceTags	region:us-east,role:analytics
maxStalenessSeconds	90

10. Handling Read Preference Failover

Scenario	Behavior
Primary lost	primaryPreferred/secondaryPreferred fall back
All members down	ServerSelectionTimeoutError after timeout
Retry	Retryable reads (default on) re-select server