Infrastructure Decisions · judgement tool

When should I add read replicas?

Primary DB strain, read-heaviness, geo-distributed reads and replication-lag tolerance decide it — and caching often comes first. Get a verdict with each factor tagged official fact / calculation / StackSays rule / estimate.

A judgement tool, not a tool ranking · thresholds are StackSays rules, not industry standards · official facts verified 2026-07-17 · share the URL to share your answers

How to read this: every factor below is tagged by where it comes from — official fact official/documented · calculation math on your inputs · StackSays rule our decision logic · StackSays estimate our judgement. The verdict cut-points are StackSays rules, not an industry threshold — there is no universal traffic number for these decisions.

Your situation

The recommendation updates as you answer. Share the URL to share it.

Primary DB is CPU/IO-bound on readsthe primary is saturated by read trafficReads vastly outnumber writesread-dominant workloadReaders in distant regionsusers far from the primary's regionHeavy reporting/analytics on the primaryexpensive read queries competing with app trafficReplica lag is acceptable for these readsreads can be a few ms–seconds behindAlready added cachinghot reads are already cached

Recommendation

Not yet

Based on the factors you set — 1 pushing toward adopting, 1 against.

Why it's leaning yes on these

+6 Replication lag tolerance — Streaming replicas are asynchronous and lag slightly (official) — your reads tolerate that, so it's safeofficial fact

Why it might be premature

-8 Primary saturated by reads — If the primary isn't strained, a replica is premature complexityStackSays rule

When to reassess

Add a replica when the primary is genuinely read-bound (after caching hasn't been enough), or you need low-latency reads in another region — not on a raw QPS number.

Minimal implementation

Most managed Postgres (RDS, Neon, Supabase, PlanetScale) add a read replica with a checkbox and give you a separate read endpoint. Route only lag-tolerant reads to it.

Risk of not doing it

A saturated primary slows every query, reads and writes alike, until it falls over under load.

Cost & complexity once adopted

A replica is a second DB instance to pay for, plus app-side read/write routing and awareness of replication lag.

This is a judgement tool, not a leaderboard. The StackSays rule and StackSays estimate factors are StackSays logic and opinion; the official fact factors link to official sources below. No industry-standard threshold is implied — the answer is driven by the conditions you set.

Official facts cited

Source	Verified
PostgreSQL streaming replication & lag (official)	2026-07-17

Only the factors tagged “official fact” rest on these sources; rules and estimates are StackSays logic and are labeled as such.

Next decision

When should I add caching? →Which database should I use? →