After DB indexes (5 Oct), this post covers read replicas: lag, write routing, read-your-writes vs stale-ok, lag SLOs, migrations, backups vs replicas, pools and multi-region.

वाचनप्रतिलिपिः: A Metrical Codification of Read Replicas (with English glosses)

What this post is about

Read replicas absorb SELECT load from a primary. Correct use needs lag awareness and split routing.

Topics covered

1. Primary overload from reads
2. Replication lag
3. Writes only on primary
4. read-your-writes vs stale-ok
5. Lag metrics and failover of reads
6. Schema migrations under lag
7. Replica vs backup
8. Separate pools
9. Multi-region lag
10. Capacity: reads vs writes

Sanskrit title

वाचनप्रतिलिपिः = vācana-pratilipiḥ “reading-copy / read replica”.

English

Minimizable; Sanskrit-first.

ग्राह्यम् / Keep

१. लेखप्राथमिकः: all writes to writer
२. पठननीतिः: explicit stale vs strong
३. lagSLO: max lag for replica use
४. पृथक्कुण्डम्: separate read pool
५. प्रवाससङ्गतिः: expand/contract under lag
६. मापनम्: lag bytes/seconds
७. HA विवेकः: replica promotion is another runbook

त्याज्यम् / Avoid

• always read replica after write without sticky
• treat replica as backup only
• one pool to mixed endpoints

पदच्छेदः
एकलेखके यदि सर्वं पठनं पतति तदा सः दह्यते । वाचनप्रतिलिपिस्तस्माद् पठनविभाजनम् ॥१॥

Word-for-word

Gloss table

English sense
Read replicas copy data so heavy SELECT traffic can leave the primary freer for writes. They are not free consistency.

Context / topic
Topic: why read replicas.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
विलम्बः अनुलिपौ सत्यं उत्पादे । तत्क्षणसत्यं मा कल्पय सर्वत्र ॥२॥

Word-for-word

Gloss table

English sense
Replica lag is normal. After a write, a read against a lagging replica may miss the new row. Design read-your-writes paths deliberately.

Context / topic
Topic: replication lag.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
लेखं सर्वदा प्राथमिके । प्रतिलिपौ लेखो द्विधा विपत्तिः ॥३॥

Word-for-word

Gloss table

English sense
Send all writes to the primary (or designated writer). Application bugs that write to replicas cause split brains or errors.

Context / topic
Topic: write routing.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
पठननीतिः लेख्या read-your-writes stale-ok । एकं pool सर्वं भ्रमयति ॥४॥

Word-for-word

Gloss table

English sense
Split connection pools or middleware rules: strong paths to primary, eventually-consistent lists to replicas.

Context / topic
Topic: read routing policy.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
lag माप्यः क्षणे क्षणे अतिविलंबे पठनं स्तम्भयेत् । अन्धपठनं मिथ्या UI ॥५॥

Word-for-word

Gloss table

English sense
Monitor lag. When it exceeds SLO, stop sending lag-sensitive reads to that replica or fail closed to primary.

Context / topic
Topic: lag SLO and shedding.

वृत्तमिति: एकादशाक्षराः पादाः।

पदच्छेदः
योजनाप्रवासः सर्वासु प्रतिलिपिषु क्रमः । एकस्यां नवं अन्यस्यां पुराणं छिद्रम् ॥६॥

Word-for-word

Gloss table

English sense
Migrations must stay expand/contract compatible across primary and replicas while lag drains. Mid-lag incompatible reads break.

Context / topic
Topic: migrations under replication. Links to expand-contract.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
बैकअपः प्रतिलिपिः न पर्यायः सर्वदा । लक्ष्यं RPO भिन्नं स्यात् ॥७॥

Word-for-word

Gloss table

English sense
A replica is for scale and sometimes HA; backups are for point-in-time recovery. Do not confuse the two.

Context / topic
Topic: replica vs backup. Links to DR post.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
संयोगकुण्डं पठने लेखे च पृथक् । एककुण्डे लेखः पठने मिलति ॥८॥

Word-for-word

Gloss table

English sense
Use separate pools or roles for primary vs replica so pool exhaustion on analytics does not block checkout writes.

Context / topic
Topic: pool split. Links to connection pool post.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
बहुक्षेत्रे अनुलिपिधर्मः भूगोलेन सह । विलम्बः समुद्रे सत्यम् ॥९॥

Word-for-word

Gloss table

English sense
Cross-region replicas add latency physics. Product promises must match lag reality.

Context / topic
Topic: multi-region lag. Links to multi-region treatise.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
क्षमतायोजना पठनभारं प्रतिलिपिभिः वहेत् । लेखभारः प्राथमिके तिष्ठेत् ॥१०॥

Word-for-word

Gloss table

English sense
Scale reads with replicas; scale writes with sharding primary capacity or redesign. Replicas do not multiply write throughput of one primary.

Context / topic
Topic: capacity link.

सन्धिः: प्रतिलिपिः पठति; प्राथमिकः लिखति।

वृत्तमिति: एकादशाक्षराः पादाः।

पदच्छेदः
मार्गः lag मापनं प्रवासः कुण्डं पञ्चकम् । एभिर्वाचनप्रतिलिपिः पठनसत्यसमन्विता ॥११॥

Word-for-word

Gloss table

English sense
Five limbs: write-to-primary routing, explicit stale policies, lag metrics, migration safety and split pools.

Context / topic
Topic: checklist.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
इति वाचनप्रतिलिपिसारः पठनसत्यसमन्वितः । मितं विलम्बं क्रियायोग्यं तन्त्रधर्मः स उच्यते ॥१२॥

Word-for-word

Gloss table

English sense
Closing: split reads with open eyes about lag not with hope of instant global sameness.

Context / topic
Topic: close.

उपसंहारन्यायः: लिख प्राथमिके; पठ विवेकेन; lag मापय।

वृत्तमिति: ८-८-८-८।

१. Why replicas
२. Lag truth
३. Write routing
४. Read policy
५. Lag metrics
६. Migrations
७. Backup contrast
८. Pools
९. Multi-region
१०. Capacity
११. Five limbs
१२. Close

1. Prior: pools 2026-09-23; migrations 2026-09-06; DR 2026-07-22; multi-region 2026-07-23.
2. Domain: OLTP read replicas.
3. English minimizable; Sanskrit-first.
4. Style: no em dash; no Oxford comma in English clauses.