After event sourcing (25 Oct), this post covers write-ahead logs inside storage engines: intent-before-mutate, fsync barriers, group commit, checkpoints, checksums, redo or undo policy, replica apply order and how WAL differs from outbox and domain event logs.

लेखनाग्रधर्मः: A Metrical Codification of Write-Ahead Logging (with English glosses)

What this post is about

Write-ahead logging records intended mutations before applying them so crash recovery can complete or roll back safely.

Topics covered

1. Why log before mutate
2. Intent-first ordering
3. fsync durability barriers
4. Group commit
5. Checkpoint and truncate
6. Checksums and torn writes
7. Redo versus undo policy
8. Replica apply order
9. Outbox is not storage WAL
10. Contrast with event sourcing

Sanskrit title

लेखनाग्रधर्मः = lekhana-agra-dharmaḥ “dharma of writing-ahead”.

English

Minimizable; Sanskrit-first.

ग्राह्यम् / Keep

१. intent before mutate: always
२. explicit fsync barriers: not cache success
३. group commit: when latency allows
४. checkpoint truncate: bound recovery
५. checksum records: reject torn tails
६. one redo/undo policy: no mixed half-work
७. replicas follow log: no side mutations

त्याज्यम् / Avoid

• treat buffered write as durable
• never truncate or checkpoint WAL
• confuse app outbox with engine WAL

पदच्छेदः
पूर्वं लेखनं पश्चात् स्थितिः यदि दुर्घटना । लेखनाग्रधर्मस्तस्माद् पुनरुद्धारः ॥१॥

Word-for-word

Gloss table

English sense
A write-ahead log records intended changes before applying them to the durable state so crash recovery can finish or undo work safely.

Context / topic
Topic: why WAL. Distinct from event sourcing app logs and from generic file journaling alone.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
intent first then mutate । mutate-first इतिहासः नश्यति ॥२॥

Word-for-word

Gloss table

English sense
Log the intent before dirtying durable structures. Mutate-first loses the story of incomplete work on power loss.

Context / topic
Topic: ordering rule.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
fsync barrier स्पष्टम् । buffered-only मिथ्यासिद्धिः ॥३॥

Word-for-word

Gloss table

English sense
Durability is a fsync or group-commit barrier not a successful write syscall to page cache.

Context / topic
Topic: durability barriers.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
group commit throughput रक्षति । प्रतिलेख fsync मन्दः ॥४॥

Word-for-word

Gloss table

English sense
Batch many logical commits behind one durability fence when latency budgets allow.

Context / topic
Topic: group commit.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
checkpoint + truncate पुरालॉग् । अनन्तपङ्क्तिः मन्दः ॥५॥

Word-for-word

Gloss table

English sense
Checkpoint applied state and truncate or archive old segments. An unbounded WAL fills disks and slows recovery.

Context / topic
Topic: checkpoint and truncation.

वृत्तमिति: एकादशाक्षराः पादाः।

पदच्छेदः
checksum CRC खण्डे । torn write मा मौन ॥६॥

Word-for-word

Gloss table

English sense
Protect log records with checksums so torn or bit-flipped tails are rejected not applied as truth.

Context / topic
Topic: integrity.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
redo undo नीति स्पष्टा । अर्धकार्यं मा भ्रम ॥७॥

Word-for-word

Gloss table

English sense
Define whether recovery redos committed intents or undoes incomplete ones. Mixed half-policies corrupt state.

Context / topic
Topic: redo/undo semantics.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
replica apply same log order । divergent mutation युद्धाय ॥८॥

Word-for-word

Gloss table

English sense
Replicas that follow the WAL stay ordered. Independent side mutations break the log as source of truth.

Context / topic
Topic: replication from log.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
app outbox != storage WAL । स्तराः भिन्नाः ॥९॥

Word-for-word

Gloss table

English sense
Application outbox is a domain pattern. Storage WAL is an engine internal. Do not conflate the two layers.

Context / topic
Topic: outbox vs WAL distinction.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
घटनास्रोतः domain इतिहासः लेखनाग्रः engine रक्षा । पटलः जन्म रक्षति ॥१०॥

Word-for-word

Gloss table

English sense
Event sourcing is domain history; WAL is engine durability; outbox guards message birth with the business transaction.

Context / topic
Topic: series distinctions.

सन्धिः: स्रोत डोमेन्; अग्र इञ्जिन्; पटल सन्देशजन्म।

वृत्तमिति: एकादशाक्षराः पादाः।

पदच्छेदः
intent barrier checkpoint integrity policy पञ्चकम् । एभिर्लेखनाग्रधर्मः पुनरुद्धारसमन्वितः ॥११॥

Word-for-word

Gloss table

English sense
Five limbs: log intent first, durable barrier, checkpoint and truncate, checksum integrity and clear redo or undo policy.

Context / topic
Topic: checklist.

वृत्तमिति: ८-८-८-८।

पदच्छेदः
इति लेखनाग्रधर्मसारः पुनरुद्धारसमन्वितः । मितं पूर्वलेखनं क्रियायोग्यं तन्त्रधर्मः स उच्यते ॥१२॥

Word-for-word

Gloss table

English sense
Closing: write the intent before the state, fence durability and keep recovery boring.

Context / topic
Topic: close.

उपसंहारन्यायः: पूर्वं लिख; पश्चात् कुरु; fsyncं स्मर।

वृत्तमिति: ८-८-८-८।

१. Why WAL
२. Intent first
३. fsync
४. Group commit
५. Checkpoint
६. Checksum
७. Redo/undo
८. Replicas
९. vs outbox
१०. vs ES
११. Five limbs
१२. Close

1. Prior: event sourcing 2026-10-25; outbox 2026-09-28; read replicas 2026-10-06.
2. Domain: storage engine write-ahead logging.
3. English minimizable; Sanskrit-first.
4. Style: no em dash; no Oxford comma in English clauses.