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Architecture

From log record
to committed row.

TwinCommit works by reading the source database's transaction log, buffering changes until their transaction commits, and applying each transaction to the target together with its checkpoint. Here's every stage.

  1. 1
    capture

    Log reader

    Reads log records through the database's native log API (db2ReadLog, redo, the SQL Server transaction log or pgoutput) and decodes row images in-process. Nothing is installed in the database.

  2. 2
    assemble

    Transaction buffer

    Groups changes by transaction ID. Rollbacks, including partial rollbacks to a savepoint, are discarded here and never reach the target.

  3. 3
    protect

    Spill manager

    Transactions larger than max_txn_memory_bytes spill to local disk, so a 50-million-row batch never exhausts memory and still applies atomically.

  4. 4
    deliver

    Commit-ordered apply

    On COMMIT, the transaction is applied to the target as one unit, in source commit order, using that platform's fastest transactional write path.

  5. 5
    guarantee

    In-target checkpoint

    The log position is written in the same target transaction as the data, so a crash can never separate "data applied" from "position saved". That is how delivery stays exactly-once.

Guarantees

What TwinCommit promises, and why it holds.

No lost transactions

Restart resumes from the checkpoint committed with the last applied transaction.

No duplicate transactions

Upsert-by-key apply makes replay idempotent, and the in-transaction checkpoint prevents replay entirely.

No partial transactions

Targets see a whole transaction or none of it.

No ordering surprises

Transactions are applied in source commit order.

Read the architecture docs →

Resilience

Built to be killed.

The TwinCommit test suite kills the process with SIGKILL mid-workload, mid-snapshot and mid-spill, then proves source and target match row for row.

$ twincommit run -config prod.yaml &
$ kill -9 $!            # mid-transaction, 1.8M rows in flight
$ twincommit run -config prod.yaml
{"level":"info","msg":"resuming from checkpoint","lsn":"0x0004F311","txn":81231}
$ twincommit verify -config prod.yaml
✓ 214 tables match  ·  0 missing  ·  0 extra  ·  0 differing

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