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mvcc

Name: Mvcc
Author: tursodatabase

// Overview of Experimental MVCC feature - snapshot isolation, versioning, limitations

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SKILL.md

readonly

name	mvcc
description	Overview of Experimental MVCC feature - snapshot isolation, versioning, limitations

MVCC Guide (Experimental)

Multi-Version Concurrency Control. Work in progress, not production-ready.

CRITICAL: Ignore MVCC when debugging unless the bug is MVCC-specific.

Enabling MVCC

PRAGMA journal_mode = 'mvcc';

Runtime configuration, not a compile-time feature flag. Per-database setting.

How It Works

Standard WAL: single version per page, readers see snapshot at read mark time.

MVCC: multiple row versions, snapshot isolation. Each transaction sees consistent snapshot at begin time.

Key Differences from WAL

Aspect	WAL	MVCC
Write granularity	Every commit writes full pages	Affected rows only
Readers/Writers	Don't block each other	Don't block each other
Persistence	`.db-wal`	`.db-log` (logical log)
Isolation	Snapshot (page-level)	Snapshot (row-level)

Versioning

Each row version tracks:

begin - timestamp when visible
end - timestamp when deleted/replaced
btree_resident - existed before MVCC enabled

Architecture

Database
  └─ mv_store: MvStore
      ├─ rows: SkipMap<RowID, Vec<RowVersion>>
      ├─ txs: SkipMap<TxID, Transaction>
      ├─ Storage (.db-log file)
      └─ CheckpointStateMachine

Per-connection: mv_tx tracks current MVCC transaction.

Shared: MvStore with lock-free crossbeam_skiplist structures.

Key Files

core/mvcc/mod.rs - Module overview
core/mvcc/database/mod.rs - Main implementation (~3000 lines)
core/mvcc/cursor.rs - Merged MVCC + B-tree cursor
core/mvcc/persistent_storage/logical_log.rs - Disk format
core/mvcc/database/checkpoint_state_machine.rs - Checkpoint logic

Checkpointing

Flushes row versions to B-tree periodically.

PRAGMA mvcc_checkpoint_threshold = <pages>;

Process: acquire lock → begin pager txn → write rows → commit → truncate log → fsync → release.

Current Limitations

Not implemented:

Garbage collection (old versions accumulate)
Recovery from logical log on restart

Known issues:

Checkpoint blocks other transactions, even reads!
No spilling to disk; memory use concerns

Testing

# Run MVCC-specific tests
cargo test mvcc

# TCL tests with MVCC
make test-mvcc

Use #[turso_macros::test(mvcc)] attribute for MVCC-enabled tests.

#[turso_macros::test(mvcc)]
fn test_something() {
    // runs with MVCC enabled
}

References

core/mvcc/mod.rs documents data anomalies (dirty reads, lost updates, etc.)
Snapshot isolation vs serializability: MVCC provides the former, not the latter

related-skills.json

نفس المستودع

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from "tursodatabase/turso"

Use when migrating Turso core code to crate::alloc, allocator-api2, cfg(nightly) allocator aliases, fallible collection APIs, try_vec!, try_collect, or LimboError::OutOfMemory handling. Covers import style, allocator-aware aliases, stable/nightly boundaries, shuttle Arc compatibility, and how to keep allocator foundation commits separate from module migrations.

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memory-benchmark.md

from "tursodatabase/turso"

How to benchmark and analyze memory usage in Turso using the memory-benchmark crate and dhat heap profiler. Use this skill whenever the user mentions memory usage, memory profiling, allocation tracking, heap analysis, memory regression, memory benchmarking, dhat, or wants to understand where memory is being allocated during SQL workloads. Also use when investigating memory growth in WAL or MVCC mode. IMPORTANT - If you modify the perf/memory crate (add profiles, change CLI flags, change output format, etc.), update this skill document to reflect those changes so it stays accurate for future agents.

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testing.md

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How to write tests, when to use each type of test, and how to run them. Contains information about conversion of `.test` to `.sqltest`, and how to write `.sqltest` and rust tests

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code-quality.md

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cdc.md

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package.json

"author": "tursodatabase"

"repository": "tursodatabase/turso"

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$ install --global

$ download --local

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مبرمجو الحاسوبمهن الحاسوب والرياضيات15-1251L4

مصممو قواعد البياناتL4

name	mvcc
description	Overview of Experimental MVCC feature - snapshot isolation, versioning, limitations

MVCC Guide (Experimental)

Multi-Version Concurrency Control. Work in progress, not production-ready.

CRITICAL: Ignore MVCC when debugging unless the bug is MVCC-specific.

Enabling MVCC

PRAGMA journal_mode = 'mvcc';

Runtime configuration, not a compile-time feature flag. Per-database setting.

How It Works

Standard WAL: single version per page, readers see snapshot at read mark time.

MVCC: multiple row versions, snapshot isolation. Each transaction sees consistent snapshot at begin time.

Key Differences from WAL

Aspect	WAL	MVCC
Write granularity	Every commit writes full pages	Affected rows only
Readers/Writers	Don't block each other	Don't block each other
Persistence	`.db-wal`	`.db-log` (logical log)
Isolation	Snapshot (page-level)	Snapshot (row-level)

Versioning

Each row version tracks:

begin - timestamp when visible
end - timestamp when deleted/replaced
btree_resident - existed before MVCC enabled

Architecture

Database
  └─ mv_store: MvStore
      ├─ rows: SkipMap<RowID, Vec<RowVersion>>
      ├─ txs: SkipMap<TxID, Transaction>
      ├─ Storage (.db-log file)
      └─ CheckpointStateMachine

Per-connection: mv_tx tracks current MVCC transaction.

Shared: MvStore with lock-free crossbeam_skiplist structures.

Key Files

core/mvcc/mod.rs - Module overview
core/mvcc/database/mod.rs - Main implementation (~3000 lines)
core/mvcc/cursor.rs - Merged MVCC + B-tree cursor
core/mvcc/persistent_storage/logical_log.rs - Disk format
core/mvcc/database/checkpoint_state_machine.rs - Checkpoint logic

Checkpointing

Flushes row versions to B-tree periodically.

PRAGMA mvcc_checkpoint_threshold = <pages>;

Process: acquire lock → begin pager txn → write rows → commit → truncate log → fsync → release.

Current Limitations

Not implemented:

Garbage collection (old versions accumulate)
Recovery from logical log on restart

Known issues:

Checkpoint blocks other transactions, even reads!
No spilling to disk; memory use concerns

Testing

# Run MVCC-specific tests
cargo test mvcc

# TCL tests with MVCC
make test-mvcc

Use #[turso_macros::test(mvcc)] attribute for MVCC-enabled tests.

#[turso_macros::test(mvcc)]
fn test_something() {
    // runs with MVCC enabled
}

References

core/mvcc/mod.rs documents data anomalies (dirty reads, lost updates, etc.)
Snapshot isolation vs serializability: MVCC provides the former, not the latter

mvcc

MVCC Guide (Experimental)

Enabling MVCC

How It Works

Key Differences from WAL

Versioning

Architecture

Key Files

Checkpointing

Current Limitations

Testing

References

المزيد من هذا المستودع

المزيد من هذا المستودع

MVCC Guide (Experimental)

Enabling MVCC

How It Works

Key Differences from WAL

Versioning

Architecture

Key Files

Checkpointing

Current Limitations

Testing

References