<div align="center">
    <h1>SKL</h1>
</div>

<div align="center">

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1. A lock-free thread-safe concurrent `SkipMap` implementation based on ARENA skiplist which helps develop MVCC memtable for LSM-Tree.
2. A lock-free thread-safe concurrent memory map based on-disk `SkipMap`.

</div>

## Installation

- Only use heap backend (suppport `no_std`)
  
    ```toml
    [dependencies]
    skl = "0.20"
    ```

- Enable memory map backend

    ```toml
    [dependencies]
    skl = { version = "0.20", features = ["memmap"] }
    ```

## Features

- **MVCC and 3D access**: Builtin MVCC (multiple versioning concurrency control) and key-value-version access support.
- **Lock-free and Concurrent-Safe:** `SkipMap` provide lock-free operations, ensuring efficient concurrent access without the need for explicit locking mechanisms.
- **Extensible for Key-Value Database Developers:** Designed as a low-level crate, `SkipMap` offer a flexible foundation for key-value database developers. You can easily build your own memtable or durable storage using these structures.
- **Memory Efficiency:** These data structures are optimized for minimal memory overhead. They operate around references, avoiding unnecessary allocations and deep copies, which can be crucial for efficient memory usage.
  - **Segment tracker:** Builtin segment recollection support, a lock-free freelist helps reuse free segments.
  - **Prefix compression:**
    - Same key will only be stored once.
    - Keys with common prefix will be stored once (longest one must be inserted first).
    - (experimental) Keys are sub-slice of negeibours will be stored once (require `CompressionPolicy::High`).
  - **Discard tracker:** Builtin discard tracker, which will track discarded bytes to help end-users decide when to compact or rewrite.
- **Efficient Iteration:** Enjoy fast forward and backward iteration through the elements in your `SkipMap`. Additionally, bounded iterators are supported, allowing you to traverse only a specified range of elements efficiently.
- **Snapshot Support:** Create snapshots of your `SkipMap`, offering a read-only view of the contents at a specific moment in time. Snapshots provide a consistent view of the data, enabling implementations of transactional semantics and other use cases where data consistency is crucial.
- **Memory Management Options:**
  - **Heap Allocation:** Memory allocation is handled by Rust's allocator, ensuring all data resides in RAM.
  - **Mmap:** Data can be mapped to a disk file by the operating system, making it suitable for durable storage.
  - **Mmap Anonymous:** Mapped to anonymous memory (virtual memory) by the OS, ideal for large in-memory memtables, optimizing memory utilization.

## Examples

Please see [examples](https://github.com/al8n/skl/tree/main/examples) folder for more details.

## Q & A

- Does the on-disk version `SkipMap` ensure crash safety or power failure resilience?
  
  No, If you really need a crash safe, power failure resilience, concurrent-safe and durable ordered write-ahead log implementation,
  see [`orderwal`](https://github.com/al8n/orderwal) project.
  
  On-disk version `SkipMap` does not ensure crash safety or power failure resilience. Hence, it is not recommended to directly
  use the `SkipMap` as a durable database. It is recommended to use the on-disk version `SkipMap` as a final frozen file for quick lookup.

## Related projects

- [`aol`](https://github.com/al8n/aol): Yet another generic purpose, append-only write-ahead log implementation.
- [`orderwal`](https://github.com/al8n/orderwal): A generic-purpose, atomic, ordered, zero-copy, concurrent-safe, pre-allocate style (memory map) write-ahead-log for developing databases.

## Tests

- `test`:

    ```sh
    cargo test --all-features
    ```

- `miri` (Stack Borrows)

    ```sh
    MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-disable-isolation -Zmiri-symbolic-alignment-check" \
    RUSTFLAGS = "--cfg all_skl_tests" \
    cargo miri test --all-features
    ```

- `miri` (Tree Borrows)

    ```sh
    MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-disable-isolation -Zmiri-symbolic-alignment-check -Zmiri-tree-borrows" \
    RUSTFLAGS = "--cfg all_skl_tests" \
    cargo miri test --all-features
    ```

## Support Platforms

See `cross` section in [GitHub CI file](https://github.com/al8n/skl/blob/main/.github/workflows/ci.yml#L69).

## Pedigree

This code is inspired and modified based on Cockroachdb's pebble arenaskl and Dgraph's badger skl code:

https://github.com/cockroachdb/pebble/tree/master/internal/arenaskl

https://github.com/dgraph-io/badger/tree/master/skl

The pebble's arenaskl code is based on Andy Kimball's arenaskl code:

https://github.com/andy-kimball/arenaskl

The arenaskl code is based on the skiplist found in Badger, a Go-based
KV store:

https://github.com/dgraph-io/badger/tree/master/skl

The skiplist in Badger is itself based on a C++ skiplist built for
Facebook's RocksDB:

https://github.com/facebook/rocksdb/tree/master/memtable

#### License

`skl` is under the terms of both the MIT license and the
Apache License (Version 2.0).

See [LICENSE-APACHE](LICENSE-APACHE), [LICENSE-MIT](LICENSE-MIT) for details.

Copyright (c) 2022 Al Liu.

[Github-url]: https://github.com/al8n/skl/
[CI-url]: https://github.com/al8n/skl/actions/workflows/ci.yml
[doc-url]: https://docs.rs/skl
[crates-url]: https://crates.io/crates/skl
[codecov-url]: https://app.codecov.io/gh/al8n/skl/
[rustc-url]: https://github.com/rust-lang/rust/blob/master/RELEASES.md
[license-apache-url]: https://opensource.org/licenses/Apache-2.0
[license-mit-url]: https://opensource.org/licenses/MIT
