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free as in lock-free embedded database
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An (alpha) modern embedded database.

use sled::Db;

let tree = Db::start_default(path)?;

// set and get
tree.set(k, v1);
assert_eq!(tree.get(&k), Ok(Some(v1)));

// compare and swap
tree.cas(k, Some(v1), Some(v2));

// scan forward
let mut iter = tree.scan(k);
assert_eq!(, Some(Ok((k, v2))));
assert_eq!(, None);

// deletion

// block until all operations are on-disk

We also support merge operators!


  • API similar to a threadsafe BTreeMap<Vec<u8>, Vec<u8>>
  • fully atomic single-key operations, supports CAS
  • zero-copy reads
  • subscription/watch semantics on key prefixes
  • multiple keyspace support
  • merge operators
  • forward and reverse iterators
  • a crash-safe monotonic ID generator capable of generating 75-125 million ID's per second
  • zstd compression (use the compression build feature)
  • cpu-scalable lock-free implementation
  • SSD-optimized log-structured storage


  1. don't make the user think. the interface should be obvious.
  2. don't surprise users with performance traps.
  3. don't wake up operators. bring reliability techniques from academia into real-world practice.
  4. don't use so much electricity. our data structures should play to modern hardware's strengths.


  • LSM tree-like write performance with traditional B+ tree-like read performance
  • MVCC, serializable transactions, and snapshots
  • forward-compatible binary format
  • concurrent snapshot delta generation and recovery
  • first-class access to replication stream
  • consensus protocol for PC/EC systems
  • pluggable conflict detection and resolution strategies for PA/EL systems
  • multiple collection types like tables, BKD trees, Merkle trees, bloom filters, etc... unified under a single transactional and operational domain

Want to prioritize a specific feature or get commercial help with using sled in your project? Ferrous Systems provides commercial support for sled, and can work with you to solve a wide variety of storage problems across the latency-throughput, consistency, and price performance spectra. Get in touch!

Ferrous Systems

known issues, warnings

  • the on-disk format is going to change in non-forward compatible ways before the 1.0.0 release! after that, we will always support forward migrations.
  • quite young, should be considered unstable for the time being
  • the C API is likely to change rapidly
  • writepath is not well optimized yet. readpath is essentially wait-free and zero-copy.
  • 32 bit architectures require Rust nightly with the nightly feature enabled.

contribution welcome!

want to help advance the state of the art in open source embedded databases? check out!


lock-free tree on a lock-free pagecache on a lock-free log. the pagecache scatters partial page fragments across the log, rather than rewriting entire pages at a time as B+ trees for spinning disks historically have. on page reads, we concurrently scatter-gather reads across the log to materialize the page from its fragments. check out the architectural outlook for a more detailed overview of where we're at and where we see things going!


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