Very rough draft, just getting some ideas down.

hmdb

An embedded database with the following properties:

• Read Optimized
• Persistent
• Transactional
• In-Memory
• Key Value Store
• Schema defined and enforced in Rust

Collection of traits that when applied to the appropriate struct allow for concurrent access to a collection of HashMaps. Writes are written to a disk using an append only log. Log can be compacted using a snapshot. Snapshots are written atomically, appends to logs are limited in scope, if they become corrupted. Compaction can happen automatically on a separate thread or when the application decides it's an appropriate time to do so. Database can be configured for different consistency guarantees (buffered logs) and can be configured for environments in which multiple non-cooperative processes are sharing a data directory (file locks + no log buffer). Optimized for latency and a compact on-disk format.

Everything you know about your schema, you can express to the database at compile time. You can specify what tables exist and what keys and values those tables have in rust. The database supports anything that implements serde traits, and uses bincode for the on-disk format (battle tested local optima for performance and compactness). No need to manage mapping to and from db-specific types yourself.

Target Usage

Define a schema:

schema! {
    SchemaV1 {
        accounts: <Username, Account>,
        files: <Uuid, EncryptedFileMetadata>
    }
}

Under the hood, this generates the struct SchemaV1 which you can use like this:

fn main() {
    let db = SchemaV1::init("data.db");
}

SchemaV1 is your type, but it will have on it impl'd various traits from this crate. These traits include Initialize which allow you to start your database from disk, initialize will use an associated type that represents your OnDiskFormat.

You can interact directly with your tables:

fn main() {
    let db = SchemaV1::init("data.db");

    db.accounts.insert(Username::from("parth"), Account { ... });
    let account = db.accounts.get(Username::from("parth"));

    db.files.insert(meta.id, meta);
    let file = db.files.get(meta.id);
}

These types are not implicitly inferred based on usage, they are specified by you in one location that represents the schema.

If you wanted to evolve your schema you would do something like this:

schema! {
    SchemaV1 {
        accounts: <Username, Account>,
        files: <Uuid, EncryptedFileMetadata>
    }
}

schema! {
    SchemaV2 {
        accounts: <Username, AccountV2>,
        files: <Uuid, EncryptedFileMetadata>
    }
}

fn main() {
    let old = SchemaV1::init("data.db");
    let new = SchemaV2::init("data-v2.db");

    old.accounts
        .iter()
        .map(|key, value| new.accounts.insert(key, value.into()));

    old.files
        .iter()
        .map(|key, value| new.files.insert(key, value.into()));

    // Migration successful, safe to delete data.db
}

Basic transaction experience:

schema! {
    SchemaV1 {
        accounts: <Username, Account>,
        files: <Uuid, EncryptedFileMetadata>
    }
}

fn main() {
    let db = SchemaV1::init("data.db");

    db.transaction(|accounts, files| {});
}

The basic and most primitive version will just lock everything for transactions. A more sophisticated implementation could allow you to specify which tables to lock. Long term target state probably involves no locks and some optimistic concurrency.

While this is already expected to be a significant speedup for use withing lockbook, further gains can be made by experimenting with state of the art:

• Zero Copy (super fast) serializers
• Lock-free Read Optimized (eventually consistent) Map