Crate extractdb

Extract DB

A thread-safe, in-memory hash store supporting concurrent fetches and writes.

This is not a traditional kv-store, in the sense that it doesn’t use any form of keys.
Specific “item” removal is not supported in favor of a fetching type system and can be thought of as a read-only dequeue database.

Table of contents

• Guarantees
• Trade-offs
• Use scenarios
• Installation
• Examples
  • Basics
    • Push, Fetch, & count
  • Multithreaded
    • Multithreaded insert & fetch
  • Disk
    • Disk loading and saving
    • Auto saving
• Testing
• Contributing
• License

Guarantees.

• All items will eventually be fetched (no duplication), but ordering is non-deterministic (Not FIFO or FILO)
• Items are never removed once inserted (append-only / reference-fetching)
• All functions are thread safe

Trade-offs.

• No item removal
• Non-deterministic fetch order
• Write throughput is prioritized over reading performance

Use scenarios:

• Concurrent queue with unique items only (HashSet + VecDeque)-like
• Fast concurrent insertions are needed over concurrent reads
• Fast reading on a single-thread with multiple concurrent writers
• Persistent in-memory hash-store

This was originally built for a web-scraper which needs to write lots of links with fewer reads.

Installation

# Cargo.toml
[dependencies]
extractdb = "0.1.0"

Examples

Push, fetch, & count

use extractdb::ExtractDb;

fn main() {
    let database: ExtractDb<i32> = ExtractDb::new(None);

    database.push(100);

    let total_items_in_db = database.internal_count();
    let mut items_in_quick_access_memory = 0;
    if total_items_in_db > 0 {
        let item: &i32 = database.fetch_next().unwrap();

        items_in_quick_access_memory = database.fetch_count();
    }
    
    println!("Total items: {} | Quick Access item count: {}", total_items_in_db, items_in_quick_access_memory);
}

Multithreaded insert & fetch

use std::sync::Arc;
use extractdb::ExtractDb;
use std::thread;

fn main() {
    let database: Arc<ExtractDb<String>> = Arc::new(ExtractDb::new(None));

    for thread_id in 0..8 {
        let local_database = Arc::clone(&database);
        thread::spawn(move || {
            local_database.push(format!("Hello from thread {}", thread_id))
        });
    }

    // Will only print some of the items... since we are not waiting for thread completion.
    for _ in 0..8 {
        if let Ok(item) = database.fetch_next() {
            println!("Item: {}", item);
        }
    }
}

Disk loading and saving

use std::path::PathBuf;
use extractdb::ExtractDb;

fn main() {
    let database: ExtractDb<String> = ExtractDb::new(Some(PathBuf::from("./test_db")));

    // `True`: Load all items back into `fetch_next` queue
    database.load_from_disk(true).unwrap();

    database.push("Hello world!".to_string());

    database.save_to_disk().unwrap();
}

Auto saving

use std::sync::Arc;
use std::path::PathBuf;
use std::sync::atomic::{AtomicBool, Ordering};
use extractdb::{CheckpointSettings, ExtractDb};

fn main() {
    let database: Arc<ExtractDb<String>> = Arc::new(ExtractDb::new(Some(PathBuf::from("./test_db"))));

    // `True`: Load all items back into `fetch_next` queue
    database.load_from_disk(true).unwrap();

    let shutdown_flag = Arc::new(AtomicBool::new(false));
    let mut save_settings = CheckpointSettings::new(shutdown_flag.clone());
    save_settings.minimum_changes = 1000;
    
    // Spawns a background watcher thread. 
    // This checks for a minimum of 1000 changes every 30 seconds (default)
    ExtractDb::background_checkpoints(save_settings, database.clone());
    
    // Perform single/multithreaded logic
    database.push("Hello world!".to_string());

    // Gracefully shutdown the background saving thread
    shutdown_flag.store(true, Ordering::Relaxed);
}

Testing + More examples

This project includes some basic tests to maintain functionality please use them.

cargo test

See internal doc-comments for more indepth information about each test:

• push
• push_multiple
• push_collided
• push_multi_thread
• push_structure
• count_empty_store
• count_loaded_store
• fetch_data
• fetch_data_multiple
• fetch_data_empty
• duplicate_fetch
• save_state_to_disk
• load_state_from_disk
• load_corrupted_state_from_disk
• load_shard_mismatch_from_disk
• load_mismatch_type_from_disk

Contributing

Pull request and issue contributions are very welcome. Please feel free to suggest changes in PRs/Issues :)

License

This project is licensed under either MIT or Apache-2.0, you choose.

Structs

CheckpointSettings

Configuration settings for the provided ExtractDb::background_checkpoints.

ExtractDb

ExtractDb is a thread-safe, in-memory hash store supporting concurrent fetches and writes.