Levelup Sync Wrapper

Provides a synchronous interface for levelup (using a cache) to avoid race conditions.

Race conditions are an easy pitfall to fall into.

const levelup = require('levelup')
const leveldown = require('leveldown')

const db = levelup(leveldown('./my-database'))

// There's a race condition in this function because we do an asynchronous get,
// increment in memory, and then do an asynchronous put. If more than one call
// to badIncrementBalance is in progress then:
//
// - badIncrementBalance 1 reads from the store and gets "0"
// - badIncrementBalance 1 increments "0" to "10" and begins to write
// - badIncrementBalance 2 reads from the store and gets "0"
// - badIncrementBalance 2 increments "0" to "10" and begins to write
// - badIncrementBalance 1 finishes writing "10" to the store
// - badIncrementBalance 2 finishes writing "10" to the store

async function badIncrementBalance (amount) {
  const balance = await db.get('balance')
  const newBalance = Number(balance) + amount

  await db.put('balance', String(newBalance))
}

async function run () {
  await db.put('balance', '0')

  await Promise.all([
    badIncrementBalance(10),
    badIncrementBalance(10)
  ])

  const finalValue = await db.get('balance')
  console.log('final value:', finalValue.toString())
}

run()
  .catch(e => {
    console.error(e)
    process.exit(1)
  })

But using a synchronous interface prevents this kind of error. Levelup sync will provide a synchronous interface to your store, while still queuing all the writes you make in the background to minimize potential for data loss.

load is called before any synchronous operations are performed, to make sure we start with the latest value from the store. It's also possible to free a key, which will wait for all writes to complete (while preventing new synchronous writes), and then erase it from the cache.

const levelup = require('levelup')
const leveldown = require('leveldown')
const { LevelupSync } = require('levelup-sync-wrapper')

const db = levelup(leveldown('./my-database'))
const syncDb = new LevelupSync(db)

function goodIncrementBalance (amount) {
  const balance = syncDb.getSync('balance')
  const newBalance = Number(balance) + amount

  syncDb.putSync('balance', String(newBalance))
}

async function run () {
  await syncDb.load('balance')
  syncDb.putSync('balance', '0')

  goodIncrementBalance(10)
  goodIncrementBalance(10)

  const finalValue = syncDb.getSync('balance')
  console.log('final value:', finalValue)
}

run()
  .catch(e => {
    console.error(e)
    process.exit(1)
  })

To run these examples yourself, you can run:

npm install
node examples/bad-increment.js
node examples/good-increment.js