Simple MVCC Example

mvcc.js

class MVCCDatabase {
  constructor() {
    this.data = {};
    this.versions = {};
  }

  async read(key, transactionId) {
    // Simulate read operation with MVCC
    if (this.versions[key]) {
      const version = this.versions[key].find(v => v.transactionId <= transactionId);
      if (version) {
        return version.value;
      }
    }
    return null; // Key not found or not visible to the transaction
  }

  async write(key, value, transactionId) {
    // Simulate write operation with MVCC
    if (!this.versions[key]) {
      this.versions[key] = [];
    }
    this.versions[key].push({ transactionId, value });

    // Update the latest value in the data store
    this.data[key] = value;
  }

  async commit(transactionId) {
    // Commit the transaction by removing old versions
    for (const key of Object.keys(this.versions)) {
      this.versions[key] = this.versions[key].filter(v => v.transactionId > transactionId);
    }
  }
}

async function exampleTransaction(database, transactionId) {
  await database.write('foo', 'initial', transactionId);
  console.log(await database.read('foo', transactionId)); // Should print 'initial'

  // Another transaction writes to the same key
  setTimeout(async () => {
    await database.write('foo', 'updated', transactionId + 1);
    console.log(await database.read('foo', transactionId)); // Should still print 'initial'
  }, 100);

  // Commit the transaction
  await database.commit(transactionId);

  console.log(await database.read('foo', transactionId)); // Should still print 'initial'
}

(async () => {
  const db = new MVCCDatabase();
  await exampleTransaction(db, 1);
})();