Class SimpleSyncRWTransaction

src/generic/key_value_filesystem.ts:147–217

 * A simple RW transaction for simple synchronous key-value stores.
 */
export class SimpleSyncRWTransaction implements SyncKeyValueRWTransaction {
  /**
   * Stores data in the keys we modify prior to modifying them.
   * Allows us to roll back commits.
   */
  private originalData: { [key: string]: Buffer | undefined } = {};
  /**
   * List of keys modified in this transaction, if any.
   */
  private modifiedKeys: string[] = [];

  constructor(private store: SimpleSyncStore) { }

  public get(key: string): Buffer | undefined {
    const val = this.store.get(key);
    this.stashOldValue(key, val);
    return val;
  }

  public put(key: string, data: Buffer, overwrite: boolean): boolean {
    this.markModified(key);
    return this.store.put(key, data, overwrite);
  }

  public del(key: string): void {
    this.markModified(key);
    this.store.del(key);
  }

  public commit(): void {/* NOP */}

  public abort(): void {
    // Rollback old values.
    for (const key of this.modifiedKeys) {
      const value = this.originalData[key];
      if (!value) {
        // Key didn't exist.
        this.store.del(key);
      } else {
        // Key existed. Store old value.
        this.store.put(key, value, true);
      }
    }
  }

  /**
   * Stashes given key value pair into `originalData` if it doesn't already
   * exist. Allows us to stash values the program is requesting anyway to
   * prevent needless `get` requests if the program modifies the data later
   * on during the transaction.
   */
  private stashOldValue(key: string, value: Buffer | undefined) {
    // Keep only the earliest value in the transaction.
    if (!this.originalData.hasOwnProperty(key)) {
      this.originalData[key] = value;
    }
  }