@Override
  public Collection<Account> getAllAccounts() {
    return primaryAccountService.getAllAccounts();
  }
An implementation of {@link AccountService} that combines two sources. The "primary" acts as the source of
truth for users of this class, but results obtained are compared against the "secondary" source. This class can be
useful for safe migrations between account service implementations using different backing stores.
ambry-account/src/main/java/com/github/ambry/account/CompositeAccountService.java