public  class LocksServiceImpl implements ILocksService {
    /**
     * acquire - Obtain a lock that prevents other transactions from executing
     */

    public void acquire(String lockItem) {

        Integer retryCount = 0;
        Exception error;

        while (retryCount < LocksService.MAX_RETRIES) {
            try {
                //  FOR UPDATE will pause this Txn until lock is freed by other transaction
                Mutex__c[] mutexes = MutexesSelector.newInstance().selectForUpdateByItem(new Set<String> {lockItem});
                if (mutexes.isEmpty()) {
                    insert new Mutex__c (Lockable_Item__c = lockItem);
                }
                return;     // this Txn now has the lock on lockItem
            }
            catch (Exception e) {
                if (Util.isTransientException(e)) { // UNABLE_TO_LOCK_ROW
                    error = e;
                    retryCount++;
                }
                else {
                    error = e;
                    break;
                }
            }
        }
        throw new LocksService.MutexException('Unable to obtain a transaction lock on ' + lockItem +
                ' retries:' + retryCount + ' v. maxRetries:' + LocksService.MAX_RETRIES + ' exception:' + Util.showException(error));
    }

}