The original implementation for VoldemortDatastoreProvider.setNextValue() totally blocks concurrency because of the exclusive lock. The second version allows concurrency but I'm not sure if it's the right way to calculate next values.

1.Original setNextValue() method with exclusive lock.

public void setNextValue(RowKey key, IntegralDataTypeHolder value, int increment, int initialValue) {
this.lock.lock();
        try {
            Versioned v = this.getValue(this.getVoldemortSequecenStoreName(),
                    this.toJSON(key.getRowKeyAsMap()), true);

            if (v == null) {
                Map<String, Integer> nextSequence = new HashMap<String, Integer>();
                nextSequence.put("nextSequence", initialValue);

                this.putValue(this.getVoldemortSequecenStoreName(),
                        this.toJSON(key.getRowKeyAsMap()), nextSequence, true,
                        false);
                value.initialize(initialValue);
                return;
            }

            int nextValue = (Integer) ((Map<String, Integer>) v.getValue())
                    .get("nextSequence") + increment;
            value.initialize(nextValue);

            Map<String, Integer> nextSequence = new HashMap<String, Integer>();
            nextSequence.put("nextSequence",
                    ((Map<String, Integer>) v.getValue()).get("nextSequence")
                            + increment);
            this.putValue(this.getVoldemortSequecenStoreName(),
                    this.toJSON(key.getRowKeyAsMap()), nextSequence, true,
                    false);
        } finally {
            this.lock.unlock();
        }
 
    }

2. Current setNextValue() method with concurrency support

public void setNextValue(RowKey key, IntegralDataTypeHolder value,
			int increment, int initialValue) {

		List<Integer> l = new LinkedList<Integer>();
		l.add(initialValue);
		if (this.nextValues.putIfAbsent(key, l) == null) {
			Map<String, Integer> nextSequence = new HashMap<String, Integer>();
			nextSequence.put(VoldemortDatastoreProvider.SEQUENCE_LABEL,
					initialValue);
			value.initialize(initialValue);

			if (this.flushToDb) {
				// this.taskQueue.offer(new PutSequenceRunnable(this, this
				// .getVoldemortSequenceStoreName(), this.toJSON(key
				// .getRowKeyAsMap()), nextSequence, true, false));

				this.putValue(this.getVoldemortSequenceStoreName(),
						this.toJSON(key.getRowKeyAsMap()), nextSequence, true,
						false);
			}
		} else {
			List<Integer> list = this.nextValues.get(key);
			synchronized (list) {
				boolean notContained = false;
				int candidate = list.get(list.size() - 1) + increment;
				if (!list.contains(candidate)) {
					this.nextValues.get(key).add(candidate);
					notContained = true;
				}
				value.initialize(candidate);
				Map<String, Integer> nextSequence = new HashMap<String, Integer>();
				nextSequence.put(VoldemortDatastoreProvider.SEQUENCE_LABEL,
						candidate);

				if (this.flushToDb) {
					if (notContained) {
						this.putValue(this.getVoldemortSequenceStoreName(),
								this.toJSON(key.getRowKeyAsMap()),
								nextSequence, true, false);
					}
				}
			}
		}
	}