Code sample provided by RavenHQ for batching up writes

RavenBatchOptimizer.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using Raven.Client;

/// <summary>
/// This class allows performing high throughput document store operations, using a batching mechanism
/// That mechanism collects up to 128 documents into a single batch and stores them in a single operation, reducing
/// network io compared to an atomic operation. The mechanism is also capable of treating single store operation failure
/// by retrying a batch, executing it item by item 
/// </summary>
internal class RavenBatchOptimizer : IDisposable
{
    private readonly BlockingCollection<Work> _pending = new BlockingCollection<Work>();
    private readonly IDocumentStore _store;
    private readonly CancellationToken _ct;
    private readonly ConcurrentQueue<ManualResetEventSlim> _mresQueue = new ConcurrentQueue<ManualResetEventSlim>();
    private readonly List<Task> _processingTasks;
    private readonly CancellationTokenSource _cts;

    public RavenBatchOptimizer(IDocumentStore store, CancellationToken ct, int parallelism = 2)
    {
        _store = store;
        _cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        _ct = _cts.Token;

        _processingTasks = new List<Task>(parallelism);
        for (var i = 0; i < parallelism; i++)
        {
            _processingTasks.Add(Task.Factory.StartNew(DoWork, TaskCreationOptions.LongRunning));
        }
    }

    /// <summary>
    /// Enqueue the entity to be stored and waits until it will be proccessed
    /// </summary>
    /// <param name="entity"></param>
    public void Write(object entity)
    {
        _cts.Token.ThrowIfCancellationRequested();
        if (_mresQueue.TryDequeue(out var mres) == false)
        {
            mres = new ManualResetEventSlim();
        }

        var work = new Work
        {
            Entity = entity,
            Event = mres
        };

        work.Event.Reset();
        _pending.Add(work, CancellationToken.None);

        // wait for entity to be stored in the next batch
        work.Event.Wait(CancellationToken.None);
        _mresQueue.Enqueue(mres);

        if (work.Exception != null)
            throw new InvalidOperationException("Failed to write", work.Exception);
    }

    public void Dispose()
    {
        if (_cts.IsCancellationRequested == false)
        {
            var proccesses = _processingTasks.ToArray();
            _processingTasks.Clear();
            _cts.Cancel();
            _pending.CompleteAdding();
            try
            {
                Task.WaitAll(proccesses);
            }
            catch (OperationCanceledException)
            {
                // do nothing
            }
        }
    }

    /// <summary>
    /// Long running method that performs the batch grouping and execution operations
    /// </summary>
    /// <returns></returns>
    async Task DoWork()
    {
        var list = new List<Work>();

        while (_cts.IsCancellationRequested == false || _pending.IsCompleted == false)
        {
            list.Clear();
            var work = _pending.Take(_ct);
            list.Add(work);

            while (list.Count < 128 && _pending.TryTake(out var item)) list.Add(item);

            using (var session = _store.OpenAsyncSession())
            {
                var failed = false;

                try
                {
                    foreach (var p in list) session.Advanced.Store(p.Entity);

                    await session.SaveChangesAsync(CancellationToken.None).ConfigureAwait(false);
                }
                catch (Exception)
                {
                    failed = true;
                }

                // if we've failed to store the batch (which was entirely rolled back), we try executing each of the
                // items seperately, in case we've failed because of a single item
                if (failed) await HandleSaveFailure(list).ConfigureAwait(false);
            }

            foreach (var p in list) p.Event.Set();
        }
    }

    /// <summary>
    /// Store entities list one by one, setting the errorous message state to the particular message
    /// </summary>
    /// <param name="list"></param>
    /// <returns></returns>
    async Task HandleSaveFailure(IEnumerable<Work> list)
    {
        foreach (var p in list)
            using (var recoverySession = _store.OpenAsyncSession())
            {
                await recoverySession.StoreAsync(p.Entity, CancellationToken.None).ConfigureAwait(false);

                try
                {
                    await recoverySession.SaveChangesAsync(CancellationToken.None).ConfigureAwait(false);
                }
                catch (Exception e)
                {
                    p.Exception = e;
                }
            }
    }

    class Work
    {
        public object Entity;
        public ManualResetEventSlim Event;
        public Exception Exception;
    }
}