private AppendContext Prepare(DateTimeOffset now)
{
var index = this.ReadIndex();
var firstLink = index.Links
.Where(l => l.IsFirstLink)
.DefaultIfEmpty(AtomLink.CreateFirstLink(this.CreateNewFeedAddress()))
.Single();
index = index.WithLinks(index.Links.Union(new[] { firstLink }));
var lastLink = index.Links.SingleOrDefault(l => l.IsLastLink);
var lastLinkAdded = false;
if (lastLink == null)
{
lastLink = firstLink.ToLastLink();
lastLinkAdded = true;
}
var lastPage = this.ReadTrueLastPage(lastLink.Href);
var lastLinkCorrected = false;
if (lastPage.Links.Single(l => l.IsSelfLink).Href != lastLink.Href)
{
lastLink = lastPage.Links.Single(l => l.IsSelfLink).ToLastLink();
lastLinkCorrected = true;
}
index = index.WithLinks(index.Links
.Where(l => !l.IsLastLink)
.Concat(new[] { lastLink }));
return(new AppendContext(
index,
lastPage,
now,
lastLinkAdded,
lastLinkCorrected));
}
/// <summary>
/// Appends an event to the event stream.
/// </summary>
/// <param name="event">
/// The event to append to the event stream.
/// </param>
/// <returns>
/// A <see cref="Task" /> representing the asynchronous operation of
/// appending the event to the event stream.
/// </returns>
/// <remarks>
/// <para>
/// This method appends <paramref name="event" /> to the current event
/// stream. Appending an event indicates that it happened
/// <em>after</em> all previous events.
/// </para>
/// <para>
/// Since this method conceptually involves writing the event to the
/// underlying <see cref="Storage" />, it may take significant time to
/// complete; for that reason, it's an asynchronous method, returning a
/// <see cref="Task" />. The operation is not guaranteed to be complete
/// before the Task completes successfully.
/// </para>
/// <para>
/// When updating the underlying Storage, the method typically only
/// updates the index feed, using
/// <see cref="IAtomEventStorage.CreateFeedWriterFor(AtomFeed)" />.
/// However, when the number of entries in the index surpasses
/// <see cref="PageSize" />, a new feed page is created for the new
/// entry. This page is also written using the CreateFeedWriterFor
/// method, and only after this succeeds is the old feed page updated
/// with a link to the new page. Since these two operations are not
/// guaranteed to happen within an ACID transaction, it's possible that
/// the new page is saved, but that the update of the old page fails.
/// If the underlying storage throws an exception at that point, that
/// exception will bubble up to the caller of the AppendAsync method.
/// It's up to the caller to retry the operation.
/// </para>
/// <para>
/// However, in that situation, an orphaned Atom feed page is likely to
/// have been left in the underlying storage. This doesn't affect
/// consistency of the system, but may take up unnecessary disk space.
/// If this is the case, a separate clean-up task should find and
/// delete orphaned pages.
/// </para>
/// </remarks>
/// <example>
/// This example shows how to create a UserCreated event and write it
/// using the AppendAsync method. Notice that since
/// AtomEventObserver<T> uses the standard Task Parallel Library
/// (TPL) model, you can use it with 'async' and 'await'.
/// <code>
/// var obs = new AtomEventObserver<IUserEvent>(
/// eventStreamId, // a Guid
/// pageSize, // an Int32
/// storage, // an IAtomEventStorage object
/// serializer); // an IContentSerializer object
///
/// var userCreated = new UserCreated
/// {
/// UserId = eventStreamId,
/// UserName = "******",
/// Password = "******",
/// Email = "*****@*****.**"
/// };
/// await obs.AppendAsync(userCreated);
/// </code>
/// </example>
/// <seealso cref="OnNext" />
public Task AppendAsync(T @event)
{
return(Task.Factory.StartNew(() =>
{
var now = DateTimeOffset.Now;
var index = this.ReadIndex();
var firstLink = index.Links
.Where(l => l.IsFirstLink)
.DefaultIfEmpty(AtomLink.CreateFirstLink(this.CreateNewFeedAddress()))
.Single();
var lastLink = index.Links.SingleOrDefault(l => l.IsLastLink);
var lastLinkChanged = false;
if (lastLink == null)
{
lastLink = firstLink.ToLastLink();
lastLinkChanged = true;
}
var lastPage = this.ReadLastPage(lastLink.Href);
if (lastPage.Links.Single(l => l.IsSelfLink).Href != lastLink.Href)
{
lastLink = lastPage.Links.Single(l => l.IsSelfLink).ToLastLink();
lastLinkChanged = true;
}
index = index.WithLinks(index.Links.Union(new[] { firstLink }));
index = index.WithLinks(index.Links
.Where(l => !l.IsLastLink)
.Concat(new[] { lastLink }));
var entry = CreateEntry(@event, now);
if (lastPage.Entries.Count() >= this.pageSize)
{
var nextAddress = this.CreateNewFeedAddress();
var nextPage = this.ReadPage(nextAddress);
nextPage = AddEntryTo(nextPage, entry, now);
var nextLink = AtomLink.CreateNextLink(nextAddress);
var previousPage = lastPage
.WithLinks(lastPage.Links.Concat(new[] { nextLink }));
var previousLink = previousPage.Links
.Single(l => l.IsSelfLink)
.ToPreviousLink();
nextPage = nextPage.WithLinks(
nextPage.Links.Concat(new[] { previousLink }));
index = index.WithLinks(index.Links
.Where(l => !l.IsLastLink)
.Concat(new[] { nextLink.ToLastLink() }));
this.Write(nextPage);
this.Write(previousPage);
this.Write(index);
}
else
{
lastPage = AddEntryTo(lastPage, entry, now);
this.Write(lastPage);
if (lastLinkChanged)
{
this.Write(index);
}
}
}));
}