/// <summary>
/// This routine fetches the list of annotations that the requester's team has made since the last
/// time the requester got a list of annotations.
/// </summary>
private async Task GetAnnotationsRequesterLacks(DecodedSyncRequest request, SyncResponse response, int puzzleId, int teamId)
{
// We get the current time (which we'll later set as the "sync time") *before* we do the
// query, so that it's a conservative estimate of when the sync happened.
DateTime now = DateTime.Now;
List <Annotation> annotations;
if (request.LastSyncTime == null)
{
// If the requester didn't specify a last-sync time, then provide all the annotations from their team
// for this puzzle.
annotations = await(from a in context.Annotations
where a.PuzzleID == puzzleId && a.TeamID == teamId
select a).ToListAsync();
}
else
{
// We know the time of the last request, so in theory we should just return only
// annotations that are from that time or later. But, it could happen that an
// update that was concurrent with the previous sync request didn't make it into the
// returned list but nevertheless got a timestamp after that sync request. Also,
// there may be multiple web servers, with slightly unsynchronized clocks. So, for
// safety, we subtract five seconds. This may cause us to unnecessarily fetch and
// return an annotation the requester already knows about, but this is harmless: The
// requester will see that the annotation has the same version number as the one
// the requester already knows about for that key, and ignore it.
var lastSyncTimeMinusSlop = request.LastSyncTime.Value.AddSeconds(-5);
annotations = await(from a in context.Annotations
where a.PuzzleID == puzzleId && a.TeamID == teamId && a.Timestamp >= lastSyncTimeMinusSlop
select a).ToListAsync();
}
response.SetSyncTimeAndAnnotations(now, annotations);
}