/// <summary>
/// Creates a Firestore document that stores the cache value and its
/// expiration info, but does not store it in Firestore.
/// </summary>
/// <param name="value">The value to be stored in the cache.</param>
/// <param name="options">Expiration info.</param>
/// <returns></returns>
private CacheDoc MakeCacheDoc(byte[] value, DistributedCacheEntryOptions options)
{
CacheDoc doc = new CacheDoc()
{
LastRefresh = _clock.GetCurrentDateTimeUtc(),
Value = value,
};
if (options.SlidingExpiration.HasValue)
{
doc.SlidingExpirationSeconds = options.SlidingExpiration.Value.TotalSeconds;
}
else
{
doc.SlidingExpirationSeconds = _forever.TotalSeconds;
}
if (options.AbsoluteExpiration.HasValue)
{
doc.AbsoluteExpiration = options.AbsoluteExpiration.Value.UtcDateTime;
}
if (options.AbsoluteExpirationRelativeToNow.HasValue)
{
doc.AbsoluteExpiration = _clock.GetCurrentDateTimeUtc() +
options.AbsoluteExpirationRelativeToNow.Value;
}
return(doc);
}
/// <summary>
/// Extracts the cache value from a Firestore document snapshot.
/// Checks all the expiration fields so never returns a stale value.
/// </summary>
/// <param name="snapshot">The snapshot to pull the cache value from.</param>
/// <returns>The cache value or null.</returns>
private byte[] ValueFromSnapshot(DocumentSnapshot snapshot)
{
if (!snapshot.Exists)
{
return(null);
}
CacheDoc doc = snapshot.ConvertTo <CacheDoc>();
var now = _clock.GetCurrentDateTimeUtc();
if (doc.AbsoluteExpiration < now)
{
return(null);
}
var slidingExpiration = doc.LastRefresh.GetValueOrDefault()
+ TimeSpan.FromSeconds(doc.SlidingExpirationSeconds.GetValueOrDefault());
if (slidingExpiration < now)
{
return(null);
}
return(doc.Value);
}
/// <summary>
/// Scans the Firestore collection for expired cache entries and
/// deletes them.
/// </summary>
/// <param name="token">A cancellation token.</param>
public async Task CollectGarbageAsync(CancellationToken token)
{
_logger.LogTrace("Begin garbage collection.");
// Purge entries whose AbsoluteExpiration has passed.
const int pageSize = 40;
int batchSize; // Batches of cache entries to be deleted.
var now = _clock.GetCurrentDateTimeUtc();
do
{
QuerySnapshot querySnapshot = await
_cacheEntries.OrderByDescending("AbsoluteExpiration")
.StartAfter(now)
.Limit(pageSize)
.GetSnapshotAsync(token)
.ConfigureAwait(false);
batchSize = 0;
WriteBatch writeBatch = _cacheEntries.Database.StartBatch();
foreach (DocumentSnapshot docSnapshot in querySnapshot.Documents)
{
if (docSnapshot.ConvertTo <CacheDoc>().AbsoluteExpiration.HasValue)
{
writeBatch.Delete(docSnapshot.Reference, Precondition.LastUpdated(
docSnapshot.UpdateTime.GetValueOrDefault()));
batchSize += 1;
}
}
if (batchSize > 0)
{
_logger.LogDebug("Collecting {0} cache entries.", batchSize);
await writeBatch.CommitAsync(token).ConfigureAwait(false);
}
token.ThrowIfCancellationRequested();
} while (batchSize == pageSize);
// Purge entries whose SlidingExpiration has passed.
do
{
QuerySnapshot querySnapshot = await
_cacheEntries.OrderBy("LastRefresh")
.Limit(pageSize)
.GetSnapshotAsync(token)
.ConfigureAwait(false);
batchSize = 0;
WriteBatch writeBatch = _cacheEntries.Database.StartBatch();
foreach (DocumentSnapshot docSnapshot in querySnapshot.Documents)
{
CacheDoc doc = docSnapshot.ConvertTo <CacheDoc>();
if (doc.SlidingExpirationSeconds.HasValue)
{
var slidingExpiration =
doc.LastRefresh.GetValueOrDefault()
+ TimeSpan.FromSeconds(doc.SlidingExpirationSeconds.Value);
if (slidingExpiration < now)
{
writeBatch.Delete(docSnapshot.Reference, Precondition.LastUpdated(
docSnapshot.UpdateTime.GetValueOrDefault()));
batchSize += 1;
}
}
}
if (batchSize > 0)
{
_logger.LogDebug("Collecting {batchSize} cache entries.", batchSize);
await writeBatch.CommitAsync(token).ConfigureAwait(false);
}
token.ThrowIfCancellationRequested();
} while (batchSize > 0);
_logger.LogTrace("End garbage collection.");
}