// return true if this caller won the race to load whatever would go at key
private static bool GotCompeteLock(LocalCache cache, string key)
{
var competeKey = key + "-cload";
if (!cache.SetNXSync(competeKey, DateTime.UtcNow))
{
var x = cache.Get <DateTime>(competeKey);
// Somebody abandoned the lock, clear it and try again
if (DateTime.UtcNow - x > TimeSpan.FromMinutes(5))
{
cache.Remove(competeKey);
return(GotCompeteLock(cache, key));
}
// Lost the lock competition
return(false);
}
// winner, go do something expensive now
return(true);
}
/// <summary>
///
/// lookup refreshes the data if necessay, passing the old data if we have it.
///
/// durationSecs is the "time before stale" for the data
/// serveStaleSecs is the maximum amount of time to serve data once it becomes stale
///
/// Note that one unlucky caller when the data is stale will block to fill the cache,
/// everybody else will get stale data though.
/// </summary>
public static T GetSet <T>(this LocalCache cache, string key, Func <T, MicroContext, T> lookup, int durationSecs, int serveStaleDataSecs)
where T : class
{
var possiblyStale = cache.Get <GetSetWrapper <T> >(key);
var localLockName = key;
var nullLoadLock = _getSetNullLocks.AddOrUpdate(localLockName, k => new object(), (k, old) => old);
if (possiblyStale == null)
{
// We can't prevent multiple web server's from running this (well, we can but its probably overkill) but we can
// at least stop the query from running multiple times on *this* web server
lock (nullLoadLock)
{
possiblyStale = cache.Get <GetSetWrapper <T> >(key);
if (possiblyStale == null)
{
T data;
using (var ctx = new MicroContext())
{
data = lookup(null, ctx);
}
possiblyStale = new GetSetWrapper <T>
{
Data = data,
StaleAfter = DateTime.UtcNow + TimeSpan.FromSeconds(durationSecs)
};
cache.Set(key, possiblyStale, durationSecs + serveStaleDataSecs);
Interlocked.Increment(ref totalGetSetSync);
}
}
}
if (possiblyStale.StaleAfter > DateTime.UtcNow)
{
return(possiblyStale.Data);
}
bool gotCompeteLock = false;
if (Monitor.TryEnter(nullLoadLock, 0))
{ // it isn't actively being refreshed; we'll check for a mutex on the cache
try
{
gotCompeteLock = GotCompeteLock(cache, key);
}
finally
{
Monitor.Exit(nullLoadLock);
}
}
if (gotCompeteLock)
{
var old = possiblyStale.Data;
var task = new Task(delegate
{
lock (nullLoadLock) // holding this lock allows us to locally short-circuit all the other threads that come asking
{
try
{
var updated = new GetSetWrapper <T>();
using (var ctx = new MicroContext())
{
updated.Data = lookup(old, ctx);
updated.StaleAfter = DateTime.UtcNow + TimeSpan.FromSeconds(durationSecs);
}
cache.Remove(key);
cache.Set(key, updated, durationSecs + serveStaleDataSecs);
}
finally
{
ReleaseCompeteLock(cache, key);
}
}
});
task.ContinueWith(t =>
{
if (t.IsFaulted)
{
Interlocked.Increment(ref totalGetSetAsyncError);
Current.LogException(t.Exception);
}
else
{
Interlocked.Increment(ref totalGetSetAsyncSuccess);
}
});
task.Start();
}
return(possiblyStale.Data);
}
// called by a winner of CompeteToLoad, to make it so the next person to call CompeteToLoad will get true
private static void ReleaseCompeteLock(LocalCache cache, string key)
{
cache.Remove(key + "-cload");
}