public async Task RunElectionsForever(CancellationToken token) {
var leaseWrapper = new BlobLeaseWrapper(_blob, 512);
while (!token.IsCancellationRequested) {
await GrabLeaseOrWait(leaseWrapper, token);
}
}
async Task GrabLeaseOrWait(BlobLeaseWrapper leaseWrapper, CancellationToken token) {
// Try to acquire the blob lease, otherwise wait for some time before we can try again.
var leaseId = await TryAcquireLeaseOrWait(leaseWrapper, token);
if (string.IsNullOrEmpty(leaseId)) {
return;
}
// Create a new linked cancellation token source, so if either the
// original token is canceled or the lease cannot be renewed,
// then the leader task can be canceled.
using (var cts =
CancellationTokenSource.CreateLinkedTokenSource(new[] {token})) {
// Run the leader task.
var leaderTask = _leaderTask.Invoke(cts.Token, _blob);
// Keeps renewing the lease in regular intervals.
// If the lease cannot be renewed, then the task completes.
var renew = KeepRenewingLease(leaseWrapper, leaseId, cts.Token);
// When any task completes (either the leader task or when it could
// not renew the lease) then cancel the other task.
await CancelAllWhenAnyCompletes(leaderTask, renew, cts);
}
}
static async Task<string> TryAcquireLeaseOrWait(BlobLeaseWrapper leaseWrapper,
CancellationToken token) {
try {
var leaseId = await leaseWrapper.AcquireLeaseAsync(token);
if (!string.IsNullOrEmpty(leaseId)) {
return leaseId;
}
await Task.Delay(AcquireAttemptInterval, token);
return null;
}
catch (OperationCanceledException) {
return null;
}
}
static async Task KeepRenewingLease(BlobLeaseWrapper leaseWrapper, string leaseId,
CancellationToken token) {
var renewOffset = new Stopwatch();
while (!token.IsCancellationRequested) {
try {
// Immediately attempt to renew the lease
// We cannot be sure how much time has passed since the lease was actually acquired
renewOffset.Restart();
var renewed = await leaseWrapper.RenewLeaseAsync(leaseId, token);
renewOffset.Stop();
if (!renewed) {
return;
}
// We delay based on the time from the start of the last renew request to ensure
var renewIntervalAdjusted = RenewInterval - renewOffset.Elapsed;
// If the adjusted interval is greater than zero wait for that long
if (renewIntervalAdjusted > TimeSpan.Zero) {
await Task.Delay(RenewInterval - renewOffset.Elapsed, token);
}
}
catch (OperationCanceledException) {
leaseWrapper.ReleaseLease(leaseId);
return;
}
}
}
