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CSHARP-3550: CSOT: Server Selection #1705

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@sanych-sun sanych-sun requested a review from a team as a code owner June 6, 2025 01:11
@sanych-sun sanych-sun removed the request for review from a team June 6, 2025 01:16
@@ -47,7 +47,7 @@ private Exception CreateTimeoutException(Stopwatch stopwatch, string message)
var checkOutsForOtherCount = checkOutsCount - checkOutsForCursorCount - checkOutsForTransactionCount;

message =
$"Timed out after {stopwatch.ElapsedMilliseconds}ms waiting for a connection from the connection pool. " +
$"Timed out after {operationContext.Elapsed.TotalMilliseconds}ms waiting for a connection from the connection pool. " +
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minor: Looks like Elapsed.TotalMilliseconds is frequently used, does this award a shortcut property:
ElapsedMilleseconds?

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Reverted to use stopwatch here.

// TODO: this static field is temporary here and will be removed in a future PRs in scope of CSOT.
public static readonly OperationContext NoTimeout = new(System.Threading.Timeout.InfiniteTimeSpan, CancellationToken.None);

private readonly Stopwatch _stopwatch;
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So we are saving the multiple stopwatches creation on operation execution path, it's minor but still nice.

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Yep, but as it was discussed had to revert the change =)

[Values(false, true)]
bool async)
{
var subject = CreateSubject();
var subject = CreateSubject(serverSelectionTimeout: TimeSpan.FromMilliseconds(10));
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This adjusted serverSelectionTimeout is an optimization of the test: we probably do not have to wait whole 2 seconds here (the default server selection for this test class, see line 49).

@@ -134,23 +134,13 @@ await Record.ExceptionAsync(() => subject.ExecuteWriteOperationAsync(null, opera
.Subject.ParamName.Should().Be("session");
}

private OperationExecutor CreateSubject(out Mock<IClusterInternal> clusterMock, out Mock<ICoreSessionHandle> implicitSessionMock)
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Leftovers of the implicit session creation, that was factored out in one of the latest commits related to OperationExecutor refactoing.

@sanych-sun sanych-sun requested a review from rstam June 7, 2025 00:55
@sanych-sun sanych-sun requested a review from BorisDog June 9, 2025 20:48
@BorisDog BorisDog requested a review from Copilot June 9, 2025 21:15
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Pull Request Overview

This PR refactors various server selection and connection pool APIs to use a new OperationContext abstraction instead of separate CancellationToken and timeout TimeSpan parameters. Key changes include:

  • Replaced all CancellationToken (and internal Stopwatch timeout logic) with OperationContext throughout connection pool helpers, cluster selection, and binding classes.
  • Updated CreateTimeoutException methods to accept a TimeSpan elapsed rather than a Stopwatch.
  • Removed legacy overloads and helper classes associated with CancellationToken/TimeSpan and simplified server selection wait logic using OperationContext.

Reviewed Changes

Copilot reviewed 141 out of 141 changed files in this pull request and generated 3 comments.

Show a summary per file
File Description
src/MongoDB.Driver/Core/ConnectionPools/ExclusiveConnectionPool.Helpers.cs Swapped Stopwatch usage and token/timeouts for OperationContext
src/MongoDB.Driver/Core/Clusters/LoadBalancedCluster.cs Updated server selection to use OperationContext and WithTimeout
src/MongoDB.Driver/Core/Clusters/IClusterExtensions.cs Changed extension methods to accept OperationContext
src/MongoDB.Driver/Core/Clusters/ICluster.cs Updated interface signatures to use OperationContext
src/MongoDB.Driver/Core/Clusters/Cluster.cs Refactored internal select/wait loops to data in OperationContext
src/MongoDB.Driver/Core/Bindings/**/*.cs Replaced all CancellationToken parameters in binding APIs with OperationContext
Comments suppressed due to low confidence (3)

src/MongoDB.Driver/Core/ConnectionPools/ExclusiveConnectionPool.Helpers.cs:50

  • [nitpick] Using TotalMilliseconds can include fractional values (e.g., 12.345ms). Consider formatting with a fixed precision or rounding (e.g., {elapsed.TotalMilliseconds:F0}) to produce a cleaner integer-based message.
message = $"Timed out after {elapsed.TotalMilliseconds}ms waiting for a connection from the connection pool. " +

src/MongoDB.Driver/Core/Clusters/LoadBalancedCluster.cs:179

  • The new WithTimeout logic and WaitTask behavior are significant changes to server selection. Please add unit tests that simulate both timeout and cancellation scenarios for SelectServer/SelectServerAsync using mocks/stubs for _serverReadyTaskCompletionSource.
var serverSelectionOperationContext = operationContext.WithTimeout(_settings.ServerSelectionTimeout);

src/MongoDB.Driver/Core/Clusters/ICluster.cs:64

  • Removing the old CancellationToken overloads is a breaking API change. Consider retaining the old overloads (marked obsolete) to preserve backward compatibility, or provide clear upgrade guidance in release notes.
IServer SelectServer(IServerSelector selector, OperationContext operationContext);

if (index != 0)
try
{
serverSelectionOperationContext.WaitTask(_serverReadyTaskCompletionSource.Task);
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The original code explicitly threw on cancellation (cancellationToken.ThrowIfCancellationRequested()), but this method only catches TimeoutException. Consider catching OperationCanceledException from WaitTask or explicitly checking operationContext.CancellationToken to ensure cancellations are surfaced correctly.

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// should be pinned.
var server = cluster.SelectServer(selector, cancellationToken);
var server = cluster.SelectServer(selector, operationContext);
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[nitpick] We added non-null checks for selector and operationContext in core cluster methods; consider adding Ensure.IsNotNull(operationContext, nameof(operationContext)); here as well to maintain consistent validation.

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For some reasons we do not validate other parameters in the IClusterExtensions methods. Will keep it as is for now.

Comment on lines 51 to 56
{
return GetReadChannelSource(null, cancellationToken);
return GetReadChannelSource(null, operationContext);
}

public Task<IChannelSourceHandle> GetReadChannelSourceAsync(CancellationToken cancellationToken)
public Task<IChannelSourceHandle> GetReadChannelSourceAsync(OperationContext operationContext)
{
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[nitpick] Most public API methods now guard against null operationContext. Add an Ensure.IsNotNull(operationContext, nameof(operationContext)); at the top of this method to match the pattern in cluster selection.

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}
}
catch (TimeoutException)
{
var message = BuildTimeoutExceptionMessage(_settings.ServerSelectionTimeout, selector, helper.Description);
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BuildTimeoutExceptionMessage doesn't inform the user whether the operation timed out or the server selection timed out.

@sanych-sun sanych-sun requested a review from rstam June 10, 2025 19:09
{
var connection = connectionCreator.CreateOpened(cancellationToken);
var connection = connectionCreator.CreateOpened(operationContext);
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The reason for tiny 20ms timeout is just try to take turn in the connecting queue without blocking checkouts.
This timeout is applied to every step individually: MaxConnections and MaxConnecting.
Seems that this code changes this and allocates 20ms to

  • MaxConnections
  • MaxConnecting
  • Whole Connection creation operation (excluding open)

If this thought is correct, it's really surprising that this was not caught by tests.

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@sanych-sun was this addressed?

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Fixed.

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What about _maxConnectingQueue inside ConnectionCreator ?
We don't want to block on that for whole timeout, just a quick try instead.

Oleksandr Poliakov added 2 commits June 10, 2025 17:06
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LGTM

@@ -50,9 +50,10 @@ public TResult ExecuteReadOperation<TResult>(
Ensure.IsNotNull(session, nameof(session));
ThrowIfDisposed();

var cancellationContext = options.ToOperationContext(cancellationToken);
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rename this variable (and other instances) to operationContext?

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Done

{
exception = Record.Exception(() => subject.SelectServer(Mock.Of<IServerSelector>(), cancellationTokenSource.Token));
}
var cancellationContext = new OperationContext(Timeout.InfiniteTimeSpan, cancellationTokenSource.Token);
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rename variable to operationContext?

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Done.

{
var connection = connectionCreator.CreateOpened(cancellationToken);
var connection = connectionCreator.CreateOpened(operationContext);
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@sanych-sun was this addressed?

Oleksandr Poliakov added 2 commits June 11, 2025 11:31
@@ -287,6 +279,15 @@ private void AcquireWaitQueueSlot()
_enteredWaitQueue = true;
}

private void EnsureTimeout(OperationContext operationContext, Stopwatch stopwatch)
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Would ThrowIfTimedOut be a better name?

Normally we use EnsureXyz for argument validation.

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Renamed

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LGTM

@sanych-sun sanych-sun requested a review from BorisDog June 11, 2025 21:28
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LGTM

}

return CreateOpenedInternal(cancellationToken);
var operationContext = new OperationContext(Timeout.InfiniteTimeSpan, cancellationToken);
return CreateOpenedInternal(operationContext);
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minor: return CreateOpenedInternal(new(Timeout.InfiniteTimeSpan, cancellationToken));

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4 participants