CSHARP-3550: CSOT: Server Selection #1705

sanych-sun · 2025-06-06T01:11:08Z

No description provided.

src/MongoDB.Driver/OperationCancellationContext.cs

BorisDog · 2025-06-06T22:11:44Z

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

@@ -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. " +


minor: Looks like Elapsed.TotalMilliseconds is frequently used, does this award a shortcut property:
ElapsedMilleseconds?

Reverted to use stopwatch here.

BorisDog · 2025-06-06T22:14:07Z

src/MongoDB.Driver/OperationContext.cs

+        // 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;


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

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

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

sanych-sun · 2025-06-06T23:48:47Z

tests/MongoDB.Driver.Tests/Core/Clusters/ClusterTests.cs

            [Values(false, true)]
            bool async)
        {
-            var subject = CreateSubject();
+            var subject = CreateSubject(serverSelectionTimeout: TimeSpan.FromMilliseconds(10));


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).

sanych-sun · 2025-06-07T00:05:06Z

tests/MongoDB.Driver.Tests/OperationExecutorTests.cs

@@ -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)


Leftovers of the implicit session creation, that was factored out in one of the latest commits related to OperationExecutor refactoing.

Copilot

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);

Copilot · 2025-06-09T21:17:04Z

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

-            if (index != 0)
+            try
+            {
+                serverSelectionOperationContext.WaitTask(_serverReadyTaskCompletionSource.Task);


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.

Copilot · 2025-06-09T21:17:05Z

src/MongoDB.Driver/Core/Clusters/IClusterExtensions.cs

            // should be pinned.
-            var server = cluster.SelectServer(selector, cancellationToken);
+            var server = cluster.SelectServer(selector, operationContext);


[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.

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

Copilot · 2025-06-09T21:17:05Z

src/MongoDB.Driver/Core/Bindings/WritableServerBinding.cs

        {
-            return GetReadChannelSource(null, cancellationToken);
+            return GetReadChannelSource(null, operationContext);
        }

-        public Task<IChannelSourceHandle> GetReadChannelSourceAsync(CancellationToken cancellationToken)
+        public Task<IChannelSourceHandle> GetReadChannelSourceAsync(OperationContext operationContext)
        {


[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.

src/MongoDB.Driver/Core/Bindings/ChannelReadWriteBinding.cs

src/MongoDB.Driver/Core/Bindings/CoreSession.cs

src/MongoDB.Driver/Core/Bindings/IBinding.cs

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

rstam · 2025-06-10T17:13:25Z

src/MongoDB.Driver/Core/Clusters/Cluster.cs

                    }
                }
+                catch (TimeoutException)
+                {
+                    var message = BuildTimeoutExceptionMessage(_settings.ServerSelectionTimeout, selector, helper.Description);


BuildTimeoutExceptionMessage doesn't inform the user whether the operation timed out or the server selection timed out.

src/MongoDB.Driver/Core/Clusters/IClusterExtensions.cs

src/MongoDB.Driver/Core/Operations/AggregateOperation.cs

BorisDog · 2025-06-10T20:00:25Z

src/MongoDB.Driver/Core/ConnectionPools/MaintenanceHelper.cs

                    {
-                        var connection = connectionCreator.CreateOpened(cancellationToken);
+                        var connection = connectionCreator.CreateOpened(operationContext);


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.

@sanych-sun was this addressed?

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

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

tests/MongoDB.Driver.Tests/OperationContextTests.cs

src/MongoDB.Driver/OperationContext.cs

src/MongoDB.Driver/OperationOptionsBase.cs

rstam

LGTM

adelinowona · 2025-06-11T17:44:59Z

src/MongoDB.Driver/OperationExecutor.cs

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

+            var cancellationContext = options.ToOperationContext(cancellationToken);


rename this variable (and other instances) to operationContext?

adelinowona · 2025-06-11T17:45:48Z

tests/MongoDB.Driver.Tests/Core/Clusters/LoadBalancedClusterTests.cs

-                    {
-                        exception = Record.Exception(() => subject.SelectServer(Mock.Of<IServerSelector>(), cancellationTokenSource.Token));
-                    }
+                    var cancellationContext = new OperationContext(Timeout.InfiniteTimeSpan, cancellationTokenSource.Token);


rename variable to operationContext?

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

BorisDog · 2025-06-11T18:27:16Z

src/MongoDB.Driver/Core/ConnectionPools/MaintenanceHelper.cs

                    {
-                        var connection = connectionCreator.CreateOpened(cancellationToken);
+                        var connection = connectionCreator.CreateOpened(operationContext);


@sanych-sun was this addressed?

tests/MongoDB.Driver.Tests/OperationContextTests.cs

rstam · 2025-06-11T20:25:48Z

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

@@ -287,6 +279,15 @@ private void AcquireWaitQueueSlot()
                _enteredWaitQueue = true;
            }

+            private void EnsureTimeout(OperationContext operationContext, Stopwatch stopwatch)


Would ThrowIfTimedOut be a better name?

Normally we use EnsureXyz for argument validation.

adelinowona

LGTM

tests/MongoDB.Driver.Tests/OperationContextTests.cs

CSHARP-3550: CSOT: Server Selection

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sanych-sun requested review from rstam, BorisDog and adelinowona June 6, 2025 01:11

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Rename OperationCancellationContext to OperationContext

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CSHARP-3550: CSOT: Server Selection #1705

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CSHARP-3550: CSOT: Server Selection #1705

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sanych-sun commented Jun 6, 2025

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Pull Request Overview

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