Get/Return pooled connections #3404

mdaigle · 2025-06-09T19:11:03Z

Description

This PR adds Get and Return functionality to the new ChannelDbConnectionPool class.
A channel and corresponding channel reader and writer are added to underpin these operations.
An array is added to hold references to all connections managed by the pool.
Logic is included to respect max pool size in all cases.

Not included: pool warm up (including respecting min pool size), pool pruning, transactions, tracing/logs. These will come in follow-up PRs.

Also includes extensive unit test coverage for the implemented functionality. Integration testing is reserved for a later state when more of the pool functionality is available.

Issues

#3356

Testing

Tests focus on the Get and Return flows with special focus on the following:

async behavior
max pool size respected
connections successfully reused
requests successfully queued and queue order respected
stress testing with many parallel operations

src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionInternal.cs

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

mdaigle · 2025-06-10T16:25:05Z

@roji tagging you here if you have any time to review. Thanks!

benrr101

In general, it looks good. A bunch of things I'd like addressed, but you know I'll accept valid arguments against fixing them :)

src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionInternal.cs

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

...lient/tests/UnitTests/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPoolTest.cs

paulmedynski

I have reviewed the ChannelDbConnectionPool changes. I will look at the tests once we have converged on an implementation. It might be a good idea to host another meeting to walk through the commentary here.

tools/props/Versions.props

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

paulmedynski · 2025-06-13T12:13:12Z

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

+
+                    return Task.FromResult<DbConnectionInternal?>(newConnection);
+                }
+                catch


Catch only what is documented to be thrown.

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

imasud00 · 2025-06-17T08:27:03Z

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

+        /// </summary>
+        private static int _instanceCount;
+
+        private readonly int _instanceID = Interlocked.Increment(ref _instanceCount);


Interlocked.Increment(ref _instanceCount);

does it matter if it becomes negative ?

it starts at zero and only ever goes up

Counters are naturally unsigned, so can we use uint or ulong instead? I'd be proud if those ever wrapped :)

Also, should this be _instanceId ?

There's no Interlocked.Increment() overload for uint. It doesn't matter if it goes negative. It's only used for identification of unique pools in trace messages.

There is for .NET:

https://learn.microsoft.com/en-us/dotnet/api/system.threading.interlocked.increment?view=net-9.0#system-threading-interlocked-increment(system-uint32@)

Framework is stuck with signed only.

Signed Increment() doesn't throw on overflow, so there's no programming problem here, just perhaps an unexpected logging issue with negative IDs. Not worth worrying about IMO.

src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionInternal.cs

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

imasud00 · 2025-06-17T19:05:51Z

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

@@ -20,52 +25,136 @@ namespace Microsoft.Data.SqlClient.ConnectionPool
    /// </summary>
    internal sealed class ChannelDbConnectionPool : IDbConnectionPool


ChannelDbConnectionPool

it might be good to add some documentation on how this class works and how this is different than the WaitHandleDbConnectionPool.

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

vonzshik · 2025-06-27T14:53:52Z

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

+                        async: true,
+                        timeout
+                    ).ConfigureAwait(false);
+                    taskCompletionSource.SetResult(connection);


Any particular reason to not use TrySetResult and TrySetException to avoid unnecessary exceptions?

I like this idea, done!

vonzshik · 2025-06-27T14:59:52Z

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

+                // We're potentially on a new thread, so we need to properly set the ambient transaction.
+                // We rely on the caller to capture the ambient transaction in the TaskCompletionSource's AsyncState
+                // so that we can access it here. Read: area for improvement.
+                ADP.SetCurrentTransaction(taskCompletionSource.Task.AsyncState as Transaction);


Looking at the current source code of WaitHandleDbConnectionPool, it seems like everything between ADP.SetCurrentTransaction and taskCompletionSource.SetResult is synchronous, unlike here, where GetInternalConnection can go async. I wonder whether it's to make the default TransactionScope work (you have to pass a parameter to support async) with OpenAsync, although it's kinda weird to do so anyway.

Thanks, this is an important point. I haven't included transaction support in the scope of work for this PR and included this line prematurely. This line is incorrect for the reason you stated. When we go down the async path, we may end up creating/retrieving a connection in a continuation and that continuation would have no reference to the ambient transaction.

This does make me wonder whether we need any additional hygiene measures to make sure we reset ambient transaction state on threads. We wouldn't want a managed thread to hold onto the ambient transaction state and incorrectly enlist other connections.

…et-return-pooled-connections

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

… connection disposal when opening new connection.

paulmedynski

We're getting closer!

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionInternal.cs

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ConnectionPoolSlots.cs

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

paulmedynski · 2025-07-02T18:29:27Z

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

-                    {
-                        cancellationToken.ThrowIfCancellationRequested();
-                        Debug.Assert(finalToken.IsCancellationRequested);
+                    // We didn't find an idle connection, try to open a new one.  


These ??= comments are a bit misleading. At line 466 we don't know whether or not we got an idle connection. We only know that if we hit the await part. Maybe this would be clearer as If we didn't find ..., then try to ....

codecov · 2025-07-16T05:48:11Z

Codecov Report

Attention: Patch coverage is 89.32806% with 27 lines in your changes missing coverage. Please review.

Project coverage is 63.10%. Comparing base (8eb9f32) to head (531215c).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
...qlClient/ConnectionPool/ChannelDbConnectionPool.cs	89.28%	21 Missing ⚠️
...Microsoft/Data/ProviderBase/DbConnectionFactory.cs	60.00%	2 Missing ⚠️
...icrosoft/Data/ProviderBase/DbConnectionInternal.cs	33.33%	2 Missing ⚠️
...ta/SqlClient/ConnectionPool/ConnectionPoolSlots.cs	95.91%	2 Missing ⚠️

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HEAD has 1 upload less than BASE

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addons 1 0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3404      +/-   ##
==========================================
- Coverage   68.86%   63.10%   -5.76%     
==========================================
  Files         280      272       -8     
  Lines       62417    62093     -324     
==========================================
- Hits        42982    39184    -3798     
- Misses      19435    22909    +3474

Flag	Coverage Δ
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netcore	`67.27% <89.32%> (-5.47%)`	⬇️
netfx	`62.57% <89.32%> (-5.47%)`	⬇️

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paulmedynski

Still looking for some changes from previous reviews. Good changes in these last 4 commits though.

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ConnectionPoolSlots.cs

...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs

…et-return-pooled-connections

…o avoid closures.

paulmedynski

Code looks good. Will look at tests shortly. Going to loop through older comments first.

paulmedynski · 2025-07-22T16:53:58Z

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ConnectionPoolSlots.cs

+                    cleanupCallback(state);
+                }
+
+                _disposed = true;


Should this go on line 42 in case cleanupCallback() throws? We don't want to try disposal again.

paulmedynski · 2025-07-22T16:57:24Z

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ConnectionPoolSlots.cs

-        /// This can occur if a reservation is not taken before adding a connection.
+        /// <param name="createCallback">Callback that provides the connection to add to the collection. This callback 
+        /// *must not* call any other ConnectionPoolSlots methods.</param>
+        /// <param name="cleanupCallback">Callback to clean up resources if an exception occurs. This callback *must 


This must also not throw, since you call it during disposal.

mdaigle added 4 commits June 9, 2025 13:44

Add get flow and basic tests.

87a1129

Add tests. Clean up diff.

4233f94

Add additional tests. Refactor to test error behavior.

fb8c9d0

Implement return flow and add return/reuse tests.

73a379b

mdaigle force-pushed the dev/mdaigle/get-return-pooled-connections branch from 9d95355 to 73a379b Compare June 9, 2025 20:45

mdaigle commented Jun 9, 2025

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src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionInternal.cs Outdated Show resolved Hide resolved

mdaigle added 2 commits June 9, 2025 15:19

Clean up comments and todos.

7ac22f6

Add test cases.

7c75930

mdaigle mentioned this pull request Jun 5, 2025

Redesign the SqlClient Connection Pool to Improve Performance and Async Support #3356

Open

mdaigle changed the title ~~Dev/mdaigle/get return pooled connections~~ Get/Return pooled connections Jun 9, 2025

mdaigle marked this pull request as ready for review June 9, 2025 23:25

mdaigle requested a review from a team as a code owner June 9, 2025 23:25

edwardneal reviewed Jun 10, 2025

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...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs Outdated Show resolved Hide resolved

benrr101 requested changes Jun 11, 2025

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mdaigle added 6 commits June 12, 2025 10:40

Enable init property accessor in netfx.

6dde5b9

Switch to auto properties.

9999bf7

Reorder class members.

931ac03

Address review comments.

7d734f9

Merge main

12a8121

Fix channels package resolution.

ec27e63

paulmedynski requested changes Jun 13, 2025

View reviewed changes

Review changes.

c4d5808

imasud00 reviewed Jun 17, 2025

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...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs Show resolved Hide resolved

imasud00 reviewed Jun 17, 2025

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src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionInternal.cs Outdated Show resolved Hide resolved

vonzshik reviewed Jun 17, 2025

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...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs Outdated Show resolved Hide resolved

imasud00 reviewed Jun 17, 2025

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...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs Outdated Show resolved Hide resolved

imasud00 reviewed Jun 17, 2025

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...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs Outdated Show resolved Hide resolved

mdaigle added 2 commits June 26, 2025 17:51

Fix timeout exception handling for async path.

f7b9465

Merge main

4b79754

vonzshik reviewed Jun 27, 2025

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mdaigle added 5 commits June 30, 2025 10:49

Review comments. Fix merge conflict.

67d0487

Merge branch 'main' of github.com:dotnet/SqlClient into dev/mdaigle/g…

0d9b606

…et-return-pooled-connections

Change object reference to full type. Fix tab.

7377e85

cleanup

901ba47

Add link to github issue for async pathways.

142f0ca

imasud00 reviewed Jul 1, 2025

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...rosoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs Outdated Show resolved Hide resolved

Remove side effects from IsLiveConnection. Fix exception handling and…

5769604

… connection disposal when opening new connection.

paulmedynski requested changes Jul 2, 2025

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mdaigle added 2 commits July 7, 2025 14:12

Minor review feedback. Make tests more reliable.

bc8b094

Fix compilation

92528d3

benrr101 previously approved these changes Jul 8, 2025

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mdaigle added 3 commits July 10, 2025 12:05

Make reservation disposable.

bffb2e7

Fix tests

339dfc0

Merge main

2633842

mdaigle dismissed benrr101’s stale review via 2633842 July 14, 2025 22:51

Make async request order test more reliable.

2c639ec

benrr101 previously approved these changes Jul 17, 2025

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paulmedynski requested changes Jul 17, 2025

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mdaigle added 3 commits July 18, 2025 13:04

Merge branch 'dev/mdaigle/disposable-reservations' into dev/mdaigle/g…

76784de

…et-return-pooled-connections

Avoid confusing double reservation release. Use params in callbacks t…

1f0e170

…o avoid closures.

Doc comment cleanup. Add unit tests for ConnectionPoolSlots.

deccaaf

mdaigle dismissed benrr101’s stale review via deccaaf July 18, 2025 22:51

mdaigle added 3 commits July 18, 2025 16:26

Fix up tests

531215c

Fix merge conflicts and adjust visibility for tests.

82e32a6

Clean up unit test proj file.

83e045f

paulmedynski reviewed Jul 22, 2025

View reviewed changes

		@@ -20,52 +25,136 @@ namespace Microsoft.Data.SqlClient.ConnectionPool
		/// </summary>
		internal sealed class ChannelDbConnectionPool : IDbConnectionPool

Get/Return pooled connections #3404

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Get/Return pooled connections #3404

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mdaigle commented Jun 9, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Description

Issues

Testing

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mdaigle commented Jun 9, 2025 •

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