Complete connection pool pruning (Story 2/3/4) by apoorvdeshmukh · Pull Request #4463 · dotnet/SqlClient

apoorvdeshmukh · 2026-07-21T15:07:29Z

Summary

Wraps up the V2 ChannelDbConnectionPool idle-pruning feature started in Story 1: derive the pruning window from Connection Idle Timeout (Story 2) plus resilience/robustness tests (Stories 3 & 4). Part of parent #37338.

What changed

Story 2 — window derived from Connection Idle Timeout (was LoadBalanceTimeout, which defaulted to 0 -> hard-coded 300s):

Both the sampling interval and sample count are derived so the window spans the full idle timeout while the sample count stays bounded:
- interval = clamp(ceil(IdleTimeout / 300), 10s, 1 day)
- sampleSize = min(300, ceil(IdleTimeout / interval))
Short/typical timeouts keep the 10s cadence; large timeouts pin sampleSize at 300 and stretch the interval (EventSource trace on stretch). The 1-day cap guards Timer.Change overflow.
Pruner constructed only when MinPoolSize < MaxPoolSize and IdleTimeout != 0.
No new keyword / AppContext switch. Default 300s -> 10s, 30 samples = Story 1 defaults (no behavior change).

Story 3 (tests): median ignores a transient demand spike; prune -> regrow -> re-arm cycle. No prune-margin change (deferred).

Story 4 (test): pruning callback racing with Shutdown() is an inert no-op.

Regression: multi-threaded checkout/return asserts Count > MinPoolSize <=> timer enabled.

Notes for reviewer (@Malcolm)

Adaptive interval per our discussion: bound both interval and sample count (max 300); stretch the interval instead of shrinking the window for large idle timeouts.
Chose to derive the window from Connection Idle Timeout rather than add a V2-specific Connection Pruning Interval keyword (ambiguous to users re: V1 vs V2 pool).

Testing

ChannelDbConnectionPoolPruningTest — 39/39 on net8.0 and net9.0; net462 driver builds.

Checklist

Tests added/updated
No public API changes
No breaking changes (defaults unchanged)
Verified against customer repro (N/A — internal pool tuning)

Suggested release note

Connection pooling (V2 ChannelDbConnectionPool): the background idle-connection pruner now sizes its sampling window from Connection Idle Timeout and is disabled when idle timeout is 0. Default behavior is unchanged.

Fixes AB#45165

Copilot

Pull request overview

Completes the V2 ChannelDbConnectionPool idle-pruning feature by deriving the pruning sampling window from the existing Connection Idle Timeout setting (instead of LoadBalanceTimeout + a fallback constant), and adds resilience/robustness unit tests for pruning behavior and shutdown races.

Changes:

Size the pruning window from PoolGroupOptions.IdleTimeout (sampleSize = ceil(idleTimeout / 10s), clamped to MaxSampleSize with a pooler trace when clamped).
Only construct the pruner when MinPoolSize < MaxPoolSize and IdleTimeout != 0.
Expand unit tests to cover spike resilience, prune→disable→regrow cycles, shutdown-race no-op behavior, and concurrent checkout/return invariants.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs	Gates pruner construction on `IdleTimeout != 0` and documents the new rationale.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/PoolPruner.cs	Computes sample window from idle-timeout seconds, clamps with EventSource trace, and uses `Timeout.InfiniteTimeSpan` consistently.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolPruningTest.cs	Updates existing tests for idle-timeout-based sizing and adds resilience/robustness/concurrency regression coverage.

+using System.Threading;
+using System.Threading.Tasks;
 using System.Transactions;


+            var threads = new Thread[threadCount];
+            for (int t = 0; t < threadCount; t++)
+            {
+                threads[t] = new Thread(() =>
+                {
+                    for (int i = 0; i < iterations; i++)
+                    {
+                        var busy = CheckOutConnections(pool, perIteration);
+                        ReturnConnections(pool, busy);
+                    }
+                });
+            }
+
+            foreach (var thread in threads)
+            {
+                thread.Start();
+            }
+            foreach (var thread in threads)
+            {
+                thread.Join();
+            }


codecov · 2026-07-21T16:42:44Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.43%. Comparing base (fdebcd2) to head (33f2900).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4463      +/-   ##
==========================================
- Coverage   65.83%   63.43%   -2.40%     
==========================================
  Files         287      283       -4     
  Lines       43763    66823   +23060     
==========================================
+ Hits        28812    42392   +13580     
- Misses      14951    24431    +9480

Flag	Coverage Δ
CI-SqlClient	`?`
PR-SqlClient-Project	`63.43% <100.00%> (?)`

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

Wrap up the V2 ChannelDbConnectionPool idle-pruning feature started in Story 1. Story 2 - derive the pruning window from Connection Idle Timeout: - Base the sampling window on PoolGroupOptions.IdleTimeout instead of LoadBalanceTimeout (which defaulted to 0 and fell back to a hard-coded 300s window). - Derive BOTH the sampling interval and the sample count from IdleTimeout so the window always spans the full idle timeout while the sample count stays bounded: interval = clamp(ceil(IdleTimeout / MaxSampleSize), 10s, 1 day) sampleSize = min(MaxSampleSize (300), ceil(IdleTimeout / interval)) Short/typical idle timeouts keep the 10s cadence and grow the sample count (e.g. default 300s -> 10s, 30 samples: identical to Story 1). Large idle timeouts pin the sample count at 300 and stretch the interval instead of shrinking the window. The 1-day interval cap is a defensive Timer.Change overflow guard that only engages above ~300-day idle timeouts. - Only construct the pruner when MinPoolSize < MaxPoolSize AND IdleTimeout != 0; when idle reclamation is disabled there is nothing to prune. Gating is on the IdleTimeout value, not on UseLegacyIdleTimeoutBehavior (which defaults to true). - Default Connection Idle Timeout (300s) yields a 10s interval and 30 samples, identical to Story 1 defaults, so shipped behavior is unchanged. - Emit an EventSource pooler trace when the interval is stretched off the default. - Use Timeout.InfiniteTimeSpan consistently in the timer-disable path. No new connection-string keyword or AppContext switch is introduced; the window is derived internally from the existing Connection Idle Timeout. Story 3 - resilience tests (tests only): median ignores a transient demand spike within a window (does not under-prune) and the prune -> disable -> regrow -> re-arm -> prune cycle. No prune-margin algorithm change. Story 4 - a pruning timer callback that races with Shutdown() is an inert no-op. Also adds a multi-threaded checkout/return regression test asserting the invariant: pool has more than MinPoolSize connections <=> pruning timer enabled. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: c28ccaf1-48e0-44cd-b2f9-8f1cc5946df7

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolPruningTest.cs:10

using System.Threading.Tasks; is unused in this test file, which will trigger CS8019 and fail the build (TreatWarningsAsErrors=true).

using System.Threading;
using System.Threading.Tasks;
using System.Transactions;

+        /// The configured Connection Idle Timeout, used as the pruning window. The sampling interval
+        /// and sample count are derived from it so the window always spans the full idle timeout while
+        /// the sample count stays bounded by <see cref="MaxSampleSize"/> (see the constructor body for
+        /// the exact formula). The pool only constructs a pruner when idle-timeout based reclamation is
+        /// enabled (idleTimeout &gt; 0); a defensive floor keeps the sizing math valid.


Copilot AI review requested due to automatic review settings July 21, 2026 15:07

github-project-automation Bot added this to SqlClient Board Jul 21, 2026

github-project-automation Bot moved this to To triage in SqlClient Board Jul 21, 2026

Copilot started reviewing on behalf of apoorvdeshmukh July 21, 2026 15:08 View session

Copilot AI reviewed Jul 21, 2026

View reviewed changes

apoorvdeshmukh force-pushed the apoorvdeshmukh-dev-automation-pool-pruning branch 2 times, most recently from 05509bf to a07449d Compare July 21, 2026 15:39

apoorvdeshmukh added this to the 7.1.0-preview3 milestone Jul 21, 2026

paulmedynski mentioned this pull request Jul 21, 2026

Standardize APIScan registration and SDL break policy in OneBranch pipelines #4462

Open

3 tasks

Copilot AI review requested due to automatic review settings July 23, 2026 14:24

apoorvdeshmukh force-pushed the apoorvdeshmukh-dev-automation-pool-pruning branch from a07449d to 33f2900 Compare July 23, 2026 14:24

Copilot started reviewing on behalf of apoorvdeshmukh July 23, 2026 14:25 View session

Copilot AI reviewed Jul 23, 2026

View reviewed changes

apoorvdeshmukh marked this pull request as ready for review July 23, 2026 14:35

apoorvdeshmukh requested a review from a team as a code owner July 23, 2026 14:35

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Complete connection pool pruning (Story 2/3/4)#4463

Complete connection pool pruning (Story 2/3/4)#4463
apoorvdeshmukh wants to merge 1 commit into
dotnet:mainfrom
apoorvdeshmukh:apoorvdeshmukh-dev-automation-pool-pruning

apoorvdeshmukh commented Jul 21, 2026 •

edited by azure-boards Bot

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

codecov Bot commented Jul 21, 2026 •

edited

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Uh oh!

Conversation

apoorvdeshmukh commented Jul 21, 2026 • edited by azure-boards Bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Summary

What changed

Notes for reviewer (@Malcolm)

Testing

Checklist

Suggested release note

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

codecov Bot commented Jul 21, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

apoorvdeshmukh commented Jul 21, 2026 •

edited by azure-boards Bot

Loading

codecov Bot commented Jul 21, 2026 •

edited

Loading