Add automatic pool size reduction (pruning) to ChannelDbConnectionPool

[apoorvdeshmukh]

Description

Summary

Implements automatic pool size reduction for ChannelDbConnectionPool (connection pool V2). When demand drops, excess idle connections are gradually closed based on observed usage patterns rather than reacting to instantaneous idle counts.

Design

• Sampling-based pruning: A one-shot timer fires every 10 seconds, recording the current idle connection count into a circular buffer.
• Median-based decision: Once the buffer fills, samples are sorted and the median is used as the prune target. This smooths out brief traffic lulls and avoids over-pruning.
• Sample window: LoadBalanceTimeout / PruningInterval samples (defaults to 300s / 10s = 30 samples when Connection Lifetime is 0).
• MinPoolSize floor: Pruning never reduces the pool below MinPoolSize.
• Fixed-size pools skip pruning: When MinPoolSize >= MaxPoolSize, no timer is created.

Changes

• ChannelDbConnectionPool.cs: Added pruning fields, timer creation in constructor, UpdatePruningTimer(), PruneIdleConnections() static callback, DivideRoundingUp() helper, and timer disposal in Shutdown().

Thread Safety

• Timer callback acquires lock(_pruningTimer) to read/write shared sample state.
• Shutdown() acquires the same lock before disposing the timer.
• UpdatePruningTimer() checks IsRunning at entry to avoid post-shutdown races.
• Prune loop checks pool.IsRunning on each iteration.

Issues

AB#44848

Testing

Tests

• ChannelDbConnectionPoolPruningTest.cs: 27 unit tests covering:
  • Timer creation and configuration (fixed-size pool skips, sample size calculation)
  • UpdatePruningTimer enable/disable logic
  • PruneIdleConnections sampling, buffer reset, MinPoolSize floor, in-use connection safety
  • DivideRoundingUp correctness
  • Shutdown behavior
  • Acceptance scenarios (excess idle pruned, no pruning at min, median prevents aggressive pruning)

Guidelines

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[Copilot]

Pull request overview

Adds an automatic “pruning” mechanism to ChannelDbConnectionPool (pool V2) to gradually reduce excess idle connections during low demand by sampling idle counts over time and pruning based on the median sample.

Changes:

• Introduces pruning state, sampling logic, and a one-shot timer callback to periodically evaluate and close excess idle connections.
• Wires pruning timer enable/disable behavior into connection add/remove paths and disposes the timer during pool shutdown.
• Adds a dedicated unit test suite validating pruning configuration, sampling/median behavior, floors (MinPoolSize), and shutdown behavior.

Reviewed changes

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

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs	Implements the pruning timer, sampling window/median computation, and integrates timer lifecycle with pool growth/shrink/shutdown.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolPruningTest.cs	Adds unit tests covering pruning timer creation/configuration, sampling/median pruning behavior, and shutdown semantics.

[Copilot]

Pull request overview

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