Replace synchronized multimap with concurrent hash map in NotificationControllerV2 for better performance

[nobodyiam]

What's the purpose of this PR

Replace synchronized multimap with concurrent hash map in NotificationControllerV2 for better performance

Which issue(s) this PR fixes:

Fixes #5450

Brief changelog

• Replace synchronized multimap usage in NotificationControllerV2 with a concurrent, case-insensitive multimap implementation to reduce lock contention under high concurrency.
• Introduce CaseInsensitiveMultimapWrapper (backed by ConcurrentMap + concurrent Set) to manage deferred results safely and efficiently; normalize keys in a locale-stable way.
• Update NotificationControllerV2Test and add CaseInsensitiveMultimapWrapperTest to cover correctness and basic concurrency behavior.

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• Read the Contributing Guide before making this pull request.
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• Update the CHANGES log.

Summary by CodeRabbit

• Chores

  • Improved notification handling with a concurrent, case-insensitive backing to boost throughput and reduce lock contention.

• New Features

  • Notification keys are now treated case-insensitively, improving matching consistency.

• Tests

  • Added comprehensive unit tests covering put/get/remove, size, unmodifiable views, custom suppliers, and concurrency scenarios.

• Documentation

  • Added a performance-focused changelog entry describing the notification handling improvements.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Replaces a synchronized TreeMultimap in NotificationControllerV2 with a new CaseInsensitiveMultimapWrapper backed by concurrent map/set implementations; adds the wrapper class and comprehensive tests, updates NotificationControllerV2 and its test, and adds a changelog entry describing the concurrency/performance change.

Changes

Cohort / File(s)	Summary
Core Implementation apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/wrapper/CaseInsensitiveMultimapWrapper.java	New generic case-insensitive multimap wrapper backed by Map<String, Set<V>> with pluggable map/set suppliers. Provides put, remove, get, containsKey, and size. Normalizes keys with Locale.ROOT.
Controller Update apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/controller/NotificationControllerV2.java	Replaces Multimap<String, DeferredResultWrapper> synchronized TreeMultimap field with CaseInsensitiveMultimapWrapper<DeferredResultWrapper> using concurrent map and concurrent-hash-set supplier; imports adjusted.
Tests — New apollo-configservice/src/test/java/com/ctrip/framework/apollo/configservice/wrapper/CaseInsensitiveMultimapWrapperTest.java	New unit tests covering put/get/remove semantics, case-insensitivity, size, unmodifiable get view, custom supplier behavior, and a concurrency race-condition scenario.
Tests — Updated apollo-configservice/src/test/java/com/ctrip/framework/apollo/configservice/controller/NotificationControllerV2Test.java	Adjusted test field type and ReflectionTestUtils casts to CaseInsensitiveMultimapWrapper<DeferredResultWrapper>.
Changelog CHANGES.md	Added Apollo 2.5.0 entry: "Perf: Replace synchronized multimap with concurrent hashmap in NotificationControllerV2 for better performance".

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Suggested labels

size:XL, lgtm

Suggested reviewers

• hezhangjian

Poem

🐰
I swapped the locks for hopping haste,
Keys downcased, no time to waste,
Sets race safe, maps run free,
Deferred results now spring with glee,
A little hop for faster spree.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.25% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the primary change: replacing synchronized multimap with concurrent data structure in NotificationControllerV2 for performance improvement.
Linked Issues check	✅ Passed	The PR successfully addresses issue #5450 by replacing synchronized multimap with CaseInsensitiveMultimapWrapper backed by ConcurrentMap, reducing lock contention and improving performance under high concurrency.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the objective: the wrapper class, controller modifications, related test updates, and changelog entry are all necessary for implementing the concurrent multimap replacement.

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✨ Finishing touches

• 📝 Generate docstrings

Warning

Review ran into problems

🔥 Problems

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In
`@apollo-configservice/src/test/java/com/ctrip/framework/apollo/configservice/wrapper/CaseInsensitiveMultimapWrapperTest.java`:
- Around line 158-167: The test fails because
CaseInsensitiveMultimapWrapper.get(...) returns a Collections.unmodifiableSet,
so asserting set instanceof HashSet is false; change the testCustomSetSupplier
to unwrap the returned unmodifiable set via reflection (the
Collections$UnmodifiableSet has a private field "c" referencing the original
set) and assert that the underlying "c" is a HashSet, or alternatively access
the wrapper's internal map entry for the key and assert that its value is a
HashSet; reference the test method testCustomSetSupplier,
CaseInsensitiveMultimapWrapper.get, and the Collections$UnmodifiableSet "c"
field when making the change.