docs: update pub-sub.md for parallel hashmap architecture (PR #7174)

[Copilot]

• Rebase on top of main (clean history based on current main)
• Update pub-sub.md to match the merged ShardedHashMap implementation:
  • Overview: ShardedHashMap with 16 shards, two-lock RCU per shard
  • Data Structures: remove ControlBlock, add ShardedHashMap, update ChannelMap typedef
  • Subscription Management: ChannelStoreUpdater batches by shard, Mutate + RCU pointer swap + freelist
  • FetchSubscribers: FindIf/ForEachShared API, non-atomic note
  • SendMessages: update pseudocode for new API
  • Cluster slot migration: RemoveAllSubscribers + ForEachShared + AwaitFiberOnAll
  • Keyspace events: global channel_store-> pointer (no more ServerState)
  • Key Files Reference: add ShardedHashMap, remove ServerState, update migration refs

[augmentcode]

🤖 Augment PR Summary

Summary: Updates docs/pub-sub.md to describe the post-#7174 Pub/Sub subscription registry design.

Changes:

• Rewrites the overview to replace the prior RCU snapshot narrative with a parallel-hashmap-backed global ChannelStore and per-submap locking
• Updates the primary data structures table (removing RCU-specific components and documenting the new map types / global instance)
• Replaces the old RCU subscription-management sections with Add() (try_emplace_l) and Remove() (erase_if) flows
• Updates the publish routing description to the new if_contains/for_each APIs and lock semantics
• Simplifies the cluster slot migration unsubscribe flow to a single scan plus AwaitFiberOnAll dispatch
• Adjusts keyspace-event and file references to use the new channel_store access pattern

Technical Notes: The updated doc assumes fine-grained contention via per-submap locks rather than a single serialized update path.

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[augmentcode]

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[augmentcode]

The doc now describes a phmap::parallel_flat_hash_map-backed global channel_store with Add()/Remove() and per-submap locking, but the current code in src/server/channel_store.h/.cc is still RCU-based with ChannelStoreUpdater and ServerState::tlocal()->channel_store(). Can we ensure this doc update lands only once the corresponding code change (PR #7174) is actually present in this branch, otherwise readers will be misled?

Severity: medium

Other Locations

• docs/pub-sub.md:45
• docs/pub-sub.md:49
• docs/pub-sub.md:82
• docs/pub-sub.md:105
• docs/pub-sub.md:155
• docs/pub-sub.md:200
• docs/pub-sub.md:420
• docs/pub-sub.md:446

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[Copilot]

Pull request overview

Updates the Pub/Sub architecture documentation to describe a new subscription registry design that uses a single global ChannelStore backed by a sharded concurrent hashmap, replacing the prior RCU-based description.

Changes:

• Replace RCU-based ChannelStore description with a phmap::parallel_flat_hash_map design (per-submap locking).
• Update subscription management and publish-path sections to reference new map APIs (try_emplace_l, erase_if, if_contains, for_each) and simplified cluster slot migration flow.
• Update examples/references to use a global channel_store instead of ServerState::tlocal()->channel_store().

[Copilot]

This snippet uses channel_store->SendMessages(...) as if there is a global channel_store, but the code currently calls ServerState::tlocal()->channel_store() (e.g. DbSlice::DeleteExpiredStep). Please align the example with the actual API used in the codebase.

Suggested change

	channel_store->SendMessages(
	ServerState::tlocal()->channel_store()->SendMessages(

[mkaruza]

Closing. New PR is generated in #7239