perf(runtime): clone only the selected instance during dispatch

[jthomson04]

Overview:

Summary

Avoid cloning the complete discovered-instance list when resolving a worker's request transport. Clone only the selected instance.

Details:

• Add a crate-private Client::instance_by_id helper that searches the current discovery snapshot and returns an owned instance.
• Use it for primary and fallback transport lookups. Release the discovery read guard before dispatch and retain the required address copy.
• Preserve the public instances() API, fallback policy, transport metadata, and error behavior. Each lookup still reads a fresh snapshot.
• Keep the linear search; add no index, cache, lock, or shared state. A successful lookup clones one instance instead of the complete list plus the selected instance. A missing lookup clones none.

Where should the reviewer start?

Client::instance_by_id in lib/runtime/src/component/client.rs, then its use in PushRouter::resolve_transport in lib/runtime/src/pipeline/network/egress/push_router.rs.

Validation

Initial validation on fa68d17ced:

• Client test module: 22 passed, including the lookup test for successful/missing IDs, updated metadata, removal, and retained instances.
• Push-router test module: 39 passed, including fallback restrictions and missing-worker cases.
• cargo check --locked -p dynamo-runtime passed.
• cargo clippy --locked -p dynamo-runtime --lib -- -D warnings passed.
• Formatting checks for both changed files and git diff --check passed.

Review cleanup on 37bf2ae47f: the focused lookup test, changed-file formatting check, and git diff --check passed. The follow-up removes only two redundant assertions and one doc comment; production logic is unchanged.

No new benchmark or profile was run; this PR makes no measured throughput claim.

Related Issues

• Confirmed — no related issue

Summary by CodeRabbit

• Bug Fixes
  • Improved instance lookup during network routing.
  • Preserved existing transport address, type, and fallback behavior when resolving destinations.
  • Ensured routing reflects current instance discovery updates and removals.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: c20ac9d3-aad5-420c-8258-63b807b8503a

📥 Commits

Reviewing files that changed from the base of the PR and between 8d81d9e and fa68d17.

📒 Files selected for processing (2)

• lib/runtime/src/component/client.rs
• lib/runtime/src/pipeline/network/egress/push_router.rs

Included review availability: Your plan provides up to 12 included reviews per hour; 11 remain after this review.

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Walkthrough

Changes

Instance transport resolution

Layer / File(s)	Summary
Snapshot instance lookup and validation lib/runtime/src/component/client.rs	Client::instance_by_id returns an owned instance from the current discovery snapshot. Tests cover successful lookup, missing IDs, ownership, updates, and removals.
Push router transport resolution lib/runtime/src/pipeline/network/egress/push_router.rs	resolve_transport uses instance_by_id while preserving TCP/NATS mapping and fallback behavior.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to fa68d

Transport resolution now retains only the selected discovered instance before dispatch, reducing unnecessary cloning while preserving existing lookup, fallback, and transport behavior. No current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly describes the main performance change: cloning only the selected instance during dispatch.
Description check	✅ Passed	The description covers the overview, implementation details, reviewer starting points, validation results, and required related-issues confirmation. It matches the pull request objectives.

• Fix all pre-merge checks with AI

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[dynamo-review-agent]

The never-present ID check is an extra missing-ID parameter that protects no distinct observable contract. instance_by_id only searches the current watch snapshot, so the later client.instance_by_id(2).is_none() after sending a snapshot without id 2 exercises the same no-match behavior while also preserving the current-snapshot removal regression; the id 2 equality assertion still protects selecting the requested non-first instance.

🤖 AI Fix

Remove the pre-update client.instance_by_id(3).is_none() assertion and keep the post-removal missing-ID assertion.

[jthomson04]

Closing as superseded by #14666, which merged into main on September 30, 2026 (99ae6e5). Every added and removed line from this PR is included in that merged patch: Client::instance_by_id(), its use in PushRouter::resolve_transport(), and the lookup test. Verified that these changes remain in main at 006e5f3.