fix(frontend): make the HTTP listen backlog configurable, default 4096

[peilii]

Overview:

The frontend HTTP listener is created with tokio::net::TcpListener::bind, which listens with a backlog of 128 (tokio copies the Rust standard library default). When a few thousand clients open connections within a few seconds, the accept queue overflows before the accept loop drains it. On hosts with net.ipv4.tcp_syncookies=1 and tcp_abort_on_overflow=0, an overflowed handshake is not retried: the final ACK is dropped and the client's first data segment is answered with a RST, which the client sees as Connection reset by peer. We found this in internal testing with a load client that opens one connection per session.

Details:

• bind_listener() in service_v2.rs builds the socket with tokio::net::TcpSocket, sets SO_REUSEADDR (what TcpListener::bind also does), binds, and listens with a configurable backlog.
• New environment variable DYN_HTTP_LISTEN_BACKLOG, default 4096. The kernel caps the effective value at net.core.somaxconn. Zero, negative or unparseable values fall back to the default.
• Only the main HTTP listener changes. Callers that pass a pre-bound listener (run_with_listener / spawn_with_listener) are unaffected.
• Docs: one ParamField in frontend-configuration.mdx; constant registered in environment_names.rs.

Where should the reviewer start?

• lib/llm/src/http/service/service_v2.rs: bind_listener, parse_listen_backlog, and the call site in run_inner.
• lib/runtime/src/config/environment_names.rs: DYN_HTTP_LISTEN_BACKLOG.

Validation

• cargo test -p dynamo-llm --lib -- listen_backlog bind_listener: test_listen_backlog_env_var (default, zero, invalid, whitespace-padded value) and test_bind_listener_accepts_connections (bind on an ephemeral port, connect, accept) pass.
• Internal testing, single frontend, a load client ramping 2,542 new TCP connections over 10 s on a host with tcp_syncookies=1: before this change 12-29 connections per run failed with Connection reset by peer; with it, 0 resets across the same ramp.
• cargo clippy -p dynamo-llm -p dynamo-runtime --no-deps --all-targets -- -D warnings and cargo fmt --check are clean.

Related Issues

🚫 This PR is NOT linked to an issue:

• Confirmed — no related issue

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Added support for configuring the frontend HTTP connection backlog with DYN_HTTP_LISTEN_BACKLOG.
  • The setting defaults to 4096; invalid, zero, or unset values use the default.
  • IPv4 and IPv6 listeners now support the configured backlog and address reuse.

• Documentation

  • Documented configuration behavior, including operating-system limits and potential connection failures when the queue is full.

[coderabbitai]

Walkthrough

The change adds DYN_HTTP_LISTEN_BACKLOG, documents its behavior, and applies it to non-TLS HTTP listeners. The listener uses a configurable backlog with a default of 4096. Tests cover parsing and connection acceptance.

Changes

HTTP listener backlog

Layer / File(s)	Summary
Backlog configuration contract lib/runtime/src/config/environment_names.rs, docs/fern/pages/reference/components/frontend-configuration.mdx	Defines and documents DYN_HTTP_LISTEN_BACKLOG, including its 4096 default and net.core.somaxconn cap.
Custom listener implementation and validation lib/llm/src/http/service/service_v2.rs	Non-TLS startup uses bind_listener. The helper parses trimmed positive u32 values, applies the backlog, enables SO_REUSEADDR, and binds IPv4 or IPv6 sockets. Tests cover fallback parsing and connection acceptance.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to 08bbe

TLS deployments may configure the documented backlog setting expecting it to increase connection capacity, but it has no effect there. Clarify the non-TLS limitation or apply the setting to TLS before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 2 files. (1 skipped: …	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 and concisely identifies the main change: configurable frontend HTTP listen backlog with a default of 4096.
Description check	✅ Passed	The description includes all required template sections, explains the implementation and scope, identifies reviewer starting points, documents validation, and confirms that no related issue exists.

Full details: Docstring Coverage

Explanation

Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 2 files. (1 skipped: 1 unsupported.)

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[coderabbitai]

Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
In `@docs/fern/pages/reference/components/frontend-configuration.mdx`:
- Around line 72-79: Update the DYN_HTTP_LISTEN_BACKLOG documentation to state
that the setting applies only to non-TLS frontend listeners, since TLS startup
uses axum_server::bind_rustls and does not consume this variable. Do not imply
that it affects TLS deployments unless the TLS listener is also updated to apply
the backlog.

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📥 Commits

Reviewing files that changed from the base of the PR and between b078517 and 08bbe55.

📒 Files selected for processing (3)

• docs/fern/pages/reference/components/frontend-configuration.mdx
• lib/llm/src/http/service/service_v2.rs
• lib/runtime/src/config/environment_names.rs

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[github-actions]

🌿 Fern Docs Preview: https://nvidia-preview-01a0b755-dfd6-77f3-96fa-2c29a96b433d.docs.buildwithfern.com/dynamo/dev

[jthomson04]

The plain HTTP listener change looks sound on source review.

The existing P2 about TLS remains open: DYN_HTTP_LISTEN_BACKLOG does not affect the axum_server::bind_rustls path. Please document the plain-HTTP-only scope or apply the setting to TLS. See the existing comment.

One non-blocking P3 documentation comment below. Source review only; I did not run tests or assess CI.

[rmccorm4]

Approving docs change