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fix(router): hash routing text when a request carries no tokens - #2148
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Only pre-tokenized generate requests reach a policy with token IDs, so chat, completions and text-form generate requests hit prefix_hash with an empty routing key. The policy returned no worker and the router answered 503, which made prefix_hash unusable for every OpenAI endpoint. Hash the leading routing text instead, budgeting four characters per configured prefix token so an untokenized request covers roughly the same span of the prompt, and fall back to the least loaded worker when a request carries neither tokens nor text. The branch label follows: no_tokens becomes no_routing_key. Signed-off-by: Simo Lin <25425177+slin1237@users.noreply.github.com>
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📝 WalkthroughSummary by CodeRabbit
WalkthroughChangesPrefix-hash routing
Estimated code review effort: 3 (Moderate) | ~20 minutes Mergeability Score: 🟡 Moderate · up to Text-only requests will now use prefix-based worker affinity instead of returning no worker, but concurrent HTTP traffic with a shared prefix may remain concentrated on one worker because the required load-accounting change is separate, potentially reducing capacity or availability; merge should wait for that fix or explicit owner acceptance, with observability labels and the precedence test updated. Possibly related PRs
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| // equivalent span of routing text. | ||
| let prefix_hash = match (info.tokens, info.request_text) { | ||
| (Some(tokens), _) if !tokens.is_empty() => self.compute_prefix_hash(tokens), | ||
| (_, Some(text)) if !text.is_empty() => self.compute_text_prefix_hash(text), |
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🔴 Important: PrefixHashPolicy now routes on info.request_text, but the LoadBalancingPolicy impl (line 259) doesn't override needs_request_text() — it inherits the default false.
The regular HTTP router and gRPC router happen to populate request_text unconditionally (when tokens are absent), so this works there. But the PD router (pd_router.rs:804-807) gates text extraction on policies_need_request_text(), which checks the trait method. With prefix_hash as a PD policy, request_text will always be None and every untokenized request will land in the NoRoutingKey fallback instead of hashing onto the ring.
Fix: add needs_request_text to the impl LoadBalancingPolicy block:
| (_, Some(text)) if !text.is_empty() => self.compute_text_prefix_hash(text), | |
| (_, Some(text)) if !text.is_empty() => self.compute_text_prefix_hash(text), |
(The actual fix belongs in the impl LoadBalancingPolicy block around line 266 — add fn needs_request_text(&self) -> bool { true } next to name().)
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Actionable comments posted: 1
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Inline comments:
In `@model_gateway/src/policies/prefix_hash.rs`:
- Around line 471-474: Update the token-precedence test around
select_worker_impl to first assert that the token-only fixture and a text-only
fixture select different workers, then assert that the tokens_and_text result
matches the token-only result, ensuring the test distinguishes token precedence
from coincidental worker selection.
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📒 Files selected for processing (8)
bindings/python/src/smg/router.pybindings/python/src/smg/router_args.pymodel_gateway/src/config/types.rsmodel_gateway/src/main.rsmodel_gateway/src/policies/prefix_hash.rsmodel_gateway/tests/common/test_config.rsmodel_gateway/tests/routing/mod.rsmodel_gateway/tests/routing/prefix_hash_test.rs
| let (result1, _) = policy.select_worker_impl(&workers, &tokens_only); | ||
| let (result2, _) = policy.select_worker_impl(&workers, &tokens_and_text); | ||
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| assert_eq!(result1, result2); |
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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win
🟡 Nit: Make the token-precedence test distinguish the two routing keys.
A text-first implementation can pass this assertion when the token-only and text-only hashes select the same worker. First assert that the selected token fixture and a text-only fixture route to different workers. Then assert that tokens_and_text matches the token-only result.
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only still-valid issues, skip the rest with a brief reason, keep changes
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In `@model_gateway/src/policies/prefix_hash.rs` around lines 471 - 474, Update the
token-precedence test around select_worker_impl to first assert that the
token-only fixture and a text-only fixture select different workers, then assert
that the tokens_and_text result matches the token-only result, ensuring the test
distinguishes token precedence from coincidental worker selection.
1 participant
Description
Problem
GenerationRequest::routing_tokens()defaults toNone(crates/protocols/src/common.rs:53), and onlyGenerateRequestoverrides it — only for theinput_ids-without-textcase (crates/protocols/src/generate.rs:239).SelectWorkerInfo::tokensis filled solely from that call.So every chat, completions, responses and text-form generate request reached
prefix_hashwith an empty routing key. The policy returnedNone, and the router answered503 no_workers(routers/http/router.rs:523).prefix_hashwas not merely degraded on the OpenAI endpoints — it served nothing at all there, and the only traffic it could route was a pre-tokenized/generate.There was no integration coverage for the policy, which is why a total outage on four endpoints went unnoticed.
Solution
Fall back to the routing text the request does carry. A text prefix is a weaker key than a token prefix, but it is the same key for the same prompt, which is all the ring needs — and it makes the policy usable on the endpoints that carry the traffic.
The character budget is
prefix_token_count * CHARS_PER_TOKENwithCHARS_PER_TOKEN = 4, the usual rule of thumb for English text, so an untokenized request hashes roughly the same span of the prompt that the configured token count would cover. Tokens still win when both are present, so pre-tokenized requests hash exactly as before.A request that carries neither tokens nor text now falls back to the least loaded healthy worker rather than returning
None. A keyless request is a routing question, not a service failure.Changes
model_gateway/src/policies/prefix_hash.rs:compute_text_prefix_hashhashes the leadingprefix_token_count * 4characters, with the budget taken on character boundaries so multi-byte input cannot panic or split a code point.select_worker_implpicks the key by precedence: non-empty tokens, then non-empty text, then neither.least_loaded_healthyextracted; both the keyless path and the existingFallbackLeastLoadpath use it.no_tokensbecomesno_routing_key, exported throughsmg_prefix_hash_policy_branch_total.prefix_token_countupdated inconfig/types.rs, the--prefix-token-countCLI help, and the Python launcher (router.py,router_args.py).model_gateway/tests/common/test_config.rs:TestRouterConfig::prefix_hash, which did not exist.model_gateway/tests/routing/prefix_hash_test.rs: new integration suite.Test Plan
Before / after, driving the router the way a client does — three healthy workers,
POST /v1/chat/completions:503 no_workers200Two integration tests, both failing on
main:test_repeated_prompt_pins_to_one_worker— six identical prompts land on exactly one of three workers, and all six arrive.test_distinct_prompts_spread_across_workers— twelve distinct prompts reach more than one worker, all twelve arrive, and each body is intact.Five policy unit tests:
test_untokenized_request_routes_consistently— text-only requests hit the ring and repeat the same workertest_shared_text_prefix_routes_same— two prompts sharing a prefix longer than the budget route togethertest_text_prefix_budget_respects_char_boundaries— the budget lands mid-emoji without panickingtest_tokens_take_priority_over_text— adding text to a tokenized request does not move ittest_no_routing_key_falls_back_to_load— a request with neither key gets the least loaded workerExisting token-path tests (
test_prefix_hash_consistent_routing,test_different_prefixes_distribute,test_shared_prefix_routes_same) are unchanged and still pass, confirming pre-tokenized behaviour is untouched.cargo +nightly fmt --all— cleancargo clippy --all-targets -- -D warnings— clean.--all-featurescannot build locally: it pullsopencv 0.99, whose build script needs a system OpenCV install; that configuration is covered by CI (pr-test-rust.yml:292).cargo test -p smg— 2104 passed, 0 failed, 23 binariespython -m py_compileon the two touched launcher filesRelated Issues
Three independent defects kept
prefix_hashfrom engaging. Each is its own PR and they do not conflict:Checklist
cargo +nightly fmtpassescargo clippy --all-targets --all-features -- -D warningspasses