UNDF: UNDF-2026-000000473

rmq-0005: rabbit_mgmt_wm_definitions export_binding O(B×Q) → O(B+Q)

Location

deps/rabbitmq_management/src/rabbit_mgmt_wm_definitions.erl Lines 54–58 (all_definitions/2) and 115–120 (vhost_definitions/2)

Severity

MEDIUM — triggered by GET /api/definitions (HTTP API export). Automation tools, monitoring pipelines, and GitOps workflows hit this endpoint on a schedule. In large deployments it serialises the entire broker definition and becomes a CPU hotspot.

Description

all_definitions/2 builds QNames as a plain list of {Name, VHost} tuples, then calls export_binding(B, QNames) for every binding B in a list comprehension. Inside export_binding/2, lists:member({Dest, VHost}, QNames) performs a linear scan of all queue-name tuples for every binding processed.

With B bindings and Q queues this is O(B × Q). In a busy broker (e.g., a topic exchange with 10 k routing keys matched against 1 k queues) the export scans 10 M tuple comparisons per API call. If called every 15 seconds by a monitoring tool, this is 666 k redundant comparisons/second sustained.

Root Cause

%% all_definitions/2 — deps/rabbitmq_management/src/rabbit_mgmt_wm_definitions.erl
Qs     = [Q || Q <- rabbit_mgmt_wm_queues:basic(ReqData), export_queue(Q)],
QNames = [{pget(name, Q), pget(vhost, Q)} || Q <- Qs],     %% ← plain list
Bs     = [B || B <- rabbit_mgmt_wm_bindings:basic(ReqData),
               export_binding(B, QNames)],                  %% ← O(B) calls

%% export_binding/2 — same file
export_binding(Binding, Qs) ->
    ...
    ( (DestType =:= queue andalso lists:member({Dest, VHost}, Qs))  %% ← O(Q) scan
    ...

vhost_definitions/2 has the identical pattern at lines 115–120 using QNames.

Fix

Convert QNames to an ordsets:ordset() (or sets:set() with {version, 2}) before the binding comprehension. Pass the set to export_binding/2 and use sets:is_element/2 in place of lists:member/2.

%% all_definitions/2 — fixed
Qs      = [Q || Q <- rabbit_mgmt_wm_queues:basic(ReqData), export_queue(Q)],
QNames  = [{pget(name, Q), pget(vhost, Q)} || Q <- Qs],
QNamesS = sets:from_list(QNames, [{version, 2}]),          %% ← O(Q log Q) once
Bs      = [B || B <- rabbit_mgmt_wm_bindings:basic(ReqData),
               export_binding(B, QNamesS)],                 %% ← O(B) calls

%% export_binding/2 — fixed
export_binding(Binding, Qs) ->
    ...
    ( (DestType =:= queue andalso sets:is_element({Dest, VHost}, Qs))  %% ← O(1)
    ...

The same change applies to vhost_definitions/2.

Complexity

	Before	After
Build QNames	O(Q)	O(Q) + O(Q log Q) set build
Per-binding check	O(Q) scan	O(1) set lookup
Total export	O(B × Q)	O(Q log Q + B)

At B=10 k bindings, Q=1 k queues: 10 M ops → 14 k ops (714x improvement). At B=100 k bindings, Q=10 k queues: 10^9 ops → 133 k ops (7500x improvement).

Context

GET /api/definitions is the standard RabbitMQ backup/export endpoint. It is called:

By rabbitmqctl export_definitions
By monitoring pipelines (Datadog, Prometheus exporters)
By GitOps automation to detect config drift

Slow exports block the HTTP listener and increase memory pressure from large response bodies held in flight while CPU-bound scan is underway.

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