KMeans

Overview

kmeans sends a request to the model assigned to its nearest learned cluster.

Implementation: Rust via Linfa (linfa-clustering).

Key Advantages

• Efficient inference: O(k×d) per query (k = clusters, d = embedding dimension).
• Natural grouping of query patterns into clusters.
• Works well when prompt traffic naturally falls into recurring categories.
• The request-time path is a direct centroid lookup with no online learning.

Algorithm Principle

1. Training: K-Means partitions training queries into num_clusters clusters using Lloyd's algorithm.
2. Cluster-Model Assignment: Each cluster is mapped to the best-performing model based on historical outcome quality.
3. Inference: New queries are embedded and assigned to the nearest cluster centroid. The cluster's mapped model is selected.

$m^* = \arg\min_{c} \| \text{embed}(q) - \mu_c \|^2 \implies \text{model}(c^*)$

Select Flow

What Problem Does It Solve?

Some prompt traffic naturally falls into recurring regions where the same model tends to win, but per-request learned ranking would be unnecessary overhead. kmeans turns those recurring regions into cluster-to-model assignments for fast, stable routing.

When to Use

• Prompt traffic naturally groups into repeatable classes (e.g., math, coding, creative writing).
• You have a cluster-based selector for candidate models.
• You need efficient O(k×d) inference per request.
• Quality-weighted cluster assignment is sufficient (vs. non-linear MLP boundaries).

Known Limitations

• Requires pre-training: clusters must be learned from historical data.
• Fixed number of clusters — too few loses granularity, too many overfits.
• Cannot adapt to new query patterns without retraining.
• Centroid-based assignment ignores cluster shape/size.

Configuration

algorithm:
  type: kmeans

Global ML Settings

global:
  router:
    model_selection:
      ml:
        models_path: ".cache/ml-models"
        embedding_dim: 768
        kmeans:
          num_clusters: 8
          efficiency_weight: 0.0
          pretrained_path: .cache/ml-models/kmeans_model.json

Parameters

Parameter	Type	Default	Description
num_clusters	int	8	Number of K-Means clusters
efficiency_weight	float	0.0	Compatibility field retained in selector configuration and artifacts; the current request-time selector does not read it
pretrained_path	string	—	Path to pre-trained KMeans model (JSON format)

Training

See ML Model Selection README for the training pipeline. KMeans models are trained using Lloyd's algorithm on historical query embeddings.

Historical prompts and outcome labels can contain sensitive data; minimize and govern the training set before producing selector artifacts. See a complete example: config/fragments/algorithm/selection/kmeans.yaml.