MLP (Multi-Layer Perceptron)

Overview

mlp runs a trained neural classifier on CPU to map a request to a candidate model.

Reference: This is part of the ML-based model selection family alongside KNN, KMeans, and SVM.

Key Advantages

• Learns complex, non-linear decision boundaries that linear methods (KNN, SVM with linear kernel) cannot capture.
• Uses the Candle inference binding.
• Supports custom hidden layer sizes to balance model capacity and inference speed.
• Integrates into the same decision.algorithm surface as other selection algorithms.

Algorithm Principle

MLP uses a feedforward neural network with configurable hidden layers to classify queries into candidate models:

1. Feature Engineering: Query embeddings (precomputed or on-demand) are concatenated with optional category one-hot encoding to form the input feature vector.
2. Forward Pass: The feature vector passes through hidden layers with ReLU activations, producing a probability distribution over candidate models.
3. Selection: The model with the highest output probability is selected.

Input: query_embedding (dim) + category_one_hot (num_categories)
  ↓
Hidden Layer 1: Linear(dim, h1) → ReLU
  ↓
Hidden Layer 2: Linear(h1, h2) → ReLU
  ↓
Output Layer: Linear(h2, num_models) → Softmax
  ↓
Output: P(model_i | query) for each candidate

Select Flow

What Problem Does It Solve?

Some routing boundaries are non-linear and cannot be captured well by static ordering or simpler linear rules. mlp learns those more complex query-to-model boundaries from historical data while keeping inference inside the selection layer.

When to Use

• You need to capture complex non-linear patterns in query-to-model mapping.
• You have a representative labeled query-to-model dataset.
• CPU inference cost is acceptable for the route.
• KNN/KMeans/SVM decision boundaries are insufficient for your workload.

Known Limitations

• Requires pre-trained model weights; cannot start from scratch without training data.
• The current decision factory always constructs the CPU selector. The accepted device field is not wired to request-time selection.
• Unlike KNN, MLP is a "black box" — harder to interpret why a specific model was chosen.
• Training requires the separate modelselection training pipeline; see ML Model Selection.

Configuration

Configure it under routing.decisions[].algorithm:

algorithm:
  type: mlp

Global ML Settings

global:
  router:
    model_selection:
      ml:
        models_path: ".cache/ml-models"
        embedding_dim: 768
        mlp:
          pretrained_path: .cache/ml-models/mlp_model.json

Parameters

Parameter	Type	Default	Description
device	string	cpu	Compatibility field; the current decision factory uses CPU regardless of this value
pretrained_path	string	—	Path to pre-trained MLP model weights (JSON format)

Feedback

MLP does not support online UpdateFeedback(). To improve selection quality, retrain the model with new query-to-model assignment data using the training pipeline.

Experimental Status

This algorithm is marked as experimental. The API may change in future releases.

Training examples and labels can contain sensitive request data; govern them and the derived artifact accordingly. See a complete example: config/fragments/algorithm/selection/mlp.yaml.