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Shadow mode

As explained in Machine Learning System Design

An operating mode in which a candidate system runs alongside the active system and produces outputs without affecting users or decisions.

Shadow mode runs a candidate model alongside the current model while keeping its predictions out of user-facing decisions. It provides real-world evidence without risking production outcomes. section 13.5.1

Use shadow mode as a controlled release checkpoint:

  • Complete rigorous offline validation before parallel production testing. section 13.5.1
  • Compare candidate and current-model performance under real-world traffic without applying candidate predictions. section 13.5.1
  • Check monitoring signals, including system health, response times, error rates, model metrics, and data drift. section 13.5.1
  • Confirm preprocessing, feature engineering, and dependent pipelines match the candidate model before rollout. section 13.5.1

A safer model release

A retail team validates a new model offline, then runs it beside the current model. The team compares predictions and performance in production while users continue receiving decisions from the current model. section 13.5.1

How Machine Learning System Design covers it

How deep it goes
Explains the release sequence and monitoring concerns but stops short of implementation details for traffic duplication or comparison logic.
Examples
Compares a newly validated model with the current model under real-world conditions while preserving current production behavior.
What it assumes
Assumes an active model, offline validation, and consistent preprocessing, feature engineering, and supporting pipelines.
What you can do after
Evaluate candidate-model performance in production without allowing candidate predictions to affect users or decisions.
How it connects
Connects model releases with offline validation, monitoring, data drift, preprocessing, feature engineering, and rollout coordination.

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This explainer was generated by Manning's knowledge graph from the text of Machine Learning System Design. The section links point to the parts of the book it draws on.