---
title: 'Single-writer control'
description: 'A control pattern in which all changes to a sensitive persistent state pass through one authorized writer.'
canonical_url: 'https://darkfactory.dev/glossary/single-writer-control'
markdown_url: 'https://darkfactory.dev/glossary/single-writer-control.md'
collection: glossary
date_published: '2026-08-03T00:00:00-04:00'
date_modified: '2026-08-26T00:00:00-04:00'
---

# Single-writer control


## Definition

A control pattern in which all changes to a sensitive persistent state pass through one authorized writer. Many agents, users, or services may read the state and propose updates. Only the designated component can commit them after applying policy, validation, ordering, and audit rules.

The pattern is valuable when competing writes would create ambiguity or when changes need a single accountable checkpoint. Examples include standing agent instructions, durable memory, identity records, promotion status, or a shared improvement policy. The writer can serialize changes, reject stale proposals, attach provenance, and keep a coherent history.

## What single means

**Single** refers to logical write authority, not necessarily one process or machine. The writer can be replicated for availability if the replicas still enforce one ordered mutation path. It also does not require one source of ideas. Optimizers and agents can propose freely while remaining unable to alter the governing state directly.

## Distinguish it from nearby terms

Leader election chooses which replica currently coordinates a distributed system. An append-only log preserves a history of events. Single-writer control is the authority rule governing mutation. A system may use all three patterns together, but none implies the others by itself.

## Check your understanding

An agent proposes changing the instruction that tells it when to stop, then writes the change directly to its own durable memory. What control is missing? Route the proposal through a separate writer that checks provenance, authorization, conflicts, evaluation evidence, and rollback before committing it.

## Related factory areas

- [Feedback, learning & controlled self-improvement](https://darkfactory.dev/factory/feedback-self-improvement)
- [Governance, audit, legal & accountability](https://darkfactory.dev/factory/governance-accountability)

## Evidence and further reading

- [The Therapist Pattern](https://blog.fsck.com/2026/07/20/the-therapist-pattern/)
- [SARC: Governance-by-Architecture](https://arxiv.org/abs/2605.07728)