# Safely rewriting history (EXPERIMENTAL)

Contents

- Current feature status
- Instability

Obsolescence markers make it possible to mark changesets that have been deleted or superseded in a new version of the changeset.

Unlike the previous way of handling such changes, by stripping the old changesets from the repository, obsolescence markers can be propagated between repositories. This allows for a safe and simple way of exchanging mutable history and altering it after the fact. Changeset phases are respected, such that only draft and secret changesets can be altered (see `hg help phases` for details).

Obsolescence is tracked using "obsolescence markers", a piece of metadata tracking which changesets have been made obsolete, potential successors for a given changeset, the moment the changeset was marked as obsolete, and the user who performed the rewriting operation. The markers are stored separately from standard changeset data can be exchanged without any of the precursor changesets, preventing unnecessary exchange of obsolescence data.

The complete set of obsolescence markers describes a history of changeset modifications that is orthogonal to the repository history of file modifications. This changeset history allows for detection and automatic resolution of edge cases arising from multiple users rewriting the same part of history concurrently.

# Current feature status

This feature is still in development.

# Instability

Rewriting changesets might introduce instability.

There are two main kinds of instability: orphaning and diverging.

Orphans are changesets left behind when their ancestors are rewritten. Divergence has two variants:

- Content-divergence occurs when independent rewrites of the same changesets lead to different results.
- Phase-divergence occurs when the old (obsolete) version of a changeset becomes public.

It is possible to prevent local creation of orphans by using the following config:

    [experimental]
    evolution.createmarkers = true
    evolution.exchange = true

You can also enable that option explicitly:

    [experimental]
    evolution.createmarkers = true
    evolution.exchange = true
    evolution.allowunstable = true