docs/obs-implementation.rst
author Pierre-Yves.David@ens-lyon.org
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Implementation of Obsolete Marker
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.. warning:: This document is still in heavy work in progress
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Main questions about Obsolete Marker Implementation
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What data should be contained in a Marker ?
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There is two critical data that **must** be stored in an obsolete Marker.
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:object:
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    the old obsoleted changeset
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:replacements:
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    list of new changeset. list size can be anything, including 0 (0..N)
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Everybody agreed on this point.
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 ---
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This is probably a good idea to have an unique Identifier, for UI, transfert and
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access.
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    :id: same as changeset but for marker.
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The field below will depends of the way we exchange obsolete marker betwenn
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changeset.
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 ---
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Having audit data will be very usefull. When it get messy you need all the
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information you can to understand the situation.
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I have the felling that we are versionning history. Therefor we will probably
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need the same kind of information than when versionning Files.
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:date: date of the marker creation
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:user: ui.username
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To go further:
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:description: "Optional reason for the rewrite (generated of added by the user)"
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:tool: the automated too that made this
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:operation: Kind of rewritting operation that created the marker (delete,
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            update, split, fold, reordering) To help conflict resolution.
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Matt see this a "too complicated". I'll wait for him to meet a very hairy
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situation to agree that they are needed.
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Leaving the door open to any addition data is an option too.
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How shall we store Marker on disk
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Requirement
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.............
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We need to quickly load the 'object' to know the "obsolete" set.
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We need quick access by object and replacements to travels along the graph.
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Common Part
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The file is store in `.hg/store/obsmarkers`. It is a binary files:
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The files starts with a Format Version string
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Minimalistic proposal
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.........................
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The core of a Marker will we stored as:
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* number of replacement (8-Bytes integer)
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* node id of the obsolete changeset (20-Bytes hash)
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* node id of replacement changeset (20-Bytes hash x number of remplacement)
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Version with ID
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.........................
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This version add a node id computed from the marker content. It will be present
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*before* other data:
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* node id of the maker (20-Bytes hash)
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Version with Metadata proposal
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...............................
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An extra files is used to old metadata (date, user, etc) `.hg/store/obs-extra`:. The format of this
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field is undefined yet. This will add the following field at the end of a marker
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* offset of the metadata in obs-extra (8-Bytes integer)
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How shall we exchange Marker over the Wire ?
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`````````````````````````````````````````````````````````
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We can have a lot of markers. We do not want to exchange data for the one we
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already know. Listkey() is not very appropriate there as you get everything.
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Moreover, we might want to only hear about Marker that impact changeset we are
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pulling.
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pushkey is not batchable yet (could be fixed)
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A dedicated discovery and exchange protocol seems mandatory here.
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Various technical details
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Some stuff that worse to note. some may deserve their own section later.
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storing old changeset
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``````````````````````
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The new general delta format allows a very efficient storage of two very similar
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changesets. Storing obsolete childrens using general delta takes no more place
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than storing the obsolete diff. Reverted file will even we reused. The whole
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operation will take much less space the strip backup.
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Abstraction from history rewriting UI
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How Mercurial handle obsolete marker is independent from who decide to create
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them and what actual operation solve error case. Any of the existing history
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rewriting UI (rebase, mq, histedit) can lay obsolete marker and resolve
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situation created by other. To go further a hook system of obsolete marker
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creation would allow each mechanism to collaborate with other though a standard
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and central mechanism.
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Obsolete marker storage
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```````````````````````````
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Obsolete marker will most likely be stored outside standard history. They are
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multiple reasons for that:
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First, obsolete markers are really perpendicular to standard history there is not
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strong reason to include it here other than convenience.
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Second, storing obsolete marker inside standard history means:
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* A changeset must be created every time an obsolete relation is added. Very
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  inconvenient for delete operation.
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* Obsolete marker must be forged at the creation of the new changeset. This
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  is very inconvenient for split operation. And in general it become
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  complicated to fix history afterward in particular when working with older
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  client.
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Storing obsolete marker outside history have several pro:
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* It ease Exchange of obsolete marker without unnecessary obsolete changeset content
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* It allow tuning the actual storage and protocol exchange while maintaining
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  compatibility with older client through the wire (as we do the repository
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  format)
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* ease the exchange of obsolete related information during discovery to exchange
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  obsolete changeset relevant to conflict resolution. Exchanging such
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  information deserve a dedicated protocol.
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Persistent
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```````````````````````
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*Extinct* changeset and obsolete marker will most likely be garbage collected as
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some point. However, archive server may decide to keep them forever in order to
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keep a fully auditable history in it's finest conception.
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Current status
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-----------------------------------------------------
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An experimental implementatione exists. What have been done so far.
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* 1-1 obsolete marker stored outside history,
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* compute obsolete-tip
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* obsolete marker exchange through pushkey,
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* compute obsolete, unstable, extinct and suspended set.
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* hidden extinct changesets for UI.
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* Use secret phase to remove from discovery obsolete and unstable changeset (to
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  be improved soon)
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* alter rebase to use obsolete marker instead of stripping. (XXX break --keep for now)
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* Have an experimental mq-like extension to rewrite history (more on that later)
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* Have an extension to update and mq repository according evolution of standard (more on that later)
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