author | Durham Goode <durham@fb.com> |
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changeset 1226 | 901d2f4b21a9 |
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.. Copyright 2011 Pierre-Yves David <pierre-yves.david@ens-lyon.org> |
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.. Logilab SA <contact@logilab.fr> |
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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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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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The new general delta format allows a very efficient storage of two very similar |
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changesets. Storing obsolete children 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 handles obsolete marker is independent from what decides |
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to create them and what actual operation solves the error case. Any of |
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the existing history rewriting UI (rebase, mq, histedit) can lay |
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obsolete markers and resolve situation created by others. To go |
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further, a hook system of obsolete marker creation would allow each |
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mechanism to collaborate with other though a standard and central |
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mechanism. |
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Obsolete marker storage |
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The Obsolete marker will most likely be stored outside standard |
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history. They are multiple reasons for this: |
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First, obsolete markers are really perpendicular to standard history |
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there is no 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 becomes |
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complicated to fix history afterward in particular when working with older |
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clients. |
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Storing obsolete marker outside history have several pros: |
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* It eases Exchange of obsolete markers without unnecessary obsolete |
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changeset contents. |
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* It allows tuning the actual storage and protocol exchange while maintaining |
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compatibility with older clients through the wire (as we do the repository |
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format). |
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* It eases the exchange of obsolete related information during |
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discovery to exchange obsolete changeset relevant to conflict |
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resolution. Exchanging such information deserves a dedicated |
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protocol. |
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Persistent |
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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 its finest conception. |
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Current status |
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Obsolete marker are partialy in core. |
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2.3: |
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- storage over obsolete marker |
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- exchange suing pushkey |
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- extinct changeset are properly hidden |
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- extinct changeset are excluded from exchange |