History Structure Git Comparison File Storage File Tracking Staging Queues (MQ) Merge Tools

1. Mercurial - Ben Pitts Hg History Structure Git Comparison File Storage File Tracking Staging Queues (MQ) Merge Tools Interfaces

2. Mercurial - Ben Pitts Mercurial / Git History • Bitmover's BitKeeper • Proprietary distributed revision control system • Used by Linux Kernel project for three years until… • April 2005 Bitmover CEO Larry McVoy pulls the free version of BitKeeper completely. • Linus Torvalds • Leaves kernel project to create the next kernel RVS • Two months later Git is born and hosts the kernel project • Matt Mackall simultaneously creates Mercurial "Mercurial is very good, but Git is better.“ Linus Torvalds “It largely comes down to taste. I guess some people just have no taste, but if Git makes them happy, I won't try to stop them.” Matt Mackall

3. Mercurial - Ben Pitts Mercurial • Project named Mercurial • Application named Hg (Mercury) • Open Source (GPL2) • http://mercurial.selenic.com/ • Mercurial is written in Python • Some machine language components • Notable users: • Mozilla, Python, NetBeans, OpenOffice, OpenJDK, rpm • Free hosting: • bitbucket.org, code.google.com, sourceforge.net

4. Mercurial - Ben Pitts Mercurial Structure - Filesystem / • Working directory • Represents one revision • Refreshed by 'hg update' /.hgignore List of ignored files Repository folder /.hg/ /.hgignore /.hgtags /.hgtags Tags are global pointers to changesets, like Git .hgignore and .hgtagskept in the working directory for version control. • A tag will always point to a revision older than the one that committed the tag. • Cloning from a tag will retrieve a repository that doesn't have the tag you cloned from. /.hg/ • Repository store • File snapshots • Manifest of files • Changesets of file deltas • Logs of all revisions

5. Mercurial - Ben Pitts Mercurial Structure – Repository b1 b2 branch • Revisions form a directed acyclic graph • Revisions stored as keyframed deltas • A revision is a snapshot of a commit. • The last revision in the chain is a head • There can be any number of heads • Heads can be merged two at a time into new revisions. v1 v2 v3 main v4

6. Mercurial - Ben Pitts Mercurial vs. Git • Largely equivalent to Git • Most functionality identical between systems • Differences in implementation and approach • Discrepancies resolved by extensions • hg-git • Mercurial plugin two-way bridge between Mercurial and Git • Allows Mercurial repository to push/pull with Git repositories • http://hg-git.github.io/ • Full command comparison at • http://mercurial.selenic.com/wiki/GitConcepts

7. Mercurial - Ben Pitts Mercurial vs. Git Extensions Extensions provide feature parity between Git and Hg • Git Stash / Mercurial Attic or Shelve • Set aside working environment for later • Side commit of working directory • Git Index / Mercurial Queue[s]

8. Mercurial - Ben Pitts Mercurial vs. Git Staging git commit git add Working Directory Git Repo Index • Git explicitly identifies what files to commit through an Index, or staging area • Mercurial treats the working directory as a snapshot of the commit, automatically managing which files are modified and need to be included. • Mercurial Queues extension can be used for more atomic control over commits, as well as the ability to push and pop patches in a system similar to Quilt. hg commit Hg Repo Working Directory

9. Mercurial - Ben Pitts Mercurial File Storage File Storage • Stores a snapshot (instance) of the current heads • Stores changesets containing commit deltas • Conflates deltas into keyframes • Compress deltas • Increase random access performance Compression • Files stored via compress (zip) • Algorithm balances compression with performance • Files stored raw if already compressed • Hg always does ‘the right thing’ when storing files. • HTTP transfers use bzip2 • SSH connections use SSH built in stream compression

10. Mercurial - Ben Pitts Mercurial File Tracking Track files not folders • Empty folders not allowed • Only files are tracked • File path is created with the file and torn down after if empty Tracking Files • ‘hg add’ – Add file[s] to the repository, and to the next commit • ‘hg addremove’ – Add all new files, removes missing files • ‘hg remove’ – Stop tracking history, file is deleted from working directory. • The complete history of the file is still stored, forever. • No garbage collection • Mercurial will track a new file with the same name as a separate entity.

11. Mercurial - Ben Pitts Mercurial History • What is past stays in the past, history moves forward. • Mercurial an append-only philosophy and makes it difficult to alter the history of commits • Avoids pitfalls of allowing history rewriting • Trades off flexibility • Guaranteed atomicity of transactions • Repository unlocked for reading at all times

12. Mercurial - Ben Pitts Mercurial vs. Git Rebasing • Git is designed to allow the graph to be modified freely. • Mercurial is designed to keep the graph moving forward. • Rewriting the commit graph is not allowed in Mercurial. • Mercurial Rebase Extension • Allows rebasing uncommitted changes • ‘hg pull’ a new head. • ‘hg rebase’ your changes onto the head. • Fails if conflicts detected • Can also be done as part of a pull • ‘hg pull –rebase’ • Hg's Rebase will not allow • rebasing to your own ancestor • rebasing to merge revision with external parents.

13. Mercurial - Ben Pitts Mercurial Queues (MQ) • To use a Git Index style layer, create a single patch • 'hg qnewpatchname‘ – Create a single patch • ‘hg qrefresh’ – Record changes into the patch • 'hg qfinish‘ – Finalize the patch • The Queue can contain multiple patches • More flexible than Git's single Index • Git is capable of handling patches too: • StGIT • http://www.procode.org/stgit/ • Guilt • http://repo.or.cz/w/guilt.git • These are scripts layered over Git • MQ is an integral extension to Hg

14. Mercurial - Ben Pitts Mercurial Queues - Patches Commit Patch 1 ‘UI Tweak’ Patch 1 ‘Bugfix’ Working Directory • ‘hg qpop’ and ‘hg qpush’ - Navigate up and down the patch chain • ‘hg refresh’ – Save changes into current patch • Changes in the commit can be separated by feature or section and tracked individually. • MQ also supports multiple parallel queues. • 'hg qqueue -c [queue name]' creates a queue. • 'hg qqueue [queue name]' switches between active queues

15. Mercurial - Ben Pitts Mercurial Queues – Versioning Patches • Patches are kept in plaintext in the repository • Single queue patches stored in: • /.hg/patches/[patchname]/ • Multiqueue patches stored in: • /.hg/patches-[queuename]/[patchname]/ • Creating a Mercurial repository inside these folders expands this intermediate area into a versioned repository. • Share queues with other people to allow others to look at your patch in progress before it is committed. • Collaborate on components of the code at a more local level before finalizing the overall commit.

16. Mercurial - Ben Pitts Mercurial Branching • Git creates branches by attaching labels to commits and growing the tree. • In Mercurial branching can be done internally, but cloning a repository is always preferred. • Cloning is safer, more modular, easy to discard. • If cloning locally, Mercurial uses hardlinks to files

17. Mercurial - Ben Pitts Mercurial Branching Mercurial supports named branching • Named branches exist in a global namespace. • Useful for team and project organization Keep a gold, beta, and alpha head. As QA tests, the heads are moved up the chain. Mercurial still doesn’t let you change history, so… • You can never delete or rename a named branch. (Of course you can, but it's not recommended.)

18. Mercurial - Ben Pitts Mercurial Branching Mercurial Bookmarks Extension • Comes in the box • Allows commits to be named less permenantly • Bookmarks are pointers into the commit graph • Can be renamed, nested and deleted at will • Bookmarks may be pushed and pulled between repositories • Global namespace still applies.

19. Mercurial - Ben Pitts Mercurial Merge Tools - Internal • internal:merge • Traditional merge with conflict markers baked into the file • ‘hg resolve’ needed before ‘hg commit’ is allowed • internal:dump • Creates three versions of the files to be merged manually: local, other and base • For the file a.txt, the conflict files will named "a.txt.local", "a.txt.other" and "a.txt.base“ • internal:fail • Marks unmerged files as unresolved, ‘hg resolve’ needed • internal:local • Uses the local version of files as the merged version • internal:other • Uses the other version of files as the merged version • internal:prompt • Asks the user to keep local or other as the merged version

20. Mercurial - Ben Pitts Mercurial Merge Tools - External • Merge tools are selected in /.hg/hgrc [merge-tools] mymergetool.priority= 100 mymergetool.premerge= False mymergetool.args= $local $other $base -o $output myimgmerge = [merge-patterns] **.jpg = myimgmerge **.exe = internal:fail

21. Mercurial - Ben Pitts Mercurial Merge Tools - External • Tool Arguments • $output – Where the merge will end up • $local- Unmerged local changes • $base - Revision from the merge ancestor • $other - Second parent revision • Tool Options • .args - The arguments to pass the merge tool • .premerge=False – Don’t first attempt internal merge (defaults to True) • .executable – Path of merge tool • .binary – Does the merge tool support binary files? • .symlinks– Does the merge tool support symlinks? • .gui– Requires a GUI, interactive merge • .priority - Priority of this merge tool (0-100)

22. Mercurial - Ben Pitts Mercurial Merge Tools - External Interactive merging tools are plentiful • KDiff3 • Merge GUI • Packaged with Hg • Meld • Merge GUI • Linux/Mac/Win • meldmerge.org • Diffuse • Text based interactive merge tool • Python • diffuse.sourceforge.net

23. Mercurial - Ben Pitts Mercurial Interfaces • SourceTree • Free Git/Hg GUI for Win/Mac • sourcetreeapp.com • MacHg • Free Hg GUI for Mac • jasonfharris.com/machg • TortoiseHg • Free file browser integration, Windows and Gnome/Nautilus • tortoisehg.bitbucket.org • EasyHg • Simple free Hg GUI for Linux/Mac/Win • easyhg.org

24. Mercurial - Ben Pitts Mercurial Integration • Eclipse • Mercurial Eclipse plugin • bitbucket.org/mercurialeclipse/main • NetBeans • Supported out of the box • Visual Studio • HgSccPackageextension • https://bitbucket.org/zzsergant/hgsccpackage/overview • Ant • ANT4HG • http://ant4hg.free.fr/ • Maven • Integrated via scm:hg namespace

25. Mercurial - Ben Pitts Hg • http://mercurial.selenic.com/ Benjamin Pitts Computer Science M.S. Student Old Dominion University