Utility to migrate code from SmalltalkHub (or any MCZ-based repo) to Git
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MCZ -> Git Migration

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Utility to migrate code from SmalltalkHub (or any MCZ-based repo) to Git


This tool has been successfully used by several people, but remains experimental. It is your responsibility to verify that the result is what you expected.


Please read #4 if you are interested in preserving mcz merges to git merges 1:1 is resolved it is not advised to used this tool, as the commit merges are not properly resolved.

Table Of Contents

Possible Issues

I am not an expert on Monticello (and I've migrated to git two years ago, so I don't know why I even wrote this tool), so it is possible that there are edge cases that I haven't considered. If you run into a problem, please open an issue (ideally with a pull request).

  • performance - Git's fast-import is used to move the data. On Pharo side (generating import file) it will take couple minutes for large repos (caching every single version, unzipping, transforming, ...). On Git side it will take about a second. (PolyMath with 800 commits accross 70 packages took ~3 minutes generating 90MB import file)
  • relying on dependencies specified in MC Versions (=NOT ConfigurationOf/BaselineOf) -- not supported
  • preserving proper merge history (see also #4)
    • after many hours I've concluded that there is no way to do a fully automated 1:1 migration; if you need to convert all MCZ commits to Git commits, you would have to guide it by hand


  • git installed in the system and available in PATH
  • Pharo 6+


Pharo 6

Metacello new
	baseline: 'GitMigration';
	repository: 'github://peteruhnak/git-migration:pharo6/repository';

Pharo 7

Metacello new
	baseline: 'GitMigration';
	repository: 'github://peteruhnak/git-migration/repository';

Usage - Fast Import

Fast Import generates a file for git-fast-import.


migration := GitMigration on: 'peteruhnak/breaking-mcz'.
migration cacheAllVersions.
migration authors: {'PeterUhnak' -> #('Peter Uhnak' '<i.uhnak@gmail.com>')}.
	fastImportCodeToDirectory: 'repository'
	initialCommit: '5793e82'
	to: 'D:/tmp/breaking-mcz2/import.txt'
# Terminal
cd D:/tmp/breaking-mcz2
git fast-import < import.txt
git reset --hard master
git gc

1. Add Source Repository

Add your source repository (SmalltalkHub) to Pharo, e.g. via Monticello Browser

2. Find The Initial Commit SHA

The migration will need to know from which commit it should start. This will be typically the SHA of the current commit of the master branch; you don't need the full 40-char SHA, an unambiguous prefix is enough.

The get the current commit, you can do the following

$ git log --oneline -n 1

3. Generating Import File

A longer description of the example above.

"Specify the name of the source repository; I am sourcing from peteruhnak/breaking-mcz project on SmalltalkHub"
migration := GitMigration on: 'peteruhnak/breaking-mcz'.

"Download all mcz files, this will take a while"
migration cacheAllVersions.

"List all authors anywhere in the project's commits"
migration allAuthors. "#('PeterUhnak')"

"You must specify name and email for _every_ author"
"You must also specify the name/email for yourself (Author fullName), even if you haven't authored any code -- git treats separately the author of a commit and the commiter of a commit"

"AuthorName (as shown in #allAuthors) -> #('Nicer Name' '<email@example.com>')"
migration authors: {
	'PeterUhnak' -> #('Peter Uhnak' '<i.uhnak@gmail.com>')

"Run the migration, this might take a while
* the code directory is where the code will be stored (common practice is to have the code in `repository` subfolder, just like this project)
* initialCommit is the commit from which the migration should start
* to is where the git-fast-import file should be stored"
	fastImportCodeToDirectory: 'repository'
	initialCommit: '5793e82'
	to: 'D:/tmp/breaking-mcz2/import.txt'

4. Running The Import

Get a terminal, go to the target git repository, and run the migration.

# import.txt is the file that you've created earlier
$ git fast-import < import.txt
# fast-import doesn't change the working directory, so we need to update it
$ git reset --hard master
# (optional) garbage collection: fast import leaves a lot of mess behind
# happens automatically on commit since Git >=2.17
$ git gc

You should see the changes, and git log should show you the entire history.

Git Tips

Forgetting all changes in the history and going back to previous state. Useful if the migration is botched and you want to rollback all changes.

$ git reset --hard SHA


If you want to play around with the version data before committing, read the following.

migration := GitMigration on: 'peteruhnak/breaking-mcz'.

Downloading all MCZs from server; this needs to happen only once and can take couple of minutes for large repos.

migration cacheAllVersions.

List all packages in the repository that have multiple roots; although rare, this could be either result of multiple people starting independently on the same package, or a mistake was made during committing. GitMigration should be able to handle this correctly regardless.

migration packagesWithMultipleRoots.

List all authors in the repository.

migration allAuthors.

Dictionary of all packages and their real (see later what's real) commits.

versionsByPackage := migration versionsByPackage.

All versions of a package, whether there is actually an MCZ or not. With Monticello it is very easy to create a commit whose ancestor is not in the repository, so it is not obvious how the commit connects the previous ones. Thankfully MCZ typically contains the hierarchy many steps back, so we can correctly reconstruct the whole tree.

allVersions := migration completeAncestryOfPackageNamed: 'Somewhere'.

The versions in mcz are random, so we need to sort them in an order in which we can commit them to git. This means that all ancestry is honored (no child is commited before its parent), and "sibling" commits are sorted by date. Note that we cannot just sort the commits by date, because the date might not follow the ancestry correctly (which can happen, especially if different timezones are involved, which MC doesn't keep track of)

sorted := migration topologicallySort: allVersions.

Get the total ordering of all commits across all packages

allVersionsOrdered := migration commitOrder.


This requires Roassal to be installed (available in catalog).

In all visualizations hovering over an item will show a popup with more information, and clicking on item will open an inspector. Keep in mind that running the command will not open a new window, so you have to either inspect it, or do-it-and-go in playground.

Single Package Ancestry

Looking at raw data is not very insightful, so couple visualization are included.

migration := GitMigration on: 'peteruhnak/breaking-mcz'.
migration cacheAllVersions.
visualization := migration visualization.

Show the complete ancestry of a single package.

visualization showAncestryTopologyOnPackageNamed: 'Somewhere'.

  • Yellow - root versions (versions with no parents, typically only a single initial commit)
  • Cyan - tail/head versions (versions with no children, typically the latest version(s))
  • Magenta - "virtual" versions that do not have a corresponding commit (this happens as mentioned earlier)

The number on the third line indicates in what order the packages will be committed (magenta packages are listed, but are not committed, because there is no code to commit). Keep in mind that the number in the commit (Somewhere-PeterUhnak.15) has no meaning, and can be easily changed (and broken by hand) when committing.

Project Ancestry

To see all packages and history, you could do.

visualization showProjectAncestry.

This is useful if you want to quickly glance at a project (and is also much faster to generate and use), but if want you can also add label

visualization showProjectAncestryWithLabels.
visualization showProjectAncestryWithLabels: true.

Limited Project Ancestry

If you have big project and want to look only at certain packages, you can do so. (In the image you can see that the longest chain has ancestry broken - red box at the end)

migration := GitMigration on: 'PolyMath/PolyMath'.
migration cacheAllVersions.
visualization := migration visualization.
"or just a collection of package names"
visualization showProjectAncestryOn: (allPackages copyWithoutAll: #('Monticello' 'ConfigurationOfSciSmalltalk'  'Math-RealInterval')).

Adding labels works the same way

visualization showProjectAncestryOn: aCollectionOfPackages withLabels: aBoolean

For Developers

Some hints and random thoughts. SmalltalkHub stores every commit in a separate MCZ file, which contains some metadata about the commit (name, ancestry, etc), as well as all the code. The code itself is not incremental, rather code in each zip is as-is.

This means that when GitFileTree is exporting, it will remove all files on the disk, unpack the MCZ file, and write all the code back to disk, and commit. Git is smart enough to only commit what has actually changed, however for GFT this operation is very IO intense - if you have 5k files in your code base and you changed just a single method (which is common), then 5k files will be removed and then added back... you can imagine what this does to the disk when performed 1000x times (once for each commit).

With fast-import I've made a workaround for this. A pseudo-repository GitMigrationMemoryTreeGitRepository is created that uses memory file system as the target directory. This way the fileout doesn't write to real disk and everything is kept in RAM, which improves the performance significantly.

Note however that instead of using MemoryStore I had to subclass it (GitMigrationMemoryStore) to properly handle path separators; on Windows, MemoryStore by itself will create files and directories with slashes (both forward and backward) in their names instead of creating a hierarchy, so my GitMigrationMemoryStore fixes this.

I am also subclassing MemoryHandle (GitMigrationMemoryHandle) and I've changed the writeStream of it to return MultiByteBinaryOrTextStream. This is because MemoryStore returns only an ordinary WriteStream which cannot handle unicode content and 那不是很好。 :)