Replicate (clone, mirror), split, merge, and rearrange parts of Subversion repositories while preserving commit history
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README.md
svn_replay.example.conf
svn_replay.pl

README.md

TL;DR

Extract once or replicate repeatedly parts of one or more Subversion repositories into another (usually new) repository.

  1. Create a configuration file:

    $ cat > extract_gem.conf
    {
        SourceRepositories =>
        [
            {
                RepoName => 'source_repo',
                RootURL => 'https://svn/repos/source_repo',
                PathMapping =>
                [
                    {
                        SourcePath => 'trunk/projects/gem',
                        TargetPath => 'trunk'
                    }
                ]
            }
        ]
    }
    
  2. Initialize the target repository:

     svn_replay.pl -i gem_repo extract_gem.conf gem_repo_checkout
    
  3. Perform the replication (depending on the number of revisions in source_repo, this process may take a long time):

     svn_replay.pl extract_gem.conf gem_repo_checkout >> gem_repo.log
    

    Optionally, the above command can be run periodically from cron to continue updating gem_repo with the latest changes from the source repository.

How It Works

This tool was created for internal use and with a single purpose. It wasn't meant to be reused, let alone open sourced. Yet I keep coming back to it whenever I need to perform a "surgery" that svnsync wasn't designed to handle. And so the tool is released into the wild in hope that someone finds it useful.

svn_replay works by reading changesets of one or more source repositories and reproducing (replaying) the same changes in the target repository. The most likely scenario is that the target repository is created from scratch by running svn_replay -i, but it is also possible to use an existing repository as the target repository. A local working copy of the target repository is maintained to prepare commits.

Physical access to the repositories is not required; the usual access protocol (https, svn+ssh, etc.) will suffice (although using the file protocol makes the replication process significantly faster).

All source repositories can be edited normally during the replication.

The target repository can also be modified externally provided that all changes happen outside the configured destination directories. In this case, consider setting the PreserveRevisionTimestamps configuration parameter to 0. Otherwise, svn_replay might violate the monotone increasing property of the commit timestamps in the target repository by making the timestamp of the replayed revision older than the timestamp of the latest manually committed revision.

When svn_replay is used to merge or cherry-pick changes from two or more source repositories, changesets that come from different source repositories are sorted by their commit dates and times. Of course, the relative order of changesets coming from each particular repository is preserved.

Installation

No installation required: simply run svn_replay.pl from the root directory. A symbolic link to the script can be created if needed.

The script has no CPAN dependencies; all modules that it uses are either bundled with the project or come standard with Perl.

Configuration

The configuration file for svn_replay is a simple Perl script, which must end with a HASH definition.

Below is an example of a very basic configuration file. It sets up replication of a single directory of one source repository to a directory inside the target repository.

{
    SourceRepositories =>
    [
        {
            RepoName => 'source_repo',
            RootURL => 'https://svn.example.org/repos/source_repo',
            PathMapping =>
            [
                {
                    SourcePath => 'path/in/source/repo',
                    TargetPath => 'path/in/target/repo'
                }
            ]
        }
    ]
}

The only required parameter in the root configuration hash is SourceRepositories. The value of this key is an array of hashes, each referring to a single source repository. The example above uses only one source repository and therefore its SourceRepositories array contains a single hash.

Each hash in the SourceRepositories array must contain the following three keys:

  • RepoName defines the name of the source repository. This name is used internally and also appears in the progress log. Because there is only one target repository, it does not need a name.

  • RootURL defines the URL that will be used to read from the repository.

  • PathMapping is the primary configuration parameter and defines a one-to-one mapping of a set of non-overlapping directories in the source repository onto a set of non-overlapping directories in the target repository.

In the example below, trunks of two source repositories become sibling directories in the target repository:

{
    SourceRepositories =>
    [
        {
            RepoName => 'red_source_repo',
            RootURL => 'https://svn/repos/red_repo',
            PathMapping =>
            [
                {
                    SourcePath => 'trunk',
                    TargetPath => 'trunk/colors/red'
                }
            ]
        }
    ],
    [
        {
            RepoName => 'blue_source_repo',
            RootURL => 'https://svn/repos/blue_repo',
            PathMapping =>
            [
                {
                    SourcePath => 'trunk',
                    TargetPath => 'trunk/colors/blue'
                }
            ]
        }
    ]
}

For each repository, multiple PathMapping elements can be defined. Here's an example where pathnames of the separated include and src directories of a C library are rewritten so that the library gets its own private directory.

{
    SourceRepositories =>
    [
        {
            RepoName => 'source_repo',
            RootURL => 'https://svn.example.org/repos/source_repo',
            PathMapping =>
            [
                {
                    SourcePath => 'trunk/include/mylib',
                    TargetPath => 'trunk/mylib/include/mylib'
                },
                {
                    SourcePath => 'trunk/src/mylib',
                    TargetPath => 'trunk/mylib/src'
                }
            ]
        }
    ]
}

Optional Parameters

Besides the three required keys (RepoName, RootURL, and PathMapping), the hash that describes a single source repository can also contain the following optional ones:

  • StopAtRevision makes the replication process stop at a certain revision number in the source repository as opposed to HEAD. The commit history will be read up to and including the specified revision.

  • DiscardSvnExternals prescribes that the svn:externals property must not be copied over to the target repository. By default, the svn:externals property is copied verbatim.

  • PreCommitHook is a Perl subroutine that is called right before the target repository modifications are committed. The commit is aborted if this subroutine returns zero, in which case it's the responsibility of the pre-commit subroutine to clean up the working copy (that is, revert all changes, including those made by svn_replay itself).

The root hash can also contain the following optional keys:

  • CommitCredentials defines authentication parameters if required by the target repository. The value of this key must be either a two-element array, in which case it's interpreted as a username-password pair or a scalar if providing only the username will suffice.

  • PreserveRevisionAuthors determines whether original revision authors are preserved when committing to the target repository. This parameter is enabled by default.

  • PreserveRevisionTimestamps determines whether commit timestamps of the original revisions are preserved. This functionality is enabled by default; set the parameter to 0 if the original commit timestamps should not be preserved (for example, when the pre-revprop-change hook in the target repository prohibits this or when there are other committers besides svn_replay).

For a complete configuration file example, see the bundled svn_replay.example.conf.

How to Run

The script has two modes of operation, each described in its own section below.

Target Repository Initialization

svn_replay.pl -i <TARGET_REPO_PATH> <CONF_FILE> <TARGET_WORKING_COPY>

When given the -i option, svn_replay creates and initializes an empty target repository. In this mode, the configuration file and the source repositories are only used to set modification date for revision zero, which is chosen as the oldest modification date among revision zero of all source repositories.

After the target repository has been created, it's checked out into the specified working copy directory, which must not exist.

Incremental Replication

svn_replay.pl <CONF_FILE> <TARGET_WORKING_COPY>

This is the normal mode of operation. When the -i option is not specified, the script iterates over the source repositories to check for new revisions. If the configured source paths have received any new changes since the last run, those changes are replicated in the target repository.

The script logs information about its progress to the standard output. To save this log to a file, use shell redirection:

svn_replay.pl my.conf workdir >> svn_replay.log
# or: svn_replay.pl my.conf workdir | tee -a svn_replay.log

Limitations

  • svn_replay cannot clone the entire source repository unless all top-level directories of that repository are listed in the PathMapping configuration section.

  • The employed path mapping scheme is not flexible enough to describe arbitrary repository restructuring (for example, merging into a single target directory). In some cases, however, the desired effect can be achieved by applying a chain of svn_replay transformations and/or using PreCommitHook.

Troubleshooting

If anything doesn't work the way it should, run prove while in the root directory of the project (or run t/test.pl if prove is not available). Then file a bug.

Disclaimer

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF PERFORMANCE, MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT SHALL ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.