NAME

VCP::Process - How vcp works

DESCRIPTION

"vcp" is designed to be a general purpose repository import/export tool. This document describes some of the techniques used to keep "vcp" general purpose.

Phases of Operation

"vcp" works in several phases:

1. Metadata Scanning: Before anything else can happen, "vcp" must take the source repository spec, something like "cvs:module/dir/..." and use the appropriate repository interface ("cvs log" in this case) to extract the metadata.
The metadata is currently kept all in memory; if you run in to a repository so big that this is troublesome, do the transfer in phases or pester us to provide a swap file capability for this data.
In the case of a RevML source, it is not practical to scan the input for metadata alone (the RevML may be coming from the standard input, for instance), so all of the files in a RevML source file are extracted during the scanning phase, as mentioned in VCP::Source::revml.

1a. Base revisions and backfilling: When sourcing from incremental RevML transfers, an additional step must be taken for each text file in the transfer. An incremental RevML file does not usually contain the entire body of any revision of a text file; it only contains deltas between revisions. This is not so for binary files, which are currently always shipped in their entirety, or for when the --bootstrap option has been provided during the extraction.
"vcp" therefore needs to be able to recreate the first revision of a text file in an incremental transfer when RevML is in use. This is addressed by a process called "backfilling the base revision".
The "base" revision of a file is the revision that immediately precedes the first revision being transfered. It is also the last revision in the previous transfer and must be the most recent revision (on the appropriate branch) in the destination repository.
"vcp" "backfills" the base revision by checking it out of the destination repository, then reconstitutes the first revision by applying the (base revision => first revision) delta to the base revision. Each revision in a RevML file contains an MD5 checksum to make sure that all backfilling and patching is implemented accurately.

1b. Selecting: In the case of VCP::Source::cvs, the initial scan often nets too much data, so the data scanned is winnowed down to the desired set (see "Files that aren't tagged" in VCP::Source::cvs for details).

2. Sorting and Change Aggregation: The order that the soruce repository presents revisions in is often not the order they need to be inserted in, so the destination driver (VCP::Dest::p4, for example) is given the opportunity to sort the revisions.
This is primarily used to do change number aggregation when converting from a repository that does not provide change set metadata (like CVS) to one that does (like p4).
This is also important when generating RevML files because the order of appearance of files in a log file may hinge on exactly when the files were inserted along with their names, at least in the case of CVS. Sorting the revisions provides for consistent RevML files, which is important in testing situations.

3. File transfer.: The final stage is to do the file transfer. When the entire source file is available, it is simply added to the result repository in the correct order.
For incremental transfers an extra step is taken to ensure that incremental transfers leave no gaps. The base revision is backfilled from the destination repository (using the process for backfilling described in phase 1 above) and compared to the base revision from the source repository.

Command Line Tools

Currently, "vcp" shells out to command line tools like "cvs" and "p4". This is a "least common denominator" approach that allows VCP to operate at a safe distance from the underlying implementations. It is also the primary bottleneck in transferring files. We will gladly accept donations of drivers that use direct library interfaces or remote procedure call (SOAP, RMI, etc., etc.) techniques to speed this process up.

AUTHOR

Barrie Slaymaker <barries@slaysys.com>

COPYRIGHT

See VCP::License ("vcp help license") for the terms of use.