Developer Document Catalog (3.3.22)

# Planned changes to the bzr core

Delivering the best possible performance requires changing the bzr core design from that present in 0.16. Some of these changes are incremental and can be done with no impact on disk format. Many of them however do require changes to the disk format, and these can be broken into two sets of changes, those which are sufficiently close to the model bzr uses today to interoperate with the 0.16 disk formats, and those that are not able to interoperate with the 0.16 disk formats - specifically some planned changes may result in data which cannot be exported to bzr 0.16’s disk formats and then imported back to the new format without losing critical information. If/when this takes place it will be essentially a migration for users to switch from their bzr 0.16 repository to a bzr that supports them. We plan to batch all such changes into one large ‘experimental’ repository format, which will be complete stable and usable before we migrate it to become a supported format. Getting new versions of bzr in widespread use at that time will be very important, otherwise the user base may be split in two - users that have upgraded and users that have not.

The following changes are grouped according to their compatibility impact: library only, disk format but interoperable, disk format interoperability unknown, and disk format, not interoperable.

## Library changes

These changes will change bzrlib’s API but will not affect the disk format and thus do not pose a significant migration issue.

> - For our 20 core use cases, we plan to add targeted API’s to bzrlib that are repository-representation agnostic. These will instead reflect the shape of data access most optimal for that case.

> - Deprecate ‘versioned files’ as a library concept. Instead of asking for information about a file-over-time as a special case, we will move to an API that assumes less coupling between the historical information and the ability to obtain texts/deltas etc. Specifically, we need to remove all API’s that act in terms of on disk representation except those within a given repository implementation.

> - Create a validator for revisions that is more amenable to use by other parts of the code base than just the gpg signing facility. This can be done today without changing disk, possibly with a performance hit until the disk formats match the validatory logic. It will be hard to tell if we have the right routine for that until all the disk changes are complete, so while this is a library only change, it’s likely one that will be delayed to near the end of the process.

> - Add an explicit API for managing cached annotations. While annotations are considered a cache this is not exposed in such a way that cache operations like ‘drop the cache’ can be performed. On current disk formats the cache is mandatory, but an API to manage would allow refreshing of the cache (e.g. after ghosts are filled in during baz conversions).

> - Use the _iter_changes API to perform merges. This is a small change that may remove the need to use inventories in merge, making a dramatic difference to merge performance once the tree shape comparison optimisations are implemented.

> - Create a network-efficient revision graph API. This is the logic at the start of push and pull operations, which currently scales O(graph size). Fixing the scaling can be done, but there are tradeoffs to latency and performance to consider, making it a little tricky to get right.

> - Working tree disk operation ordering. We plan to change the order in which some operations are done (specifically TreeTransform ones) to improve performance. There is already a 66% performance boost in that area going through review.

> - Stop requiring full memory copies of files. Currently bzr requires that it can hold 3 copies of any file it’s versioning in memory. Solving this is tricky, particularly without performance regressions on small files, but without solving it versioning of .iso and other large objects will continue to be extremely painful.

> - Add an API for per-file graph access that alllows incremental access and is suitable for on-demand generation if desired.

> - Repository stacking API. Allowing multiple databases to be stacked to give a single ‘repository’ will allow implementation of some long desired features like history horizons, and bundle usage where the bundle is not added to the local repository just to examine its contents.

> - Revision data manipulation API. We need a single streaming API for adding data to or getting it from a repository. This will need to allow hints such as ‘optimise for size’, or ‘optimise for fast-addition’ to meet the various users planned, but it is a core part of the library today, and it’s not sufficiently clean to let us simplify/remove a lot of related code today.

## Interoperable disk changes

> - New container format to allow single-file description of multiple named objects. This will provide the basis for transmission of revisions over the network, the new bundle format, and possibly a new repository format as well. [Core implemented]

> - Separate the annotation cache from the storage of actual file texts and make the annotation style, and when to do it, configurable. This will reduce data sent over the wire when repositories have had ‘needs-annotations’ turned off, which very large trees may choose to do - generating just-in-time annotations may be desirable for those trees (even when performing annotation based merges).

> - Repository disk operation ordering. The order that tasks access data within the repository and the layout of the data should be harmonised. This will require disk format changes but does not inherently alter the model, so it’s straight forward to export from a repository that has been optimised in this way to a 0.16 based repository.

> - Inventory representation. An inventory is a logical description of the shape of a version controlled tree. Currently we operate on the whole inventory as a tree broken down per directory, but we store it as a flat file. This scale very poorly as even a minor change between inventories requires us to scan the entire file, and in large trees this is many megabytes of data to consider. We are investigating the exact form, but the intent is to change the serialisation of inventories so that comparing two inventories can be done in some smaller time - e.g. O(log N) scaling. Whatever form this takes, a repository that can export it directly will be able to perform operations between two historical trees much more efficiently than the current repositories.

> - Greatest distance from origin cache. This is a possible change to introduce, but it may be unnecessary - listed here for completeness till it has been established as [un]needed.

## Possibly non-interoperable disk changes

> - Removing of derivable data from the core of bzr. Much of the data that bzr stores is derivable from the users source files. For instance the annotations that record who introduced a line. Given the full history for a repository we can recreate that at any time. We want to remove the dependence of the core of bzr on any data that is derivable, because doing this will give us the freedom to:

>   - Improve the derivation algorithm over time.

>   - Deal with bugs in the derivation algorithms without having ‘corrupt repositories’ or such things.

>   However, some of the data that is technically derived, like the per-file merge graph, is both considered core, and can be generated differently when certain circumstances arive, by bzr 0.16. Any change to the ‘core’ status of that data will discard data that cannot be recreated and thus lead to the inability to export from a format where that is derived data to bzr 0.16’s formats without errors occuring in those circumstances. Some of the data that may be considered for this includes:

>   - Per file merge graphs

>   - Annotations

## Non-interoperable disk changes

> - Drop the per-file merge graph ‘cache’ currently held in the FILE-ID.kndx files. A specific case of removing derivable data, this may allow smaller inventory metadata and also make it easier to allow two different trees (in terms of last-change made, e.g. if one is a working tree) to be compared using a hash-tree style approach.

> - Use hash based names for some objects in the bzr database. Because it would force total-knowledge-of-history on the graph revision objects will not be namable via hash’s and neither will revisio signatures. Other than that though we can in principle use hash’s e.g. SHA1 for everything else. There are many unanswered questions about hash based naming related to locality of reference impacts, which need to be answered before this becomes a definite item.