~bzr-pqm/bzr/bzr.dev

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 compatability 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, its 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 in 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 its 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 its 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 its

   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.

 * Delta storage optimisation. We plan to change the delta storage logic to use

   a binary delta like xdelta rather than using line based deltas from python.

   These binary deltas could be done along ancestry ordering, or other

   arbitrary patterns chosen for their intended use. Line based deltas will

   still be created for cached annotations. This is still under some discussion.

   http://bazaar-vcs.org/PerformanceRoadmap/Xdelta

 * 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.