~bzr-pqm/bzr/bzr.dev : revision 2508

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Planned changes to the bzr core

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Delivering the best possible performance requires changing the bzr core design

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from that present in 0.16. Some of these changes are incremental and can be

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done with no impact on disk format. Many of them however do require changes to

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the disk format, and these can be broken into two sets of changes, those which

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are sufficiently close to the model bzr uses today to interoperate with the

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0.16 disk formats, and those that are not able to interoperate with the 0.16

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disk formats - specifically some planned changes may result in data which

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cannot be exported to bzr 0.16's disk formats and then imported back to the new

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format without losing critical information. If/when this takes place it will be

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essentially a migration for users to switch from their bzr 0.16 repository to a

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bzr that supports them. We plan to batch all such changes into one large

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'experimental' repository format, which will be complete stable and usable

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before we migrate it to become a supported format. Getting new versions of bzr

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in widespread use at that time will be very important, otherwise the user base

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may be split in two - users that have upgraded and users that have not.

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The following changes are grouped according to their compatability impact:

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library only, disk format but interoperable, disk format interoperability

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unknown, and disk format, not interoperable.

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Library changes

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===============

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These changes will change bzrlib's API but will not affect the disk format and

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thus do not pose a significant migration issue.

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* For our 20 core use cases, we plan to add targeted API's to bzrlib that are

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repository-representation agnostic. These will instead reflect the shape of

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data access most optimal for that case.

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* Deprecate 'versioned files' as a library concept. Instead of asking for

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information about a file-over-time as a special case, we will move to an API

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that assumes less coupling between the historical information and the

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ability to obtain texts/deltas etc. Specifically, we need to remove all

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API's that act in terms of on disk representation except those within a

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given repository implementation.

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* Create a validator for revisions that is more amenable to use by other parts

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of the code base than just the gpg signing facility. This can be done today

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without changing disk, possibly with a performance hit until the disk

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formats match the validatory logic. It will be hard to tell if we have the

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right routine for that until all the disk changes are complete, so while

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this is a library only change, its likely one that will be delayed to near

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the end of the process.

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* Add an explicit API for managing cached annotations. While annotations are

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considered a cache this is not exposed in such a way that cache operations

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like 'drop the cache' can be performed. On current disk formats the cache is

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mandatory, but an API to manage would allow refreshing of the cache (e.g.

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after ghosts are filled in in baz conversions).

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* Use the _iter_changes API to perform merges. This is a small change that may

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remove the need to use inventories in merge, making a dramatic difference to

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merge performance once the tree shape comparison optimisations are

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implemented.

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* Create a network-efficient revision graph API. This is the logic at the

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start of push and pull operations, which currently scales O(graph size).

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Fixing the scaling can be done, but there are tradeoffs to latency and

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performance to consider, making it a little tricky to get right.

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* Working tree disk operation ordering. We plan to change the order in which

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some operations are done (specifically TreeTransform ones) to improve

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performance. There is already a 66% performance boost in that area going

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through review.

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* Stop requiring full memory copies of files. Currently bzr requires that it

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can hold 3 copies of any file its versioning in memory. Solving this is

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tricky, particularly without performance regressions on small files, but

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without solving it versioning of .iso and other large objects will continue

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to be extremely painful.

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* Add an API for per-file graph access that alllows incremental access and is

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suitable for on-demand generation if desired.

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* Repository stacking API. Allowing multiple databases to be stacked to give a

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single 'repository' will allow implementation of some long desired features

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like history horizons, and bundle usage where the bundle is not added to the

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local repository just to examine its contents.

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* Revision data manipulation API. We need a single streaming API for adding

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data to or getting it from a repository. This will need to allow hints such

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as 'optimise for size', or 'optimise for fast-addition' to meet the various

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users planned, but it is a core part of the library today, and its not

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sufficiently clean to let us simplify/remove a lot of related code today.

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Interoperable disk changes

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==========================

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* New container format to allow single-file description of multiple named

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objects. This will provide the basis for transmission of revisions over the

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network, the new bundle format, and possibly a new repository format as

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well.

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* Separate the annotation cache from the storage of actual file texts and make

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the annotation style, and when to do it, configurable. This will reduce data

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sent over the wire when repositories have had 'needs-annotations' turned

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off, which very large trees may choose to do - generating just-in-time

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annotations may be desirable for those trees (even when performing

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annotation based merges).

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* Repository disk operation ordering. The order that tasks access data within

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the repository and the layout of the data should be harmonised. This will

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require disk format changes but does not inherently alter the model, so its

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straight forward to export from a repository that has been optimised in this

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way to a 0.16 based repository.

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* Inventory representation. An inventory is a logical description of the shape

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of a version controlled tree. Currently we operate on the whole inventory as

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a tree broken down per directory, but we store it as a flat file. This scale

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very poorly as even a minor change between inventories requires us to scan

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the entire file, and in large trees this is many megabytes of data to

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consider. We are investigating the exact form, but the intent is to change

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the serialisation of inventories so that comparing two inventories can be

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done in some smaller time - e.g. O(log N) scaling. Whatever form this takes,

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a repository that can export it directly will be able to perform operations

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between two historical trees much more efficiently than the current

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repositories.

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

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

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

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

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

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

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* Greatest distance from origin cache. This is a possible change to introduce,

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but it may be unnecessary - listed here for completeness till it has been

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established as [un]needed.

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Possibly non-interoperable disk changes

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=======================================

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* Removing of derivable data from the core of bzr. Much of the data that bzr

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stores is derivable from the users source files. For instance the

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annotations that record who introduced a line. Given the full history for a

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repository we can recreate that at any time. We want to remove the

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dependence of the core of bzr on any data that is derivable, because doing

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this will give us the freedom to:

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* Improve the derivation algorithm over time.

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* Deal with bugs in the derivation algorithms without having 'corrupt

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repositories' or such things.

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However, some of the data that is technically derived, like the per-file

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merge graph, is both considered core, and can be generated differently when

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certain circumstances arive, by bzr 0.16. Any change to the 'core' status of

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that data will discard data that cannot be recreated and thus lead to the

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inability to export from a format where that is derived data to bzr 0.16's

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formats without errors occuring in those circumstances. Some of the data

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that may be considered for this includes:

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* Per file merge graphs

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* Annotations

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Non-interoperable disk changes

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==============================

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* Drop the per-file merge graph 'cache' currently held in the FILE-ID.kndx

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files. A specific case of removing derivable data, this may allow smaller

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inventory metadata and also make it easier to allow two different trees (in

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terms of last-change made, e.g. if one is a working tree) to be compared

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using a hash-tree style approach.

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* Use hash based names for some objects in the bzr database. Because it would force

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total-knowledge-of-history on the graph revision objects will not be namable

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via hash's and neither will revisio signatures. Other than that though we

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can in principle use hash's e.g. SHA1 for everything else. There are many

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unanswered questions about hash based naming related to locality of

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reference impacts, which need to be answered before this becomes a definite

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item.