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view tests/test-contrib.t @ 26623:5a95fe44121d
clonebundles: support for seeding clones from pre-generated bundles
Cloning can be an expensive operation for servers because the server
generates a bundle from existing repository data at request time. For
a large repository like mozilla-central, this consumes 4+ minutes
of CPU time on the server. It also results in significant network
utilization. Multiplied by hundreds or even thousands of clients and
the ensuing load can result in difficulties scaling the Mercurial server.
Despite generation of bundles being deterministic until the next
changeset is added, the generation of bundles to service a clone request
is not cached. Each clone thus performs redundant work. This is
wasteful.
This patch introduces the "clonebundles" extension and related
client-side functionality to help alleviate this deficiency. The
client-side feature is behind an experimental flag and is not enabled by
default.
It works as follows:
1) Server operator generates a bundle and makes it available on a
server (likely HTTP).
2) Server operator defines the URL of a bundle file in a
.hg/clonebundles.manifest file.
3) Client `hg clone`ing sees the server is advertising bundle URLs.
4) Client fetches and applies the advertised bundle.
5) Client performs equivalent of `hg pull` to fetch changes made since
the bundle was created.
Essentially, the server performs the expensive work of generating a
bundle once and all subsequent clones fetch a static file from
somewhere. Scaling static file serving is a much more manageable
problem than scaling a Python application like Mercurial. Assuming your
repository grows less than 1% per day, the end result is 99+% of CPU
and network load from clones is eliminated, allowing Mercurial servers
to scale more easily. Serving static files also means data can be
transferred to clients as fast as they can consume it, rather than as
fast as servers can generate it. This makes clones faster.
Mozilla has implemented similar functionality of this patch on
hg.mozilla.org using a custom extension. We are hosting bundle files in
Amazon S3 and CloudFront (a CDN) and have successfully offloaded
>1 TB/day in data transfer from hg.mozilla.org, freeing up significant
bandwidth and CPU resources. The positive impact has been stellar and
I believe it has proved its value to be included in Mercurial core. I
feel it is important for the client-side support to be enabled in core
by default because it means that clients will get faster, more reliable
clones and will enable server operators to reduce load without
requiring any client-side configuration changes (assuming clients are
up to date, of course).
The scope of this feature is narrowly and specifically tailored to
cloning, despite "serve pulls from pre-generated bundles" being a valid
and useful feature. I would eventually like for Mercurial servers to
support transferring *all* repository data via statically hosted files.
You could imagine a server that siphons all pushed data to bundle files
and instructs clients to apply a stream of bundles to reconstruct all
repository data. This feature, while useful and powerful, is
significantly more work to implement because it requires the server
component have awareness of discovery and a mapping of which changesets
are in which files. Full, clone bundles, by contrast, are much simpler.
The wire protocol command is named "clonebundles" instead of something
more generic like "staticbundles" to leave the door open for a new, more
powerful and more generic server-side component with minimal backwards
compatibility implications. The name "bundleclone" is used by Mozilla's
extension and would cause problems since there are subtle differences
in Mozilla's extension.
Mozilla's experience with this idea has taught us that some form of
"content negotiation" is required. Not all clients will support all
bundle formats or even URLs (advanced TLS requirements, etc). To ensure
the highest uptake possible, a server needs to advertise multiple
versions of bundles and clients need to be able to choose the most
appropriate from that list one. The "attributes" in each
server-advertised entry facilitate this filtering and sorting. Their
use will become apparent in subsequent patches.
Initial inspiration and credit for the idea of cloning from static files
belongs to Augie Fackler and his "lookaside clone" extension proof of
concept.
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Fri, 09 Oct 2015 11:22:01 -0700 |
parents | ef1eb6df7071 |
children | c3e9269d9602 |
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Set vars: $ CONTRIBDIR="$TESTDIR/../contrib" Prepare repo-a: $ hg init repo-a $ cd repo-a $ echo this is file a > a $ hg add a $ hg commit -m first $ echo adding to file a >> a $ hg commit -m second $ echo adding more to file a >> a $ hg commit -m third $ hg verify checking changesets checking manifests crosschecking files in changesets and manifests checking files 1 files, 3 changesets, 3 total revisions Dumping revlog of file a to stdout: $ python "$CONTRIBDIR/dumprevlog" .hg/store/data/a.i file: .hg/store/data/a.i node: 183d2312b35066fb6b3b449b84efc370d50993d0 linkrev: 0 parents: 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000 length: 15 -start- this is file a -end- node: b1047953b6e6b633c0d8197eaa5116fbdfd3095b linkrev: 1 parents: 183d2312b35066fb6b3b449b84efc370d50993d0 0000000000000000000000000000000000000000 length: 32 -start- this is file a adding to file a -end- node: 8c4fd1f7129b8cdec6c7f58bf48fb5237a4030c1 linkrev: 2 parents: b1047953b6e6b633c0d8197eaa5116fbdfd3095b 0000000000000000000000000000000000000000 length: 54 -start- this is file a adding to file a adding more to file a -end- Dump all revlogs to file repo.dump: $ find .hg/store -name "*.i" | sort | xargs python "$CONTRIBDIR/dumprevlog" > ../repo.dump $ cd .. Undumping into repo-b: $ hg init repo-b $ cd repo-b $ python "$CONTRIBDIR/undumprevlog" < ../repo.dump .hg/store/00changelog.i .hg/store/00manifest.i .hg/store/data/a.i $ cd .. Rebuild fncache with clone --pull: $ hg clone --pull -U repo-b repo-c requesting all changes adding changesets adding manifests adding file changes added 3 changesets with 3 changes to 1 files Verify: $ hg -R repo-c verify checking changesets checking manifests crosschecking files in changesets and manifests checking files 1 files, 3 changesets, 3 total revisions Compare repos: $ hg -R repo-c incoming repo-a comparing with repo-a searching for changes no changes found [1] $ hg -R repo-a incoming repo-c comparing with repo-c searching for changes no changes found [1] Test simplemerge command: $ cp "$CONTRIBDIR/simplemerge" . $ echo base > base $ echo local > local $ cat base >> local $ cp local orig $ cat base > other $ echo other >> other changing local directly $ python simplemerge local base other && echo "merge succeeded" merge succeeded $ cat local local base other $ cp orig local printing to stdout $ python simplemerge -p local base other local base other local: $ cat local local base conflicts $ cp base conflict-local $ cp other conflict-other $ echo not other >> conflict-local $ echo end >> conflict-local $ echo end >> conflict-other $ python simplemerge -p conflict-local base conflict-other base <<<<<<< conflict-local not other end ======= other end >>>>>>> conflict-other [1] 1 label $ python simplemerge -p -L foo conflict-local base conflict-other base <<<<<<< foo not other end ======= other end >>>>>>> conflict-other [1] 2 labels $ python simplemerge -p -L foo -L bar conflict-local base conflict-other base <<<<<<< foo not other end ======= other end >>>>>>> bar [1] 3 labels $ python simplemerge -p -L foo -L bar -L base conflict-local base conflict-other base <<<<<<< foo not other end ||||||| base ======= other end >>>>>>> bar [1] too many labels $ python simplemerge -p -L foo -L bar -L baz -L buz conflict-local base conflict-other abort: can only specify three labels. [255] binary file $ $PYTHON -c "f = file('binary-local', 'w'); f.write('\x00'); f.close()" $ cat orig >> binary-local $ python simplemerge -p binary-local base other warning: binary-local looks like a binary file. [1] binary file --text $ python simplemerge -a -p binary-local base other 2>&1 warning: binary-local looks like a binary file. \x00local (esc) base other help $ python simplemerge --help simplemerge [OPTS] LOCAL BASE OTHER Simple three-way file merge utility with a minimal feature set. Apply to LOCAL the changes necessary to go from BASE to OTHER. By default, LOCAL is overwritten with the results of this operation. options: -L --label labels to use on conflict markers -a --text treat all files as text -p --print print results instead of overwriting LOCAL --no-minimal no effect (DEPRECATED) -h --help display help and exit -q --quiet suppress output wrong number of arguments $ python simplemerge simplemerge: wrong number of arguments simplemerge [OPTS] LOCAL BASE OTHER Simple three-way file merge utility with a minimal feature set. Apply to LOCAL the changes necessary to go from BASE to OTHER. By default, LOCAL is overwritten with the results of this operation. options: -L --label labels to use on conflict markers -a --text treat all files as text -p --print print results instead of overwriting LOCAL --no-minimal no effect (DEPRECATED) -h --help display help and exit -q --quiet suppress output [1] bad option $ python simplemerge --foo -p local base other simplemerge: option --foo not recognized simplemerge [OPTS] LOCAL BASE OTHER Simple three-way file merge utility with a minimal feature set. Apply to LOCAL the changes necessary to go from BASE to OTHER. By default, LOCAL is overwritten with the results of this operation. options: -L --label labels to use on conflict markers -a --text treat all files as text -p --print print results instead of overwriting LOCAL --no-minimal no effect (DEPRECATED) -h --help display help and exit -q --quiet suppress output [1]