Mercurial > hg
view tests/test-obsolete-changeset-exchange.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 | fab9dda0f2a3 |
children | f20533623833 |
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Test changesets filtering during exchanges (some tests are still in test-obsolete.t) $ cat >> $HGRCPATH << EOF > [experimental] > evolution=createmarkers > EOF Push does not corrupt remote ---------------------------- Create a DAG where a changeset reuses a revision from a file first used in an extinct changeset. $ hg init local $ cd local $ echo 'base' > base $ hg commit -Am base adding base $ echo 'A' > A $ hg commit -Am A adding A $ hg up 0 0 files updated, 0 files merged, 1 files removed, 0 files unresolved $ hg revert -ar 1 adding A $ hg commit -Am "A'" created new head $ hg log -G --template='{desc} {node}' @ A' f89bcc95eba5174b1ccc3e33a82e84c96e8338ee | | o A 9d73aac1b2ed7d53835eaeec212ed41ea47da53a |/ o base d20a80d4def38df63a4b330b7fb688f3d4cae1e3 $ hg debugobsolete 9d73aac1b2ed7d53835eaeec212ed41ea47da53a f89bcc95eba5174b1ccc3e33a82e84c96e8338ee Push it. The bundle should not refer to the extinct changeset. $ hg init ../other $ hg push ../other pushing to ../other searching for changes adding changesets adding manifests adding file changes added 2 changesets with 2 changes to 2 files $ hg -R ../other verify checking changesets checking manifests crosschecking files in changesets and manifests checking files 2 files, 2 changesets, 2 total revisions Adding a changeset going extinct locally ------------------------------------------ Pull a changeset that will immediatly goes extinct (because you already have a marker to obsolete him) (test resolution of issue3788) $ hg phase --draft --force f89bcc95eba5 $ hg phase -R ../other --draft --force f89bcc95eba5 $ hg commit --amend -m "A''" $ hg --hidden --config extensions.mq= strip --no-backup f89bcc95eba5 $ hg pull ../other pulling from ../other searching for changes adding changesets adding manifests adding file changes added 1 changesets with 0 changes to 1 files (+1 heads) (run 'hg heads' to see heads, 'hg merge' to merge) check that bundle is not affected $ hg bundle --hidden --rev f89bcc95eba5 --base "f89bcc95eba5^" ../f89bcc95eba5.hg 1 changesets found $ hg --hidden --config extensions.mq= strip --no-backup f89bcc95eba5 $ hg unbundle ../f89bcc95eba5.hg adding changesets adding manifests adding file changes added 1 changesets with 0 changes to 1 files (+1 heads) (run 'hg heads' to see heads)