Mercurial > hg
view tests/test-rollback.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 | e78a80f8f51e |
children | 4414d500604f |
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setup repo $ hg init t $ cd t $ echo a > a $ hg commit -Am'add a' adding a $ hg verify checking changesets checking manifests crosschecking files in changesets and manifests checking files 1 files, 1 changesets, 1 total revisions $ hg parents changeset: 0:1f0dee641bb7 tag: tip user: test date: Thu Jan 01 00:00:00 1970 +0000 summary: add a rollback to null revision $ hg status $ hg rollback repository tip rolled back to revision -1 (undo commit) working directory now based on revision -1 $ hg verify checking changesets checking manifests crosschecking files in changesets and manifests checking files 0 files, 0 changesets, 0 total revisions $ hg parents $ hg status A a Two changesets this time so we rollback to a real changeset $ hg commit -m'add a again' $ echo a >> a $ hg commit -m'modify a' Test issue 902 (current branch is preserved) $ hg branch test marked working directory as branch test (branches are permanent and global, did you want a bookmark?) $ hg rollback repository tip rolled back to revision 0 (undo commit) working directory now based on revision 0 $ hg branch default Test issue 1635 (commit message saved) $ cat .hg/last-message.txt ; echo modify a Test rollback of hg before issue 902 was fixed $ hg commit -m "test3" $ hg branch test marked working directory as branch test (branches are permanent and global, did you want a bookmark?) $ rm .hg/undo.branch $ hg rollback repository tip rolled back to revision 0 (undo commit) named branch could not be reset: current branch is still 'test' working directory now based on revision 0 $ hg branch test working dir unaffected by rollback: do not restore dirstate et. al. $ hg log --template '{rev} {branch} {desc|firstline}\n' 0 default add a again $ hg status M a $ hg bookmark foo $ hg commit -m'modify a again' $ echo b > b $ hg bookmark bar -r default #making bar active, before the transaction $ hg commit -Am'add b' adding b $ hg log --template '{rev} {branch} {desc|firstline}\n' 2 test add b 1 test modify a again 0 default add a again $ hg update bar 1 files updated, 0 files merged, 1 files removed, 0 files unresolved (activating bookmark bar) $ cat .hg/undo.branch ; echo test $ hg rollback -f repository tip rolled back to revision 1 (undo commit) $ hg id -n 0 $ hg branch default $ cat .hg/bookmarks.current ; echo bar $ hg bookmark --delete foo bar rollback by pretxncommit saves commit message (issue1635) $ echo a >> a $ hg --config hooks.pretxncommit=false commit -m"precious commit message" transaction abort! rollback completed abort: pretxncommit hook exited with status * (glob) [255] $ cat .hg/last-message.txt ; echo precious commit message same thing, but run $EDITOR $ cat > editor.sh << '__EOF__' > echo "another precious commit message" > "$1" > __EOF__ $ HGEDITOR="\"sh\" \"`pwd`/editor.sh\"" hg --config hooks.pretxncommit=false commit 2>&1 transaction abort! rollback completed note: commit message saved in .hg/last-message.txt abort: pretxncommit hook exited with status * (glob) [255] $ cat .hg/last-message.txt another precious commit message test rollback on served repository #if serve $ hg commit -m "precious commit message" $ hg serve -p $HGPORT -d --pid-file=hg.pid -A access.log -E errors.log $ cat hg.pid >> $DAEMON_PIDS $ cd .. $ hg clone http://localhost:$HGPORT u requesting all changes adding changesets adding manifests adding file changes added 3 changesets with 2 changes to 1 files (+1 heads) updating to branch default 1 files updated, 0 files merged, 0 files removed, 0 files unresolved $ cd u $ hg id default 068774709090 now rollback and observe that 'hg serve' reloads the repository and presents the correct tip changeset: $ hg -R ../t rollback repository tip rolled back to revision 1 (undo commit) working directory now based on revision 0 $ hg id default 791dd2169706 #endif update to older changeset and then refuse rollback, because that would lose data (issue2998) $ cd ../t $ hg -q update $ rm `hg status -un` $ template='{rev}:{node|short} [{branch}] {desc|firstline}\n' $ echo 'valuable new file' > b $ echo 'valuable modification' >> a $ hg commit -A -m'a valuable change' adding b $ hg update 0 1 files updated, 0 files merged, 1 files removed, 0 files unresolved $ hg rollback abort: rollback of last commit while not checked out may lose data (use -f to force) [255] $ hg tip -q 2:4d9cd3795eea $ hg rollback -f repository tip rolled back to revision 1 (undo commit) $ hg status $ hg log --removed b # yep, it's gone same again, but emulate an old client that doesn't write undo.desc $ hg -q update $ echo 'valuable modification redux' >> a $ hg commit -m'a valuable change redux' $ rm .hg/undo.desc $ hg update 0 1 files updated, 0 files merged, 0 files removed, 0 files unresolved $ hg rollback rolling back unknown transaction $ cat a a corrupt journal test $ echo "foo" > .hg/store/journal $ hg recover rolling back interrupted transaction couldn't read journal entry 'foo\n'! checking changesets checking manifests crosschecking files in changesets and manifests checking files 1 files, 2 changesets, 2 total revisions