view tests/test-bundle2-multiple-changegroups.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.

Create an extension to test bundle2 with multiple changegroups

  $ cat > bundle2.py <<EOF
  > """
  > """
  > from mercurial import changegroup, exchange
  > 
  > def _getbundlechangegrouppart(bundler, repo, source, bundlecaps=None,
  >                               b2caps=None, heads=None, common=None,
  >                               **kwargs):
  >     # Create two changegroups given the common changesets and heads for the
  >     # changegroup part we are being requested. Use the parent of each head
  >     # in 'heads' as intermediate heads for the first changegroup.
  >     intermediates = [repo[r].p1().node() for r in heads]
  >     cg = changegroup.getchangegroup(repo, source, heads=intermediates,
  >                                      common=common, bundlecaps=bundlecaps)
  >     bundler.newpart('output', data='changegroup1')
  >     bundler.newpart('changegroup', data=cg.getchunks())
  >     cg = changegroup.getchangegroup(repo, source, heads=heads,
  >                                      common=common + intermediates,
  >                                      bundlecaps=bundlecaps)
  >     bundler.newpart('output', data='changegroup2')
  >     bundler.newpart('changegroup', data=cg.getchunks())
  > 
  > def _pull(repo, *args, **kwargs):
  >   pullop = _orig_pull(repo, *args, **kwargs)
  >   repo.ui.write('pullop.cgresult is %d\n' % pullop.cgresult)
  >   return pullop
  > 
  > _orig_pull = exchange.pull
  > exchange.pull = _pull
  > exchange.getbundle2partsmapping['changegroup'] = _getbundlechangegrouppart
  > EOF

  $ cat >> $HGRCPATH << EOF
  > [experimental]
  > bundle2-exp=True
  > [ui]
  > logtemplate={rev}:{node|short} {phase} {author} {bookmarks} {desc|firstline}
  > EOF

Start with a simple repository with a single commit

  $ hg init repo
  $ cd repo
  $ cat > .hg/hgrc << EOF
  > [extensions]
  > bundle2=$TESTTMP/bundle2.py
  > EOF

  $ echo A > A
  $ hg commit -A -m A -q
  $ cd ..

Clone

  $ hg clone -q repo clone

Add two linear commits

  $ cd repo
  $ echo B > B
  $ hg commit -A -m B -q
  $ echo C > C
  $ hg commit -A -m C -q

  $ cd ../clone
  $ cat >> .hg/hgrc <<EOF
  > [hooks]
  > pretxnchangegroup = sh -c "printenv.py pretxnchangegroup"
  > changegroup = sh -c "printenv.py changegroup"
  > incoming = sh -c "printenv.py incoming"
  > EOF

Pull the new commits in the clone

  $ hg pull
  pulling from $TESTTMP/repo (glob)
  searching for changes
  remote: changegroup1
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  pretxnchangegroup hook: HG_NODE=27547f69f25460a52fff66ad004e58da7ad3fb56 HG_PENDING=$TESTTMP/clone HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  remote: changegroup2
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  pretxnchangegroup hook: HG_NODE=f838bfaca5c7226600ebcfd84f3c3c13a28d3757 HG_PENDING=$TESTTMP/clone HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  changegroup hook: HG_NODE=27547f69f25460a52fff66ad004e58da7ad3fb56 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=27547f69f25460a52fff66ad004e58da7ad3fb56 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  changegroup hook: HG_NODE=f838bfaca5c7226600ebcfd84f3c3c13a28d3757 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=f838bfaca5c7226600ebcfd84f3c3c13a28d3757 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  pullop.cgresult is 1
  (run 'hg update' to get a working copy)
  $ hg update
  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg log -G
  @  2:f838bfaca5c7 public test  C
  |
  o  1:27547f69f254 public test  B
  |
  o  0:4a2df7238c3b public test  A
  
Add more changesets with multiple heads to the original repository

  $ cd ../repo
  $ echo D > D
  $ hg commit -A -m D -q
  $ hg up -r 1
  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
  $ echo E > E
  $ hg commit -A -m E -q
  $ echo F > F
  $ hg commit -A -m F -q
  $ hg up -r 1
  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
  $ echo G > G
  $ hg commit -A -m G -q
  $ hg up -r 3
  2 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo H > H
  $ hg commit -A -m H -q
  $ hg log -G
  @  7:5cd59d311f65 draft test  H
  |
  | o  6:1d14c3ce6ac0 draft test  G
  | |
  | | o  5:7f219660301f draft test  F
  | | |
  | | o  4:8a5212ebc852 draft test  E
  | |/
  o |  3:b3325c91a4d9 draft test  D
  | |
  o |  2:f838bfaca5c7 draft test  C
  |/
  o  1:27547f69f254 draft test  B
  |
  o  0:4a2df7238c3b draft test  A
  
New heads are reported during transfer and properly accounted for in
pullop.cgresult

  $ cd ../clone
  $ hg pull
  pulling from $TESTTMP/repo (glob)
  searching for changes
  remote: changegroup1
  adding changesets
  adding manifests
  adding file changes
  added 2 changesets with 2 changes to 2 files (+1 heads)
  pretxnchangegroup hook: HG_NODE=b3325c91a4d916bcc4cdc83ea3fe4ece46a42f6e HG_PENDING=$TESTTMP/clone HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  remote: changegroup2
  adding changesets
  adding manifests
  adding file changes
  added 3 changesets with 3 changes to 3 files (+1 heads)
  pretxnchangegroup hook: HG_NODE=7f219660301fe4c8a116f714df5e769695cc2b46 HG_PENDING=$TESTTMP/clone HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  changegroup hook: HG_NODE=b3325c91a4d916bcc4cdc83ea3fe4ece46a42f6e HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=b3325c91a4d916bcc4cdc83ea3fe4ece46a42f6e HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=8a5212ebc8527f9fb821601504794e3eb11a1ed3 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  changegroup hook: HG_NODE=7f219660301fe4c8a116f714df5e769695cc2b46 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=7f219660301fe4c8a116f714df5e769695cc2b46 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=1d14c3ce6ac0582d2809220d33e8cd7a696e0156 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=5cd59d311f6508b8e0ed28a266756c859419c9f1 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  pullop.cgresult is 3
  (run 'hg heads' to see heads, 'hg merge' to merge)
  $ hg log -G
  o  7:5cd59d311f65 public test  H
  |
  | o  6:1d14c3ce6ac0 public test  G
  | |
  | | o  5:7f219660301f public test  F
  | | |
  | | o  4:8a5212ebc852 public test  E
  | |/
  o |  3:b3325c91a4d9 public test  D
  | |
  @ |  2:f838bfaca5c7 public test  C
  |/
  o  1:27547f69f254 public test  B
  |
  o  0:4a2df7238c3b public test  A
  
Removing a head from the original repository by merging it

  $ cd ../repo
  $ hg merge -r 6 -q
  $ hg commit -m Merge
  $ echo I > I
  $ hg commit -A -m H -q
  $ hg log -G
  @  9:9d18e5bd9ab0 draft test  H
  |
  o    8:71bd7b46de72 draft test  Merge
  |\
  | o  7:5cd59d311f65 draft test  H
  | |
  o |  6:1d14c3ce6ac0 draft test  G
  | |
  | | o  5:7f219660301f draft test  F
  | | |
  +---o  4:8a5212ebc852 draft test  E
  | |
  | o  3:b3325c91a4d9 draft test  D
  | |
  | o  2:f838bfaca5c7 draft test  C
  |/
  o  1:27547f69f254 draft test  B
  |
  o  0:4a2df7238c3b draft test  A
  
Removed heads are reported during transfer and properly accounted for in
pullop.cgresult

  $ cd ../clone
  $ hg pull
  pulling from $TESTTMP/repo (glob)
  searching for changes
  remote: changegroup1
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 0 changes to 0 files (-1 heads)
  pretxnchangegroup hook: HG_NODE=71bd7b46de72e69a32455bf88d04757d542e6cf4 HG_PENDING=$TESTTMP/clone HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  remote: changegroup2
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  pretxnchangegroup hook: HG_NODE=9d18e5bd9ab09337802595d49f1dad0c98df4d84 HG_PENDING=$TESTTMP/clone HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  changegroup hook: HG_NODE=71bd7b46de72e69a32455bf88d04757d542e6cf4 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=71bd7b46de72e69a32455bf88d04757d542e6cf4 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  changegroup hook: HG_NODE=9d18e5bd9ab09337802595d49f1dad0c98df4d84 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  incoming hook: HG_NODE=9d18e5bd9ab09337802595d49f1dad0c98df4d84 HG_PHASES_MOVED=1 HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/repo (glob)
  pullop.cgresult is -2
  (run 'hg update' to get a working copy)
  $ hg log -G
  o  9:9d18e5bd9ab0 public test  H
  |
  o    8:71bd7b46de72 public test  Merge
  |\
  | o  7:5cd59d311f65 public test  H
  | |
  o |  6:1d14c3ce6ac0 public test  G
  | |
  | | o  5:7f219660301f public test  F
  | | |
  +---o  4:8a5212ebc852 public test  E
  | |
  | o  3:b3325c91a4d9 public test  D
  | |
  | @  2:f838bfaca5c7 public test  C
  |/
  o  1:27547f69f254 public test  B
  |
  o  0:4a2df7238c3b public test  A