~bzr-pqm/bzr/bzr.dev

Advanced Topics

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

System Monitoring

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Capacity Planning Tips

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Clustering

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Multi-site Setups

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The "distributed" in distributed version control system should indicate that

Bazaar is  well suited for multi-site development situations and indeed, that

is the case.  The advantage comes from the ease and transparency of managing

merges between branches with divergent history.  Note that there are many,

many different ways to manage widely-flung development setups using Bazaar and

its branching and merging capabilities.  These can be discovered and tested

before being implemented as policy.  We will describe one such possible setup

here.

Consider ProjectX Corp's international expansion with a new branch office in

Darwin, Australia, in addition to the company's headquarters in Austin, Texas,

USA.  One of the difficulties of a far-flung multi-site development

environment such as this is that the network connection between Australia and

Texas is slow and unreliable.  So, each branch office would like the master

branch to be local to them.  (In situations with good network connectivity, a

local branch bound to the remote master may be all that is needed to support

multi-site development.)

Of course, with two master branches, there is always the question of which one

is authoritative.  Given Bazaar's facility at managing multiple branches, we

suggest that it is best not to privilege either the Texas or Australia

branches, but to merge both of them into a separate master branch (which may

reside at either site).  For definiteness, we will locate the master branch at

the Texas site.  So, we will have three branches stored on two servers:

trunk and texas-integration at the Texas site and australia-integration at the

Darwin site.  These branches are named in terms of the sites where the 

development takes place, but in many cases it may make more sense to name

branches after the functional teams rather their geographical locations.

Since we are trying illustrate the issues with multi-*site* development, we

will persist in this naming scheme.

Setup

~~~~~

Using our previous setup at the Texas site, we will simply rename the old

trunk branch as trunk and branch a copy as texas-integration.

::

  $ cd /srv/bzr/projectx

  $ mv trunk trunk              # can simply rename on the filesystem

  $ bzr branch trunk texas-integration   # very fast in a shared repository

In Australia, we need to set up the ``/srv/bzr/projectx`` directory and get a

copy of the current trunk as australia-integration::

  $ mkdir -p /srv/bzr

  $ cd /srv/bzr

  $ bzr init-repo --no-trees projectx

  $ cd projectx

  $ bzr branch bzr+ssh://server.example.com/srv/bzr/trunk

  $ bzr branch trunk australia-integration

Merging to master

~~~~~~~~~~~~~~~~~

Then, each office works with their local copy of the trunk.  At some point,

sooner or later depending on the pace of development in the two locations, the

two local trunks need to be merged.  (In general, sooner beats later when

merging, since there is no penalty for multiple merges.)  In this example,

Alice at the Texas office will do the merging on her local machine using

branches on the server::

  # Get a copy of the Australia branch in Texas.  After the initial branch

  # command, use pull to keep the branch up to date.  With a slow network,

  # this is the only slow part

  $ bzr branch bzr+ssh://autralia.example.com/srv/bzr/projectx/australia-integration \

    bzr+ssh://server.example.com/srv/bzr/projectx/australia-integration

  # Check out the master branch locally for doing the merge

  $ cd ~/projectx

  $ bzr checkout bzr+ssh://server.example.com/srv/bzr/projectx/trunk

  $ cd trunk

  $ bzr merge bzr+ssh://server.example.com/srv/bzr/projectx/texas-integration

  # Run the test suite and resolve any conflicts

  $ bzr commit -m "Merge Texas branch to master"

  # Now, merge from Australia using the local copy of that branch

  $ bzr merge bzr+ssh://server.example.com/srv/bzr/projectx/australia-integration

  # Run the test suite and resolve any conflicts between the two offices

  $ bzr commit -m "Merge Australia branch to master"

Note that Bazaar does not commit even cleanly applied merges by default.  This

is because although a merge may apply cleanly, the merged state still needs to

be checked before it is committed.  (Just because there are no text conflicts

does not mean that everything will work after a merge.)  An alternative that 

can pull when possible and merge otherwise is available with 

``bzr merge --pull``.

Merging back to local trunks

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now the trunk branch is the most up-to-date version of the software and

both of the local trunks need to reincorporate the changes from the master.

If no new commits have been made to texas-integration, then that can happen using

``bzr pull``::

  $ cd ~/projectx

  $ bzr checkout bzr+ssh://server.example.com/srv/bzr/projectx/texas-integration

  $ cd texas-integration

  $ bzr pull ../trunk  # Use trunk from the local disk

                              # No need to commit

If new changes have happened on texas-integration since the integration with

trunk, then the above pull will produce an error suggesting to use

merge::

  $ bzr merge ../trunk

  # Run test suite, resolve conflicts

  $ bzr commit -m "Merging Australian changes"

In Australia, they will need to update their local copy of trunk::

  $ cd /srv/bzr/projectx/trunk

  $ bzr pull     # parent location is used by default

Then, they need to pull or merge the changes from trunk into the local trunk.

This should be done by a developer with a checkout of australia-integration so

that they can run the test suite::

  $ cd ~/projectx

  $ bzr co bzr+ssh://australia.example.com/srv/bzr/projectx/australia-integration

  $ cd australia-integration

  $ bzr merge bzr+ssh://australia.example.com/srv/bzr/projectx/trunk

  # Run test suite and integrate Texan changes with only recent local

  # development

  $ bzr commit -m "Integrate work from Texas"

Other Considerations

~~~~~~~~~~~~~~~~~~~~

Multi-site deployments can be complicated, due to the many possible variations

of development velocity, divisions of labor, network connectivity, resources

for integration, etc.  The preceding description is meant to be one possible

way to do fairly symmetric multi-site development.  (Neither Texas or

Australia is privileged in this structure.)  In a situation where there is one

main site and other smaller sites, one of the local trunk branches can be

eliminated and trunk can be used directly for development at the main

site.

It is also up to the particular situation how frequently the local trunks are

integrated into the master trunk.  Given resources specifically for

integration, it is conceivable that a developer may be constantly responsible

for integrating changes from the two teams.  Alternatively, the two sites

could work on well-separated, well-defined features and merge to the master

trunk only when their respective features are complete.  Given the difficulty

of resolving conflicts in very large merges and the ease of merge handling in

Bazaar, we suggest that merges be done more frequently, rather than less.