概要

一種專案開發流程：

（1）各開發成員建立自己的分支，並在此分支上開發。
（2）各開發成員把分支合併到主幹上，形成較為穩定的版本。
（3）各個成員重新從主幹上建立新的分支，回到第一步。
（4）迴圈，直到工程結束。

分支

（一）分支（branch）的定義

 A line of development that exists independently of another line, yet still shares a common history
if you look far enough back in time. A branch always begins life as a copy of someting, and moves
on from there, generating its own history. 

（二）為什麼要有分支

建立分支最大的目的是跟主線進行並行開發而不影響主線的開發。
This allows you to save your half-broken work frequently without interfering with others, yet you
can still selectively share information with your collaborators.
For projects that have a large number of contributors, it's common for most people to have working
copies of the trunk. Whenever someone needs to make a long-running change that is likely to
disrupt the trunk, a standard procedure is to create a private branch and commit changes there
until all the work is complete.
需要開發新的特性時，直接在主線trunk上開發可能導致其不穩定或不可用，這時可以建立分支來開發。 

（三）分支的生命週期

1. 何時建立？
For projects that have a large number of contributors, it's common for most people to have working
copies of the trunk. Whenever someone needs to make a long-running change that is likely to
disrupt the trunk, a standard procedure is to create a private branch and commit changes there
until all the work is complete.

2. 分支的維護
The good news it that you won't interfere with others. The bad news is that it's very easy to drift too
far apart. Remember that one of the problems with the "crawl in a hole" strategy is that by the time
you're finished with your branch, it may be near-impossible to merge your changes back into the
trunk without a huge number of conflicts.
Instead, you and other menbers can continue to share changes as you work. It's up to you to
decide which changes are worth sharing.Subversion gives you the ability to selective copy
changes between branches. And when you're completely finished with your branch, your entire
set of branch changes can be copied back into the trunk. 

（四）分支的建立

專案project1的檔案放在trunk目錄下，現在在branches目錄下建立分支zhangskd：
svn copy http://localhost/svn/project1/trunk http://localhost/svn/project1/branches/zhangskd \
-m "Creating a private branch of /project1/trunk."
顯示：Committed revision ***.
表示新建立的分支zhangskd的版本號為***。

合併

（一）合併（merge）的定義 

Instead of printing the differences to your terminal, it applies them directly to your working
copy as local modifications.

（二）合併的原理

A better name for the command might have been svn diff-and-apply, because that's all that
happens: two repository trees are compared, and the differences are applied to a working
copy. 

svn merge有三個引數：
1）An initial repository tree (often called the left side of the comparsion)
2）An final repository tree (often called the right side of the comparsion)
3）A working copy to accept the differences as local changes (often called the target)

所以合併的格式：svn merge leftside rightside target 

為什麼merge應該叫diff-and-apply？
leftside和rightside分別是兩個版本樹，對應於兩個不同的版本，即[email protected]，如果它們的URL
相同，還可以表示為-r REV1：REV2 URL。可以用diff來檢視這兩個版本樹的區別：
svn diff [email protected] [email protected]
svn diff -r REV1 : REV2 URL
svn diff leftside rightside ，假設得到的修改集B。
target是要合併修改集B的分支。假設target在上次合併後，又產生了本地修改集A。
那麼本次合併要做的事情就是：把修改集B也當成本地修改集，應用到target上！

1）如果修改集A和修改集B不重疊，可以順利合併。此時target和上次合併時的版本相比，就有了
兩個本地修改集A和B，commit後可以得到新的版本。

2）如果修改集A和B有重疊，會產生衝突。這個時候就需要解決衝突後，才能順利合併。
這個時候會產生三個檔案：filename.working，filename.left，filename.right。這三個檔案
和svn update檢測到衝突後產生的三個檔案有著相同的作用。 

當本地修改集A為空的時候，不會有衝突出現。
To begin with, assume that your working copy has no local edits. When you svn update to a
particular revision, the changes set by the server will always apply "cleanly" to your working
copy. The server produces the delta by comparing two trees: a virtual snapshot of your
working copy, and the revision tree you're interested in. Because the lefthand side of the
comparision is exactly equal to what you already have, the delta is guaranteed to correctly
convert your working copy into the righthand tree. 

（三）分支的合併

合併分支的時候，需要指定要一個目標，用來接收這些修改集，如果沒有，則預設：
svn merge requires a working-copy path as a target, i.e. a place where it should apply the
tree-changes. If the target isn't specified, it assumes you are trying to perform one of the
following common operations:
1）You want to merge directory changes into your current working directory.
2）You want to merge the changes in a specific file into a file by the same name which
exists in your current working directory. 

和取消本地修改一樣，可以用svn revert來取消合併。
可以用svn delete來刪除分支。

（四）合併的最佳使用方式

1）手動記錄合併
主線和分支都在不斷的變化著，當我們要把主線的一些特性合併到分支上，應該從哪個版本開始合併？
假設分支的建立版本為10，且它已經合併主線版本10~20的變化了，這個時候應該從主線版本21開始
合併，才不會導致重複合併。
因為SVN並不會自動為我們記錄合併的資訊，所以我們需要手動記錄。
當每次提交合並時，可以在日誌中說明本次合併的範圍，比如此次分支合併了主線的哪些版本。
這樣，當下次分支要合併主線的變化時，就可以知道要從主線的哪個版本開始。

2）預覽合併
可以用--dry-run和svn status來預覽合併，提前知道本次合併是否會產生衝突。
例 svn merge --dry-run -r 10 : 20 http://192.168.1.1/svn/project/trunk
      svn status
      # nothing printed, working copy is still unchanged, no conflict will happen

The --dry-run option doesn't apply any local changes to the working copy. It only shows status
code that would be printed in a real merge.

3）常用分支模式
3.1) 釋出分支
釋出分支(Release Branch)指的是建立分支主要為了釋出新的版本，其過程如下：
開發者提交所有的新特性到主幹，在主幹上開發。
當小組認為軟體已經做好釋出準備(1.0版)，主幹/trunk被拷貝到分支/branches/1.0。
專案組並行工作：一個小組對分支進行測試，另一小組在主幹上繼續新的工作(2.0)。如果一個bug
在任何位置被發現，錯誤修正則需來回合併。
當測試結束時，分支作為標籤釋出，/branches/1.0拷貝到/tags/1.0.0，這個標籤被打包釋出給客戶。
繼續在/trunk上為版本2.0工作，bug修正繼續從/trunk運送到/branches/1.0，如果積累了足夠的bug
修正，則可以釋出1.0.1版本：拷貝/branches/1.0到/tags/1.0.1，標籤被打包釋出。
當2.0完成時，重複上述測試、打標籤、釋出。

3.2) 特徵分支
特徵分支(Feature Branch)主要用於開發新的特性，同時不影響主線的穩定性和可用性。
Unlike release branches (which may need to be supported forever), feature branches are born,
used for a while, merged back to the trunk, then ultimately deleted. They have a finite span of
usefulness.
要注意保持特徵分支同步於主幹，因為在一個分支上工作數週或幾個月有很大風險，這個時候主幹
和分支可能會有大量的衝突！
This situation is best avoided by regularly merging trunk changes to the branch. Make up a
policy: once a week, merge the last week's worth of trunk changes to the branch.

Author

zhangskd @ csdn

Reference

svn book