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Git And Hadoop

A lot of people use Git with Hadoop because they have their own patches to make to Hadoop, and Git helps them manage it.

This page tells you how to work with Git. See HowToContribute for instructions on building and testing Hadoop.

Key Git Concepts

The key concepts of Git.

  • Git doesn't store changes, it snapshots the entire source tree. Good for fast switch and rollback, bad for binaries. (as an enhancement, if a file hasn't changed, it doesn't re-replicate it).
  • Git stores all "events" as SHA1 checksummed objects; you have deltas, tags and commits, where a commit describes the status of items in the tree.
  • Git is very branch centric; you work in your own branch off local or central repositories
  • You had better enjoy merging.

Checking out the source

You need a copy of git on your system. Some IDEs ship with Git support; this page assumes you are using the command line.

Clone a local Git repository from the Apache repository. The Hadoop subprojects (common, HDFS, and MapReduce) live inside a combined repository called hadoop.git.

git clone git://git.apache.org/hadoop.git

Committers: for read/write access use


The total download is a few hundred MB, so the initial checkout process works best when the network is fast. Once downloaded, Git works offline -though you will need to perform your initial builds online so that the build tools can download dependencies.

Grafts for complete project history

The Hadoop project has undergone some movement in where its component parts have been versioned. Because of that, commands like git log --follow needs to have a little help. To graft the history back together into a coherent whole, insert the following contents into hadoop/.git/info/grafts:

# Project split
5128a9a453d64bfe1ed978cf9ffed27985eeef36 6c16dc8cf2b28818c852e95302920a278d07ad0c
6a3ac690e493c7da45bbf2ae2054768c427fd0e1 6c16dc8cf2b28818c852e95302920a278d07ad0c
546d96754ffee3142bcbbf4563c624c053d0ed0d 6c16dc8cf2b28818c852e95302920a278d07ad0c
# Project un-split in new writable git repo
a196766ea07775f18ded69bd9e8d239f8cfd3ccc 928d485e2743115fe37f9d123ce9a635c5afb91a
cd66945f62635f589ff93468e94c0039684a8b6d 77f628ff5925c25ba2ee4ce14590789eb2e7b85b

You can then use commands like git blame --follow with success.

Migrating private branches to the new git commit history

The migration from svn to git changed the commit ids for anyone tracking the history of the project via the svn to git bridge. This means that private forks/branches will not rebase to the new versions. Follow the MigratingPrivateGitBranches instructions.

Forking onto GitHub

You can create your own fork of the ASF project, put in branches and stuff as you desire. GitHub prefer you to explicitly fork their copies of Hadoop.

  1. Create a GitHub login at http://github.com/ ; Add your public SSH keys

  2. Go to http://github.com/apache and search for the Hadoop and other Apache projects you want (avro is handy alongside the others)

  3. For each project, fork in the github UI. This gives you your own repository URL which you can then clone locally with git clone

  4. For each patch, branch.

At the time of writing (December 2009), GitHub was updating its copy of the Apache repositories every hour. As the Apache repositories were updating every 15 minutes, provided these frequencies are retained, a GitHub-fork derived version will be at worst 1 hour and 15 minutes behind the ASF's Git repository. If you are actively developing on Hadoop, especially committing code into the Git repository, that is too long -work off the Apache repositories instead.

  1. Clone the read-only repository from Github (their recommendation) or from Apache (the ASF's recommendation)
  2. in that clone, rename that repository "apache": git remote rename origin apache

  3. Log in to [http://github.com]

  4. Create a new repository (e.g hadoop-fork)
  5. In the existing clone, add the new repository :

    git remote add -f github git@github.com:MYUSERNAMEHERE/hadoop.git

This gives you a local repository with two remote repositories: "apache" and "github". Apache has the trunk branch, which you can update whenever you want to get the latest ASF version:

 git checkout trunk
 git pull apache

Your own branches can be merged with trunk, and pushed out to git hub. To generate patches for submitting as JIRA patches, check everything in to your specific branch, merge that with (a recently pulled) trunk, then diff the two:  git diff --no-prefix trunk > ../hadoop-patches/HADOOP-XYX.patch 

If you are working deep in the code it's not only convenient to have a directory full of patches to the JIRA issues, it's convenient to have that directory a git repository that is pushed to a remote server, such as this example. Why? It helps you move patches from machine to machine without having to do all the updating and merging. From a pure-git perspective this is wrong: it loses history, but for a mixed workflow it doesn't matter so much.


Git makes it easy to branch. The recommended process for working with Apache projects is: one branch per JIRA issue. That makes it easy to isolate development and track the development of each change. It does mean if you have your own branch that you release, one that merges in more than one issue, you have to invest some effort in merging everything in. Try not to make changes in different branches that are hard to merge, and learn your way round the git rebase command to handle changes across branches. Better yet: do not use rebase once you have created a chain of branches that each depend on each other

Creating the branch

Creating a branch is quick and easy

#start off in the apache trunk
git checkout trunk
#create a new branch from trunk
git branch HDFS-775
#switch to it
git checkout HDFS-775
#show what's branch you are in
git branch

Remember, this branch is local to your machine. Nobody else can see it until you push up your changes or generate a patch, or you make your machine visible over the network to interested parties.

Creating Patches for attachment to JIRA issues

Assuming your trunk repository is in sync with the Apache projects, you can use git diff to create a patch file. First, have a directory for your patches:

mkdir ../hadoop-patches

Then generate a patch file listing the differences between your trunk and your branch

git diff --no-prefix trunk > ../hadoop-patches/HDFS-775-1.patch

The patch file is an extended version of the unified patch format used by other tools; type git help diff to get more details on it. Here is what the patch file in this example looks like

cat ../outgoing/HDFS-775-1.patch
diff --git src/java/org/apache/hadoop/hdfs/server/datanode/FSDataset.java src/java/org/apache/hadoop/hdfs/server/datanode/FSDataset.java
index 42ba15e..6383239 100644
--- src/java/org/apache/hadoop/hdfs/server/datanode/FSDataset.java
+++ src/java/org/apache/hadoop/hdfs/server/datanode/FSDataset.java
@@ -355,12 +355,14 @@ public class FSDataset implements FSConstants, FSDatasetInterface {
       return dfsUsage.getUsed();

+    /**
+     * Calculate the capacity of the filesystem, after removing any
+     * reserved capacity.
+     * @return the unreserved number of bytes left in this filesystem. May be zero.
+     */
     long getCapacity() throws IOException {
-      if (reserved > usage.getCapacity()) {
-        return 0;
-      }
-      return usage.getCapacity()-reserved;
+      long remaining = usage.getCapacity() - reserved;
+      return remaining > 0 ? remaining : 0;

     long getAvailable() throws IOException {

It is essential that patches for JIRA issues are generated with the --no-prefix option. Without that an extra directory path is listed, and the patches can only be applied with a patch -p1 call, which Hudson does not know to do. If you want your patches to take, this is what you have to do. You can of course test this yourself by using a command like patch -p0 << ../outgoing/HDFS-775.1 in a copy of the Git source tree to test that your patch takes.

Updating your patch

If your patch is not immediately accepted, do not be offended: it happens to us all. It introduces a problem: your branches become out of date. You need to check out the latest apache version, merge your branches with it, and then push the changes back to github

 git checkout trunk
 git pull apache
 git checkout mybranch
 git merge trunk
 git push github mybranch

Your branch is up to date, and new diffs can be created and attached to patches.

Deriving Branches from Branches

If you have one patch that depends upon another, you should have a separate branch for each one. Simply merge the changes from the first branch into the second, so that it is always kept up to date with the first changes. To create a patch file for submission as a JIRA patch, do a diff between the two branches, not against trunk.

do not play with rebasing once you start doing this as you will make merging a nightmare

What to do when your patch is committed

Once your patch is committed into Git, you do not need the branch any more. You can delete it straight away, but it is safer to verify the patch is completely merged in

Pull down the latest release and verify that the patch branch is synchronized

 git checkout trunk
 git pull apache
 git checkout mybranch
 git merge trunk
 git diff trunk

the output of the last command should be nothing: the two branches should be identical. You can then prove to git that this is true by switching back to the trunk branch and merging in the branch, an operation which will not change the source tree, but update Git's branch graph.

 git checkout trunk
 git merge mybranch

Now you can delete the branch without being warned by git

 git branch -d mybranch

Finally, propagate that deletion to your private github repository

 git push github :mybranch

This odd syntax says "push nothing to github/mybranch".

GitAndHadoop (last edited 2015-05-15 08:10:22 by AkiraAjisaka)