Thursday, June 19, 2014

Revert a patch in most awesome way

If you are a Release Engineer, System Admin or Support Engineer you have definitely come across a requirement where you have to apply patches to the target systems be it production or non-production. I'm assuming that you are using some automated system to manage the patches i.e applying them and reverting them. In this blog I would be discussing about the standard way of patch management and how you can have an out of the box solution to revert your patch in most simplistic way and without much fuss. At the end of the blog I would like to see an expression where you will say what the hell it's so awesome yet so simple :).

People usually use some tool to apply patch to a target system which in addition to applying a patch also manage the history the patches so that it can be reverted in case the patch goes wrong. The patch history usually contains below details:
  1. The new files that were added in the patch, while reverting the patch those files should be deleted.
  2. The files that were deleted by the patch, while reverting the patch the deleted files should be restored back.
  3. The files that were modified by the patch, while reverting the patch the modified files should be restored back.

You can definitely create a tool that can revert the patch for you as the use cases are not much, but do you really need to put this much effort if you can have an out of the box solution for this. What if I tell you that we use git for managing our patch history and reverting them. As git comes with a local repository concept so we created a local git repository at our app server codebase location only. Git comes with all the file level tracking we map each patch with one git commit, so at the time of reverting a specific patch you can ask git to simply revert the commit for you.

Extra steps to be done after applying patch:
To make git track the changes done in patch, you just need to perform 2 extra commands

git add . : This command will track all the files that have been modififed, added or deleted in the system.
git commit -m "Applying Patch" : This command actually adds the files information tracked by previous command with a message in the git system


Steps to be done in reverting changes done by a patch:
Once you have all the information tracked in git it will become no-brainer to revert the patches.

To view the details of all the patches: You can use git log command that will provide you the list of all the patches that you have applied or reverts that you have done

sandy@sandy:~/test/app1$ git log
commit f622f1f97fc44f6897f9edc25f9c6aab8e425049
Author: sandy
Date:   Thu Jun 19 15:19:53 2014 +0530

    Patch 1 on release2

commit 9a1dd81c7799c2f83d897eed85914eecef304bf0
Author: sandy
Date:   Thu Jun 19 15:16:52 2014 +0530

    Release 2

commit 135e04c00b3c3d5bc868f7774a5f284c3eb8cb29
Author: sandy
Date:   Thu Jun 19 15:16:28 2014 +0530

    Release 1


Now Reverting a patch is as simple as executing a simple command git revert, with the commit id of the patch

git revert f622f1f97fc44f6897f9edc25f9c6aab8e425049
[master 0ba533f] q Revert "Patch 1 on release2"
 1 file changed, 1 deletion(-)


If you run git log command, you will see the patch revert history as well

sandy@sandy:~/test/app1$ git log
commit 0ba533fda95ed4d7fcf0b7e6b23cd1a5589946a7
Author: sandy
Date:   Thu Jun 19 15:20:24 2014 +0530

    Revert "Patch 1 on release2"
   
    This reverts commit f622f1f97fc44f6897f9edc25f9c6aab8e425049.

commit f622f1f97fc44f6897f9edc25f9c6aab8e425049
Author: sandy
Date:   Thu Jun 19 15:19:53 2014 +0530

    Patch 1 on release2

commit 9a1dd81c7799c2f83d897eed85914eecef304bf0
Author: sandy
Date:   Thu Jun 19 15:16:52 2014 +0530

    Release 2

commit 135e04c00b3c3d5bc868f7774a5f284c3eb8cb29
Author: sandy
Date:   Thu Jun 19 15:16:28 2014 +0530

    Release 1



I hope this blog has given you a very different perspective of managing the patches, let me know your thoughts about this. Also if you have such more ideas do share with me.