SVN Tagging – Substantial Problems and Solutions

svntagging

A common practice for SVN tags is to tag different releases in order to be able to find them with ease later. As I understand it, tagging is the same thing as branching, that is in two cases, it just creates a copy of the trunk in a separate directory (would it be branches/something or tags/1.2.3), which means that:

There is no impact on the amount of data stored by SVN itself (excluding a few bytes which indicate that a specific tag corresponds to a specific revision). If the dump was 800 MB without the tag, it will still be 800 MB plus a few bytes with the new tag.
Files are duplicated on disk when checked out. A project which takes 30 MB on disk will take 60 MB once the tag is created.

It means that:

Once I create a tag, every developer of my team will get a lot of data from SVN the next time he will update.
With dozens or hundreds of tags (like it's usually the case when tagging every release), checkouts can take hours even for a small project.

Isn't that problematic? Couldn't it implemented differently, for instance through svn propedit? Are there valid cases where it makes sense to checkout every tag, instead of just one?

Note: as someone who uses continuous deployment consistently, I never used tags in practice, since every commit is potentially deployed in production if it passes the tests. This means that I may have missed something fundamental about tags or their internals, and I'm sorry if this is the case. My interest towards tags rises from the fact that I spent the last two hours checking out a third-party small project which has over two hundred tags.

Best Answer

Why would it be a problem over having trunk and branches? A tag is just another branch, but (and perhaps this is the important bit) you do not have to checkout the entire SVN repo.

Instead of calling svn co http://repo/ call svn co http://repo/trunk Then, if you want to see a particular branch use svn switch.

Everyone I know who uses the trunk/tags/branches repo setup will checkout trunk or a branch as the root. Doing this, they will never see the other branches or tags unless they use svn switch to swap the view of the repo to that root.

What you describe is even more a problem with git as it checks out the entire repo and then presents you with a view of it based on a single branch or master. All the files are present (but hidden 'underneath' the current view), so technically SVN is superior to git in this regard (ie being able to partially checkout a fraction of a repo is particularly good when you multi-gigabyte repositories).

You don't need to use tags if you don't want to, they are only a marker in the repo anyway, being a cheap copy of the trunk that you have tagged. You could, if you preferred, remember the revision number (as this is a globally unique value) for the revision you tagged, but there is no way in SVN to mark a revision number with some human-readable moniker.

Related Solutions

Version Control – Creating a Version Control Strategy for SVN

If you want a unified build process, then be sure to put branches/tags/trunk at the root, like this:

branches/
tags/
trunk/
  dev/
    ...

If you don't need a unified build process, then you can put branches/tags/trunks within each project if you want. However, it might be difficult to migrate to a unified build after having put them within each project. A unified build has advantages, such as eliminating the need to publish shared components among projects -- they're all part of the build.

Personally, I like a unified build process. Furthermore, I don't think you should have a "dev" project. You should have projects directly under trunk, and then branch trunk into a dev branch. Use tags for releases. For example, I would do it like this:

branches/
  dev/
    Site1/
    Site2/
    WebService/
    SharedCode/
tags/
  release1/
    Site1/
    Site2/
    WebService/
    SharedCode/
trunk/
  Site1/
  Site2/
  WebService/
  SharedCode/

SVN Tagging – Tags or Specify Revision?

It depends on how cleanly all these elements are separated.

In order to put clarification on how exactly i see things - i am trying to put general

As you highlighted you can deal with this in two ways -

A. Single Product Approach
The entire space is one projects and while they are fairly independent - the repo doesn't recognize it.

In this case few things can be observed.

The same release number of any RC is applied to all object products. So if the repo moves from 1.2 to 1.3 via 1.2.1 and 1.2.3 - there is a corresponding revision of each element i.e. project1.1.2.1.so as well as project2.1.2.1.so.
Version number of tags of all projects and application has the same tag.
Between two versions of a particular singular repo, whether the projectA or projectB has any changes or not, the tag keeps advancing as per total progress.
In such case, when you take a branch from any point in trunk, the entire trunk must be mirrored.
Assuming that testing has been good, all projects and application with same version/tag are always perfectly compatible with each other.

B. Independent Product approach
Here each project is an independent product -

They must have different repo (or at least separate trunks for themselves).
The tagging is limited to the projects individually each part of repo will produce end product like - projectA.1.2.1.so OR projectB.1.5.lib
In such cases - when you branch - the branch is always only for the given project and any tags before/after the merge only applies to those projects.
Each project can have independent number of tags depending on the actual number of bugs within it.
In this case each different project advances with different amount - you always need to find which set of tags - for each other works compatibly with other projects. For example - projectA-1.2 works for projectB-1.2 all the way upto projectB-2.3 but breaks after projectB-2.5 or higher. Such nested dependencies is what you need to now manage through explicitly integration testing.

So what are the pros and cons?

In a simple terms - the single product approach is clumsy because all elements are copied when a branch is created. Now if before committing if someone changes something on another person's module it becomes a conflict and that too after some significant time in future when other person decides to commit/merge. This problem essentially grows as the number of people grow in the team.
One of the primary reason why single product approach helps is the simplicity. Ideally if you have a single authority for release management - every full release is not only tested to see that pieces work individually but also that all versions work together. The integration testing is always more painful when independent products grow and you need to keep a tab on multiple versions of each different products that works together and that is the job by itself.
Support cost increases in case of multiple product situation. This is understood in this way - suppose projectB-1.2 and projectB-1.7 depends on different compatible versions of projectA-0.8 and projectA-0.9. Now, if the projectA has got a bug which could have been affecting all its' clients (irrespective of compatibility) it has to be fixed twice - both in projectA-0.8 and projectA-0.9. This doubles the effort on bug support for projectA.
Further suppose if implementations in projectA-0.8 is found to be a bug (say already solved in projectA-0.9) but you realize that solution to this bug under 0.8 will help part of its clients, but break others; in this case projectA-0.8 needs to be further branched out separately to see that now both groups are supported.
Last but most important - when people work in independent systems the communication between them reduces. They establish their work based on their individual tests; later on when the application folk needs to put in all the modules together, they have to put effort to bring pieces together, you realize a lot of issues of integration and further, if the responsibility of the bugs are not appropriately fixed based on root cause, system will tend to accumulate tech debt over time.

In a nutshell the more break-up you do in terms of a projects/modules, more of the nesting dependencies issues arise.

However this complexity is worth it if the size of the repo is too large. In a single repo, everyday people may end-up stuck dealing with conflicts or mergers.

One of the practical solution
In our place this has been a long time debate with all of us.

A simple approach we try to follow is as follows. Initially we all start with a single product approach. As we realize that as APIs become more stable, and certain key objects becomes fairly stable, we remove them slowly to another repo so that it becomes black-box for everyone. This approaches balances the initial need to remain in close communication and integration, and longer term need to keep repo manageable.

Sorry, i wrote too much?

NOTE: From your question - i felt that your current practice and proposed one are both slightly different from how i described; however, i have used the above scenarios since they are well outlined methods so easier to follow.

Best Answer

Related Solutions

Version Control – Creating a Version Control Strategy for SVN

SVN Tagging – Tags or Specify Revision?

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