Git Branch Divergence Calculation

Copado Divergence Calculation

A git branch divergence consists of the difference between the current state of two branches and the list of changes (commits) performed to each branch from the common state to get the current divergence (commits ahead and behind). This definition accepts two different approaches, the common state can be taken as the state where the branches start to differ, that means where the first commit ahead/behind happens, or the last common state where the branches were in sync (known as merge base).

In Copado the second approach is used, the branch divergence is calculated from the merge base (last common state) including only those commits chronologically placed between that point and the current head as the ahead and behind differences. This approach is from performance view much more efficient than the first one as only the history from the merge base is needed and this means to Copado to execute all the git operations in a much more efficiently way.

Let’s analyze the example above to understand how it works and the difference with the other approach used by Github and other git providers. 

As we can see in the picture we have in this case one promotion branch with only one merge commit from a feature branch with 3 commits. The merge base between the promotion branch and the environment branch is A2 while the feature branch is merged from the commit A1 placed before in the git history. If we execute a git log command to see the history all this commits in the graph will be chronologically sorted resulting in the history line on the right in the picture.

With the first approach A1 will be the common state used to obtain the ahead or behind differences what means to include the commits C1 and C2  to the list of divergence commits, in this case we would have 5 commits ahead or behind. Note that to find the commit A1 the whole git history is needed what means time consumption to retrieve commits what are not needed at all to see a correct state difference. 

On the other hand if we use the second approach the merge base A2 will be used to obtain the differences and only the commits C3 B1 and A3 will be included to the list of ahead or behind commits, the result would be then only 3 commits instead of the 5 from the first approach. In this case only the history until merge base is needed what means less time for the calculation and working in a more efficiently manner. 

The divergence obtained as result is correct in both cases because no matter the way we follow to reach the head from a common state the resulting divergence will be the same as we can see in the picture, the only difference is that the number of commits will be greater if we take an older common state as point of reference.