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MagicDraw 2021x Documentation


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The 3-way merge is the default method used to merge selected Teamwork Cloud projects; the ancestor version is calculated automatically depending on the selected ancestor calculation algorithm. Once the correct ancestor for the two selected project versions is calculated, the Merge dialog opens.

You need to start the model merge from the Tools (Tools > Project Merge) menu to be able to change the ancestor selection (Lowest Common Parent or Branch Point) for the model merge. By default, Lowest Common Parent is considered an ancestor of the selected project versions.

Lowest Common Parent (Default)

If Lowest Common Parent is selected, the lowest common version for both the Target and Source is considered an ancestor. The lowest common parent of two revisions of the Source and Target in a directed acyclic graph is the nearest revision that has both the Target and Source as descendants. The algorithm considers both the direct parent path and the merged path when traversing and searching for the lowest common parent. In other words, during the repeated merge, the Contributor from the Source that was participating in the last merge is considered to be the common ancestor.

As shown in the image below, version #1 is considered to be a common parent during the first merge between versions #4 and #5. During the second merge between version #7 and #8, we traverse graphs back until we find the common parent for #7 and #8, which, in this case, is #4.

Visual representation of the model merge when Lowest Common Parent is considered an ancestor

Branch Point

If Branch Point is selected, the version from which the branches separate is considered an ancestor. As shown in the image below, version #1 is considered an ancestor in the first (version #4 is merged with version #3) and subsequent merges (version #6 is merged with version #5).  

Visual representation of the model merge when Branch Point is considered an ancestor

Both ancestor calculation algorithms work in an identical way during the very first merge of the selected branches because the same ancestor is taken during the very first merge, no matter if the Branch Point or the Lowest Common Parent is specified. However, it is crucial to understand the importance of the ancestor selection during subsequent merges because the changes may differ greatly from those you expect to see (see examples below).


Repetitive merge

Lowest Common Parent

During the repeated merge, the Lowest Common Parent algorithm always chooses the latest merged Source version as the ancestor. Consequently, you do not need to review historical decisions made during previous merges.

Example #1

You create a Class Machine in the Source branch and merge it with the Target (merge version #3 with version #4). Then continue your development process in the Source and rename Machine to Washing Machine. When you merge these two branches for the second time (merge version #6 with version #5), the algorithm recognizes that Machine was already created in the Target and Source, and, therefore, the only change that needs to be merged is renaming the Class Machine to Washing Machine

On the other hand, the Lowest Common Parent calculation algorithm may cause confusion, since rejections from the Source branch are considered to be the inverted changes in the Target branch. For example, the created Class that was previously rejected in the Source will appear as a deletion in the Target. As a result, it may seem that some changes that are shown have never been made in the project.

Example #2

You create two packages Requirements and Design in your development branch (Source). You decide to do the merge but you know that the Design package is incomplete. Therefore, you reject the addition of this particular package and continue your merge with just the Requirements package. You continue your work with the Design of your system in the Source branch. When you are done, you decide to merge this package into the Target branch (merge version #6 with version #5). However, during the merge, you may notice that this Design package is shown as deleted in the Target (because it does not exist in the Target but exists in the selected ancestor). In this case, you have to go through the Target changes and reject the deletion of this package to be able to add it.

Branch Point

During the repeated merge, the branch point always compares the two branches as if they were developed completely separately and were never merged before. As a consequence, you can easily understand what changes have been made in the Source and Target since the branch creation. This eliminates any confusion that may occur in the case of the Lowest Common Parent algorithm and the rejected changes (see above).

Example #3

You create two packages (Requirements and Design) in your development branch (Source). You decide to do the merge but you know that the Design package is incomplete. Therefore, you reject the addition of that particular package and continue your merge with just the Requirements package. Then you continue your work with the Design of your system in the Source branch. When you are done, you decide to merge this package into the Target branch. During the merge, you will see that the Design package is added and you will just carry on to merge it.

The default filter hides all the equivalent changes from Target and Source. Therefore, the creation of the Requirements package is hidden by default. 

However, this algorithm has other drawbacks that can emerge if the elements that were already merged are edited.

Example #4

You create a class Machine in the Source and merge it to the Target. Then you continue your work in the Source branch and decide to rename it to Washing Machine. If the Branch Point is specified as an ancestor, you have to solve the conflict of whether you want to keep the addition of a Class Washing Machine or Machine, because neither of these Classes existed in the branch point (the version from which the branch was created). This may require a lot of manual work since you will have to make sure that the conflicting changes are resolved in the desired direction.

Set as Latest

Lowest Common Parent

If an earlier version of the project is set to the latest version in the Target branch, e.g., version #8 is reverted to version #7 (version #9 is created), the ancestor is calculated taking the version that was set as the latest (version #7). If there is a preceding merge (version #3 is merged with version #4), the Source version from the last merge (version #3) is considered the ancestor; however, if no merges precede the version that was set as the latest, the version where the branch splits off the trunk (version #1) is considered the ancestor. 

Example #5

Branch Point

If an earlier version of the project is set to the latest version in the Target branch, e.g., version #8 is reverted to version #7 (version #9 was created), and then the merge of the two selected project versions occurs, the version from which the branches separate (i.e., a branch point) is considered an ancestor (version #1 in the image above). 

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