# Consistency¶

Because of the very nature of scientific research, it becomes indispensable to be able to both delete parts of a database (e.g., if errors are made, inputs are misspelled, or useless calculations are performed) or export it (for collaboration or publication purposes). Both these features, which are provided by AiiDA, have one aspect in common: they can easily lead to a provenance graph with incomplete information. To better understand why, let’s take a look at the following basic provenance graph:

Even in this simple case, if we were to export only the calculation node and the output data node (or, equivalently, delete just the input data node), then we would have lost part of the critical information needed to run the calculation (the D1 node), thus losing the reproducibility of the calculation C1. In this simple case, therefore, in order to have a consistent provenance, whenever you export a calculation node you must also import all of its input nodes (or, symmetrically, whenever you delete a data node you must also delete all calculations that used it as an input).

This is just one of the many rules that must be considered when trying to manually edit a provenance database. The key message to remember is that AiiDA will not only delete or export the nodes explicitly targeted by the user, but will also include any other nodes that are needed for keeping a consistent provenance in the resulting database.

It is also worth noting that if you do successive exports of partial information, AiiDA will be able to reconstruct links that might have been broken when dividing the data for export. So if you first where to export the previous graph, and then you exported the next section of your full database:

Then AiiDA will be able to automatically identify the shared node D2 and connect both sections back together during the import process. For this kind of recognition it doesn’t matter which sub-graph was exported first.

In the following section we will explain in more detail the criteria for including other nodes and the corresponding traversal rules.

## Traversal Rules¶

When you run verdi node delete [NODE_IDS] or verdi archive create -N [NODE_IDS], AiiDA will look at the links incoming or outgoing from the nodes that you specified and decide if there are other nodes that are critical to keep.

For this decision, it is not only important to consider the type of link, but also if we are following it along its direction (we will call this forward direction) or in the reversed direction (backward direction). To clarify this, in the example above, when deleting data node D1, AiiDA will follow the input_calc link in the forward direction (in this case, it will decide that the linked node (C1) must then also be deleted). If the initial target node was, instead, C1 the input_calc link would be followed in the backward direction (and in this case the node D1 will not be deleted, as we will explain below).

This process will be repeated recursively for every node that has just been included for deletion or export, until no more nodes need to be added. The rules defining whether a linked node should be added or not to the delete/export list (based on the kind and direction of the link) are called traversal rules. In the following section we will describe these rules both for the export and delete procedures.

The tables below are grouped according to the type of nodes and links involved. We also provide illustrations of the cases considered, where the encircled node is the one being targeted, and the other node (to which the red arrow is pointing) is the one that is being considered for addition into the delete/export list.

### Data and Calculation Nodes¶

The first example above already discusses the case of deleting an input node: in this case, it is necessary to also delete any calculation that uses it as an input.

In AiiDA, we apply the same criterion also when deleting an output: in this case, we follow the create link in the backward direction and we mark for deletion also the calculation that created it. The reason for this is that a calculation with missing outputs could be misleading. For instance, some calculations produce optional outputs depending on the combination of input flags that are used. A missing output might be interpreted as if that piece of information was not computed by the calculation. In the case of export, the rules are typically the reverse of those used for deletion. Therefore, in this case, the following rule applies: when exporting a calculation node, all its input data nodes and created output nodes must be exported as well.

On the other hand, when exporting a data node, users typically do not need to also export all the calculations that used it as an input. These may represent further work that, by default, does not need to be exported as well (unless explicitly specified by the user in the list of nodes). Equivalently, when deleting a calculation, one typically wants to keep its inputs, as they might be used by other unrelated calculations.

What should happen instead for the outputs of a calculation to be deleted? Often, one might want to delete (recursively) all the outputs generated by it. However, we leave the option to users to just delete the calculation, keeping its outputs in the database. While we emphasize that this operation removes all provenance information for the output nodes, there are cases in which this is useful or even needed (removal of inputs that are protected by copyright, or creating a smaller archive file to transfer to collaborators who want to work with the output data).

Illustrative diagram (explicitly targeted node is encircled)

Name of Rule

Behavior when exporting target node

Behavior when deleting target node

input_calc_forward

• Default Value: False

• Linked node won’t be exported by default.

• Fixed Value: True

• Linked node will always be deleted.

input_calc_backward

• Fixed Value: True

• Linked node will always be exported.

• Fixed Value: False 1

• Linked node will never be deleted.

create_forward

• Fixed Value: True

• Linked node will always be exported.

• Default Value: True

• Linked node will be deleted by default.

create_backward

• Default Value: True.

• Linked node will be exported by default.

• Fixed Value: True

• Linked node will always be deleted.

1

Although we provide the option to automatically export all calculations that use as input any targeted data node (by specifying input_calc_forward=True) we currently do not provide the reciprocal option to delete all the data node inputs when targeting calculation nodes. This is mainly for the potential danger that would imply automatically enabling upwards traversal of the data provenance when deleting, which would make it extremely hard to predict or control the nodes that will be ultimately affected.

### Data and Workflow Nodes¶

The behavior when considering input_work links is exactly the same as when considering input_calc links for the same reasons. The case for return links is partially similar to the one for create one. Indeed, it isn’t desirable to have a resulting database with missing outputs, so when exporting a workflow the returned data nodes will also be included (and when deleting a data node, the returning workflow will also be removed). However, when exporting a returned node, the default behavior is not to traverse backwards through the return links, since a data node might be returned by several unrelated workflows (representing selection procedures for other studies, for example) that are unrelated to its creation. The workflow responsible for coordinating its creation will be included in the export, not directly, but through the chain effect of including the creating calculation (through create_backward) and then including its calling workflows (through call_calc_backward and call_work_backward, see next sections).

Illustrative diagram (explicitly targeted node is encircled)

Name of Rule

Behavior when exporting target node

Behavior when deleting target node

input_work_forward

• Default Value: False

• Linked node won’t be exported by default.

• Fixed Value: True

• Linked node will always be deleted.

input_work_backward

• Fixed Value: True

• Linked node will always be exported.

• Fixed Value: False

• Linked node will never be deleted.

return_forward

• Fixed Value: True

• Linked node will always be exported.

• Fixed Value: False 2

• Linked node will never be deleted.

return_backward

• Default Value: False.

• Linked node won’t be exported by default.

• Fixed Value: True

• Linked node will always be deleted.

2

The reason to prevent the deletion of returned data nodes is that, since the logical provenance can be cyclical, this might end up deleting inputs and thus propagating the deletion process to other unrelated parts of the database. In most cases where you will want to delete a returned data node, you will be able to do so by setting call_calc_forward=True (see below) and create_forward=True (which is the default value).

### Workflows and Calculation Nodes¶

Finally, we will consider the possible (call) links between processes. The results of a parent workflow depend critically on the sub-workflows or calculations launched by it. When exporting a workflow node, we therefore always traverse its call links (both call_calc and call_work) in the forward direction to include all children processes (i.e. processes directly called by it). Since the traversal rules are applied recursively, this means that also the children processes of any workflow that was a child of the targeted one will be exported as well, and so on. Analogously, when deleting a process the same applies but in the opposite direction (backward), including the parent workflow of the targeted node (if there is one), and the parent of that parent, etc.

Since call links are followed backward by default, targeting one process for either export or deletion results in selecting not only all of its child processes but also all children of any of its parent processes. As a result of all call links being traversed in both directions, targeting any of the process nodes in a workflow will mean the inclusion of the other processes of that workflow as well. Users can disable the traversal of call links in one of the directions (forward for deletion, backward for export) for fine-grained control (see examples below).

Illustrative diagram (explicitly targeted node is encircled)

Name of Rule

Behavior when exporting target node

Behavior when deleting target node

call_calc_forward

• Fixed Value: True

• Linked node will always be exported.

• Default Value: True

• Linked node will be deleted by default.

call_calc_backward

• Default Value: True

• Linked node will be exported by default.

• Fixed Value: True

• Linked node will always be deleted.

call_work_forward

• Fixed Value: True

• Linked node will always be exported.

• Default Value: True

• Linked node will be deleted by default.

call_work_backward

• Default Value: True.

• Linked node will be exported by default.

• Fixed Value: True

• Linked node will always be deleted.

In the previous sections we have described the basic rules used by AiiDA to decide which nodes should also be included from an initial list of nodes to delete or export. These rules are applied recursively: as new nodes are included in the deletion (or export)list, the rules are applied to them as well until no new nodes are included. Therefore, the consequence of using these features on a given set of nodes may not always be straightforward, and the final set might include more nodes than naively expected.

Let us first focus on the data provenance only (i.e., only input_calc and create links). The following two rules apply when going in the forward direction:

• If you delete a data node, any calculation that uses it as input will always be deleted as well (input_calc_forward=True).

• If you delete a calculation node, any output data node will be deleted by default (create_forward=True).

The consequence of these two together is a “chain reaction” in which every node that can be traced back through the data provenance to any of the initial targeted nodes will end up being deleted as well. The reciprocal is true for the export: the default behavior is that every ancestor will also be exported by default (because create_backward is True by default and input_calc_backward is always True).

In regards to the connection between data provenance and logical provenance, the most important thing to understand is how the default behavior of the program treats the highest-level workflows as the units to be handled. The logic behind this is the assumption that the typical user of the program will be dealing with it mostly in an interactive way, running pre-defined workflows through the verdi command line without needing a detailed knowledge of their internal procedures. The default behavior then was designed to reproduce the most intuitive outcomes for this type of usage.

This behavior is basically the result of the settings of call_calc_forward=True and call_work_forward=True, which makes that the inclusion of a process node will also imply the inclusion of any child or parent process node as well. Following these rules in a recursive way leads to the command affecting all the processes within any given workflow: in this way, nodes that are sub-processes of a given highest-level workflow will end up grouped together, in the sense that (by default) they will all be affected in the same way when deleting or exporting.

More freedom to further customize the selection of sections to export or delete is available through the specific switchable flags for each functionality (although the final sections must always comply with the non-switchable rules, see above). However, this usually requires a deeper understanding of the traversal rules and may imply a more thorough analysis of the particular graph. To better illustrate this, we will now consider the application of the deletion procedure to the following graph:

As you can see, W1 and W2 describe two similar but independent procedures that were launched by a single parent workflow W0. A typical user would have obtained this by directly running this workflow W0 to obtain the results D3 and D4 from the inputs D1 and D2, and may even be unaware of the internal division of W0 into two sub-Workflows W1 and W2. Hence, if the user considers the workflow (meaning, the whole set of nodes produced by it) no longer necessary, the intuitive thing to do in order to remove it from its database would be by targeting the workflow node W0 for deletion. Indeed, this would produce the desired result:

The nodes W1 and W2 would be included because W0 is being targeted (call_work_forward=True), then the nodes C1 and C2 would also be included (call_calc_forward=True), and finally the nodes D3 and D4 would end up being included as well (create_forward=True). In the end, only the inputs D1 and D2 remain (since input_work_backward=False always and input_calc_backward=False by default).

The same result would occur if the user were to target the output nodes instead (intending to delete everything associated with the obtention of those results). It is important to notice that even if the user deletes only one of the outputs, the whole set of nodes generated by the workflow would be deleted, and not just the ones associated to the targeted data node. As the results D3 and D4 where obtained from the same high-level process W0, then the default behavior has the underlying assumption that they are interconnected and not independent from one another (as if they were two different outputs of a single calculation).

In this case, the node C1 would first be included because the data node D3 is being targeted (create_reverse=True), and this in turn would include the node W1 (call_calc_reverse=True) and then its parent workflow W0 (call_work_reverse=True). Then nodes W2, C2 and D4 will be included because W0 was included, for the same reasons that were explained in the paragraphs above.

### Customizing the graph traversal (for deletion or export)¶

This dependency between nodes becomes particularly relevant when, for example, a user with more knowledge of the internal procedures of the parent workflow W0 wants to only delete the calculations and results associated to workflow W1. The intuitive action of targeting W1 does not produce the desired outcome:

Indeed C1 and D4 will be deleted (through call_calc_forward from W1 to C1 and create_forward from C1 to D3), but so will W0 (through call_work_reverse from W1), W2 (call_work_forward from W0), C2 (call_calc_forward from W2) and D4 (create_forward from C2). The way to achieve the desired outcome is not trivial, although in some situations like this, one could propose case-specific solutions such as targeting W1 with the switchable flag call_work_forward=False (preventing the traversal from W0 to W2):

However, this approach is not generally applicable, and wouldn’t work if W1 had sub-workflows that needed to be deleted as well. A more general approach is to first sever the connection to W2 by deleting node W0 with all switchable traversal rules turned off. Then, once the independence of W1 and W2 is explicitly reflected in the graph, node W1 can be deleted with the default settings.

It is worth noting that if the workflow W0 was itself part of a higher-level workflow, all that higher-level logic would be deleted due to the non-switchable rule call_work_reverse=True. This is an inevitable outcome of deleting part of a workflow, since due to the loss of that information it has become incomplete and it makes no sense to keep it.