How to recursively update a Project Planning Graph with Cypher - neo4j

Question

I am refering to this graphgist : https://neo4j.com/graphgist/project-management
I'm actually trying to update a project plan when a duration on one task changes.
In the GraphGist, the whole project is always calculated from the initial activity to the last activity. This doesn't work great for me in a multi-project environment where I don't really know what the starting point is, and I don't know either the end point. What I would like for now, is just to update the earliest start of any activity which depends on a task I just updated.
The latest I have is the following :
MATCH p1=(:Activity {description:'Perform needs analysis'})<-[:REQUIRES*]-(j:Activity)
UNWIND nodes(p1) as task
MATCH (pre:Activity)<-[:REQUIRES]-(task:Activity)
WITH MAX(pre.duration+pre.earliest_start) as updateEF,task
SET task.earliest_start = updateEF
The intent is to get all the paths in the projects which depends on the task I just updated (in this case : "perform needs analysis"), also at every step of the path I'm checking if there aren't other dependencies which would override my duration update.
So, of course it only works on the direct connections.
if I have A<-[:requires]-B<-[:requires]-C
if I increase duration A, I believe it updates B based on A, but then C is calculated with the duration of B before B duration was updated.
How can I make this recursive? maybe using REDUCE?
(still searching...)

Answer 1

This is a very interesting issue.
You want to update the nodes 1 step away from the originally updated node, and then update the nodes 2 steps away (incorporating the previously-updated values as appropriate), and then 3 steps away, and so on until every node reachable from the original node has been updated.
The Cypher planner does not generate code that performs this kind of query/update pattern, where new values are propagated step by step through paths.
However, there is a workaround using the APOC plugin. For example, using apoc.periodic.iterate:
CALL apoc.periodic.iterate(
"MATCH p=(:Activity {description:'Perform needs analysis'})<-[:REQUIRES*]-(task:Activity)
RETURN task ORDER BY LENGTH(p)",
"MATCH (pre:Activity)<-[:REQUIRES]-(task)
WITH MAX(pre.duration+pre.earliest_start) as updateEF, task
SET task.earliest_start = updateEF",
{batchSize:1})
The first Cypher statement passed to the procedure generates the task nodes, ordered by distance from the original node. The second Cypher statement gets the pre nodes for each task, and sets the appropriate earliest_start value for that task. The batchSize:1 option tells the procedure to perform every iteration of the second statement in its own transaction, so that subsequent iterations will see the updated values.
NOTE: If the same task can be encountered multiple times at different distances, you will have to determine if this approach is right for you. Also, you cannot have other operations writing to the DB at the same time, as that could lead to inconsistent results.