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Consider for example the case with a FeatureMerger or a FeatureJoiner.

I have always been wondering how the groupBy is executed here. Is this on a single port (e.g. Requestor or Left) or across multiple ports. Especially in combination with the option 'Process when group changes'.

 

I made a small illustration workspace, with the merging application inspired on 'a hiccup' I came across today where I wanted to accumulate incoming attributes at a 2DGridAccumulator, but there is no default field/option for this (an idea on this was already created, see; https://community.safe.com/s/idea/0874Q000000TlLbQAK/detail).

 

To see differences in results, I changed the order of the features entering the supplier input port of the FeatureMerger, and for whether features should first route to the Requestor, or first to the Supplier port.

In the cases when the feature order going to the supplier is changed (case 1 and 2), I see that most features don't merge, but I don't understand why it is feature 9 and 10 that merge in case 10 (I would expect only feature 10), and why only feature 9 is merged in case 2 (if any, I would again expect only feature 10).

 

Curious to hear if others have some feedback/ideas on the general scope of this question.

​@thijsknapen​  Input Group Mode: Process When Group Changes (Advanced) was introduced in FME 2019:

Pretty messy to explain for transformers that have multiple input ports. But in essence the groups on the 'primary' input wait for their matching group on the 'secondary' input port. If another 'secondary' group arrives first, then those are 'unpaired' and don't get a match. So in the image below, the red & purple on the top row find their groups on the bottom row, but the green, yellow & final red do not.

GroupByModeNot really the desired results, so in this case using Group By Mode: Process At End would give the expected results.

So why bother...? What this means is that provided the groups on the primary & secondary inputs are in the same relative order, it doesn't matter if the primary feature (left) or secondary (right) features arrive at the transformer first, they will be grouped properly.

GroupByMode2The performance advantage is that as soon as a pair of groups are matched, they can be released from the transformer so less data is cached. But... notice how the red groups are separated and so the first group of red data does not get paired with the last group of red data. If you use Group By Mode: Process At End then all the reds will be grouped. So there is a difference in behaviour. In the above example:

  • Group By Mode: Process At End would result in 4 groups (one for each colour)
  • Group By Mode: Process When Group Changes (Advanced) will give 5 groups (red, purple, green, yellow, red again)

 

Prior to FME 2019, the groups would have to arrive at the input ports in the correct order, so the reds for the primary and the red for the secondary would have to arrive at the same time, then the purple and so on. this was very hard to accomplish.

 

I've attached an example workspace that matches the colours:

@Mark Stoakes​ Thanks for the explanation!

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