RE: [tosca] "occurrences" in requirement assignments

[Arturo Martin De Nicolas <arturo.martin-de-nicolas@ericsson.com>]

Hello Tal, all,

 

I would like to jump in in relation to the capabilities occurrences. I don't think a node template can express the "requirement" that a capability must be used, i.e. if a topology template contains a node template that has a capability which is not used to match a requirement, I wouldn't consider that as an error, even if the capability definition indicates an occurrences minimum value > 0.

 

If we read the description of the occurrences key name in the capability definition, it refers to the number of relationships "allowed" to be formed, not "required" to be formed.

 

For requirements it is of course different.

 

Best regards,

Arturo

 

 

 

From: tosca@lists.oasis-open.org <tosca@lists.oasis-open.org> On Behalf Of Tal Liron
Sent: Sunday, June 7, 2020 7:20 PM
To: Calin Curescu <calin.curescu@ericsson.com>
Cc: tosca@lists.oasis-open.org
Subject: Re: [tosca] "occurrences" in requirement assignments

 

On Thu, Jun 4, 2020 at 11:41 AM Calin Curescu <calin.curescu@ericsson.com> wrote:

But, with that understanding, I have no idea what "occurrences" could mean in the requirement assignment. It seems the thinking here is that "occurrences" has something to do with the instance model. So, for now, Puccini parses this keyword as proper syntax, but never does anything with it.

[CC] For requirements: In the assignment the occurrences itsâs only relevant if we use node-filters to connect to several nodes. Then if the same node filter should be reused, itâs easier to just put occurrences to â3â instead of repeating 3 times the requirement assignment

 

This could make sense, but remember that it's more complicated because it's a range. What would [ 4, 6 ] mean? Let's say that there 10 possible matches. Would we gobble up the top of our range, 6 connections? Or do we emit an error about there being too many possible matches? And what if there are 2 possible matches? Does that mean an error, too, because we haven't reached our minimum?

 

(By the way, it's not just an aspect of node filters. Even the most minimal requirement, e.g. specified via a capability type name, could have >1 matches.)

 

This could possibly make sense, though it still seems convoluted to me. One thing is for sure is that "occurrences" in requirement definitions are entirely different from "occurrences" in requirement assignment. The latter cannot be a refinement of the former. So if we decide to keep this in 2.0 (PLEASE LET'S NOT) then we need to do a lot more to clarify the meaning of both keywords and remove any refinement rules.

 

[CC] For capabilities: Itâs the final decision of how many connections to that capability are allowed. In the type we can specify a range, but in the end a final number needs to be chosen. Note, that this does not require a fixed number of connections, but fixes the maximum number of connections that can be supported. Of course, if no assignment is provided, the orchestrator could automatically choose (say the max of the interval from the type).

 

A few things to say about this:

 

1) I don't see any of this as an "orchestrator" issue, because it's a grammatical feature, all in design-time. We again are going back to discussing the instance model. :) I interpret capability "occurrences" to be about counting how many design requirements can be "satisfied" by connecting to this capability. (Similar to how you described "occurrences" for requirement assignments.)

 

2) There's less confusion regarding capabilities, because capabilities are syntactically maps, so there is only ever one capability of that name in both the node type and the node template. Requirements are far more complicated because they are syntactically sequenced lists, both in the node type and the node template. That's why "counting" could have multiple meanings.

 

3) For capabilities, I do treat the lower bound as a minimum. E.g. if your capability "occurrences" is [ 2, 3 ] but you only have 1 incoming requirement then it is indeed an error.

 

All this means is that in Puccini the requirement satisfying algorithm is a bit complex (see the source code here). When choosing a node to match a requirement it tries to satisfy those capabilities that have minimums first, to make sure that they are "filled up". Once they all have their minimums it is possible to choose any arbitrary capability, up to the maximum specified by the "occurrences" range. When all maximums are reached, the requirement cannot be satisfied and you get an error.