Ok, so summing up the suggestion would be to go with 'target_filter' which would look something like this:

mysql:
type: tosca.nodes.DBMS.MySQL
properties:

1. omitted here for sake of brevity
   requirements:

• host: tosca.nodes.Compute
  target_filter:
• num_cpus: { in_range: [ 1, 4 ] }
• mem_size: { greater_or_equal: 2 }
• os_arch: { equal: x86_64}
• os_type: { equal: linux}
• os_distribution: { equal: ubuntu}

We will have to add a section to the spec that defines the grammer for the target_filter statement. It is basically a list of filter constraints on properties of the entity to be selected. Each entry is a key-value pair where the key is the name of a property and the value is the filter criterion. Note that there can be multiple entries for the same property (since we are using a list), so one would be able to to defined multiple additive criteria.

I will work with Matt on including this into the spec, if everyone is ok.

To extend this to include capabilities we could express it this way:
mysql:
type: tosca.nodes.DBMS.MySQL
properties:

1. omitted here for sake of brevity
   requirements:

• host: tosca.nodes.Compute
  target_filter:

1. node innate
   properties:

• num_cpus: { in_range: [ 1, 4 ] }
• mem_size: { greater_or_equal: 2 }
• os_arch: { equal: x86_64}
• os_type: { equal: linux}
• os_distribution: { equal: ubuntu}

  capability:

• tosca.capabilities.encryption
  properties:
  algorithm: aes
  keylength: 128
• tosca.capabilities.foo
  properties:
  x: y

Jacques: Are the filter expressions an <or> semantic (either or?) if multiple options exist? We need to express combinations at some point.
Derek: allow target filter to allow multiple, could use conjunctive logic in some way. Current interp. is it is a conjunction. Filters <could> be named at some point
Jacques: evolutionary approach can be taken.
Matt: perhaps advantage a regex. (as we have loosely defined so far as an allowed constraint clause)
Derek: as long as the domain of the property is usable (accessible) to expression it should be allowed.
Jacques: if only 1 property is truly needed, need a way to express this (or even 1 from an allowed set of values).

These discussions seems to make us work on https://issues.oasis-open.org/browse/TOSCA-152 ("Extend Requirement grammar to support "Optional/Best Can" Capability Type matching ") as a base capability to enable more expressivenes of target filters.h

TOSCA TC approve closure of this issue at the 2014-11-20 meeting. See https://www.oasis-open.org/apps/org/workgroup/tosca/download.php/54595/TOSCA%20Minutes%202014-11-20.docx