Add dedicated client nodes as an option for TransportClient sniffing

[ppf2]

Currently TransportClient's sniffing feature only adds data nodes to the list of available hosts. For clusters that have dedicated client nodes, it will be nice for the sniffing feature to be able to honor client nodes so that the reduce phase can occur on the dedicated client node machines.

[makeyang]

pelase also consider gateway node

[javanna]

would you add client nodes and data nodes, or client nodes only if there's at least a certain number of them (how many?) ? Or did you mean to add an option to the transport client to control this? I think we should do here what the language clients do, not sure if they add client nodes to their internal list. @clintongormley can comment on this.

[clintongormley]

The lang clients are inconsistent here. Really you want a number of options available, eg:

• client nodes
• data nodes
• tribe nodes
• nodes with a particular tag (or tags)

and you probably want to prefer eg client nodes but allow fallback to eg data nodes...

[clintongormley]

Discussed in FiF - we should add an overridable method which allows the user to specify whether a sniffed node should be used or not. Currently we only sniff data notes, but the predicate should allow client nodes to be accepted too.

[martijnvg]

Closing in favour of #21888