Allow Multi-Link on Input Ports

[aglinxinyuan]

Feature Summary

Currently, the system allows multiple links (multi-link) on output ports, but input ports are restricted to a single connection by default. Multi-link refers to the ability for multiple links to connect to the same port.

Proposed Solution or Design

To ensure consistency and improve the overall design semantics, we propose enabling multi-link support for input ports. This change would align input port behavior with output ports and provide greater flexibility in connection patterns. This implicitly assumes that multiple links on the same port share the same schema and that their data will be combined.

Impact / Priority

(P2)Medium – useful enhancement

Affected Area

No response

[Yicong-Huang]

Thanks for the proposal. But I think we need some more deeper reasoning here.

I think we need to clarify in the proposal about the behavior when multiple links connects to the same input port. It is not just about align input and output port behavior, or flexibility. Recall that input and output are NOT semantically symmetric.

I assume this has an implicitly assumption that multi links on the same port need to share same schema, also their data will be combined (like an implicit Union operator). This could be a good design, if we choose to get rid of union operator. Is this the intention/direction?

[Yicong-Huang]

In fact, currently Union operator does allow multiple input links on the same input port. So we have this mechanism implemented already and it is controlled by a config. Do you propose to extend this to all input ports?

[chenlica]

We want to keep the Union operator, and generalize its design (allowing multiple links to go into the same input port) to other operators. The schemas of all the input links are required to be the same.

[aglinxinyuan]

In fact, currently Union operator does allow multiple input links on the same input port. So we have this mechanism implemented already and it is controlled by a config. Do you propose to extend this to all input ports?

Yes. I totally agree that this implicitly assumes that multiple links on the same port share the same schema and that their data will be combined, and we want to extend this to all input ports.

[Yicong-Huang]

Ok. I see. @aglinxinyuan please update the original proposal with this information.

On this direction, maybe we don't need Union operator (logical and physical). We essentially just embed this to all operators. This could be a good idea.

[Yicong-Huang]

For "the same schema requirement", do we already have validation during compilation?

[aglinxinyuan]

For "the same schema requirement", do we already have validation during compilation?

No. We can add one.

[chenlica]

It's an interesting idea to get rid of the Union operator after this generalization. There are several "set operators" such as "Union", "Intersect", and "Difference." I feel we need to keep Union at least for the reason of the completeness of this family of set operators.

[Yicong-Huang]

Other set-operators are different from Union. They need two ports to differentiate left and right relation, even though they also require same schema.

[chenlica]

Then we need to have a new Union operator with two input ports. We keep this new Union operator, and remove the old single-port Union operator. I believe it's important to have a "complete" group of set operators.