What is a validation oracle, and how do its internal and external forms map onto push and pull?

A validation oracle is a trusted operator (automatic or human) that vouches for data: an internal one periodically pushes verified external data on-chain (push-based inbound), an external one validates transactions against outside information on request (pull-based inbound).

The idea (Xu et al.) is to treat a trusted third party as an inbound oracle whose job is to validate:

• Internal validation oracle — proactively and periodically transmits externally-verified data into the chain. Since it volunteers the data, it is push-based.
• External validation oracle — acts as a trusted external validator of transactions, checking them against information that lives outside the chain, when asked. Since it responds to a request, it is pull-based.

It's a neat demonstration that the push/pull dimension cleanly classifies even pre-existing oracle concepts that weren't designed with the framework in mind.

Go deeper:

• Foundational Oracle Patterns (Mühlberger et al., arXiv) — maps prior oracle notions (including validation oracles) onto the push/pull dimension.