How does a SegWit node verify a block? I know that non-SegWit node could verify a block without witness.

For SegWit node, is witness necessary to verify a block? Because I know witness are committed as wtxid merkle node in coinbase. What's the purpose of the wtxid merkle node?

If witenss is necessary for SegWit node, then can I say witness is an part of blockchain in SegWit node's view?
For SegWit node, is witness necessary to verify a block? Because I know witness are committed as wtxid merkle node in coinbase. What's the purpose of the wtxid merkle node?


If a block contains transactions that have witnesses, if the block then does not have a witness, the Segwit node will still validate it, but it will find it to be invalid. Because segwit is a soft fork, Segwit nodes still know what blocks without witnesses look like and can verify them as such. This is because the block structure actually did not change, what changed was the transaction structure.


  If witenss is necessary for SegWit node, then can I say witness is an part of blockchain in SegWit node's view?


Yes.