https://trychroma.com/engineering/wal3

(generally with S3 but this is fine)

1. GET (current_node, cksum) FROM head_pointer
1. INSERT node (previous_node) VALUES (head_pointer) RETURNING node.id
1. UPDATE head_pointer (current_node, cksum) VALUES (node.id, blockchain) IF head_pointer.cksum=cksum