nginx module dev question

[Delta Yeh]

Hi all,
  I want to implement the following feature:
 1. client HEAD request arrive , my module send it to an upstream.
 2. the upstream return response.
 3. my module check the response, if it is not the data I want, send the
origin request to another upstream

 I know the block mode will work. I want to know if there is asynchronous
mode which will fit into nginx better?

BR,
DeltaY
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