Varnish vcl_backend_response not called - varnish-4

Question

I came across inconsistencies with Varnish 4.x documentation.
According to the documentation vcl_backend_response will be called each time an object is fetched from the backend and the backend doesn't return an error (and is alive and healthy).
However, I noticed many of the 'fetch' requests (resulting from misses) are not going through this function at all.
Here is an example output:
- VCL_call HASH
- VCL_return lookup
- VCL_call MISS
- VCL_return fetch
- Link bereq 294915 fetch
- Timestamp Fetch: 1504198046.101306 0.003644 0.003644
- RespProtocol HTTP/1.1
- RespStatus 200
- RespReason OK
- RespHeader Server: nginx/1.10.3 (Ubuntu)
- RespHeader Date: Thu, 31 Aug 2017 16:47:26 GMT
- RespHeader Vary: Accept-Encoding
- RespHeader Last-Modified: Thu, 31 Aug 2017 16:42:58 GMT
- RespHeader Expires: Thu, 31 Aug 2017 16:47:26 GMT
- RespHeader Cache-Control: public, max-age=90, s-maxage=332
- RespHeader Pragma: cache
- RespHeader X-Lift-Version: 2.6.3
- RespHeader X-Frame-Options: SAMEORIGIN
- RespHeader Content-Encoding: gzip
- RespHeader Content-Type: application/json; charset=utf-8
- RespHeader X-Varnish: 294914
- RespHeader Age: 0
- RespHeader Via: 1.1 varnish-v4
- VCL_call DELIVER
- RespHeader grace: none
- VCL_return deliver
as opposed to a flow that adheres the documentation:
- VCL_call BACKEND_FETCH
- VCL_return fetch
- BackendOpen 22 boot.default 127.0.0.1 9919 127.0.0.1 22536
- Timestamp Bereq: 1504198040.603601 0.000222 0.000222
- Timestamp Beresp: 1504198040.659723 0.056345 0.056123
- BerespProtocol HTTP/1.1
- BerespStatus 200
- BerespReason OK
- BerespHeader Server: nginx/1.10.3 (Ubuntu)
- BerespHeader Date: Thu, 31 Aug 2017 16:47:20 GMT
- BerespHeader Content-Type: text/html;charset=utf-8
- BerespHeader Transfer-Encoding: chunked
- BerespHeader Connection: keep-alive
- BerespHeader Vary: Accept-Encoding
- BerespHeader Last-Modified: Thu, 31 Aug 2017 16:42:58 GMT
- BerespHeader Expires: Thu, 31 Aug 2017 16:47:20 GMT
- BerespHeader Cache-Control: public, max-age=21600, s-maxage=21600
- BerespHeader Pragma: cache
- BerespHeader X-Lift-Version: 2.6.3
- BerespHeader X-Frame-Options: SAMEORIGIN
- BerespHeader Content-Encoding: gzip
- TTL RFC 21600 10 -1 1504198041 1504198041 1504198040 1504198040 21600
- VCL_call BACKEND_RESPONSE
So - according to Varnish log - when there is a miss and the object is fetched from the backed vcl_backend_response is not invoked.
Unless I'm missing something - this contradicts the documentation.

Answer 1

There are no inconsistencies in the documentation. What you see is the normal behaviour since Varnish 4.0. During a cache miss, you'll see (at least) two transactions in varnishlog: (1) the one handling the client request (usual flow is vcl_recv → vcl_hash → vcl_miss → vcl_deliver; i.e. the first log excerpt in your question); and (2) the one handling the request to the backend side (usual flow is vcl_backend_fetch → vcl_backend_response; i.e. the second log excerpt in your question). That's why you don't see references to vcl_backend_response in the client transaction.
Obviously both transactions are connected. At some point the client transaction creates and waits for completion of the backend transaction. That's shown in the log of the client transaction:
- ...
- Link bereq 294915 fetch
- ...
That means a backend transaction was created (VXID 294915) in order to fetch an object from the origin.