Add exception to no-upcall note of cells doc

The cells v2 layout documentation clearly states that there are no
upcalls from cells back to the central API services. This mislead
me for sometime as I could not fathom how a compute node in a cell
was supposed to report its resource info.

It turns out nova looks up the placement service in the keystone
catalogue and contacts it directly which to my mind is an upcall. I
wonder if the author of the not felt that the placement service is
not really part of nova?

Change-Id: If14be8b182f0af4e4e6641046fec638c07e26546
Closes-Bug: #1742421
changes/91/532491/1
Liam Young 5 years ago
parent 90a92d33ed
commit 30786b892e
  1. 19
      doc/source/user/cellsv2-layout.rst

@ -231,15 +231,16 @@ to distinguish its place and purpose from the per-cell conductor nodes.
conductor -> mq2
}
It is important to note that services in the lower cell boxes do not
have the ability to call back to the API-layer services via RPC, nor
do they have access to the API database for global visibility of
resources across the cloud. This is intentional and provides security
and failure domain isolation benefits, but also has impacts on some
things that would otherwise require this any-to-any communication
style. Check the release notes for the version of Nova you are using
for the most up-to-date information about any caveats that may be
present due to this limitation.
It is important to note that services in the lower cell boxes only
have the ability to call back to the placement API but cannot access
any other API-layer services via RPC, nor do they have access to the
API database for global visibility of resources across the cloud.
This is intentional and provides security and failure domain
isolation benefits, but also has impacts on some things that would
otherwise require this any-to-any communication style. Check the
release notes for the version of Nova you are using for the most
up-to-date information about any caveats that may be present due to
this limitation.
Caveats of a Multi-Cell deployment
----------------------------------

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