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Support sort and pagination together will be the biggest challenge: it's up to how many cells will be involved in the query, 3,5 may be OK, you can search each cells, and cached data. But how about 20, 50 or more, and how many data will be cached?

More over, during the query there are instances operation( create, delete)  in parallel during the pagination/sort query, there is situation some cells may not provide response in time, or network connection broken, etc, many abnormal cases may happen. How to deal with some of cells abnormal query response is also one great factor to be considered. 

It's not good idea to support pagination and sort at the same time (may not provide exactly the result end user want) if searchlight should not be integrated.

In fact in Tricircle, when query ports from neutron where tricircle central plugin is installed, the tricircle central plugin do the similar cross local Neutron ports query, and not support pagination/sort together.

Best Regards
Chaoyi Huang (joehuang)

________________________________________
From: Matt Riedemann [mriedemos at gmail.com]
Sent: 19 May 2017 5:21
To: openstack-dev at lists.openstack.org
Subject: [openstack-dev] [nova] Boston Forum session recap - searchlight        integration

Hi everyone,

After previous summits where we had vertical tracks for Nova sessions I
would provide a recap for each session.

The Forum in Boston was a bit different, so here I'm only attempting to
recap the Forum sessions that I ran. Dan Smith led a session on Cells
v2, John Garbutt led several sessions on the VM and Baremetal platform
concept, and Sean Dague led sessions on hierarchical quotas and API
microversions, and I'm going to leave recaps for those sessions to them.

I'll do these one at a time in separate emails.


Using Searchlight to list instances across cells in nova-api
------------------------------------------------------------

The etherpad for this session is here [1]. The goal for this session was
to explain the problem and proposed plan from the spec [2] to the
operators in the room and get feedback.

Polling the room we found that not many people are deploying Searchlight
but most everyone was using ElasticSearch.

An immediate concern that came up was the complexity involved with
integrating Searchlight, especially around issues with latency for state
changes and questioning how this does not redo the top-level cells v1
sync issue. It admittedly does to an extent, but we don't have all of
the weird side code paths with cells v1 and it should be self-healing.
Kris Lindgren noted that the instance.usage.exists periodic notification
from the computes hammers their notification bus; we suggested he report
a bug so we can fix that.

It was also noted that if data is corrupted in ElasticSearch or is out
of sync, you could re-sync that from nova to searchlight, however,
searchlight syncs up with nova via the compute REST API, which if the
compute REST API is using searchlight in the backend, you end up getting
into an infinite loop of broken. This could probably be fixed with
bypass query options in the compute API, but it's not a fun problem.

It was also suggested that we store a minimal set of data about
instances in the top-level nova API database's instance_mappings table,
where all we have today is the uuid. Anything that is set in the API
would probably be OK for this, but operators in the room noted that they
frequently need to filter instances by an IP, which is set in the
compute. So this option turns into a slippery slope, and is potentially
not inter-operable across clouds.

Matt Booth is also skeptical that we can't have a multi-cell query
perform well, and he's proposed a POC here [3]. If that works out, then
it defeats the main purpose for using Searchlight for listing instances
in the compute API.

Since sorting instances across cells is the main issue, it was also
suggested that we allow a config option to disable sorting in the API.
It was stated this would be without a microversion, and filtering/paging
would still be supported. I'm personally skeptical about how this could
be consider inter-operable or discoverable for API users, and would need
more thought and input from users like Monty Taylor and Clark Boylan.

Next steps are going to be fleshing out Matt Booth's POC for efficiently
listing instances across cells. I think we can still continue working on
the versioned notifications changes we're making for searchlight as
those are useful on their own. And we should still work on enabling
searchlight in the nova-next CI job so we can get an idea for how the
versioned notifications are working by a consumer. However, any major
development for actually integrating searchlight into Nova is probably
on hold at the moment until we know how Matt's POC works.

[1]
https://etherpad.openstack.org/p/BOS-forum-using-searchlight-to-list-instances
[2]
https://specs.openstack.org/openstack/nova-specs/specs/pike/approved/list-instances-using-searchlight.html
[3] https://review.openstack.org/#/c/463618/

--

Thanks,

Matt

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