on-demand lazy loading for S/RDS

[stevenzzzz]

Issue Template

Title: * On demand S/RDS

Description:

Issue for tracking on-demand SRDS implementation.
to make better use of resources (RAM, CPU), make RDS on-demand from SRDS.An example use scenario is that in a MT proxy, many long-tail route tables arent that active, in stead of fetching and storing these route tables via SRDS, we can put them into a "secondary tier", and only fetch the route table when there are such requests hitting the proxy.

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issue for on-demand routes #2500

[htuch]

@stevenzzzz I've tagged this as an open xDS/API issue, but could you elaborate a little more on this one for folks who don't have the same context that we share?

[stevenzzzz]

sure, what's missing here is to make on-demand RDS work w/ the SRDS: depends on how we design it, we could either:

1. when we have a scope-key and there is no scope defined for it, fire a on-demand SRDS request to management server to get back the scopedrouteconfiguration for that key, then issue a RDS request.
2. or since SRDS is light weight enough, we always send SotW for SRDS, in this case, if scope is not found, it's a 404. if corresponding Route table is not found, make the existing on-demand RDS to work with SRDS.

I think likely we want to pick option 2.

[htuch]

If you anticipate SRDS SoTW not being a scaling bottleneck, then I agree that (2) makes the most sense. One thing to keep in mind is that there is not a full blown existing on-demand RDS, rather we have on-demand VHDS, which is most of what is needed for on-demand RDS but there is a missing last mile there. CC @dmitri-d

[stevenzzzz]

agreed. The current RDS is "on-demand-RDS when no route entry", w/ SRDS supporting on-demand, what we need is actually "on-demand RDS for no route-table". for users w/. many route-tables configured, on-demand SRDS makes sense as a measure of lazy loading route tables.

…

On Sun, Apr 5, 2020 at 7:40 PM htuch ***@***.***> wrote: If you anticipate SRDS SoTW not being a scaling bottleneck, then I agree that (2) makes the most sense. One thing to keep in mind is that there is not a full blown existing on-demand RDS, rather we have on-demand VHDS, which is most of what is needed for on-demand RDS but there is a missing last mile there. CC @dmitri-d <https://github.com/dmitri-d> — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#10641 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AA2I6TDD4KPYNSS3XKJUHJDRLEJILANCNFSM4L4JJ36Q> .

-- Xin Zhuang

[mattklein123]

+1 for building this into the on-demand VHDS filter/support.