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From Markus Thömmes <markusthoem...@apache.org>
Subject Re: Kafka and Proposal on a future architecture of OpenWhisk
Date Wed, 22 Aug 2018 21:55:11 GMT
Hi Tyson,

Am Mi., 22. Aug. 2018 um 23:37 Uhr schrieb Tyson Norris
<tnorris@adobe.com.invalid>:

> Hi -
>     >
>     > When exactly is the case that a ContainerRouter should put a blocking
>     > activation to a queue for stealing? Since a) it is not spawning
> containers
>     > and b) it is not parsing request/response bodies, can we say this
> would
>     > only happen when a ContainerRouter maxes out its incoming request
> handling?
>     >
>
>     That's exactly the idea! The work-stealing queue will only be used if
> the
>     Router where to request landed cannot serve the demand right now. For
>     example, if it maxed out the slots it has for a certain action (all
>     containers are working to their full extent) it requests more
> resources and
>     puts the request-token on the work-stealing queue.
>
> So to clarify, ContainerRouter "load" (which can trigger use of queue) is
> mostly (only?) based on:
> * the number of Container references
> * the number of outstanding inbound  HTTP requests, e.g. when lots of
> requests can be routed to the same container
> * the number of outstand outbound HTTP requests to remote action
> containers (assume all are remote)
> It is unclear the order of magnitude considered for "maxed out slots",
> since container refs should be simple (like ip+port, action metadata,
> activation count, and warm state), inbound connection handling is basically
> a http server, and outbound is a connection pool per action container
> (let's presume connection reuse for the moment).
> I think it will certainly need testing to determine these and to be
> configurable in any case, for each of these separate stats.. Is there
> anything else that affects the load for ContainerRouter?
>

"Overload" is determined by the availability of free slots on any container
being able to serve the current action invocation (or rather the absence
thereof). An example:
Say RouterA has 2 containers for action X. Each container has an allowed
concurrency of 10. On each of those 2 there are 10 active invocations
already running (the ContainerRouter knows this, these are open connections
to the containers). If another request comes in for X, we know we don't
have capacity for it. We request more resources and offer the work we got
for stealing.

I don't think there are tweaks needed here. The Router keeps an
"activeInvocations" number per container and compares that to the allowed
concurrency on that container. If activeInvocations == allowedConcurrency
we're out of capacity and need more.

We need a work-stealing queue here to dynamically rebalance between the
Routers since the layer above the Routers has no idea about capacity and
(at least that's my assumption) schedules randomly.


>
>     That request-token will then be taken by any Router that has free
> capacity
>     for that action (note: this is not simple with kafka, but might be
> simpler
>     with other MQ technologies). Since new resources have been requested,
> it is
>     guaranteed that one Router will eventually become free.
>
> Is "requests resources" here requesting new action containers, which it
> won't be able to process itself immediately, but should startup + warm and
> be provided to "any ContainerRouter"? This makes, sense, just want to
> clarify that "resources == containers".
>

Yes, resources == containers.


>
>     >
>     > If ContainerManager has enough awareness of ContainerRouters'
> states, I'm
>     > not sure where using a queue would be used (for redirecting to other
>     > ContainerRouters) vs ContainerManager responding with a
> ContainerRouters
>     > reference (instead of an action container reference) - I'm not
> following
>     > the logic of the edge case in the proposal - there is mention of
> "which
>     > controller the request needs to go", but maybe this is a typo and
> should
>     > say ContainerRouter?
>     >
>
>     Indeed that's a typo, it should say ContainerRouter.
>
>     The ContainerManager only knows which Router has which Container. It
> does
>     not know whether the respective Router has capacity on that container
> (the
>     capacity metric is very hard to share since it's ever changing).
>
>     Hence, in an edge-case where there are less Containers than Routers,
> the
>     ContainerManager can hand out references to the Routers it gave
> Containers
>     to the Routers that have none. (This is the edge-case described in the
>     proposal).
>
> I'm not sure why in this case the ContainerManager does not just create a
> new container, instead of sending to another Router? If there is some
> intended limit on "number of containers for a particular action", that
> would be a reason, but given that the ContainerManager cannot know the
> state of the existing containers, it seems like sending to another Router
> which has the container, but may not be able to use it immediately, may
> cause failures in some cases.
>

The edge-case here is for very slow load. It's minimizing the amount of
Containers needed. Another example:
Say you have 3 Routers. A request for action X comes in, goes to Router1.
It requests a container, puts the work on the queue, nobody steals it, as
soon as the Container gets ready, the work is taken from the queue and
executed. All nice and dandy.

Important remark: The Router that requested more Containers is not
necessarily the one that's getting the Containers. We need to make sure to
evenly distribute Containers across the system.

So back to our example: What happens if requests for action X are made one
after the other? Well, the layer above the Routers (something needs to
loadbalance them, be it DNS or another type of routing layer) isn't aware
of the locality of the Container that we created to execute action X. As it
schedules fairly randomly (round-robin in a multi-tenant system is
essentially random) the action will hit each Router once very soon. As
we're only generating one request after the other, arguably we only want to
create only one container.

That's why in this example the 2 remaining Routers with no container get a
reference to Router1.

In the case you mentioned:
> it seems like sending to another Router which has the container, but may
not be able to use it immediately, may cause failures in some cases.

I don't recall if it's in the document or in the discussion on the
dev-list: The router would respond to the proxied request with a 503
immediatly. That tells the proxying router: Oh, apparently we need more
resources. So it requests another container etc etc.

Does that clarify that specific edge-case?

Memo to self: I need to incooperate all the stuff from these discussions
into the document.


>
>
>     The work-stealing queue though is used to rebalance work in case one
> of the
>     Routers get overloaded.
>
> Got it.
>
> Thanks
> Tyson
>
>
>

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