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Re: Architecture for Shared Components


Hi Jorge,

On Sat, Jul 31, 2010 at 1:22 PM, Jorge Williams <
jorge.williams@xxxxxxxxxxxxx> wrote:

> Guys,
> I like this idea a lot.  I hadn't thought about the concept of using a
> language binding to communicate with upstream proxies, but it makes sense.

Just to clarify: I am not advocating a downstream (in the request chain)
server calling back to an upstream server.  Instead, if both servers need to
do something re: authentication, they would both call "sideways" to an
authentication service sitting out-of-request-chain that can answer their
query and hang up.

(It sounded like you maybe were advocating server A proxying to server B
proxying to server C, then C calls A to asks a question, hangs up after
getting the answer, then proxies to server D.  That makes A have to behave
as both a proxy and a regular server, probably makes the scaling analysis a
little trickier, and if nothing else makes the diagrams harder to draw :) )

FWIW, this is Google's architecture in all the services that I was familiar
with when I worked there -- a stack of servers in the request chain, each
handling the minimum amount of work, and calling out sideways to other
services as needed.  If two servers needed to ask about authentication, they
could call out to Gaia the authentication service, get an answer, hang up on
Gaia, and continue servicing the request.  What's great about this is that
it becomes so normal as a way to solve the problem that it becomes a design
pattern: you can "smell" problems with the architecture if the owner can't
draw it on a whiteboard as a stack of servers, from netscaler down to final
request server, with some arrows out sideways to caches, Gaia, GFS, etc.

 Being able to purge something from an HTTP cache by simply making a "purge"
> call in whatever language I'm using to write my API is a win.  That said,
> I'm not envisioning a lot of communication going upstream in this manner.
>  An authentication proxy service, for example, may need to communicate with
> an IDM system, but should require no input from the API service itself.  In
> fact, I would try to discourage such
communication just to avoid chatter.

I want to push back a little on that point -- I don't know think we should
optimize for low network chatter as much as for simplicity of design.  The
example that started this thread was authentication having to happen at two
points in the request chain.  If we tried to eliminate the deeper of the two
requests to the auth server in order to reduce network chatter, the trade
off is having to bubble state up to the shallower server, making that server
more complicated and making it harder to separate what each server in the
chain does.  If we find that we're saturating the network with calls to a
particular service, only then do I think we should start looking at
alternatives like changing the request flow.

> In cases where this can't be avoided  I would require the proxy services
> expose a rest endpoint so we can take advantage of it even if a binding
> isn't available.
I would definitely advocate for using REST for *all* our service
communication unless there were a really strong case for doing otherwise in
an individual case.  Makes the system more consistent, makes it easy to
interface with it if you need to, makes it easy to write a unified logging
module, lets us easily tcpdump the internal traffic if we ever needed to,
etc.  Putting a language binding on top of that is just added convenience.



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