Open Stack

My comments are inline below...


On Fri, Jun 6, 2014 at 8:47 AM, Jamie Hannaford <
jamie.hannaford at rackspace.com> wrote:

>    Whether the same one is used for each service or a new one is used for
> each service doesn't matter.
>
>
>  Yes, it does matter IMO - and here are the reasons why:
>
>  1. By sharing a global transport object you’re introducing the risk of
> side effects. A transport object contains state that can be modified by its
> service object. Somewhere along the line, a Swift service could introduce a
> state modification that’s completely incompatible with a Nova service.
> What’s worse is that it will be a nightmare to debug - you’d have to trawl
> the entire service to see points where it interacts with the transport
> client. This is why people don’t use singletons - it’s incredibly risky and
> hard to debug. Object instantiations, on the other hand, are cheap and they
> offer protection through isolation.
>

There are two things here.

First, if the transport client has no state for the service than it doesn't
get mixed up on state. A Swift client would never introduce state for swift
to the transport client because the transport client has no state for this.
It's for transporting.

Second, it's not a singleton. You could have the same transport client for
all of them, a different transport client for each, or any permutation in
between. If the transport client contains no state to a service than it
doesn't matter.

To quote wikipedia, "the singleton pattern is a design pattern that
restricts the instantiation of a class to one object". A singleton is an
intended restriction. This isn't a restriction. It's about options.

If the service client is responsible for state for the service and the
transport client is responsible for transporting information and the state
of transport (e.g., is the info going through a proxy) than you don't run
into issues where the transport client knows state of a service because
that's the responsibility of the service client not the transport client.



>
>   2. Certain services rely on custom transport configurations. Each
> transport client has a base URL that is used for issuing HTTP requests -
> every time you execute a request, you’re effectively adding relevant paths
> for that API operation. A Swift service will have different URL endpoints
> from a Nova one - so there’s no point sharing. Another example is custom
> headers. Marconi requests custom headers to be sent, as does Glance. You
> save these as default headers on the transport client, that are sent for
> all requests that the service executes. These custom headers are not
> applicable to any other service except Marconi/Glance.
>

If a transport client know the base URL than it knows state about the
service. The separation of concerns is broken. Why does it need to know the
URL? Why does it need to know about custom headers? Customizations and
state for a service are the responsibility of the service client and not
the transport client.

Why does a service client and transport client need to both know the state
of the service? The responsibility become blurred here.



>
>  In the use-cases you mentioned, you’d easily handle that. You’d pass in
> proxy settings through the OpenStack entry point (like you do with your
> username and password), which would then percolate down into the transport
> clients as they’re created. These settings would be injected into each
> transport client. So if you require a different set-up for public clouds -
> that’s fine - you define different settings and fire up another $openstack
> object.
>

How things get passed around isn't an issues. I don't think we need to
debase how we pass settings around right now. The issue is separation of
concerns between the service clients and the transport clients.



>
>  *-OR-* you could define different transport settings for different
> services - by passing them into the $openstack->get(‘compute’,
> [‘custom_settings’ => true]); call. This is great because it gives users
> the ability to apply custom transport options to certain services. So if I
> want to interact with a private Compute instance, I’d pass in a custom
> transport configuration for that service; if I wanted to use a proxy with
> my Swift service, I can pass details into that service when creating it.
> You can only do this (provide custom transport settings for 1 service) if
> each transport client is isolated, i.e. if there’s a 1-to-1 relationship
> between service and transport client. If you have a global one, you
> couldn’t introduce custom settings per service because it’d affect ALL
> others, which is a bad user experience.
>

We're not talking about an application. We're talking about an SDK with a
simple entry point for ease and building blocks you can do a lot with. This
isn't about a 1-to-1 relationship between a service and transport client OR
a global one. It's different than that.

They should have different responsibilities. Entirely different. A
transport client moves data. It doesn't know about a service. A service
client knows about a service but not about moving data. They have their own
scope.

A transport client is used to move data. Since it's scope is about
transporting the different configurations for it are about the different
ways an application needs to transport things. If it's scoped at
transporting it doesn't need or care to know anything else. It doesn't know
state on a service (and state includes a URL to a service).

A service client knows about a service. So, for each service you connect to
you'd need one. You'd need as many service clients as services you'd
connect to.

A singleton (forcing just one) would be a bad thing. A 1-to-1 relationship
between the two where the transport client knows about state of the service
breaks the separation of concerns.

They have separate jobs. The number of each you need depends on what's
happening in the scope of each space.

Does that make sense?


>
>  Jamie
>
> On June 5, 2014 at 6:33:34 PM, Matthew Farina (matt at mattfarina.com) wrote:
>
>
> > My opinion is that we create a *new* transport client instance for
> every service client, not re-use existing instances. What’s your take on
> this?
>
>  I'm not in agreement and here is why (with a use case).
>
>  A transport client is concerned with transporting only. Whether the same
> one is used for each service or a new one is used for each service doesn't
> matter.
>
>  An example of using two transport clients would be a case where an
> application communicates with two different OpenStack clouds. One is a
> public cloud and the application communicates through a proxy. A transport
> client would know how to talk through the proxy to the public cloud. A
> second OpenStack cloud is a private cloud that is on the same company
> network. A second transport client would know how to talk to that without
> communicating through the proxy.
>
>  The service clients communicating with each cloud would use the
> appropriate transport client.
>
>  The mapping of transport client to service client doesn't need to be 1:1
> if they operate in different spaces. Only having instances of a transport
> client as needed decreases the use of resources or the time needed to
> manage those.
>
>  If a transport client is only concerned with transporting than what is
> the need to have more than one per case to transport?
>
> - Matt
>
> On Thu, Jun 5, 2014 at 12:09 PM, Jamie Hannaford <
> jamie.hannaford at rackspace.com> wrote:
>
>>  I completely agree with you regarding separation of concerns.
>>
>>  I also agree with your definitions: a transport client is for managing
>> HTTP transactions, a service client contains all the domain logic for an
>> API service (Swift, Nova, etc.). A service knows nothing about HTTP, a
>> transport client knows nothing about Swift. A transport client is injected
>> into the service client, satisfying the type hint. So any transport client
>> implementing our interface is fine.
>>
>>  Up to this point I’m in 100% agreement. The area which I think I
>> misunderstood was the *creation process* of service clients. My take was
>> that you were advocating a shared transport client instance - i.e. a
>> transport client instantiated once, and re-used for every service client.
>> If we did that, there would be global state.
>>
>>  My opinion is that we create a *new* transport client instance for
>> every service client, not re-use existing instances. What’s your take on
>> this?
>>
>>  Jamie
>>
>> On June 5, 2014 at 5:17:57 PM, Matthew Farina (matt at mattfarina.com)
>> wrote:
>>
>>  We've started to talk about the interactions between transport
>> clients, service clients, and state. I've noticed we're not on the
>> same page so I wanted to start a dialog. Here's my starting point...
>>
>> A Transport Client is about transporting data. It sends and receives data.
>>
>> A Service Client handles the interactions with a service (e.g., swift,
>> nova, keystone).
>>
>> A Service Client uses a Transport Client when it needs to transport
>> data to and from a service.
>>
>> When it comes to state, a Transport Client knows about transporting
>> things. That means it knows things like if there is a proxy and how to
>> work with it. A Service Client knows about a service which includes
>> and state for that service.
>>
>> In the realm of separation of concerns, a Service Client doesn't know
>> about transport state and a Transport Client doesn't know about
>> service state. They are separate.
>>
>> A Service Client doesn't care what Transport Client is used as long as
>> the API (interface) is compliant. A Transport Client doesn't care what
>> code calls it as long as it uses the public API defined by an
>> interface.
>>
>> This is my take. If someone has a different take please share it with
>> the reasoning.
>>
>> - Matt
>>
>>
>>   Jamie Hannaford
>> Software Developer III - CH [image: experience Fanatical Support]  [image:
>> LINE]    Tel: +41434303908  Mob: +41791009767   [image: Rackspace]
>>
>>
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>
>
>
>   Jamie Hannaford
> Software Developer III - CH [image: experience Fanatical Support]  [image:
> LINE]    Tel: +41434303908  Mob: +41791009767   [image: Rackspace]
>
>
>
> Rackspace International GmbH a company registered in the Canton of Zurich,
> Switzerland (company identification number CH-020.4.047.077-1) whose
> registered office is at Pfingstweidstrasse 60, 8005 Zurich, Switzerland.
> Rackspace International GmbH privacy policy can be viewed at
> www.rackspace.co.uk/legal/swiss-privacy-policy
> -
> Rackspace Hosting Australia PTY LTD a company registered in the state of
> Victoria, Australia (company registered number ACN 153 275 524) whose
> registered office is at Suite 3, Level 7, 210 George Street, Sydney, NSW
> 2000, Australia. Rackspace Hosting Australia PTY LTD privacy policy can be
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> -
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> -
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> Rackspace Asia Limited privacy policy can be viewed at
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> -
> This e-mail message (including any attachments or embedded documents) is
> intended for the exclusive and confidential use of the individual or entity
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> dissemination, distribution or copying of the enclosed material is
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