Open Stack

So what you’re proposing is effectively a stateless transport client that accepts input and then works on it? I didn’t realize you meant this until your previous e-mail - the more I think about your idea, the more I like it. I think it’s great for two reasons:

- It really ties in with our idea of a pluggable transport layer. My idea of having transport state doesn’t actually go against this (nor does it break single responsibility/blur separation of concerns like you mention - but that’s another debate!) - but your idea enforces it in a stronger way.

- In OOP, many people believe that a class should either hold state or perform functionality - but not both. If we remove state from our client, we’d have a smaller and more coherent scope of responsibility.

However, as much as I like the idea, I do have some major reservations which we need to think about:

- Nearly all HTTP clients out there hold state - whether it’s base URLs, default headers, whatever. Guzzle does this, ZF2 does this, Symfony does this, Python’s base HTTP client does this. Do we really feel confident building our transport client like nobody else has before? Maybe there are good reasons why they chose to maintain state in their clients for reasons we can’t immediately see now.

- Do we really expect our end-users to create, manage and inject the transport clients? You yourself admitted that most of our users will not understand or use dependency injection - so why push a pattern that will hardly be used by the majority of our user base? I’ve worked on our current SDK for over a year - it has almost 40k downloads - and I’ve never been asked about custom transport configurations like proxy settings or whatever.

- Related to the previous point: how are you going to lower the barriers for users who don’t want to inject their own HTTP clients? Will you offer a default transport client, for example?

Jamie


On June 7, 2014 at 9:58:07 PM, Matthew Farina (matt at mattfarina.com<mailto:matt at mattfarina.com>) wrote:

My comments are inline below...


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