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Parse's reliance on globals (singletons) grates on my quite a bit, and has largely prevented me from bothering to read further. [1] I think of global variables

by nupark2 15y ago

Parse's reliance on globals (singletons) grates on my quite a bit, and has largely prevented me from bothering to read further. [1] I think of global variables as a cardinal sin of OO API design. They create unnecessary dependencies, lock down architecture, and reduce/eliminate composability. Taken to the extreme demonstrated in Parse, they remove most of the advantages of using an OO language that supports polymorphism. [1] That, and serious questions about whether something so general purpose can adequately meet the needs of anything but the most basic application. For instance, the lack of schemas and the lack of server-side code means that upgrading of data (and validation of data) is pushed to the client-side application. I'm not sure I'd like to be the one debugging the kinds of issues that arise when you have different end-user application versions potentially corrupting one another's data. [Edit] Can someone that's downvoted me into the serious negatives please explain what portion of my architectural / API / OO arguments they disagree with? This perfectly reasonable technical complaint was also voted into the negatives (I just voted it up), which implies some odd voting behavior by Parse proponents: http://news.ycombinator.com/item?id=3151570 http://news.ycombinator.com/item?id=3151570

csmajorfive 15y ago
What singletons are we using besides one for the current user? I think you may be confusing singletons with class methods.
- nupark2 15y ago
  I was actually referring to the use of global variables (but I'll tackle the class method issue below). Take, for example, this instance (not class) method: -[PFObject delete]; Where is the object deleted from? Where was it created? It uses a global variable to achieve this behavior. Likewise, there are methods like: +[PFObject saveAll:] This also relies on a global variable somewhere to do its magic. Where are the objects saved? How do I interject different behavior? How do I modify the behavior for a specific subsystem of my application? That said, I can also address the use of class methods. Similar to how singletons make it quite difficult to leverage OO polymorphism and decoupled architecture, class methods literally throw away OO polymorphism by providing direct bindings to what amount to global functions. One can not leverage polymorphism with class methods, as the bindings are direct and fixed -- one can not modify the behavior of class method by configuring a specific instance (since there's only one global instance). One also can not modify the behavior of a method by providing a different instance that conforms to the required protocol.
  - csmajorfive 15y ago
    You're right, delete is an instance method. You call it like so: [myObject delete]; So the object referred to by the variable myObject gets deleted from our servers. It was created whenever it was instantiated using a query. What global is it using? saveAll is a static (class) method. It takes in a collection of instantiated objects and saves them to our servers. What global is it using? As for interjecting custom behavior, perhaps you're missing that all of these methods take in a variety of callbacks that can modify behavior.
    - nupark2 15y ago
      So the object referred to by the variable myObject gets deleted from our servers. It was created whenever it was instantiated using a query. What global is it using? How does myObject acquire a reference of some kind to your servers, and the context in which it is stored? (That's the answer). saveAll is a static (class) method. It takes in a collection of instantiated objects and saves them to our servers. What global is it using? As for interjecting custom behavior, perhaps you're missing that all of these methods take in a variety of callbacks that can modify behavior. How do I arbitrarily modify the saveAll method so that I can perform arbitrary operations on objects before they're saved, so that I can implement a custom undo manager (but only when working on objects in the context of the user editing the document. Elsewhere I want different behavior)? ---- These approaches (class methods, singletons/globals) are breaking OO polymorphism, and significantly hamstringing your APIs. I'll try to provide an example that might matter to you: Let's pretend that I want to add support for sharing data between users of my application. It's important to me to not repeat myself, so I want to have the same code be able to write data to the user's personal storage, write data to one of several possible shared storage destinations. You decide you want to support this API. How do you make your current global/singleton API work for me?
      - csmajorfive 15y ago
        The hostname of our servers is indeed hardcoded in the framework. So, yes, that's a 'singleton' in the same way that configuring your database hostname in Rails is a singleton. So far our developers have had a lot of success building upon Parse. There are certainly some missing pieces (like the one pointed out in another comment on this post) but none of them have related to our overall architecture. I invite you to let us know what it should look like ideally at founders@parse.com or even stop by our office. We love to talk with developers.
deleted 15y ago
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by nupark2 15y ago

csmajorfive 15y ago

What singletons are we using besides one for the current user? I think you may be confusing singletons with class methods.

nupark2 15y ago
I was actually referring to the use of global variables (but I'll tackle the class method issue below). Take, for example, this instance (not class) method: -[PFObject delete]; Where is the object deleted from? Where was it created? It uses a global variable to achieve this behavior. Likewise, there are methods like: +[PFObject saveAll:] This also relies on a global variable somewhere to do its magic. Where are the objects saved? How do I interject different behavior? How do I modify the behavior for a specific subsystem of my application? That said, I can also address the use of class methods. Similar to how singletons make it quite difficult to leverage OO polymorphism and decoupled architecture, class methods literally throw away OO polymorphism by providing direct bindings to what amount to global functions. One can not leverage polymorphism with class methods, as the bindings are direct and fixed -- one can not modify the behavior of class method by configuring a specific instance (since there's only one global instance). One also can not modify the behavior of a method by providing a different instance that conforms to the required protocol.
- csmajorfive 15y ago
  You're right, delete is an instance method. You call it like so: [myObject delete]; So the object referred to by the variable myObject gets deleted from our servers. It was created whenever it was instantiated using a query. What global is it using? saveAll is a static (class) method. It takes in a collection of instantiated objects and saves them to our servers. What global is it using? As for interjecting custom behavior, perhaps you're missing that all of these methods take in a variety of callbacks that can modify behavior.
  - nupark2 15y ago
    So the object referred to by the variable myObject gets deleted from our servers. It was created whenever it was instantiated using a query. What global is it using? How does myObject acquire a reference of some kind to your servers, and the context in which it is stored? (That's the answer). saveAll is a static (class) method. It takes in a collection of instantiated objects and saves them to our servers. What global is it using? As for interjecting custom behavior, perhaps you're missing that all of these methods take in a variety of callbacks that can modify behavior. How do I arbitrarily modify the saveAll method so that I can perform arbitrary operations on objects before they're saved, so that I can implement a custom undo manager (but only when working on objects in the context of the user editing the document. Elsewhere I want different behavior)? ---- These approaches (class methods, singletons/globals) are breaking OO polymorphism, and significantly hamstringing your APIs. I'll try to provide an example that might matter to you: Let's pretend that I want to add support for sharing data between users of my application. It's important to me to not repeat myself, so I want to have the same code be able to write data to the user's personal storage, write data to one of several possible shared storage destinations. You decide you want to support this API. How do you make your current global/singleton API work for me?
    - csmajorfive 15y ago
      The hostname of our servers is indeed hardcoded in the framework. So, yes, that's a 'singleton' in the same way that configuring your database hostname in Rails is a singleton. So far our developers have had a lot of success building upon Parse. There are certainly some missing pieces (like the one pointed out in another comment on this post) but none of them have related to our overall architecture. I invite you to let us know what it should look like ideally at founders@parse.com or even stop by our office. We love to talk with developers.

deleted 15y ago

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