#resources #hot-reloading

warmy

Hot-reloading loadable and reloadable resources

14 releases (7 breaking)

0.8.0 Aug 13, 2018
0.7.3 Jul 23, 2018
0.7.2 Apr 30, 2018
0.6.0 Feb 24, 2018
0.3.0 Nov 26, 2017

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warmy, hot-reloading loadable and reloadable resources

Build Status dependency status crates.io docs.rs License

warmy is a crate that helps you introduce hot-reloading in your software.

The crate has an embedded tutorial in its documentation that will provide you with enough details to both understand what warmy is about and how you’re supposed to use it.


lib.rs:

Hot-reloading, loadable and reloadable resources.

Foreword

Resources are objects that live in a store and can be hot-reloaded – i.e. they can change without you interacting with them. There are currently two types of resources supported:

  • Filesystem resources, which are resources that live on the filesystem and have a real representation (i.e. a file for short).
  • Logical resources, which are resources that are computed and don’t directly require any I/O.

Resources are referred to by keys. A key is a typed index that contains enough information to uniquely identify a resource living in a store. You will find filesystem keys and logical keys.

This small introduction will give you enough information and examples to get your feet wet with warmy. If you want to know more, feel free to visit the documentation of submodules.

Loading a resource

Loading is the action of getting an object out of a given location. That location is often your filesystem but it can also be a memory area – mapped files or memory parsing. In warmy, loading is implemented per-type: this means you have to implement a trait on a type so that any object of that type can be loaded. The trait to implement is [Load]. We’re interested in four items:

  • The [Store], which holds and caches resources.
  • The Load::Key associated type, used to tell warmy which kind of resource your type represents and what information the key must contain.
  • The Load::Error associated type, that is the error type used when loading fails.
  • The Load::load method, which is the method called to load your resource in a given store.

Store

A [Store] is responsible for holding and caching resources. Each [Store] is associated with a root, which is a path on the filesystem all filesystem resources will come from. You create a [Store] by giving it a [StoreOpt], which is used to customize the [Store] – if you don’t need it, use Store::default.

use warmy::{Store, StoreOpt};

let res = Store::<()>::new(StoreOpt::default());

match res {
  Err(e) => {
    eprintln!("unable to create the store: {:#?}", e);
  }

  Ok(store) => ()
}

As you can see, the [Store] has a type variable. This type variable refers to the type of context you want to use with your resource. For now we’ll use () as we don’t want contexts, but more to come. Keep on reading.

Load::Key

This associated type must implement [Key], which is the class of types recognized as keys by warmy. In theory, you shouldn’t worry about that trait because warmy already ships with some key types.

If you really want to implement [Key], have a look at its documentation for further details.

Keys are a core concept in warmy as they are objects that uniquely represent resources – should they be on a filesystem or in memory. You will refer to your resources with those keys.

Let’s dig in some key types.

The classic: FSKey, the filesystem key

[FSKey] is the type of key to choose if you want to refer to a resource on a filesystem. It’s very easy to build one:

use warmy::FSKey;

let my_key = FSKey::new("/foo/bar/zoo.json");

The paths you use in [FSKey] are always relative to the store’s root, which implements some kind of a VFS for those keys.

Note: if you don’t use the leading '/', the [FSKey] is still considered as if it was expressed with a leading '/'. Both FSKey::new("/zulu.json") and FSKey::new("zulu.json") refer to the exact same resource.

Flexibility: LogicalKey, the memory key

This type of key is a bit hard to wrap your finger around at first, because you might not need it. This type of key enables you to create unique identifiers for resources that do not necessarily exist on a filesystem. Those are like keys in a key-value store (think of the local storage of your web browser, for instance).

However, they come in very handy when coping with dependency graphs. More on that in a few minutes – keep on reading!

use warmy::LogicalKey;

let my_key = LogicalKey::new("586e6452-4bac-11e8-842f-0ed5f89f718b");

Logical keys are very simple to use and may contain any kind of information. However, for now, they must be encoded with strings.

Special case: dependency key

A dependency key (a.k.a. [DepKey]) is a key used to express dependencies. Any type of key that implements [Key] also implements Into<DepKey>, which comes in handy when you want to build heterogenous lists of dependency keys.

[DepKey] is either akin to a [FSKey] or [LogicalKey].

Load::Error

This associated type must be set to the type of error your loading implementation might generate. For instance, if you load something with serde-json, you might want to set it to °serde_json::Error`.

On a general note, you should always try to stick precise and accurate errors.Avoid simple types such as String or u64 and prefer to use detailed, algebraic datatypes.

Load::load

This is the entry-point method. Load::load must be implemented in order for warmy to know how to read the resource. Let’s implement it for two types: one that represents a resource on the filesystem, one computed from memory.

use std::fs::File;
use std::io::{self, Read};
use std::path::PathBuf;
use warmy::{FSKey, Load, Loaded, LogicalKey, Storage};

// The resource we want to take from a file.
struct FromFS(String);

// The resource we want to compute from memory.
struct FromMem(usize);

impl<C> Load<C> for FromFS {
  type Key = FSKey;

  type Error = io::Error;

  fn load(
    key: Self::Key,
    storage: &mut Storage<C>,
    _: &mut C
  ) -> Result<Loaded<Self>, Self::Error> {
    let mut fh = File::open(key.as_path())?;
    let mut s = String::new();
    fh.read_to_string(&mut s);

    Ok(FromFS(s).into())
  }
}

impl<C> Load<C> for FromMem {
  type Key = LogicalKey;

  type Error = io::Error;

  fn load(
    key: Self::Key,
    storage: &mut Storage<C>,
    _: &mut C
  ) -> Result<Loaded<Self>, Self::Error> {
    // this is a bit dummy, but why not?
    Ok(FromMem(key.as_str().len()).into())
  }
}

As you can see here, there’re a few new concepts:

  • [Loaded]: A type you have to wrap your object in to express dependencies. Because it implements From<T> for Loaded<T>, you can use .into() to state you don’t have any dependencies.
  • [Storage]: This is the minimal structure that holds and caches your resources. A [Store] is actually the interface structure you will handle in your client code.

Express your dependencies with Loaded

An object of type [Loaded] gives information to warmy about your dependencies. Upon loading – i.e. your resource is successfully loaded – you can tell warmy which resources your loaded resource depends on. This is a bit tricky, though, because a diffference is important to make there.

When you implement Load::load, you are handed a [Storage]. You can use that [Storage] to load additional resources and gather them in your resources. When those additional resources get reloaded, if you directly embed the resources in your object, you will automatically see the automated resources. However, if you don’t express a dependency relationship to those resources, your former resource will not reload – it will just use automatically-synced resources, but it will not reload itself. This is a bit touchy but let’s take an example of a typical situation where you might want to use dependencies and then dependencies graphs:

  1. You want to load an object that is represented by aggregation of several values / resources.
  2. You choose to use a logical resource and guess all the files to load from a [LogicalKey].
  3. When you implement Load::load, you open several files, load them into memory, compose them and finally end up with your object.
  4. You return your object from Load::load with no dependencies (i.e. you use .into() on it).

What is going to happen here is that if any of the files your resource depends on changes, since they don’t have a proper resource in the store, your object will see nothing. A typical solution there is to load those files as proper resources (by using [FSKey]) and put those keys in the returned [Loaded] object to express that you depend on the reloading of the objects referred by these keys. It’s a bit touchy but you will eventually find yourself in a situation when this [Loaded] thing will help you. You will then use Loaded::with_deps. See the documentation of [Loaded] for further information.

Fun fact: [LogicalKey] was introduced to solve that problem along with dependency graphs.

Let’s get some things!

When you have implemented [Load], you’re set and ready to get (cached) resources. You have several functions to achieve that goal:

  • Store::get, used to get a resource. This will effectively load it if it’s the first time it’s asked. If it’s not, it will use a cached version.
  • Store::get_proxied, a special version of Store::get. If the initial loading (non-cached) fails to load (missing resource, fail to parse, whatever), a proxy will be used – passed in to Store::get_proxied. This value is lazy though, so if the loading succeeds, that value won’t ever be evaluated.

Let’s focus on Store::get for this tutorial.

use std::fs::File;
use std::io::{self, Read};
use std::path::PathBuf;
use warmy::{FSKey, Load, Loaded, LogicalKey, Res, Store, StoreOpt, Storage};

// The resource we want to take from a file.
struct FromFS(String);

impl<C> Load<C> for FromFS {
  type Key = FSKey;

  type Error = io::Error;

  fn load(
    key: Self::Key,
    storage: &mut Storage<C>,
    _: &mut C
  ) -> Result<Loaded<Self>, Self::Error> {
    let mut fh = File::open(key.as_path())?;
    let mut s = String::new();
    fh.read_to_string(&mut s);

    Ok(FromFS(s).into())
  }
}

fn main() {
  // we don’t need a context, so we’re using this mutable reference to unit
  let ctx = &mut ();
  let mut store: Store<()> = Store::new(StoreOpt::default()).expect("store creation");

  let my_resource = store.get::<_, FromFS>(&FSKey::new("/foo/bar/zoo.json"), ctx);

  //

  // imagine that you’re in an event loop now and the resource has changed
  store.sync(ctx); // synchronize all resources (e.g. my_resource) with the filesystem
}

Reloading a resource

Most of the interesting concept of warmy is to enable you to hot-reload resources without having to re-run your application. This is done via two items:

  • Load::reload, a method called whenever an object must be reloaded.
  • Store::sync, a method to synchronize a [Store] with the part of the filesystem it’s responsible for.

The Load::reload function is very straight-forward: it’s called when the resource changes. This situation happens:

  • Either when the resource is on the filesystem (the file changes).
  • Or if it’s a dependent resource of one that has reloaded.

See the documentation of Load::reload for further details.

Context inspection

A context is a special value you can access to via a mutable reference when loading or reloading. If you don’t need any, it’s highly recommended not to use () when implementing Load<C> but leave it as type variable so that it compose better – i.e. impl<C> Load<C>.

If you’re writing a library and need to have access to a specific value in a context, it’s also recommended not to set the context type variable to the type of your context directly. If you do that, no one will be able to use your library because types won’t match – or people will accept to be restrained to your only types. A typical way to deal with that is by constraining a type variable. The [Inspect] trait was introduced for this very purpose. For instance:

use std::io;
use warmy::{Inspect, Load, Loaded, LogicalKey, Storage};

struct Foo;

struct Ctx {
  nb_res_loaded: usize
}

impl<C> Load<C> for Foo where Foo: for<'a> Inspect<'a, C, &'a mut Ctx> {
  type Key = LogicalKey;

  type Error = io::Error;

  fn load(
    key: Self::Key,
    storage: &mut Storage<C>,
    ctx: &mut C
  ) -> Result<Loaded<Self>, Self::Error> {
    Self::inspect(ctx).nb_res_loaded += 1; // magic happens here!

    Ok(Foo.into())
  }
}

fn main() {
  use warmy::{Res, Store, StoreOpt};

  let mut store: Store<Ctx> = Store::new(StoreOpt::default()).unwrap();
  let mut ctx = Ctx { nb_res_loaded: 0 };

  let r: Res<Foo> = store.get(&LogicalKey::new("test-0"), &mut ctx).unwrap();
}

In this example, because the context value we want is the same as the [Store]’s context, a universal implementor of [Inspect] enables you to directly inspect the context. However, if you wanted to inspect it more precisely, like with &mut usize, you would need to write an implementation of [Inspect] for your types:

use std::io;
use warmy::{Inspect, Load, Loaded, LogicalKey, Storage};

struct Foo;

struct Ctx {
  nb_res_loaded: usize
}

// this implementor states how the inspection should occur for Foo when the context has type
// Ctx: by targetting a mutable reference on a usize (i.e. the counter)
impl<'a> Inspect<'a, Ctx, &'a mut usize> for Foo {
  fn inspect(ctx: &mut Ctx) -> &mut usize {
    &mut ctx.nb_res_loaded
  }
}

// notice the usize instead of Ctx here
impl<C> Load<C> for Foo where Foo: for<'a> Inspect<'a, C, &'a mut usize> {
  type Key = LogicalKey;

  type Error = io::Error;

  fn load(
    key: Self::Key,
    storage: &mut Storage<C>,
    ctx: &mut C
  ) -> Result<Loaded<Self>, Self::Error> {
    *Self::inspect(ctx) += 1; // direct access to the counter

    Ok(Foo.into())
  }
}

Load methods

warmy supports load methods. Those are used to specify several ways to load an object of a given type. By default, [Load] is implemented with the default method – which is (). If you want more methods, you can set the type parameter to something else when implementing [Load].

You can also find several methods centralized in here, but you definitely don’t have to use them. In theory, those will be removed and placed into other crates to add automatic implementations.

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Dependencies

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