The issue with app providers

Vapor has a factor referred to as providers, you may add new performance to the system by following the sample described within the documentation. Learn-only providers are nice there is no such thing as a subject with them, they all the time return a brand new occasion of a given object that you simply wish to entry.

The issue is once you wish to entry a shared object or in different phrases, you wish to outline a writable service. In my case I wished to create a shared cache dictionary that I might use to retailer some preloaded variables from the database.

My preliminary try was to create a writable service that I can use to retailer these key-value pairs. I additionally wished to make use of a middleware and cargo every thing there upfront, earlier than the route handlers. 💡

import Vapor

personal extension Software {
    
    struct VariablesStorageKey: StorageKey {
        typealias Worth = [String: String]
    }

    var variables: [String: String] {
        get {
            self.storage[VariablesStorageKey.self] ?? [:]
        }
        set {
            self.storage[VariablesStorageKey.self] = newValue
        }
    }
}

public extension Request {
    
    func variable(_ key: String) -> String? {
        utility.variables[key]
    }
}

struct CommonVariablesMiddleware: AsyncMiddleware {

    func reply(to req: Request, chainingTo subsequent: AsyncResponder) async throws -> Response {
        let variables = strive await CommonVariableModel.question(on: req.db).all()
        var tmp: [String: String] = [:]
        for variable in variables {
            if let worth = variable.worth {
                tmp[variable.key] = worth
            }
        }
        req.utility.variables = tmp
        return strive await subsequent.reply(to: req)
    }
}

Now you would possibly assume that hey this appears to be like good and it will work and you might be proper, it really works, however there’s a HUGE downside with this resolution. It isn’t thread-safe in any respect. ⚠️

While you open the browser and kind http://localhost:8080/ the web page will load, however once you begin bombarding the server with a number of requests utilizing a number of threads (wrk -t12 -c400 -d30s http://127.0.0.1:8080/) the appliance will merely crash.

There’s a related subject on GitHub, which describes the very same downside. Sadly I used to be unable to resolve this with locks, I do not know why nevertheless it tousled much more issues with unusual errors and since I am additionally not in a position to run devices on my M1 Mac Mini, as a result of Swift packages will not be code signed by default. I’ve spent so many hours on this and I’ve bought very pissed off.

Constructing a customized world storage

After a break this subject was nonetheless bugging my thoughts, so I’ve determined to do some extra analysis. Vapor’s discord server is often an awesome place to get the precise solutions.

I’ve additionally seemed up different net frameworks, and I used to be fairly shocked that Hummingbird presents an EventLoopStorage by default. Anyway, I am not going to change, however nonetheless it is a good to have function.

As I used to be trying on the recommendations I noticed that I want one thing just like the req.auth property, so I’ve began to research the implementation particulars extra intently.

First, I eliminated the protocols, as a result of I solely wanted a plain [String: Any] dictionary and a generic solution to return the values primarily based on the keys. For those who take a better look it is fairly a easy design sample. There’s a helper struct that shops the reference of the request and this struct has an personal Cache class that can maintain our tips that could the situations. The cache is accessible by way of a property and it’s saved contained in the req.storage.

import Vapor

public extension Request {

    var globals: Globals {
        return .init(self)
    }

    struct Globals {
        let req: Request

        init(_ req: Request) {
            self.req = req
        }
    }
}

public extension Request.Globals {

    func get<T>(_ key: String) -> T? {
        cache[key]
    }
    
    func has(_ key: String) -> Bool {
        get(key) != nil
    }
    
    func set<T>(_ key: String, worth: T) {
        cache[key] = worth
    }
    
    func unset(_ key: String) {
        cache.unset(key)
    }
}


personal extension Request.Globals {

    closing class Cache {
        personal var storage: [String: Any]

        init() {
            self.storage = [:]
        }

        subscript<T>(_ kind: String) -> T? {
            get { storage[type] as? T }
            set { storage[type] = newValue }
        }
        
        func unset(_ key: String) {
            storage.removeValue(forKey: key)
        }
    }

    struct CacheKey: StorageKey {
        typealias Worth = Cache
    }

    var cache: Cache {
        get {
            if let current = req.storage[CacheKey.self] {
                return current
            }
            let new = Cache()
            req.storage[CacheKey.self] = new
            return new
        }
        set {
            req.storage[CacheKey.self] = newValue
        }
    }
}

After altering the unique code I’ve give you this resolution. Possibly it is nonetheless not one of the simplest ways to deal with this subject, nevertheless it works. I used to be in a position to retailer my variables inside a world storage with out crashes or leaks. The req.globals storage property goes to be shared and it makes attainable to retailer knowledge that must be loaded asynchronously. 😅

import Vapor

public extension Request {
    
    func variable(_ key: String) -> String? {
        globals.get(key)
    }
}

struct CommonVariablesMiddleware: AsyncMiddleware {

    func reply(to req: Request, chainingTo subsequent: AsyncResponder) async throws -> Response {
        let variables = strive await CommonVariableModel.question(on: req.db).all()
        for variable in variables {
            if let worth = variable.worth {
                req.globals.set(variable.key, worth: worth)
            }
            else {
                req.globals.unset(variable.key)
            }
        }
        return strive await subsequent.reply(to: req)
    }
}

After I’ve run a number of extra checks utilizing wrk I used to be in a position to affirm that the answer works. I had no points with threads and the app had no reminiscence leaks. It was a aid, however nonetheless I am undecided if that is one of the simplest ways to deal with my downside or not. Anyway I wished to share this with you as a result of I imagine that there’s not sufficient details about thread security.

The introduction of async / await in Vapor will remedy many concurrency issues, however we will have some new ones as effectively. I actually hope that Vapor 5 will likely be an enormous enchancment over v4, persons are already throwing in concepts and they’re having discussions about the way forward for Vapor on discord. That is only the start of the async / await period each for Swift and Vapor, nevertheless it’s nice to see that lastly we’re going to have the ability to do away with EventLoopFutures. 🥳