Constructing a world storage for Vapor

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The issue with app providers


Vapor has a factor known as providers, you possibly can add new performance to the system by following the sample described within the documentation. Learn-only providers are nice there isn’t any subject with them, they all the time return a brand new occasion of a given object that you just wish to entry.

The issue is whenever you wish to entry a shared object or in different phrases, you wish to outline a writable service. In my case I needed to create a shared cache dictionary that I may 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 needed to make use of a middleware and cargo every little thing there upfront, earlier than the route handlers. 💡


import Vapor

personal extension Utility {
    
    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? {
        software.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.software.variables = tmp
        return strive await subsequent.reply(to: req)
    }
}


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


If you open the browser and kind http://localhost:8080/ the web page will load, however whenever 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 applying will merely crash.

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



Constructing a customized international 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 appeared up different internet frameworks, and I used to be fairly shocked that Hummingbird presents an EventLoopStorage by default. Anyway, I am not going to modify, however nonetheless it is a good to have function.


As I used to be trying on the solutions I spotted that I would like one thing much like the req.auth property, so I’ve began to analyze the implementation particulars extra carefully.


First, I eliminated the protocols, as a result of I solely wanted a plain [String: Any] dictionary and a generic option to return the values primarily based on the keys. In the event you take a more in-depth 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 out there by means 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 {

    remaining 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 present = req.storage[CacheKey.self] {
                return present
            }
            let new = Cache()
            req.storage[CacheKey.self] = new
            return new
        }
        set {
            req.storage[CacheKey.self] = newValue
        }
    }
}


After altering the unique code I’ve provide you with this answer. Possibly it is nonetheless not one of the best ways to deal with this subject, however it works. I used to be capable of retailer my variables inside a world storage with out crashes or leaks. The req.globals storage property goes to be shared and it makes potential to retailer information 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 assessments utilizing wrk I used to be capable of verify that the answer works. I had no points with threads and the app had no reminiscence leaks. It was a reduction, however nonetheless I am undecided if that is one of the best ways to deal with my downside or not. Anyway I needed 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’ll have some new ones as effectively. I actually hope that Vapor 5 can 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 just the start of the async / await period each for Swift and Vapor, however it’s nice to see that lastly we’re going to have the ability to eliminate EventLoopFutures. 🥳


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