Constructing a world storage for Vapor


The issue with app companies

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

The issue is if you need to entry a shared object or in different phrases, you need to outline a writable service. In my case I needed 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 needed to make use of a middleware and cargo all the things 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? {
        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 assume that hey this seems good and it will work and you’re proper, it really works, however there’s a HUGE drawback with this resolution. It isn’t thread-safe in any respect. ⚠️

Once you open the browser and sort http://localhost:8080/ the web page will load, however if 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 problem on GitHub, which describes the very same drawback. Sadly I used to be unable to resolve this with locks, I do not know why but it surely 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 should not code signed by default. I’ve spent so many hours on this and I’ve received very pissed off.

Constructing a customized world storage

After a break this problem was nonetheless bugging my thoughts, so I’ve determined to do some extra analysis. Vapor’s discord server is normally a fantastic place to get the correct solutions.

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

As I used to be trying on the solutions I spotted 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 approach to return the values primarily based on the keys. In case you 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 offered via 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 {

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

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

        subscript<T>(_ sort: 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 provide you with this resolution. Perhaps it is nonetheless not one of the simplest ways to deal with this problem, but it surely 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 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 exams 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 reduction, however nonetheless I am unsure if that is one of the simplest ways to deal with my drawback or not. Anyway I needed to share this with you as a result of I consider that there’s not sufficient details about thread security.

The introduction of async / await in Vapor will clear up many concurrency issues, however we’ll have some new ones as effectively. I actually hope that Vapor 5 shall be an enormous enchancment over v4, individuals 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, but it surely’s nice to see that lastly we’re going to have the ability to eliminate EventLoopFutures. 🥳

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