The issue with app providers
Vapor has a factor known as providers, you may add new performance to the system by following the sample described within the documentation. Learn-only providers are nice there isn’t a difficulty with them, they at all times return a brand new occasion of a given object that you simply need to entry.
The issue is whenever 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 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 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 may suppose that hey this appears good and it will work and you’re proper, it really works, however there’s a HUGE downside with this answer. It is not 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 appliance will merely crash.
There’s a related difficulty on GitHub, which describes the very same downside. Sadly I used to be unable to unravel this with locks, I do not know why but it surely 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 should not code signed by default. I’ve spent so many hours on this and I’ve acquired very pissed off.
Constructing a customized world storage
After a break this difficulty was nonetheless bugging my thoughts, so I’ve determined to do some extra analysis. Vapor’s discord server is often a fantastic place to get the correct solutions.
I’ve additionally appeared up different internet frameworks, and I used to be fairly stunned that Hummingbird provides 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 ideas I spotted that I want one thing much 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 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 to the cases. The cache is on the market by 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(_ key: String) -> T? {
cache[key]
}
func has(_ key: String) -> Bool {
get(key) != nil
}
func set(_ key: String, worth: T) {
cache[key] = worth
}
func unset(_ key: String) {
cache.unset(key)
}
}
personal extension Request.Globals {
last class Cache {
personal var storage: [String: Any]
init() {
self.storage = [:]
}
subscript(_ 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 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 difficulty, but it surely works. I used to be capable of retailer my variables inside a worldwide storage with out crashes or leaks. The req.globals storage property goes to be shared and it makes doable 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 exams 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 unsure 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 consider that there’s not sufficient details about thread security.
The introduction of async / await in Vapor will resolve many concurrency issues, however we will have some new ones as properly. I actually hope that Vapor 5 might 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, but it surely’s nice to see that lastly we’re going to have the ability to do away with EventLoopFutures. 🥳
