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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 isn’t any difficulty with them, they at all times return a brand new occasion of a given object that you simply wish to entry.
The issue is while 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 the whole lot there upfront, earlier than the route handlers. 💡
import Vapor
non-public 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 assume that hey this appears to be like good and it will work and you’re proper, it really works, however there’s a HUGE drawback with this resolution. It is not thread-safe in any respect. ⚠️
Once you open the browser and sort http://localhost:8080/ the web page will load, however while 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 related difficulty on GitHub, which describes the very same drawback. 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 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 obtained very annoyed.
Constructing a customized international 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 normally an awesome place to get the suitable solutions.
I’ve additionally regarded up different internet frameworks, and I used to be fairly shocked that Hummingbird affords 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 wanting on the ideas I noticed that I would like 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 method to return the values primarily based on the keys. If you happen to 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 non-public Cache class that can maintain our tips that could the situations. The cache is out there 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)
}
}
non-public extension Request.Globals {
remaining class Cache {
non-public 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 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 resolution. Perhaps it is nonetheless not one of the simplest ways to deal with this difficulty, nevertheless 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 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 assessments utilizing wrk I used to be capable of 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 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 resolve many concurrency issues, however we’ll have some new ones as properly. I actually hope that Vapor 5 shall 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, nevertheless it’s nice to see that lastly we’re going to have the ability to do away with EventLoopFutures. 🥳
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