MetricKit and Vapor

I recently learned about MetricKit from Apple and I thought this would be a good fit to learn something about my iOS code in the wild.

Using MetricKit could not be easier. After you conformed to MXMetricManagerSubscriber you can add yourself to the MXMetricManager

MXMetricManager.shared.add(self)

The most obvious place would be the AppDelegate's didFinishLaunchingWithOptions function. You can use applicationWillTerminate to remove yourself from the MXMetricManager

MXMetricManager.shared.remove(self)

The only thing that's left is implementing the delegate method:

extension AppDelegate: MXMetricManagerSubscriber {
    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads {
            let url = URL(string: "https://your.vapor.server/collect")!

            var request = URLRequest(url: url)
            request.httpMethod = "POST"
            request.httpBody = payload.jsonRepresentation()

            let task = URLSession.shared.dataTask(with: request)
            task.resume()
        }
    }
}

All this and more is nicely documented on NSHipster – MetricKit.

Vapor

So in Vapor we need a /collect-route that takes the payload. The easiest solution would be to build a struct that takes some (or all) of the information from the payload, but I wanted to do the same thing Matt did (in Ruby or JS) and just save the payload in a jsonb-column.

With the help of the really great people in the Vapor-Discord (namely: TypeBeta) I was able to achieve what I wanted with the following model-class

import Vapor
import Fluent
import PostgresNIO

struct JsonWrapper: Codable, PostgresDataConvertible {
    let payload: String

    static var postgresDataType: PostgresDataType {
        .jsonb
    }

    init(_ payload: String) {
        self.payload = payload
    }

    init(from decoder: Decoder) throws {
        self.payload = try String.init(from: decoder)
    }

    func encode(to encoder: Encoder) throws {
        try payload.encode(to: encoder)
    }

    init?(postgresData: PostgresData) {
        guard let data = postgresData.jsonb else { return nil }
        guard let payload = String(data: data, encoding: .utf8) else { return nil }
        self.init(payload)
    }
    
    var postgresData: PostgresData? {
        guard let jsonString = self.payload.data(using: .utf8) else { return nil }
        return .init(jsonb: jsonString)
    }
}

final class Metric: Model, Content {
    static let schema = "metrics"

    @ID(key: .id)
    var id: UUID?
    
    @Field(key: "payload")
    var payload: JsonWrapper

    init() { }

    init(id: UUID? = nil, payload: JsonWrapper) {
        self.id = id
        self.payload = payload
    }
}

In the Metric-class the member payload is of type JsonWrapper. This type tells postgres how to get the wrapped payload into postgres (var postgresData) and how to initialise the payload from the data that is saved in postgres (init?(postgresData: PostgresData)). The PostgresData-initializer (.init(jsonb: jsonString)) tells the type that it should treat jsonString as json.

Now the controller becomes a piece of cake 😎

struct MetricController: RouteCollection {
    func boot(routes: RoutesBuilder) throws {
        let collect = routes.grouped("collect")
        collect.post(use: create)
    }

    func create(req: Request) throws -> EventLoopFuture<HTTPStatus> {
        let metric = Metric()
        let payload_string = try req.content.decode(String.self)
        metric.payload = JsonWrapper(payload_string)
        return metric.save(on: req.db).transform(to: HTTPStatus.noContent)
    }
}

Heads up

Simulating MetricKit payload is only enabled on a real device. So you need to run your iOS code on a real device. Once you have data in the database you can query "into" your payload like this:

select payload->'appVersion' from metrics;

Or how about building a view:

CREATE VIEW app_versions AS SELECT id,payload->'appVersion' AS app_version,payload->'metaData'->>'deviceType' AS device_type FROM metrics;

Now you can do something like this:

select distinct(app_version) from app_versions;

Question

This is completely unrelated, but: Is it a good idea to have UUID as primary keys? Does this have any performance implications?