ccn/Services/Website/Sources/App/Extensions/App+Build.swift

import Configuration
import Hummingbird
import HummingbirdCompression
import Logging
import Persistence
import Infrastructure
import WebsiteLibrary

/// Builds the website application.
///
/// Reads the log level, server name, static files location, minimum response size to compress, and security headers from the configuration, then assembles
/// the router, server configuration, and logger. It also builds the persistence driver, registers its migrations, and attaches the `Fluent` service so it starts
/// and stops alongside the HTTP server; the ephemeral in-memory backend is migrated on startup, while a MySQL/MariaDB backend is migrated out of
/// band (so a shared database is never migrated on boot).
/// - Parameter reader: the configuration reader the values are read from.
/// - Returns: the configured application, ready to run as a service.
func application(
    reader: ConfigReader
) async -> some ApplicationProtocol {
    let logger = logger(
        serverName: reader.serverName,
        logLevel: reader.logLevel
    )
    let persistence = Service(
        driver: reader.driver,
        logger: logger
    )
    let fluent = persistence()

    let fingerprintAssets = FingerprintAssets(logger: logger)
    let prepareDB = PrepareDB()

    await prepareDB(for: fluent)

    var app = Application(
        router: router(
            staticFilesPath: reader.staticFilesPath,
            assetVersion: fingerprintAssets(reader.staticFilesPath),
            cacheControl: reader.cacheControl,
            compressionMinResponseSize: reader.compressionMinResponseSize,
            rateLimit: reader.rateLimit,
            securityHeaders: reader.securityHeaders,
            logLevel: reader.logLevel,
            probe: Probe(fluent: fluent)
        ),
        configuration: ApplicationConfiguration(
            reader: reader.scoped(to: "http")
        ),
        logger: logger
    )

    app.addServices(fluent)

    // The in-memory backend is recreated on every launch, so it is migrated on startup. The MySQL/MariaDB
    // backend is left untouched here: a shared database is migrated out of band to avoid multi-instance races.
    if case .inMemory = reader.driver {
        app.beforeServerStarts {
            try await fluent.migrate()
        }
    }

    return app
}

/// Runs every registered migration against the configured backend, then exits.
///
/// This is the out-of-band migration path selected by the `database.migrate` flag: it builds the same driver the service would run against, applies the
/// migrations, and shuts the database down — so a shared MySQL/MariaDB database is migrated by a single deliberate invocation rather than by every
/// booting instance.
/// - Parameter reader: the configuration reader the values are read from.
func migration(
    reader: ConfigReader
) async throws {
    let logger = logger(
        serverName: reader.serverName,
        logLevel: reader.logLevel
    )
    let service = Service(
        driver: reader.driver,
        logger: logger
    )

    let fluent = service()
    let prepareDB = PrepareDB()

    await prepareDB(for: fluent)

    do {
        try await fluent.migrate()
    }
    catch {
        try? await fluent.shutdown()

        throw error
    }

    try await fluent.shutdown()
}

// MARK: - Helpers

/// The request context type the application serves its routes with.
private typealias AppRequestContext = WebsiteRequestContext

/// Builds the application's logger.
/// - Parameters:
///   - serverName: the label applied to the logger.
///   - logLevel: the minimum level the logger emits.
/// - Returns: the configured logger.
private func logger(
    serverName: String,
    logLevel: Logger.Level
) -> Logger {
    var logger = Logger(label: serverName)

    logger.logLevel = logLevel

    return logger
}

/// Builds the application's router.
///
/// Registers the request-logging middleware, the security-headers middleware that stamps the given `securityHeaders` onto every response, the
/// vary middleware that marks every response as varying on `Accept-Encoding`, the response-compression middleware that compresses responses
/// larger than `minimumResponseSizeToCompress` when the client advertises support, the localization middleware that negotiates the request's
/// language from its `Accept-Language` header, the not-found middleware that serves the error page, and the static file middleware that serves the
/// contents of `staticFilesPath` (tagging responses with the given `cacheControl` directives), then adds the `RootController` routes that
/// render the landing page, the `SubscriptionController` routes that register newsletter subscriptions, and the `HealthController` routes
/// that serve the health check.
///
/// The security-headers middleware sits just inside request logging so it covers every response that reaches a client — the landing page, the compressed
/// responses, the rendered error page, and the served static files.
/// - Parameters:
///   - staticFilesPath: the folder, relative to the working directory, the static files are served from.
///   - assetVersion: the version token the pages append to their asset URLs, or `nil` to leave them unversioned.
///   - cacheControl: the cache-control directives applied to the served static files.
///   - compressionMinResponseSize: the minimum response body size, in bytes, before compression is applied.
///   - rateLimit: the rate limit applied to the subscription endpoint.
///   - securityHeaders: the security headers applied to every response.
///   - logLevel: the level the request-logging middleware logs at.
///   - probe: the probe consulted by the `HealthController` readiness route.
/// - Returns: the configured router.
private func router(
    staticFilesPath: String,
    assetVersion: String?,
    cacheControl: CacheControl,
    compressionMinResponseSize: Int,
    rateLimit: RateLimitMiddleware<AppRequestContext>.Configuration,
    securityHeaders: SecurityHeadersMiddleware<AppRequestContext>.Configuration,
    logLevel: Logger.Level,
    probe: Probe
) -> Router<AppRequestContext> {
    // HEAD siblings are generated for every GET route, so uptime monitors and crawlers probing
    // with HEAD requests get the page's status and headers instead of a 404.
    let router = Router(
        context: AppRequestContext.self,
        options: .autoGenerateHeadEndpoints
    )

    router.addMiddleware {
        LogRequestsMiddleware(logLevel)
        SecurityHeadersMiddleware(
            configuration: securityHeaders
        )
        VaryMiddleware()
        ResponseCompressionMiddleware(
            minimumResponseSizeToCompress: compressionMinResponseSize
        )
        LocalizationMiddleware()
        NotFoundMiddleware(
            assetVersion: assetVersion
        )
        FileMiddleware(
            staticFilesPath,
            cacheControl: cacheControl
        )
    }

    router.addController {
        RootController<AppRequestContext>(
            assetVersion: assetVersion
        )
        HealthController<AppRequestContext>(
            probe: probe
        )
    }

    return router
}