Static file caching for the Website service (#6)

This PR contains the work done to add cache-control headers to static file responses by configuring the `FileMiddleware` middleware to tag served static files with *Cache-Control* directives, tuned per media type.

To provide further details about the work done:

* Added a cache control that sets per-type policies: text assets (CSS/JS) get public, max-age, must-revalidate; images get public, max-age; everything else gets a default public, max-age.
* Files stay validated via `ETag/Last-Modified` header.
* Made the max-age values configurable, with defaults, via new cache constants.

Reviewed-on: rock-n-code/loud-amsterdam#6
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

.swift-format

@@ -39,7 +39,7 @@
     "NeverForceUnwrap": true,
     "NeverUseForceTry": true,
     "NeverUseImplicitlyUnwrappedOptionals": true,
-    "NoAccessLevelOnExtensionDeclaration": true,
+    "NoAccessLevelOnExtensionDeclaration": false,
     "NoAssignmentInExpressions": true,
     "NoBlockComments": true,
     "NoCasesWithOnlyFallthrough": true,

Services/Website/Sources/App/App+build.swift

@@ -12,6 +12,20 @@ import WebsiteCore
 func application(
     reader: ConfigReader
 ) async -> some ApplicationProtocol {
+    let cacheControl = cacheControl(
+        textMaxAge: reader.int(
+            forKey: .Cache.maxAgeText,
+            default: .Cache.maxAgeText
+        ),
+        imageMaxAge: reader.int(
+            forKey: .Cache.maxAgeImage,
+            default: .Cache.maxAgeImage
+        ),
+        defaultMaxAge: reader.int(
+            forKey: .Cache.maxAgeDefault,
+            default: .Cache.maxAgeDefault
+        )
+    )
     let logLevel = reader.string(
         forKey: .Log.level,
         as: Logger.Level.self,
@@ -29,6 +43,7 @@ func application(
     return Application(
         router: router(
             staticFilesPath: staticFilesPath,
+            cacheControl: cacheControl,
             logLevel: logLevel
         ),
         configuration: ApplicationConfiguration(
@@ -46,6 +61,29 @@ func application(
 // Request context used by application
 private typealias AppRequestContext = BasicRequestContext
 
+/// Builds the cache-control policy applied to the served static files.
+///
+/// Static files are public and validated by `FileMiddleware` through their `ETag` and
+/// `Last-Modified` headers, so each media type is given a `max-age` after which the browser
+/// revalidates. Text-based assets (CSS, JavaScript) are additionally marked `must-revalidate`
+/// since they change between deployments while keeping their filenames.
+/// - Parameters:
+///   - textMaxAge: the max-age, in seconds, applied to text-based static files (CSS, JavaScript, plain text).
+///   - imageMaxAge: the max-age, in seconds, applied to image static files (ICO, PNG, SVG).
+///   - defaultMaxAge: the max-age, in seconds, applied to all other static files (e.g. the web manifest).
+/// - Returns: the configured cache-control policy.
+private func cacheControl(
+    textMaxAge: Int,
+    imageMaxAge: Int,
+    defaultMaxAge: Int
+) -> CacheControl {
+    .init([
+        (.text, [.public, .maxAge(textMaxAge), .mustRevalidate]),
+        (.image, [.public, .maxAge(imageMaxAge)]),
+        (.init(type: .any), [.public, .maxAge(defaultMaxAge)]),
+    ])
+}
+
 /// Builds the application's logger.
 /// - Parameters:
 ///   - serverName: the label applied to the logger.
@@ -65,14 +103,17 @@ private func logger(
 /// Builds the application's router.
 ///
 /// Registers the request-logging middleware, the not-found middleware that serves the error
-/// page, and the static file middleware that serves the contents of `staticFilesPath`, then
-/// adds the `RootController` routes that render the landing page.
+/// 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.
 /// - Parameters:
 ///   - staticFilesPath: the folder, relative to the working directory, the static files are served from.
+///   - cacheControl: the cache-control directives applied to the served static files.
 ///   - logLevel: the level the request-logging middleware logs at.
 /// - Returns: the configured router.
 private func router(
     staticFilesPath: String,
+    cacheControl: CacheControl,
     logLevel: Logger.Level
 ) -> Router<AppRequestContext> {
     let router = Router(context: AppRequestContext.self)
@@ -80,7 +121,10 @@ private func router(
     router.addMiddleware {
         LogRequestsMiddleware(logLevel)
         NotFoundMiddleware()
-        FileMiddleware(staticFilesPath)
+        FileMiddleware(
+            staticFilesPath,
+            cacheControl: cacheControl
+        )
     }
 
     router.addRoutes(RootController<AppRequestContext>().routes)

Services/Website/Sources/Library/Public/Extensions/AbsoluteConfigKey+Constants.swift

@@ -1,6 +1,15 @@
 import Configuration
 
 extension AbsoluteConfigKey {
+    /// A namespace for the static files cache configuration keys, as absolute keys.
+    public enum Cache {
+        /// The absolute configuration key for the max-age, in seconds, applied to text-based static files.
+        public static let maxAgeText: AbsoluteConfigKey = .init(.Cache.maxAgeText)
+        /// The absolute configuration key for the max-age, in seconds, applied to image static files.
+        public static let maxAgeImage: AbsoluteConfigKey = .init(.Cache.maxAgeImage)
+        /// The absolute configuration key for the max-age, in seconds, applied to all other static files.
+        public static let maxAgeDefault: AbsoluteConfigKey = .init(.Cache.maxAgeDefault)
+    }
     /// A namespace for the HTTP server configuration keys, as absolute keys.
     public enum HTTP {
         /// The absolute configuration key for the host the server binds to.

Services/Website/Sources/Library/Public/Extensions/ConfigKey+Constants.swift

@@ -1,6 +1,15 @@
 import Configuration
 
 extension ConfigKey {
+    /// A namespace for the static files cache configuration keys.
+    public enum Cache {
+        /// The configuration key for the max-age, in seconds, applied to text-based static files (CSS, JavaScript, plain text).
+        public static let maxAgeText: ConfigKey = "cache.maxAge.text"
+        /// The configuration key for the max-age, in seconds, applied to image static files (ICO, PNG, SVG).
+        public static let maxAgeImage: ConfigKey = "cache.maxAge.image"
+        /// The configuration key for the max-age, in seconds, applied to all other static files (e.g. the web manifest).
+        public static let maxAgeDefault: ConfigKey = "cache.maxAge.default"
+    }
     /// A namespace for the HTTP server configuration keys.
     public enum HTTP {
         /// The configuration key for the host the server binds to.

Services/Website/Sources/Library/Public/Extensions/Int+Constants.swift

@@ -0,0 +1,11 @@
+extension Int {
+    /// A namespace for the cache's default configuration values.
+    public enum Cache {
+        /// The default max-age, in seconds, applied to text-based static files (1 hour).
+        public static let maxAgeText = 3_600
+        /// The default max-age, in seconds, applied to image static files (1 week).
+        public static let maxAgeImage = 604_800
+        /// The default max-age, in seconds, applied to all other static files (1 day).
+        public static let maxAgeDefault = 86_400
+    }
+}

Services/Website/Tests/App/AppTests.swift

@@ -13,6 +13,12 @@ import Testing
 struct AppTests {
 
     // MARK: Constants
+    
+    private let textExtensions: [StaticFile.Extension] = [
+        .css,
+        .js,
+        .txt
+    ]
 
     // Absolute path to the package's "Resources/Static" folder, derived from this
     // file's location so the static files resolve regardless of the working directory.
@@ -35,7 +41,7 @@ struct AppTests {
                 let body = String(buffer: response.body)
 
                 #expect(response.status == .ok)
-                #expect(response.headers[.contentType]?.hasPrefix("text/html") == true)
+                #expect(response.headers[.contentType] == "text/html; charset=utf-8")
                 #expect(body.contains("Hello world!"))
             }
         }
@@ -52,6 +58,15 @@ struct AppTests {
             ) { response in
                 #expect(response.status == .ok)
                 #expect(response.headers[.contentType] == file.contentType)
+                
+                let cacheControl = try #require(response.headers[.cacheControl])
+
+                #expect(cacheControl.contains("public") == true)
+                #expect(cacheControl.contains("max-age=") == true)
+                
+                if textExtensions.contains(file.fileExtension) {
+                    #expect(cacheControl.contains("must-revalidate") == true)
+                }
             }
         }
     }
@@ -66,7 +81,7 @@ struct AppTests {
                 let body = String(buffer: response.body)
 
                 #expect(response.status == .notFound)
-                #expect(response.headers[.contentType]?.hasPrefix("text/html") == true)
+                #expect(response.headers[.contentType] == "text/html; charset=utf-8")
                 #expect(body.contains("Page Not Found"))
             }
         }