add prelude service to run PG initialization code

Author: sebsto

Examples/ServiceLifecycle+Postgres/Sources/Lambda.swift

@@ -47,12 +47,21 @@ struct LambdaFunction {
     private func main() async throws {
 
         // Instantiate LambdaRuntime with a handler implementing the business logic of the Lambda function
-        let runtime = LambdaRuntime(logger: self.logger, body: self.handler)
+        let lambdaRuntime = LambdaRuntime(logger: self.logger, body: self.handler)
+
+        // Use a prelude service to execute PG code before setting up the Lambda service
+        // the PG code will run only once and will create the database schema and populate it with initial data
+        let preludeService = PreludeService(
+            service: lambdaRuntime,
+            prelude: {
+                try await prepareDatabase()
+            }
+        )
 
         /// Use ServiceLifecycle to manage the initialization and termination
         /// of the PGClient together with the LambdaRuntime
         let serviceGroup = ServiceGroup(
-            services: [self.pgClient, runtime],
+            services: [self.pgClient, preludeService],
             gracefulShutdownSignals: [.sigterm],
             cancellationSignals: [.sigint],
             logger: self.logger
@@ -76,12 +85,6 @@ struct LambdaFunction {
             // This is why there is a timeout, as suggested Fabian
             // See: https://github.com/vapor/postgres-nio/issues/489#issuecomment-2186509773
             result = try await timeout(deadline: .seconds(3)) {
-                // check if table exists
-                // TODO: ideally, I want to do this once, after serviceGroup.run() is done
-                // but before the handler is called
-                logger.trace("Checking database")
-                try await prepareDatabase()
-
                 // query users
                 logger.trace("Querying database")
                 return try await self.queryUsers()

Examples/ServiceLifecycle+Postgres/Sources/PreludeService.swift

@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftAWSLambdaRuntime open source project
+//
+// Copyright (c) 2025 Apple Inc. and the SwiftAWSLambdaRuntime project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftAWSLambdaRuntime project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Hummingbird server framework project
+//
+// Copyright (c) 2024 the Hummingbird authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See hummingbird/CONTRIBUTORS.txt for the list of Hummingbird authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+// Copied from https://github.com/hummingbird-project/hummingbird/blob/main/Sources/Hummingbird/Utils/PreludeService.swift
+
+import ServiceLifecycle
+
+/// Wrap another service to run after a prelude closure has completed
+struct PreludeService<S: Service>: Service, CustomStringConvertible {
+    let prelude: @Sendable () async throws -> Void
+    let service: S
+
+    var description: String {
+        "PreludeService<\(S.self)>"
+    }
+
+    init(service: S, prelude: @escaping @Sendable () async throws -> Void) {
+        self.service = service
+        self.prelude = prelude
+    }
+
+    func run() async throws {
+        try await self.prelude()
+        try await self.service.run()
+    }
+}
+
+extension Service {
+    /// Build existential ``PreludeService`` from an existential `Service`
+    func withPrelude(_ prelude: @escaping @Sendable () async throws -> Void) -> Service {
+        PreludeService(service: self, prelude: prelude)
+    }
+}