Add deferred hostname resolution functionality

 - If immediate hostname resolution on creation of RedisConfiguration
   fails, retry gracefully on pool creation and on boot

Issue: 212

Author: eric-musliner

Sources/Redis/Application.Redis+configuration.swift

@@ -15,4 +15,18 @@ extension Application.Redis {
             self.application.redisStorage.use(newConfig, as: self.id)
         }
     }
+
+    /// Attempts to resolve any pending hostname resolutions.
+    /// This can be used if hostname resolution failed during initial configuration
+    /// and you want to retry after DNS becomes available
+    ///
+    /// This only updates the configuration and any existing connection pools are not affected.
+    public func retryHostnameResolution() throws {
+        try self.application.redisStorage.retryHostnameResolution(for: self.id)
+    }
+
+    /// Indicates whether this Redis configuration has unresolved hostnames
+    public var hasUnresolvedHostname: Bool {
+        return self.configuration?.hasUnresolvedHostname ?? false
+    }
 }

Sources/Redis/RedisConfiguration.swift

@@ -16,6 +16,13 @@ public struct RedisConfiguration: Sendable {
     public var tlsConfiguration: TLSConfiguration?
     public var tlsHostname: String?
 
+    internal var deferredHostname: String?
+    internal var deferredPort: Int?
+
+    public var hasUnresolvedHostname: Bool {
+        return deferredHostname != nil
+    }
+
     public struct PoolOptions: Sendable {
         public var maximumConnectionCount: RedisConnectionPoolSize
         public var minimumConnectionCount: Int
@@ -83,14 +90,22 @@ public struct RedisConfiguration: Sendable {
     ) throws {
         if database != nil && database! < 0 { throw ValidationError.outOfBoundsDatabaseID }
 
-        try self.init(
-            serverAddresses: [.makeAddressResolvingHost(hostname, port: port)],
-            password: password,
-            tlsConfiguration: tlsConfiguration,
-            tlsHostname: hostname,
-            database: database,
-            pool: pool
-        )
+        do {
+            let resolvedAdress = try SocketAddress.makeAddressResolvingHost(hostname, port: port)
+            self.serverAddresses = [resolvedAdress]
+            self.deferredHostname = nil
+            self.deferredPort = nil
+        } catch {
+            self.serverAddresses = []
+            self.deferredHostname = hostname
+            self.deferredPort = port
+        }
+
+        self.password = password
+        self.tlsConfiguration = tlsConfiguration
+        self.tlsHostname = hostname
+        self.database = database
+        self.pool = pool
     }
 
     public init(
@@ -102,18 +117,48 @@ public struct RedisConfiguration: Sendable {
         pool: PoolOptions = .init()
     ) throws {
         self.serverAddresses = serverAddresses
+        self.deferredHostname = nil
+        self.deferredPort = nil
         self.password = password
         self.tlsConfiguration = tlsConfiguration
         self.tlsHostname = tlsHostname
         self.database = database
         self.pool = pool
     }
+
+    /// Attempts to resolve any pending hostname resolution
+    ///  - Returns: new configuration with resolved addresses, or throws if resolution fails
+    public func resolveServerAddresses() throws -> RedisConfiguration {
+        guard let hostname = deferredHostname, let port = deferredPort else {
+            return self
+        }
+
+        var resolved = self
+        let resolvedAddress = try SocketAddress.makeAddressResolvingHost(hostname, port: port)
+        resolved.serverAddresses = [resolvedAddress]
+        resolved.deferredHostname = nil
+        resolved.deferredPort = nil
+        return resolved
+    }
 }
 
 extension RedisConnectionPool.Configuration {
     internal init(_ config: RedisConfiguration, defaultLogger: Logger, customClient: ClientBootstrap?) {
+        // Handle deferred hostname resolution at pool creation time
+        var addresses = config.serverAddresses
+
+        if let hostname = config.deferredHostname, let port = config.deferredPort {
+            do {
+                let resolvedAddress = try SocketAddress.makeAddressResolvingHost(hostname, port: port)
+                addresses = [resolvedAddress]
+            } catch {
+                defaultLogger.notice("Hostname '\(hostname)' could not be resolved at pool creation time: \(error). Redis connections will fail until hostname becomes resolvable.")
+                // Placeholder address so Redis operations fail gracefully
+                addresses = [try! SocketAddress.makeAddressResolvingHost("0.0.0.0", port: 1)]
+            }
+        }
         self.init(
-            initialServerConnectionAddresses: config.serverAddresses,
+            initialServerConnectionAddresses: addresses,
             maximumConnectionCount: config.pool.maximumConnectionCount,
             connectionFactoryConfiguration: .init(
                 connectionInitialDatabase: config.database,

Sources/Redis/RedisStorage.swift

@@ -57,6 +57,19 @@ final class RedisStorage: Sendable {
         }
         return pool
     }
+
+    func retryHostnameResolution(for id: RedisID = .default) throws {
+        guard let configuration = self.configuration(for: id) else {
+            throw RedisConfiguration.ValidationError.missingURLHost
+        }
+
+        guard configuration.hasUnresolvedHostname else {
+            return
+        }
+
+        let resolvedConfiguration = try configuration.resolveServerAddresses()
+        self.use(resolvedConfiguration, as: id)
+    }
 }
 
 extension RedisStorage {
@@ -73,12 +86,26 @@ extension RedisStorage {
             for eventLoop in application.eventLoopGroup.makeIterator() {
                 redisStorage.box.withLockedValue { storageBox in
                     for (redisID, configuration) in storageBox.configurations {
+                        // Attempt to resolve any deferred hostnames at boot time
+                        let resolvedConfiguration: RedisConfiguration
+                        if configuration.hasUnresolvedHostname {
+                            do {
+                                resolvedConfiguration = try configuration.resolveServerAddresses()
+                                application.logger.info("Successfully resolved Redis hostname for \(redisID) during application boot")
+                                storageBox.configurations[redisID] = resolvedConfiguration
+                            } catch {
+                                application.logger.warning("Redis hostname resolution failed for \(redisID) during boot: \(error)")
+                                resolvedConfiguration = configuration
+                            }
+                        } else {
+                            resolvedConfiguration = configuration
+                        }
 
                         let newKey: PoolKey = PoolKey(eventLoopKey: eventLoop.key, redisID: redisID)
 
                         let redisTLSClient: ClientBootstrap? = {
-                            guard let tlsConfig = configuration.tlsConfiguration,
-                                    let tlsHost = configuration.tlsHostname else { return nil }
+                            guard let tlsConfig = resolvedConfiguration.tlsConfiguration,
+                                    let tlsHost = resolvedConfiguration.tlsHostname else { return nil }
 
                             return ClientBootstrap(group: eventLoop)
                                 .channelOption(ChannelOptions.socket(SocketOptionLevel(SOL_SOCKET), SO_REUSEADDR), value: 1)
@@ -97,7 +124,7 @@ extension RedisStorage {
                         }()
 
                         let newPool = RedisConnectionPool(
-                            configuration: .init(configuration, defaultLogger: application.logger, customClient: redisTLSClient),
+                            configuration: .init(resolvedConfiguration, defaultLogger: application.logger, customClient: redisTLSClient),
                             boundEventLoop: eventLoop)
 
                         newPools[newKey] = newPool

Tests/RedisTests/RedisTests.swift

@@ -68,6 +68,39 @@ extension RedisTests {
         XCTAssertEqual(redisConfiguration.password, "password")
         XCTAssertEqual(redisConfiguration.database, 0)
     }
+
+    func testDeferredHostnameResolution() throws {
+        let config = try RedisConfiguration(
+            hostname: "nonexistent.hostname",
+            port: 6379,
+            pool: .init(connectionRetryTimeout: .milliseconds(100))
+        )
+
+        XCTAssertTrue(config.hasUnresolvedHostname)
+
+        let localhostConfig = try RedisConfiguration(
+            hostname: "localhost",
+            port: 6379
+        )
+
+        XCTAssertFalse(localhostConfig.hasUnresolvedHostname)
+    }
+
+    func testHostnameResolutionRetry() throws {
+        let config = try RedisConfiguration(
+            hostname: "nonexistent.hostname",
+            port: 6379
+        )
+
+        let app = Application(.testing)
+        defer { app.shutdown() }
+
+        app.redis.configuration = config
+
+        if app.redis.hasUnresolvedHostname {
+            XCTAssertThrowsError(try app.redis.retryHostnameResolution())
+        }
+    }
 }
 
 // MARK: Redis extensions