Adding 'github.com/lib/pq' library to use our PostgreSQL database. Updating our main.go file to establish a connection with the database pool.

cmd/api/main.go

@@ -1,12 +1,17 @@
 package main
 
 import (
+	"context"
+	"database/sql"
 	"flag"
 	"fmt"
 	"log/slog"
 	"net/http"
 	"os"
 	"time"
+
+	// Import the pq driver so that it can register itself with the database/sql package. Note that we alias this import to the blank identifier, to stop the Go compiler complaining that the package isn't being used.
+	_ "github.com/lib/pq"
 )
 
 /*
@@ -22,9 +27,13 @@ For now, the only configuration settings will be the network port that we want t
 and the name of the current operating environment for the application (development, staging, production, etc.)
 We'll read in the configuration settings from command-line flags when the application starts.
 */
+// Add a db struct field to hold the configuration settings for our database connection pool. For now, this only holds the DSN, which we will read in from a command-line flag.
 type config struct {
 	port int
 	env  string
+	db   struct {
+		dsn string
+	}
 }
 
 /*
@@ -46,11 +55,29 @@ func main() {
 	*/
 	flag.IntVar(&cfg.port, "port", 4000, "API server port")
 	flag.StringVar(&cfg.env, "env", "development", "Environment (development|staging|production")
+
+	// Read the DSN value from the db-dsn command-line flag into the config struct. We default to using our development DSN if no flag is provided.
+	// Use the value of the GREENLIGHT_DB_DSN environment variable as the default value for our db-dsn command-line flag.
+	flag.StringVar(&cfg.db.dsn, "db-dsn", os.Getenv("GREENLIGHT_DB_DSN"), "PostgreSQL DSN")
+
 	flag.Parse()
 
 	// Initialize a new structured logger which writes log entries to standard out stream.
 	logger := slog.New(slog.NewTextHandler(os.Stdout, nil))
 
+	// Call the openDB() helper function to create the connection pool, passing in the config struct. If this returns an error, we log it and exit the application immediately.
+	db, err := openDB(cfg)
+	if err != nil {
+		logger.Error(err.Error())
+		os.Exit(1)
+	}
+
+	// Defer a call to db.Close() so that the connection pool is closed before the main() function exists
+	defer db.Close()
+
+	// Also log a message to say that the connection pool has been successfully established.
+	logger.Info("database connection pool established")
+
 	// Declare an instance of the application struct, containing the config struct and the logger.
 	app := &application{
 		config: cfg,
@@ -74,9 +101,33 @@ func main() {
 	// Start the HTTP server.
 	logger.Info("starting server", "addr", srv.Addr, "env", cfg.env)
 
-	err := srv.ListenAndServe()
+	// Because the err variable is now already declared in the code above, we need to user = operator here, instead of the := operator.
+	err = srv.ListenAndServe()
 	if err != nil {
 		logger.Error(err.Error())
 		os.Exit(1)
 	}
 }
+
+// openDB() function returns a sql.DB connection pool.
+func openDB(cfg config) (*sql.DB, error) {
+	// Use sql.Open() to create an empty connection pool, using the DSN from the config struct.
+	db, err := sql.Open("postgres", cfg.db.dsn)
+	if err != nil {
+		return nil, err
+	}
+
+	// Create a context with a 5-second timeout deadline.
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// Use PingContext() to establish a new connection to the database, passing in the context we created above as parameter. If the connection couldn't be established successfully within the 5-second deadline, then this will return an error. If we get this error, or any other, we close the connection pool and return the error.
+	err = db.PingContext(ctx)
+	if err != nil {
+		db.Close()
+		return nil, err
+	}
+
+	// Return the sql.DB connection pool.
+	return db, nil
+}