Add RunTransactions() approach to application design

example4/database.go

@@ -0,0 +1,96 @@
+// This file is part of https://github.com/racingmars/go3270/
+// Copyright 2025 by Matthew R. Wilson, licensed under the MIT license. See
+// LICENSE in the project root for license information.
+
+// This example demonstrates using the RunTransactions() approach to
+// structuring a go3270 application.
+
+// This file contains a mock in-memory database for the purposes of our
+// example application.
+
+package main
+
+import (
+	"errors"
+	"sync"
+	"time"
+)
+
+type User struct {
+	Username   string
+	Password   string
+	Name       string
+	SignupDate time.Time
+}
+
+type DB interface {
+	GetUser(username string) (User, error)
+	CreateUser(user User) (User, error)
+	UpdateUser(user User) (User, error)
+}
+
+type dbstate struct {
+	lock  sync.Mutex
+	users map[string]User
+}
+
+func Connect() DB {
+	return &dbstate{
+		users: make(map[string]User),
+	}
+}
+
+var ErrUserExists = errors.New("username already exists")
+var ErrNotFound = errors.New("record not found")
+
+func (db *dbstate) GetUser(username string) (User, error) {
+	db.lock.Lock()
+	defer db.lock.Unlock()
+
+	user, ok := db.users[username]
+	if !ok {
+		return User{}, ErrNotFound
+	}
+
+	return user, nil
+}
+
+func (db *dbstate) CreateUser(user User) (User, error) {
+	db.lock.Lock()
+	defer db.lock.Unlock()
+
+	if _, ok := db.users[user.Username]; ok {
+		return User{}, ErrUserExists
+	}
+
+	user.SignupDate = time.Now().UTC().Truncate(time.Second)
+
+	// Obviously in a real application you wouldn't store plaintext password,
+	// but this is a simple example for the purposes of demonstrating go3270,
+	// not demonstrating how to build a secure application.
+	db.users[user.Username] = user
+
+	return user, nil
+}
+
+func (db *dbstate) UpdateUser(user User) (User, error) {
+	db.lock.Lock()
+	defer db.lock.Unlock()
+
+	olduser, ok := db.users[user.Username]
+	if !ok {
+		return User{}, ErrNotFound
+	}
+
+	// Make sure original signup date is maintained
+	user.SignupDate = olduser.SignupDate
+
+	// Carry over existing password if we aren't setting a new password
+	if user.Password == "" {
+		user.Password = olduser.Password
+	}
+
+	db.users[user.Username] = user
+
+	return user, nil
+}