Local Data Storage and Caching in Mobile

In mobile app development, managing data and state effectively is essential for delivering consistent, high-performance user experiences. Unlike web applications, mobile apps must gracefully handle varying network conditions, offline access, and device limitations. This chapter explores the full spectrum of strategies for handling data and state in mobile apps. These include storing information locally, implementing caching mechanisms, managing offline synchronization, and resolving conflicts between local and remote data.

We begin with the foundation of any resilient data strategy: local data storage and caching. Since mobile apps often operate in environments with unreliable or intermittent connectivity, storing critical information directly on the device is key to ensuring smooth performance. This also enhances user trust by making the app feel fast and reliable.

The diagram below illustrates the key data layers that mobile apps interact with:

In this lesson, we will explore the local layers of this architecture in detail. We will learn how to identify different storage types and their use cases, implement in-memory caching for quick access, and design effective caching strategies. Additionally, we’ll explore how to manage persistence across app restarts and balance the critical trade-offs between performance, power, and memory.

Let’s begin by exploring the types of local storage available on mobile devices.

Types of local storage

Choosing the right type of storage is essential for building a reliable mobile app. When a user interacts with our application, some data is only needed temporarily, while other data must be saved for future sessions. This is the fundamental difference between the two main categories of local storage. Think of it like the difference between jotting a note on a chalkboard that will be erased at the end of the day (in-memory storage) and saving a document to your hard drive, where it remains even after a restart (persistent storage).

• In-memory storage is temporary and exists only while the app is running. It’s perfect for short-term data that makes the app feel snappy, like the current scroll position in a list or temporarily fetched content. Because it’s held in the device’s fast RAM, access is nearly instant, but all of it is lost the moment the app closes.

• Persistent storage, on the other hand, retains data even after the app is closed or the device is restarted. It’s ideal for information that should not be lost, like user preferences, offline content, or saved game progress. When approaching persistent storage, there are generally two methods a developer can choose: a simple key-value system or a more robust structured database.

  • Key-value storage is ideal for small, standalone pieces of data. It operates like a dictionary, where data is saved and retrieved using a unique key. This approach is perfect for simple use cases like storing a user’s name, notification preferences, or dark mode choice. On Android, this is commonly handled by SharedPreferencesA SharedPreferences object points to a file containing key-value pairs and provides simple methods to read and write them., while iOS uses UserDefaultsUserDefaults is a key-value storage mechanism in iOS (and macOS) that allows developers to store small amounts of persistent data..

The following illustration shows how the simple key-value pairs can control the settings in a user interface:

• For more complex or related data, structured storage is a better choice. These systems, like relational databases, allow for creating tables and data models that support querying, sorting, and filtering. For example, a to-do list app would use structured storage to manage all the tasks, their due dates, and their completion status. The most common technology for this on both Android and iOS is SQLite. To make it easier to work with, Android offers an abstraction layer called Room, while the iOS ecosystem has its own framework called ...