Realm

Realm is an object database for local persistence in mobile, wearable, and edge applications. It suits developers who want native-feeling object access across several SDKs and need data to remain useful through unreliable connectivity. Its open-source price is attractive, but the announced deprecation of the SDKs and Device Sync makes long-term adoption a careful decision.

Overview

Realm stores application data locally and exposes that data as objects queried directly in code, avoiding an object-relational mapping layer. That model can reduce translation between application code and persistence logic, particularly for mobile teams working with structured objects and relationships. Support for relationships, generics, and vectorization gives the database a broader data model than a minimal key-value store.

The local-first design is most relevant when an app must continue operating with a weak signal, no connection, or an outage. Realm's documentation describes those scenarios as supported use cases, so it is a sensible fit for field apps, travel-oriented software, and edge workloads where connectivity cannot be assumed. The trade-off is strategic rather than merely technical: MongoDB acquired Realm in 2019, renamed the technology as MongoDB Atlas Device SDKs in 2023, and the Realm JavaScript repository records a September 2024 announcement that Atlas Device Sync and Realm SDKs are being deprecated.

Key features

Object-based local database

Realm presents records as objects that can be queried in application code without an ORM. Developers who want relationships and typed structures close to their domain model get a direct programming interface, while teams committed to SQL workflows may find the abstraction less familiar.

Offline operation and synchronization

Local persistence is designed for weak-signal and offline operation. The former Device Sync service connected data among users, devices, and a backend in real time, a useful architecture for coordinated multi-device applications. Because that service and the SDKs are being deprecated, new projects should treat sync-dependent plans as migration-sensitive rather than as a stable long-term foundation.

Security and tooling

Realm Swift supports encryption in flight and at rest, giving Swift applications stated protection for stored and transmitted data. Realm Studio can open and edit local or synced Realms and administer Realm Object Server instances, which is useful for inspecting data and managing server deployments. Help is directed through the forums, Stack Overflow, and GitHub issues, so support is community- and issue-tracker-led.

Pricing

Realm Database (open source) costs free. It is free and open source under the Apache License 2.0, with no paid tier stated. That makes it straightforward for individuals, startups, and teams that need a local database without a license bill. The free status does not remove the product-lifecycle concern: the announced SDK and Device Sync deprecation can create future migration work even when current software costs nothing.

Platforms

Realm is available on Android, iOS, Linux, macOS, self-hosted deployments, and Windows. Its mobile offering names iOS, Android, and Xamarin, with SDKs for Java, Objective-C, JavaScript, Swift, and Xamarin. The JavaScript implementation covers React Native on iOS and Android, plus Node.js and Electron on Windows, macOS, and Linux. This breadth suits organizations sharing a persistence approach across mobile and desktop or server-adjacent JavaScript projects, although each team should account for the deprecation announcement when choosing an SDK.

Who it's for

Realm is best for developers building offline-capable mobile or edge applications that benefit from object queries, relationships, and a local database spanning several language ecosystems. It is also a reasonable choice for an existing Realm installation whose maintainers understand the announced transition and can plan for it. New products that require dependable, ongoing vendor-backed SDK and real-time sync investment should look elsewhere, because the deprecation changes the risk profile despite the capable local database.

Pros and cons

Pros

• Free open-source license: the Apache 2.0 database has no stated usage charge.
• Direct object queries: applications can work with persisted objects without an ORM layer.
• Offline-oriented design: local data remains usable during weak signal, offline periods, or outages.
• Broad SDK and platform coverage: Java, Swift, Objective-C, JavaScript, and .NET-oriented ecosystems are represented across mobile, desktop, Linux, and self-hosted environments.
• Operational tools and stated encryption: Realm Studio supports local and synced data administration, while Realm Swift supports encryption in transit and at rest.

Cons

• SDK and Device Sync deprecation: new implementations face a material maintenance and migration risk.
• Sync uncertainty: real-time connectivity across users, devices, and a backend was a former capability, not a dependable basis for future projects.
• Community-led support: assistance is routed through forums, Stack Overflow, and GitHub issues rather than a stated dedicated support plan.

Alternatives

For a wider comparison, see Mobile Databases.

• Espresso is a free Android UI-testing option for a single target app, rather than a general Realm-style object database.
• Hive offers a free-forever plan with 200MB storage, 1,000 AI credits per month, and up to 10 workspace members for teams wanting those workspace limits.
• RxDB is a freemium alternative whose free plan includes core functionality, replication and realtime sync, default storages, and up to 13 open collections.
• Ditto suits teams seeking cloud connectivity limits, with a free tier covering 10 cloud device connections, 2GB storage, and 5GB transfer, but no SLA.
• ObjectBox provides a free core database plan and a separately priced Sync offering for teams separating local storage from synchronization.
• Sparksee has a free academic licence for qualifying users who can provide university or school identification.
• SQLDelight is a free open-source Kotlin SQL code-generation library for teams that prefer SQL-shaped development.
• WCDB is a free open-source mobile database framework for teams wanting a conventional mobile-focused option.

Verdict

Choose Realm when free Apache-licensed local object persistence, offline behavior, and multi-language SDK coverage are the priority, especially in an existing application. Look elsewhere for a new system whose roadmap depends on maintained SDKs or real-time Device Sync: the deprecation announcement is the decisive reason not to treat Realm as a low-risk long-term default.