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As mentioned elsewhere in the documentation, we wrote LoopBack 4 from the ground up and therefore the migration requires more effort than in previous major versions. The migration guide presented in the nested pages describe steps to migrate various features used by typical LoopBack 3 applications.

Unless your project is very small, upgrading everything in one go is most likely not feasible. It would require too much time and introduce too many breaking changes to consumers of your API.

We are recommending the following incremental approach:

  1. Before you start, learn more about the differences between LoopBack versions 3 and 4 and build a good understanding of how LoopBack 3 concepts are translated to LoopBack 4. See Understanding the differences between LoopBack 3 and LoopBack 4.

  2. Start with mounting your existing LoopBack 3 application in a new LoopBack 4 project, as described in Mounting a LoopBack 3 application. This will allow you to build new features in LoopBack 4, while keeping your existing APIs unchanged.

  3. Next migrate your global (application-level) Express middleware, so that both your old and your new APIs can use the same set of middleware. Learn more in Migrating Express middleware.

  4. Before you can define new models (or import existing ones), you need datasources to attach them to. Follow the steps in Migrating datasources to bring them over to LoopBack 4.

  5. With this setup in place, it’s time to migrate models. In theory, you can migrate models one-by-one, but in practice you will need to migrate related models together. We have a dedicated section for model migration, start with the overview in Migrating models.

Besides models and their APIs, there are few more LoopBack 3 features that require migration:

  1. In LoopBack 3, boot scripts allow the application to run custom code at startup. In LoopBack 4, Lifecycle observers allow application to run code not only at startup, but also before shutting down. Learn how to convert your boot scripts to lifecycle observers in Migrating boot scripts.

  2. To make your application secure, LoopBack 3 offers several ways how to implement authentication and authorization. Check out Authentication & authorization for an overview and links to further resources.

  3. LoopBack version 4 has its own CLI tool lb4 (see Command-line interface). We preserved many of the commands you are familiar with in lb from LoopBack 3, but some of them are different in version 4. Learn more in Migrating CLI usage patterns.

  4. LoopBack 4 introduced a very different programming model for components and extensions. If you are maintaining or using LoopBack 3 components, then you should read Migrating components and extensions.

  5. There are several client SDKs for LoopBack 3 that make it easier for different clients to consume REST APIs provided by LoopBack applications. Please refer to Migrating clients to learn how to upgrade clients built using the SDKs.

Last but not least, we took the opportunity presented by this big release to abandon certain features and components that we don’t have bandwidth to maintain and improve going forward. Learn more in LoopBack 3 features not planned in LoopBack 4.