> For the complete documentation index, see [llms.txt](https://docs.umbraco.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.umbraco.com/umbraco-cms/13.latest/implementation.md).

# Implementation

- [Learn how Umbraco works](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/learn-how-umbraco-works.md): Get to know the Umbraco codebase.
- [Routing](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/default-routing.md)
- [Controller & Action Selection](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/default-routing/controller-selection.md)
- [Execute Request](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/default-routing/execute-request.md)
- [Request Pipeline](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/default-routing/inbound-pipeline.md)
- [Custom Routing](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/custom-routing.md): Learn everything you need to know about custom routing in Umbraco CMS.
- [Adding a hub with SignalR and Umbraco](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/custom-routing/signalr.md): Umbraco ships with signalR installed, find out how to add your own hub(s) to the existing setup
- [Controllers](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/controllers.md): An Umbraco API Controller is an ASP.NET WebApi controller that is used for creating REST services.
- [Data Persistence (CRUD)](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/data-persistence.md)
- [Composing](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/composing.md): This article covers the topic of composing in Umbraco.
- [Integration Testing](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/integration-testing.md): A guide to getting started with integration testing in Umbraco
- [Nullable Reference Types](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/nullable-reference-types.md): In this article we describe what Nullable reference types is.
- [Services and Helpers](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/services.md): Umbraco has a range of 'Core' Services and Helpers that act as a 'gateway' to Umbraco data and functionality to use when extending or implementing an Umbraco site.
- [Circular Dependencies](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/services/circular-dependencies.md)
- [Unit Testing](https://docs.umbraco.com/umbraco-cms/13.latest/implementation/unit-testing.md): A guide to getting started with unit testing in Umbraco


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