hi Karthik

1. To facilitate and promote the development of robust, error-free code, Apex Code requires the creation and execution of unit tests. Unit tests are class methods that verify whether a particular piece of code is working properly.

2. Multitenancy refers to a standard in software architecture where a single instance of the software runs on a server, serving multiple client organizations (tenants). A software application is designed to virtually partition its data and configuration, and each client association works with a customized virtual application instance. Multitenancy is the fundamental technology that clouds use to share IT resources cost-efficiently and securely.

3. REST API is a simple and powerful web service based on RESTful principles. It exposes all sorts of Salesforce functionality via REST resources and HTTP methods. For example, you can create, read, update, and delete (CRUD) records, search or query your data, retrieve object metadata, and access information about limits in your org. REST API supports both XML and JSON.Because REST API has a lightweight request and response framework and is easy to use, it’s great for writing mobile and web apps.
                                                          A REST request consists of four components: a resource URI, an HTTP method, request headers, and a request body. Request headers specify metadata for the request. The request body specifies data for the request, when necessary. If there’s no data to specify, the body is omitted from the request

Thanks
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