Code on demand

In distributed computing, code on demand (COD) is any technology that enables the server to send the software code to the clients to be executed on the client computer upon request from the client's software. Some well-known examples of the COD paradigm on the web are Java applets, the Adobe ActionScript language for the Flash player, and JavaScript.

The following can also be called the advantages of COD:

• COD is the optional constraint of REST and intends to allow business logic within the client web browser, applets, JavaScript, and ActionScript (Flash). I think video on demand sites are good examples of COD, as the video data files are downloaded and played according to the client system's specifications.
• Only one optional constraint according to REST architectural style and it is COD. COD allow it clients to be flexible because the server that decides how specific items need to be handled on the client side. For instance, with COD, a client may download action scripts such as JavaScript, Applets (not widely used these days), Flex scripts to encrypt the client-server communication, so the underlying servers won’t aware of any specific encryption methods used in the process.
• COD can also be applied to services and service consumers. For instance, service design can enable the servers to dynamically defer some portions of logic to the service client programs. This approach of delaying code execution to the client side is justifiable when service logic can be executed by the consumer more efficiently or effectively.
• RESTful applications may very well be able to utilize clients that support COD. For instance, web browsers can allow servers to return scripts or links that can be executed at the client side. This sort of additional code execution helps to expand the capabilities of the client, without needing the user to install new client software.
• In the COD style, a client component has access to a set of resources, but not the know-how of how to process them. It sends a request to a remote server for the code representing that know-how, receives that code, and executes it locally.

However, the down side of using COD is reduces the visibility of the underlying API, and not every API prefers these kind of flexibility.