What is a Shim?

Question

What's the definition of a Shim?

Answer 1

As we could see in many responses here, a shim is a sort of adapter that provides functionality at API level which was not necessarily part of that API. This thread has a lot of good and complete responses, so I'm not expanding the definition further.

However, I think I can add a good example, which is the Javascript ES5 Shim (https://github.com/es-shims/es5-shim):

Javascript has evolved a lot during the last few years, and among many other changes to the language specification, a lot of new methods have been added to its core objects.

For example, in the ES2015 specification (aka ES5), the method find has been added to the Array prototype. So let's say you are running your code using a JavasScript engine prior to this specification (ex: Node 0.12) which doesn't offer that method yet. By loading the ES5 shim, these new methods will be added to the Array prototype, allowing you to make use of them even if you are not running on a newer JavaScript specification.

You might ask: why would someone do that instead of upgrading the environment to a newer version (let's say Node 8)?

There is a lot of real cases scenarios where that approach makes sense. One good example:

Let's say you have a legacy system that is running in an old environment, and you need to use such new methods to implement/fix a functionality. The upgrade of your environment still a work in progress because there are compatibility issues that require a lot of code changes and tests (a critical component).

In this example, you could try to craft your own version of such functionality, but that would make your code harder to read, more complex, can introduce new bugs and will require tons of additional tests just to cover a functionality that you know it will be available in the next release.

Instead, you can use this shim and make use of these new methods, taking advantage of the fact that this fix/functionality will be compatible after the upgrade, because you are already using the methods known to be available in the next specification. And there is a bonus reason: since these methods are native to the next language specification, there is a good chance that they will run faster than any implementation that you could have done if you tried to make your own version.

Another real scenario where such approach is welcome is at browser level. Let's say you need to support old browser and want to take advantage of these newer features. Javascript is a language that allows you to add/modify methods in its core objects (like adding methods to Array prototype), and those shim libraries are smart enough to add such methods only if the current implementation is lacking of them.

PS: 1) You will see the term "Polyfill" related to these Javascript shims. Polyfill is a more specialized type of shim that is used to provide forward compatibility in different browser level specifications. By the way, my example above refers exactly to such example.

2) Shims are not limited to this example (adding functionality that will be available in a future release). There are different use cases that would be considered to be a shim as well.

3) If you are curious about how this specific polyfill is implemented, you can open Javascript Array.find specs and scroll to the end of the page where you will find a canonical implementation for this method.

Answer 2

SHIM is another level of security check which is done for all the services, to protect upstream systems. SHIM Server validates every incoming request, with Headers User credentials, against the user credentials, which are passed in the request(SOAP / RESTFUL).