Using Decorators to Set and Get Request Data

• Applying a Policy to a CSM Design
• Create the AsyncLocalStorage and Decorator
• Get the User Data

Applying a Policy to a CSM Design

src/
 |- user.controller.ts
 |- user.service.ts
 '- user.model.ts

Consider the use case where we want to apply a policy that users can only see the projects they own. Often we first query, then prune. This is a sub-par db query and is prone to data leaks if we don't prune at the right level - in the service.

Not only that, but in vanilla javascript, we have to pass in the data we need for filtering all the way down to the Model level.

We can elegantly solve this with ALS and Decorators.

src/index.ts simply simulates a request coming in and running through the controller.

Create the AsyncLocalStorage and Decorator

src/
 |- app-context.ts
 '- user-data.decorator.ts

app-context uses AsyncLocalStorage to setup a simple data bucket that can hold user data.

user-data.decorator.ts pulls off the user data from the request object and pushes it into the als.

Get the User Data

There are two ways demonstrated in looking up data.

user.service.ts uses the WithUserData decorator, this feels nice but gets dicey since you have to decorate the classes property - I'll be looking for a better way to do this.

The second approach is in user.model.ts where we simply use the als to fetch the user data from the context.