Go + gRPC + OPA - A Perfect Union - Part 3

I finished my last post with the following issue -

Now, here one problem arises, how to make sure that the search results will
not return any book which the user is not authorized to access. We will solve
this problem using OPA in the next and last post of this series.

Let's solve this issue now. We will use OPA's declarative language, Rego, to implement policies which will decide on the basis of some user-provided data, which all objects are to be returned to the user.

We will also define a list of all the users who are part of this library. Here we are hardcoding this data, as I did not want to waste my time in implementing a user registration service, but this functionality is not very important from our point of view. We will require only one field from this users data - the user_type field. This field will determine what the access level for the user is. We have already added the access_level field in the Book definition of our proto file.

When the user wants to search for a particular book, it will provide its user_type the ISBN of the book to our service. Our service will take that ISBN and pass it to the OPA server. OPA server already has the Book data and the User data. Now it has the required ISBN to query the Book data. The Rego policy will query the Book data by ISBN. It will also check for the access_level condition. Moreover, after this operation, it will return the resultant set of books that satisfy both the requirements.

Here is the Rego policy -

package library
import data.books
import data.users
import input
search_books[book] {
  input.book.isbn:= books[i].isbn
  input.user.user_type >= books[i].access_level
  book: books[i]
list_all_books[books[i]] {
  input.user.user_type >= books[i].access_level

The user data is here and the book data is here .

A sample input request is shown below -

  "input": {
    "book": {
      "isbn": "1128959038"
    "user": {
      "user_type": 3

The input is the data that the user is providing. In search_books function, the input ISBN is matched with the ISBN of all books one by one. Then the resultant set of books is filtered by user_type and access_level (these two fields are essentially the same). In the last, the resultant set of books is assigned to the variable book which will be returned to the gRPC service.

The list_all_books function is implemented similarly. The only difference is that we do not need to filter the books by ISBN. Filtering by access_level is enough.

Now our library service is completed. It is a very basic service. The intention was to show that the decision-making process can be offloaded to the OPA to reduce the complexity of the services. In this example, the advantages might not be obvious, but in large production environments, where many services are running, it can make a significant difference.

The code for this series can be found on my Github account.

I hope you liked the article. Share your views and suggestions in the comments.

Thanks for reading. Cheers :)