roles-and-annotations.md

Roles / namespaces / annotations in v3-engine

In V2 we created a schema per role.

In V3 we create one big schema at build time, where some nodes are annotated with either generic info, or namespaced info that only applies to specific roles.

The types for these annotations come from the lang-graphql crate, where they are associated types on the SchemaContext trait.

This example used in tests for lang-graphql shows a schema with no annotations. It uses a Namespace type to differentiate different scopes, however since it has no annotations this won't do anything meaningful:

impl SchemaContext for SDL {
    type Namespace = Namespace;
    type GenericNodeInfo = ();
    type NamespacedNodeInfo = ();

The engine itself uses the GDS type, and so we can use the following annotations:

impl gql_graphql_schema::SchemaContext for GDS {
    type Namespace = Role;
    type GenericNodeInfo = types::Annotation;
    type NamespacedNodeInfo = Option<types::NamespaceAnnotation>;

Note that our Namespace type is Role - this means that the engine attached useful information per role, so we can add select permissions for a field that only apply to the user-1 role, for instance.

(incidentally, there is a comment on the SchemaContext trait suggesting Namespace is renamed to Scope or Role - this seems like a useful move to make all this clearer, personally Namespace makes me think about different subgraphs or something)

What is an annotation then, concretely?

The change I have recently been working on is to add preset arguments to commands. This means that given a delete_user command with a user_id: Int argument, we can preset the value for certain roles.

For instance, we might want a user-1 role to only be able to delete themselves, so we'd preset user_id with { "sessionVariable": "x-hasura-user-id" }.

Previous to this change, when building the schema for the Command, we'd use builder.allow_all_namespaced to create it, like

builder.allow_all_namespace(command_schema_stuff, None)

Instead, for our example, we'd use conditional_namespaced like this (excuse my pseudo-Rust):

let role_annotations = HashMap::new();

role_annotations.insert("user-1", Some(ArgumentPresets { "user_id":
"x-hasura-role-id" }));

role_annotations.insert("admin", None);

builder.conditional_namespaced(command_schema_stuff, role_annotations)

Here, we've added an annotation for user-1 with some useful information we can use later when receiving and running queries. We've also added a None annotation for admin - this means there'll be no useful information later, but we're still signalling that we want this Command to work for admin users. If we had a user-2 role in the schema, they wouldn't be able to use the command.

Removing items from the GraphQL schema, per role

By adding or omitting keys from the role_annotations above, we've made our command appear or disappear from the GraphQL schema. This "snipping" as such, happens in the normalize_request function in lang-graphql.

The important thing to know here is that lang-graphql code does not know about GDS or our NamespaceAnnotation type. All it has to act on is whether a Role (or Namespace, to it's eyes) has a key in any namespaced annotations or not. Because we're using associated types the contents are "protected" from lang-graphql and so it can't peek inside.

We're going to want the user_id argument to disappear from user-1's schema

• we do this by using conditional_namespaced when contructing schema for the user_id command argument itself:

let role_annotations = HashMap::new();

// insert an empty annotation for `admin` to make sure the argument remains in
the schema
role_annotations.insert("admin", None);

// don't add one for `user-1`, so it disappears from the schema (as it has been
replaced with the preset value)
builder.conditional_namespaced(command_argument_schema_stuff, role_annotations)

Reading the annotations at request time

Everything before here happens at "compile time" for the engine, so we do it all once at startup and it remains static for the lifetime of the application.

At some point we are going to need to serve a request though. The steps are as follows:

• Receive request
• Parse into a GraphQL query
• Normalize the query
• Generate IR
• Construct a query plan
• Execute / explain the query plan

The normalize step was mentioned earlier - it takes place in lang_graphql and combines the query with the role information to remove all irrelevant namespace information from the query, and snip any parts of the schema that the current role can't see.

Generally the helpful place for our annotations is in generating IR (intermediate representation, compiler fans). For our commands change, we can look up the ArgumentPresets we stored earlier like this.

match field_call.info.namespaced {
  None => {}
  Some(NamespaceAnnotation::ArgumentPresets(argument_presets)) => {
    // use `argument_presets` for current role to generate IR
    ...
  }
}

The IR will then be created from a mixture of the schema for the user's role, and any arguments etc from their request.