Before we go into the details of the authorization flow, it's important to understand the subsystems involved. The authorization (AuthZ) system shown in the figure below consists of several logical subsystems, which we will elaborate on in the following section.
ZMS is the source of truth for domains, roles, and policies for centralized authorization. In addition to allowing CRUD operations on the basic entities, ZMS provides an API to replicate the entities, per domain, to ZTS. ZMS supports a centralized call to check if a principal has access to a resource both for internal management system checks, as well as a simple centralized deployment. Because ZMS supports service identities, ZMS can authenticate services.
For centralized authorization, ZMS may be the only Athenz subsystem that you need to interact with.
ZTS, the authentication token service, is only needed to support decentralized or data plane functionality. In many ways, ZTS is like a local replica of ZMS's data to check a principal's authentication and confirm membership in roles within a domain. The authentication is in the form of a signed ZToken that can be presented to any decentralized service that wants to authorize access efficiently. Multiple ZTS instances can be distributed to different locations as needed to scale for issuing tokens.
SIA (Service Identity Agent) Provider is part of the container, although likely built with Athenz libraries. As services are authenticated by their private keys, the job of the SIA Provider is to generate a NToken and sign it with the given private key so that the service can present that NToken to ZMS/ZTS as its identity credentials. The corresponding public key must be registered in ZMS so Athenz services can validate the signature.
Like ZTS, ZPE, the authorization policy engine is only needed to support decentralized authorization. ZPE is the subsystem of Athenz that evaluates policies for a set of roles to yield an allowed or a denied response.
ZPE is a library that your service calls and only refers to a local policy cache for your services domain (a small amount of data).
Like ZTS and ZPE, ZPU is only needed to support decentralized authorization. The policy updater is the utility that retrieves from ZTS the policy files for provisioned domains on a host, which ZPE uses to evaluate access requests.