Architecture of Harbor: An Open Source Enterprise-class Registry Server

About Project Harbor

VMware has initiated an enterprise-class Registry called Project Harbor, which helps users rapidly build a private enterprise-class registry service. It extends the open source Docker Distibution by adding the functionality usually required by an enterprise, such as management UI, Role Based Access Control(RBAC), AD/LDAP integration, image replication and auditing. The project has received over 1100 stars and been forked over 290 times since it was released 6 months ago. This article introduces the main modules of the Project Harbor and describes the operational principles behind Harbor.

Architecture

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As depicted in the above diagram, Harbor comprises 6 components:

Proxy: Components of Harbor, such as registry, UI and token services, are all behind a reversed proxy. The proxy forwards requests from browsers and Docker clients to various backend services.

Registry: Responsible for storing Docker images and processing Docker push/pull commands. As Harbor needs to enforce access control to images, the Registry will direct clients to a token service to obtain a valid token for each pull or push request.

Core services: Harbor’s core functions, which mainly provides the following services:

  • UI: a graphical user interface to help users manage images on the Registry
  • Webhook: Webhook is a mechanism configured in the Registry so that image status changes in the Registry can be populated to the Webhook endpoint of Harbor. Harbor uses webhook to update logs, initiate replications, and some other functions.
  • Token service: Responsible for issuing a token for every docker push/pull command according to a user’s role of a project. If there is no token in a request sent from a Docker client, the Registry will redirect the request to the token service.

Database: Database stores the meta data of projects, users, roles, replication policies and images.

Job services: used for image replication, local images can be replicated(synchronized) to other Harbor instances.

Log collector: Responsible for collecting logs of other modules in a single place.

Implementation

Each component of Harbor is wrapped as a Docker container. Naturally, Harbor is deployed by Docker Compose.

In the source code (https://github.com/vmware/harbor), the Docker Compose template used to deploy Harbor is located at /Deployer/docker-compse.yml. Opening this template file reveals the 6 container components making up Harbor:

proxy: Reverse-proxy formed by the Nginx Server.

registry: Container instance created from the official image of Docker distribution.

ui: Core services within the architecture. This container is the main part of Project Harbor.

mysql: Database container created from the official MySql image.

job services: Replicating images to a remote registry via state machines. Image deletion can also be synchronized to a remote Harbor instance.

log: Container that runs rsyslogd, used for collecting logs from other containers through the log-driver mode.

These containers are linked via DNS service discovery in Docker. By this means, each container can be accessed by their names. For the end user, only the service port of the proxy (Nginx) needs to be revealed.

The following two examples of Docker command illustrate the interaction between Harbor’s components.

docker login

Suppose Harbor is deployed on a host with IP 192.168.1.10. A user runs the docker command to send a login request to Harbor:

$ docker login 192.168.1.10

After the user enters the required credentials, the Docker client sends an HTTP GET request to the address “192.168.1.10/v2/”. The different containers of Harbor will process it according to the following steps:

docker login(a) First, this request is received by the proxy container listening on port 80. Nginx in the container forwards the request to the Registry container at the backend.

(b) The Registry container has been configured for token-based authentication, so it returns an error code 401, notifying the Docker client to obtain a valid token from a specified URL. In Harbor, this URL points to the token service of Core Services;

(c) When the Docker client receives this error code, it sends a request to the token service URL, embedding username and password in the request header according to basic authentication of HTTP specification;

(d) After this request is sent to the proxy container via port 80, Nginx again forwards the request to the UI container according to pre-configured rules. The token service within the UI container receives the request, it decodes the request and obtains the username and password;

(e) After getting the username and password, the token service checks the database and authenticates the user by the data in the MySql database. When the token service is configured for LDAP/AD authentication, it authenticates against the external LDAP/AD server. After a successful authentication, the token service returns a HTTP code that indicates the success. The HTTP response body contains a token generated by a private key.

At this point, one docker login process has been completed. The Docker client saves the encoded username/password from step (c) locally in a hidden file.

docker Push

article1_image4(We have omitted proxy forwarding steps. The figure above illustrates communication between different components during the docker push process)

After the user logs in successfully, a Docker Image is sent to Harbor via a Docker Push command:

# docker push 192.168.1.10/library/hello-world

(a) Firstly, the docker client repeats the process similar to login by sending the request to the registry, and then gets back the URL of the token service;

(b) Subsequently, when contacting the token service, the Docker client provides additional information to apply for a token of the push operation on the image (library/hello-world);

(c) After receiving the request forwarded by Nginx, the token service queries the database to look up the user’s role and permissions to push the image. If the user has the proper permission, it encodes the information of the push operation and signs it with a private key and generates a token to the Docker client;

(d) After the Docker client gets the token, it sends a push request to the registry with a header containing the token. Once the Registry receives the request, it decodes the token with the public key and validates its content. The public key corresponds to the private key of the token service. If the registry finds the token valid for pushing the image, the image transferring process begins.

For more information about enterprise registry Harbor, take a look at Github: https://github.com/vmware/harbor

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