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GitLab to Kubernetes communication

The goal of this document is to define how GitLab can communicate with Kubernetes and in-cluster services through the GitLab agent.

Challenges

Lack of network connectivity

For various features that exist today, GitLab communicates with Kubernetes by directly or indirectly calling its API endpoints. This works well, as long as a network path from GitLab to the cluster exists, which isn't always the case:

  • GitLab.com and a self-managed cluster, where the cluster is not exposed to the Internet.

  • GitLab.com and a cloud-vendor managed cluster, where the cluster is not exposed to the Internet.

  • Self-managed GitLab and a cloud-vendor managed cluster, where the cluster is not exposed to the Internet and there is no private peering between the cloud network and the customer's network.

    This last item is the hardest to address, as something must give to create a network path. This feature gives the customer an extra option (exposing the gitlab-kas domain but not the whole GitLab) in addition to the existing options (peering the networks, or exposing one of the two sides).

Even if technically possible, it's almost always undesirable to expose a Kubernetes cluster's API to the Internet for security reasons. As a result, our customers are reluctant to do so, and are faced with a choice of security versus the features GitLab provides for connected clusters.

This choice is true not only for Kubernetes' API, but for all APIs exposed by services running on a customer's cluster that GitLab may need to access. For example, Prometheus running in a cluster must be exposed for the GitLab integration to access it.

Cluster-admin permissions

Both current integrations - building your own cluster (certificate-based) and GitLab-managed cluster in a cloud - require granting full cluster-admin access to GitLab. Credentials are stored on the GitLab side and this is yet another security concern for our customers.

For more discussion on these issues, read issue #212810.

GitLab agent epic

To address these challenges and provide some new features, the Configure group is building an active in-cluster component that inverts the direction of communication:

  1. The customer installs an agent into their cluster.
  2. The agent connects to GitLab.com or their self-managed GitLab instance, receiving commands from it.

The customer does not need to provide any credentials to GitLab, and is in full control of what permissions the agent has.

For more information, visit the GitLab agent repository or the epic.

Request routing

Agents connect to the server-side component called GitLab agent server (gitlab-kas) and keep an open connection that waits for commands. The difficulty with the approach is in routing requests from GitLab to the correct agent. Each cluster may contain multiple logical agents, and each may be running as multiple replicas (Pods), connected to an arbitrary gitlab-kas instance.

Existing and new features require real-time access to the APIs of the cluster and (optionally) APIs of components, running in the cluster. As a result, it's difficult to pass the information back and forth using the more traditional polling approach.

A good example to illustrate the real-time need is Prometheus integration. If we wanted to draw real-time graphs, we would need direct access to the Prometheus API to make queries and quickly return results. gitlab-kas could expose the Prometheus API to GitLab, and transparently route traffic to one of the correct agents connected at the moment. The agent then would stream the request to Prometheus and stream the response back.

Proposal

Implement request routing in gitlab-kas. Encapsulate and hide all related complexity from the main application by providing a clean API to work with Kubernetes and the agents.

The above does not necessarily mean proxying Kubernetes' API directly, but that is possible should we need it.

What APIs gitlab-kas provides depends on the features developed, but first we must solve the request routing problem. It blocks any and all features that require direct communication with agents, Kubernetes or in-cluster services.

Detailed implementation proposal with all technical details is in kas_request_routing.md.

flowchart LR
  subgraph "Kubernetes 1"
    agentk1p1["agentk 1, Pod1"]
    agentk1p2["agentk 1, Pod2"]
  end

  subgraph "Kubernetes 2"
    agentk2p1["agentk 2, Pod1"]
  end

  subgraph "Kubernetes 3"
    agentk3p1["agentk 3, Pod1"]
  end

  subgraph kas
    kas1["kas 1"]
    kas2["kas 2"]
    kas3["kas 3"]
  end

  GitLab["GitLab Rails"]
  Redis

  GitLab -- "gRPC to any kas" --> kas
  kas1 -- register connected agents --> Redis
  kas2 -- register connected agents --> Redis
  kas1 -- lookup agent --> Redis

  agentk1p1 -- "gRPC" --> kas1
  agentk1p2 -- "gRPC" --> kas2
  agentk2p1 -- "gRPC" --> kas1
  agentk3p1 -- "gRPC" --> kas2

Iterations

Iterations are tracked in the dedicated epic.