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| stage | group | info |
|---|---|---|
| Configure | Configure | To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers |
Routing kas requests in the Kubernetes Agent (PREMIUM SELF)
This document describes how kas routes requests to concrete agentk instances.
GitLab must talk to GitLab Kubernetes Agent Server (kas) to:
- Get information about connected agents. Read more.
- Interact with agents. Read more.
- Interact with Kubernetes clusters. Read more.
Each agent connects to an instance of kas and keeps an open connection. When
GitLab must talk to a particular agent, a kas instance connected to this agent must
be found, and the request routed to it.
System design
For an architecture overview please see architecture.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
For this architecture, this diagram shows a request to agentk 3, Pod1 for the list of pods:
sequenceDiagram
GitLab->>+kas1: Get list of running<br />Pods from agentk<br />with agent_id=3
Note right of kas1: kas1 checks for<br />agent connected with agent_id=3.<br />It does not.<br />Queries Redis
kas1->>+Redis: Get list of connected agents<br />with agent_id=3
Redis-->-kas1: List of connected agents<br />with agent_id=3
Note right of kas1: kas1 picks a specific agentk instance<br />to address and talks to<br />the corresponding kas instance,<br />specifying which agentk instance<br />to route the request to.
kas1->>+kas2: Get the list of running Pods<br />from agentk 3, Pod1
kas2->>+agentk 3 Pod1: Get list of Pods
agentk 3 Pod1->>-kas2: Get list of Pods
kas2-->>-kas1: List of running Pods<br />from agentk 3, Pod1
kas1-->>-GitLab: List of running Pods<br />from agentk with agent_id=3
Each kas instance tracks the agents connected to it in Redis. For each agent, it
stores a serialized protobuf object with information about the agent. When an agent
disconnects, kas removes all corresponding information from Redis. For both events,
kas publishes a notification to a Redis pub-sub channel.
Each agent, while logically a single entity, can have multiple replicas (multiple pods)
in a cluster. kas accommodates that and records per-replica (generally per-connection)
information. Each open GetConfiguration() streaming request is given
a unique identifier which, combined with agent ID, identifies an agentk instance.
gRPC can keep multiple TCP connections open for a single target host. agentk only
runs one GetConfiguration() streaming request. kas uses that connection, and
doesn't see idle TCP connections because they are handled by the gRPC framework.
Each kas instance provides information to Redis, so other kas instances can discover and access it.
Information is stored in Redis with an expiration time,
to expire information for kas instances that become unavailable. To prevent
information from expiring too quickly, kas periodically updates the expiration time
for valid entries. Before terminating, kas cleans up the information it adds into Redis.
When kas must atomically update multiple data structures in Redis, it uses
transactions to ensure data consistency.
Grouped data items must have the same expiration time.
In addition to the existing agentk -> kas gRPC endpoint, kas exposes two new,
separate gRPC endpoints for GitLab and for kas -> kas requests. Each endpoint
is a separate network listener, making it easier to control network access to endpoints
and allowing separate configuration for each endpoint.
Databases, like PostgreSQL, aren't used because the data is transient, with no need to reliably persist it.
GitLab : kas external endpoint
GitLab authenticates with kas using JWT and the same shared secret used by the
kas -> GitLab communication. The JWT issuer should be gitlab and the audience
should be gitlab-kas.
When accessed through this endpoint, kas plays the role of request router.
If a request from GitLab comes but no connected agent can handle it, kas blocks
and waits for a suitable agent to connect to it or to another kas instance. It
stops waiting when the client disconnects, or when some long timeout happens, such
as client timeout. kas is notified of new agent connections through a
pub-sub channel to avoid frequent polling.
When a suitable agent connects, kas routes the request to it.
kas : kas internal endpoint
This endpoint is an implementation detail, an internal API, and should not be used
by any other system. It's protected by JWT using a secret, shared among all kas
instances. No other system must have access to this secret.
When accessed through this endpoint, kas uses the request itself to determine
which agentk to send the request to. It prevents request cycles by only following
the instructions in the request, rather than doing discovery. It's the responsibility
of the kas receiving the request from the external endpoint to retry and re-route
requests. This method ensures a single central component for each request can determine
how a request is routed, rather than distributing the decision across several kas instances.
Reverse gRPC tunnel
This section explains how the agentk -> kas reverse gRPC tunnel is implemented.
For a video overview of how some of the blocks map to code, see GitLab Kubernetes Agent reverse gRPC tunnel architecture and code overview .
High level schema
In this example, Server side of module A exposes its API to get the Pod list
on the Public API gRPC server. When it receives a request, it must determine
the agent ID from it, then call the proxying code which forwards the request to
a suitable agentk that can handle it.
The Agent side of module A exposes the same API on the Internal gRPC server.
When it receives the request, it needs to handle it (such as retrieving and returning
the Pod list).
This schema describes how reverse tunneling is handled fully transparently for modules, so you can add new features:
graph TB
subgraph kas
server-internal-grpc-server[Internal gRPC server]
server-api-grpc-server[Public API gRPC server]
server-module-a[Server side of module A]
server-module-b[Server side of module B]
end
subgraph agentk
agent-internal-grpc-server[Internal gRPC server]
agent-module-a[Agent side of module A]
agent-module-b[Agent side of module B]
end
agent-internal-grpc-server -- request --> agent-module-a
agent-internal-grpc-server -- request --> agent-module-b
server-module-a-. expose API on .-> server-internal-grpc-server
server-module-b-. expose API on .-> server-api-grpc-server
server-internal-grpc-server -- proxy request --> agent-internal-grpc-server
server-api-grpc-server -- proxy request --> agent-internal-grpc-server
Implementation schema
HandleTunnelConnection() is called with the server-side interface of the reverse
tunnel. It registers the connection and blocks, waiting for a request to proxy
through the connection.
HandleIncomingConnection() is called with the server-side interface of the incoming
connection. It registers the connection and blocks, waiting for a matching tunnel
to proxy the connection through.
After it has two connections that match, Connection registry starts bi-directional
data streaming:
graph TB
subgraph kas
server-tunnel-module[Server tunnel module]
connection-registry[Connection registry]
server-internal-grpc-server[Internal gRPC server]
server-api-grpc-server[Public API gRPC server]
server-module-a[Server side of module A]
server-module-b[Server side of module B]
end
subgraph agentk
agent-internal-grpc-server[Internal gRPC server]
agent-tunnel-module[Agent tunnel module]
agent-module-a[Agent side of module A]
agent-module-b[Agent side of module B]
end
server-tunnel-module -- "HandleTunnelConnection()" --> connection-registry
server-internal-grpc-server -- "HandleIncomingConnection()" --> connection-registry
server-api-grpc-server -- "HandleIncomingConnection()" --> connection-registry
server-module-a-. expose API on .-> server-internal-grpc-server
server-module-b-. expose API on .-> server-api-grpc-server
agent-tunnel-module -- "establish tunnel, receive request" --> server-tunnel-module
agent-tunnel-module -- make request --> agent-internal-grpc-server
agent-internal-grpc-server -- request --> agent-module-a
agent-internal-grpc-server -- request --> agent-module-b