adk-architecture

name

adk-architecture

description

Explains how the ADK runtime fits together: the node and graph execution model, Context and Event flow, checkpoint and resume, tracing, and the rules governing the public API surface. Use when answering "how does X work" about ADK internals, tracing where an event or a piece of state comes from, deciding where a new capability belongs, reviewing a change to BaseNode, Workflow, Runner, Agent, Event or Context, working out why a node re-ran or stayed waiting after a resume, or judging whether a change breaks the public API. Don't use for assembling an agent from existing pieces (use adk-agent-builder), diagnosing one failing run or test (use adk-debug), or formatting and naming conventions (use adk-style).

ADK Architecture

The runtime is a graph of nodes. BaseNode is the unit of execution. Workflow is a node that schedules other nodes along declared edges. NodeRunner executes exactly one node. Runner owns the invocation and the session. Agents are nodes too — BaseAgent extends BaseNode.

A node communicates with its parent through a per-execution Context, and with the session through Events it yields. Those are two separate channels: ctx carries the result upward, events carry persistence and streaming.

Read the source before relying on any signature here. These notes drift; the code does not. Paths below are relative to src/google/adk/.

Pick a reference

Question	Reference
What must a node implement? What may it yield? Which config fields exist?	BaseNode
How does the graph schedule nodes, dedup dynamic children, propagate interrupts?	Workflow
How does a caller start an invocation?	Runner
What is Agent, and which methods do I call on it?	Agent
I am subclassing an agent — what do I override?	BaseAgent
What is on an Event, and what may I assume about its lifetime?	Event
What does a node read and write on ctx?	Context
Who creates the child Context, stamps events, retries, catches errors?	NodeRunner
Why are Runner, NodeRunner and Workflow three separate things?	Runner roles
How does a human-in-the-loop pause and resume work for one node?	Checkpoint and resume
How does a whole workflow survive a pause, and what does is_resumable change?	Workflow resumability
How are spans created, and what attributes do they carry?	Observability
Why does the model not see the raw event log?	LLM context orchestration
Is this change a breaking change? Where does a new export belong?	API principles

Where the code lives

Concept	Module
BaseNode, START	workflow/_base_node.py
Workflow, _LoopState	workflow/_workflow.py
Graph, edge compilation	workflow/_graph.py
NodeRunner	workflow/_node_runner.py
DynamicNodeScheduler	workflow/_dynamic_node_scheduler.py
ReplayManager (resume scan)	workflow/utils/_replay_manager.py
NodeInterruptedError, NodeTimeoutError	workflow/_errors.py
Context, ctx.run_node()	agents/context.py
ReadonlyContext	agents/readonly_context.py
InvocationContext	agents/invocation_context.py
BaseAgent, LlmAgent (aliased Agent)	agents/base_agent.py, agents/llm_agent.py
Event, NodeInfo	events/event.py
EventActions	events/event_actions.py
Branch paths (parent.child@1)	events/_branch_path.py
Node paths (wf@1/child@2)	events/_node_path_builder.py
Runner, InMemoryRunner	runners.py
Node spans, TelemetryContext

Everything under workflow/ is a leading-underscore module. Treat those names as internal — they can change without a major version bump, so a change there is not automatically a breaking change.