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layout default
title Chapter 2: System Architecture
nav_order 2
parent HAPI Tutorial

Chapter 2: System Architecture

Welcome to Chapter 2: System Architecture. In this part of HAPI Tutorial: Remote Control for Local AI Coding Sessions, you will build an intuitive mental model first, then move into concrete implementation details and practical production tradeoffs.

HAPI is a control plane around local coding agents: CLI wrapper, hub service, and remote clients.

Architecture Diagram

graph TD
    CLI[HAPI CLI + Agent] <--> HUB[Hub API + Socket + SSE]
    HUB --> DB[SQLite]
    HUB <--> WEB[PWA/Web Client]
    HUB <--> TG[Telegram Mini App]
    RUN[Runner Service] <--> HUB
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Component Roles

Component Responsibilities
CLI wraps agent process, relays messages, emits permission events
Hub session persistence, real-time transport, auth, notifications
PWA/Web remote session control and approval UX
Runner background machine service for remote session spawning

Protocol Boundaries

  • CLI to hub: Socket.IO for low-latency bidirectional events
  • hub to UI: REST for actions, SSE for live updates
  • external users: relay/tunnel ingress with token-based auth

Summary

You now understand where HAPI stores state and routes interactive control.

Next: Chapter 3: Session Lifecycle and Handoff

What Problem Does This Solve?

Most teams struggle here because the hard part is not writing more code, but deciding clear boundaries for graph, HAPI, Agent so behavior stays predictable as complexity grows.

In practical terms, this chapter helps you avoid three common failures:

  • coupling core logic too tightly to one implementation path
  • missing the handoff boundaries between setup, execution, and validation
  • shipping changes without clear rollback or observability strategy

After working through this chapter, you should be able to reason about Chapter 2: System Architecture as an operating subsystem inside HAPI Tutorial: Remote Control for Local AI Coding Sessions, with explicit contracts for inputs, state transitions, and outputs.

Use the implementation notes around Socket, SQLite, Client as your checklist when adapting these patterns to your own repository.

How it Works Under the Hood

Under the hood, Chapter 2: System Architecture usually follows a repeatable control path:

  1. Context bootstrap: initialize runtime config and prerequisites for graph.
  2. Input normalization: shape incoming data so HAPI receives stable contracts.
  3. Core execution: run the main logic branch and propagate intermediate state through Agent.
  4. Policy and safety checks: enforce limits, auth scopes, and failure boundaries.
  5. Output composition: return canonical result payloads for downstream consumers.
  6. Operational telemetry: emit logs/metrics needed for debugging and performance tuning.

When debugging, walk this sequence in order and confirm each stage has explicit success/failure conditions.

Chapter Connections

Source Code Walkthrough

web/src/router.tsx

The SessionsIndexPage function in web/src/router.tsx handles a key part of this chapter's functionality:

}

function SessionsIndexPage() {
    return null
}

function SessionPage() {
    const { api } = useAppContext()
    const { t } = useTranslation()
    const goBack = useAppGoBack()
    const navigate = useNavigate()
    const queryClient = useQueryClient()
    const { addToast } = useToast()
    const { sessionId } = useParams({ from: '/sessions/$sessionId' })
    const {
        session,
        refetch: refetchSession,
    } = useSession(api, sessionId)
    const {
        messages,
        warning: messagesWarning,
        isLoading: messagesLoading,
        isLoadingMore: messagesLoadingMore,
        hasMore: messagesHasMore,
        loadMore: loadMoreMessages,
        refetch: refetchMessages,
        pendingCount,
        messagesVersion,
        flushPending,
        setAtBottom,
    } = useMessages(api, sessionId)
    const {

This function is important because it defines how HAPI Tutorial: Remote Control for Local AI Coding Sessions implements the patterns covered in this chapter.

web/src/router.tsx

The SessionPage function in web/src/router.tsx handles a key part of this chapter's functionality:

}

function SessionPage() {
    const { api } = useAppContext()
    const { t } = useTranslation()
    const goBack = useAppGoBack()
    const navigate = useNavigate()
    const queryClient = useQueryClient()
    const { addToast } = useToast()
    const { sessionId } = useParams({ from: '/sessions/$sessionId' })
    const {
        session,
        refetch: refetchSession,
    } = useSession(api, sessionId)
    const {
        messages,
        warning: messagesWarning,
        isLoading: messagesLoading,
        isLoadingMore: messagesLoadingMore,
        hasMore: messagesHasMore,
        loadMore: loadMoreMessages,
        refetch: refetchMessages,
        pendingCount,
        messagesVersion,
        flushPending,
        setAtBottom,
    } = useMessages(api, sessionId)
    const {
        sendMessage,
        retryMessage,
        isSending,
    } = useSendMessage(api, sessionId, {

This function is important because it defines how HAPI Tutorial: Remote Control for Local AI Coding Sessions implements the patterns covered in this chapter.

web/src/router.tsx

The SessionDetailRoute function in web/src/router.tsx handles a key part of this chapter's functionality:

}

function SessionDetailRoute() {
    const pathname = useLocation({ select: location => location.pathname })
    const { sessionId } = useParams({ from: '/sessions/$sessionId' })
    const basePath = `/sessions/${sessionId}`
    const isChat = pathname === basePath || pathname === `${basePath}/`

    return isChat ? <SessionPage /> : <Outlet />
}

function NewSessionPage() {
    const { api } = useAppContext()
    const navigate = useNavigate()
    const goBack = useAppGoBack()
    const queryClient = useQueryClient()
    const { machines, isLoading: machinesLoading, error: machinesError } = useMachines(api, true)
    const { t } = useTranslation()

    const handleCancel = useCallback(() => {
        navigate({ to: '/sessions' })
    }, [navigate])

    const handleSuccess = useCallback((sessionId: string) => {
        void queryClient.invalidateQueries({ queryKey: queryKeys.sessions })
        // Replace current page with /sessions to clear spawn flow from history
        navigate({ to: '/sessions', replace: true })
        // Then navigate to new session
        requestAnimationFrame(() => {
            navigate({
                to: '/sessions/$sessionId',
                params: { sessionId },

This function is important because it defines how HAPI Tutorial: Remote Control for Local AI Coding Sessions implements the patterns covered in this chapter.

web/src/router.tsx

The NewSessionPage function in web/src/router.tsx handles a key part of this chapter's functionality:

}

function NewSessionPage() {
    const { api } = useAppContext()
    const navigate = useNavigate()
    const goBack = useAppGoBack()
    const queryClient = useQueryClient()
    const { machines, isLoading: machinesLoading, error: machinesError } = useMachines(api, true)
    const { t } = useTranslation()

    const handleCancel = useCallback(() => {
        navigate({ to: '/sessions' })
    }, [navigate])

    const handleSuccess = useCallback((sessionId: string) => {
        void queryClient.invalidateQueries({ queryKey: queryKeys.sessions })
        // Replace current page with /sessions to clear spawn flow from history
        navigate({ to: '/sessions', replace: true })
        // Then navigate to new session
        requestAnimationFrame(() => {
            navigate({
                to: '/sessions/$sessionId',
                params: { sessionId },
            })
        })
    }, [navigate, queryClient])

    return (
        <div className="flex h-full min-h-0 flex-col">
            <div className="flex items-center gap-2 border-b border-[var(--app-border)] bg-[var(--app-bg)] p-3 pt-[calc(0.75rem+env(safe-area-inset-top))]">
                {!isTelegramApp() && (
                    <button

This function is important because it defines how HAPI Tutorial: Remote Control for Local AI Coding Sessions implements the patterns covered in this chapter.

How These Components Connect

flowchart TD
    A[SessionsIndexPage]
    B[SessionPage]
    C[SessionDetailRoute]
    D[NewSessionPage]
    E[createAppRouter]
    A --> B
    B --> C
    C --> D
    D --> E
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