understanding-streamlit-architecture

name	understanding-streamlit-architecture
description	Explains Streamlit's internal architecture including backend runtime, frontend rendering, and WebSocket communication. Use when debugging cross-layer issues, understanding how features work end-to-end, planning architectural changes, or onboarding to the codebase. Covers ForwardMsg/BackMsg protocol, script rerun model, element tree, widget state management, and more.

Understanding Streamlit architecture

Streamlit is a client-server application with bidirectional WebSocket communication using Protocol Buffers. Use this file as a quick mental model and navigation index; use references/backend.md, references/frontend.md, and references/communication.md for implementation details.

Concepts glossary

Concept	Description	Key files
Command	Any function exposed in Streamlit's public API (st.* namespace). Commands can create elements/widgets, control execution flow, or configure app behavior.	lib/streamlit/delta_generator.py, lib/streamlit/elements/, lib/streamlit/commands/
Element	Umbrella term for all UI components in Streamlit: widgets, containers, and display elements. Represented in Element.proto as a oneof union of ~50+ types.	proto/streamlit/proto/Element.proto, lib/streamlit/elements/
Widget	Interactive element (button, slider, text_input) that triggers reruns on user interaction. Value accessible via return value or st.session_state. Some elements become widgets conditionally (e.g., dataframe/chart with on_select).	lib/streamlit/elements/widgets/, frontend/lib/src/components/widgets/
Display Element	Non-interactive element (text, markdown, image, chart) that renders content without triggering reruns by itself.	lib/streamlit/elements/, frontend/lib/src/components/elements/
Container	Layout block that groups elements spatially (sidebar, columns, expander, tabs, form). Represented as BlockNode in the element tree.	lib/streamlit/elements/layouts.py, Block.proto
DeltaGenerator	The st object; API entry point that queues UI deltas. Uses mixin pattern to compose all st.* commands.	lib/streamlit/delta_generator.py
Session State	Per-session dictionary (st.session_state) persisting data across reruns. Stores widget values and user variables.	lib/streamlit/runtime/state/session_state.py
Rerun	Re-execution of the user script for the current page. Triggered by widget interactions, st.rerun(), file changes, or fragment timers. Rebuilds the element tree while preserving session state.	lib/streamlit/runtime/scriptrunner/script_runner.py, lib/streamlit/commands/execution_control.py
Form	Container (st.form) that batches widget inputs, deferring reruns until form submission.	lib/streamlit/elements/form.py, WidgetStateManager.ts
Fragment	Decorator (@st.fragment) enabling partial reruns of specific UI sections without full script re-execution.	lib/streamlit/runtime/fragment.py
Caching	Decorators (@st.cache_data, @st.cache_resource) that memoize function results to avoid redundant computation.	lib/streamlit/runtime/caching/
Pages	Multipage app system using st.navigation() and st.Page() or auto-discovery from pages/ directory.	lib/streamlit/navigation/, lib/streamlit/runtime/pages_manager.py
Config	App configuration via .streamlit/config.toml controlling server, client, and theme settings.	lib/streamlit/config.py, lib/streamlit/config_option.py
Theming	Customizable UI themes (Light/Dark/Custom) defined in config or via theme editor.	lib/streamlit/runtime/theme_util.py, frontend/lib/src/theme/
Secrets	Secure credential storage via .streamlit/secrets.toml (local) or platform settings (deployed). Accessed via st.secrets.	lib/streamlit/runtime/secrets.py
Connection	Database/service abstraction (st.connection) with built-in caching and secrets integration.	lib/streamlit/connections/
Custom Components	User-built extensions using React/iframe. v1 (legacy): declare_component() API. v2 (current): Bidirectional components with improved state management.	frontend/component-lib/, frontend/component-v2-lib/, lib/streamlit/components/v1/, lib/streamlit/components/v2/
Static File Serving	Files in static/ directory served directly via /app/static/* (when static serving is enabled).	lib/streamlit/web/server/server.py, lib/streamlit/web/server/starlette/starlette_routes.py
App Testing	Testing framework (AppTest) for simulating user interactions and inspecting rendered output.	lib/streamlit/testing/

Core mental model

flowchart TB
    subgraph Backend["Backend (Python)"]
        Script[User Script]
        DG[DeltaGenerator]
        Runtime[Runtime/AppSession]
    end

    subgraph Protocol["WebSocket + Protobuf"]
        FM[ForwardMsg]
        BM[BackMsg]
    end

    subgraph Frontend["Frontend (React)"]
        App[App.tsx]
        Tree[AppRoot Tree]
        Renderer[ElementNodeRenderer]
        WSM[WidgetStateManager]
    end

    Script --> DG
    DG --> FM
    FM --> App
    App --> Tree
    Tree --> Renderer
    Renderer --> WSM
    WSM -->|User interaction| BM
    BM --> Runtime
    Runtime -->|Rerun| Script

Key insight: Script execution is rerun-driven: most widget interactions trigger reruns (full app or fragment-scoped). State persists via st.session_state and caching decorators.

Execution model

Streamlit's execution model differs from traditional web frameworks:

Rerun triggers:

1. Widget interaction: User clicks button, moves slider, etc.
2. Source code change: File watcher detects script modification
3. st.rerun(): Explicit programmatic rerun
4. Fragment timer: @st.fragment(run_every=...) periodic reruns

Execution order nuances:

• Scripts execute top-to-bottom on every rerun
• Callbacks first: on_change/on_click handlers run before the main script body
• Fragments typically isolate reruns: Widget interactions inside @st.fragment usually rerun only that fragment
• Control flow exceptions: st.stop(), st.rerun(), st.switch_page() raise exceptions to halt/redirect execution

Session isolation:

• Each browser tab = separate AppSession with its own SessionState
• Refreshing the page creates a new session (unless reconnecting within TTL)
• No shared state between sessions (use external storage for multi-user state)

What persists across reruns (within a session):

• st.session_state values
• Cached function results (@st.cache_data, @st.cache_resource)
• Uploaded files
• Fragment registrations

What resets on each rerun:

• Local variables in script
• Widget return values (re-read from SessionState)
• Element tree (rebuilt from scratch, then diffed)

Architecture layers

Backend (Python)

Component	File	Purpose
Runtime	lib/streamlit/runtime/runtime.py	Singleton managing app lifecycle and sessions
AppSession	lib/streamlit/runtime/app_session.py	Per-browser-tab: ScriptRunner + SessionState + ForwardMsgQueue
ScriptRunner	lib/streamlit/runtime/scriptrunner/script_runner.py	Executes user scripts in separate thread
DeltaGenerator	lib/streamlit/delta_generator.py	API entry point using mixin pattern
SessionState	lib/streamlit/runtime/state/session_state.py	Widget values and user variables
Elements	lib/streamlit/elements/	Backend implementation of st.* commands

For backend deep dive: See references/backend.md

Frontend (TypeScript/React)

Component	File	Purpose
App	frontend/app/src/App.tsx	Central orchestrator
AppRoot	frontend/lib/src/render-tree/AppRoot.ts	Immutable element tree with 4 containers
ElementNodeRenderer	frontend/lib/src/components/core/Block/ElementNodeRenderer.tsx	Maps protos to React components
WidgetStateManager	frontend/lib/src/WidgetStateManager.ts	Widget state, forms, query params
ConnectionManager	frontend/connection/src/ConnectionManager.ts	WebSocket state machine

For frontend deep dive: See references/frontend.md

Communication (Protobuf)

Proto	Purpose
ForwardMsg.proto	Server to client: deltas, session events, navigation
BackMsg.proto	Client to server: rerun requests with widget states
Element.proto	~50+ element types in oneof type union
WidgetStates.proto	Widget values: trigger_value, string_value, bool_value, etc.

Location: proto/streamlit/proto/

For protocol deep dive: See references/communication.md

Essential concepts (quick map)

• Layout system: Width/height parameters for elements, flexbox containers, and responsive sizing.
  • Deep dive: references/layout.md
• Script rerun model: Widget interaction -> BackMsg (ClientState) -> backend updates SessionState -> script rerun -> ForwardMsg deltas.
  • Deep dive: references/communication.md#widget-interaction-to-script-rerun
• Delta path system: Elements are addressed by delta paths (for example [0, 2, 3]) to support efficient tree updates.
  • Deep dive: references/communication.md#delta-ui-changes
• active_script_hash semantics: ForwardMsg.metadata.active_script_hash scopes node ownership across multipage and fragment reruns.
  • Deep dive: references/communication.md#forwardmsg-metadata-active_script_hash
• Element tree structure: Frontend AppRoot maintains main, sidebar, event, and bottom containers with BlockNode/ElementNode.
  • Deep dive: references/frontend.md#element-tree-frontendlibsrcrender-tree
• Element identity semantics: Delta paths decide where a node lives; element IDs decide whether stateful elements can reconnect to prior state after remounts.
  • Deep dive: references/element-identity.md
• Fragments (@st.fragment): Fragment interactions usually trigger fragment-scoped reruns and update only affected regions.
  • Deep dive: references/backend.md

Element identity

• Delta path controls where a node is placed in the render tree.
• Element ID controls whether a stateful element can reconnect to prior state after remounts.
• Widgets use the ID to connect frontend WidgetStateManager state, BackMsg.WidgetStates, backend SessionState, callbacks, and st.session_state.
• Some non-widgets also use IDs for frontend-only reconstruction (for example chart view state or media autoplay guards).
• Delta-path changes can remount elements, but a stable element.id can still let React preserve a leaf when it remains under the same rendered parent list.

Deep dive: references/element-identity.md

Key implementation patterns

• Backend mixin composition: DeltaGenerator composes st.* API via mixins.
  • Deep dive: references/backend.md#deltagenerator-libstreamlitdelta_generatorpy
• Frontend visitor pattern: Render-tree updates/staleness cleanup use visitors.
  • Deep dive: references/frontend.md#visitor-pattern
• Message deduplication: ForwardMsg.hash + ref_hash with frontend ForwardMsgCache reduces bandwidth.
  • Deep dive: references/communication.md#message-caching

Quick reference: adding features

1. Proto definition: proto/streamlit/proto/<Element>.proto
2. Register in Element.proto: Add to oneof type
3. Backend mixin: lib/streamlit/elements/<element>.py
4. Frontend component: frontend/lib/src/components/elements/<Element>/ or frontend/lib/src/components/widgets/<Element>/ (depending on element vs widget)
5. Register in ElementNodeRenderer: Add case to switch statement
6. Compile protos: make protobuf

See wiki/new-feature-guide.md for a detailed implementation guide.

Maintaining this documentation

When making changes that impact any of the concepts documented here (Runtime, AppSession, DeltaGenerator, element tree, communication protocol, element identity, etc.), update the relevant sections in this skill's files (SKILL.md, references/backend.md, references/frontend.md, references/communication.md, references/element-identity.md) to keep them accurate.