Reactive Engine

Reactive Engine

A reactive state system built on a graph of typed nodes. Five packages compose:

It also powers @virtuoso.dev/data-table internally — the same concepts apply when remote-controlling the table, and data-table's architecture is the reference for building on the engine (building-on-the-engine).

Mental model: nodes are definitions, engines hold state

Node constructors return inert references (symbols at runtime), defined at module scope with a $ suffix. They hold no state. An Engine instance activates nodes lazily on first use and owns their values — two engines using the same node have independent state. This is what makes engine-based libraries reusable: one module-scope graph, one engine per component instance.

import { Cell, Stream, Engine, e } from '@virtuoso.dev/reactive-engine-core'

const count$ = Cell(0)
const engine = new Engine()
engine.sub(count$, (value) => console.log('count:', value))
engine.pub(count$, 1) // logs 'count: 1'

Node types — pick by statefulness:

Nodes are distinct by default using reference equality (===): publishing the same reference (or an equal primitive) does not re-emit. Always publish new references from updates (new Map(old), spread). Pass false to always emit, or a comparator (prev, next) => boolean (true = equal = suppress); prev is undefined on first emission.

The three API flavors (most important distinction)

The same verbs exist in three forms — using the wrong one is the main source of errors:

Calling a context-bound utility elsewhere throws No active engine found. You can use getValue only in the context of node subscription callbacks. — fix by using engine.* methods, React hooks, or moving the code into a subscription. Note the asymmetry: standalone sub/link/changeWith are safe at module scope (they defer); standalone pub/getValue are not.

The e export bundles all combinators, operators, and context utilities into one namespace — idiomatic for wiring blocks:

e.link(
  e.pipe(
    toggleTheme$,
    e.withLatestFrom(theme$),
    e.map(([, t]) => (t === 'light' ? 'dark' : 'light'))
  ),
  theme$
)

Wiring the graph

• link(source$, sink$) — forward emissions; pipe(source$, ...ops) — derive a new node
• combine(a$, b$, ...) — emit [a, b, ...] when ANY source emits; withLatestFrom(...) (operator) — read other nodes passively, emit only when the source emits
• changeWith(cell$, source$, (cell, value) => next) — reducer-style cell update from a stream (return a new reference)
• sub, subMultiple, singletonSub (replaces the previous singleton subscription — use for callbacks driven by React renders)
• Operators: map(fn, distinct?), filter (supports type guards), mapTo, scan, once, throttleTime, debounceTime, delayWithMicrotask, onNext, handlePromise

Publishing is transactional: engine.pubIn({ [a$]: 1, [b$]: 2 }) batches into one cycle; the graph executes in topological order, so diamond dependencies emit downstream exactly once with consistent inputs. Emissions are synchronous; there are no error/completion channels (exceptions abort the cycle — handle errors at the edges). Subscribing never replays a current value; read cells with getValue when needed. See transactions.

React integration

import { EngineProvider, useCellValue, usePublisher } from '@virtuoso.dev/reactive-engine-react'

const Counter = () => {
  const count = useCellValue(count$)
  const setCount = usePublisher(count$)
  return <button => setCount(count + 1)}>{count}</button>
}

• useCellValue(cell$) subscribes and re-renders; useCellValues(a$, b$, ...) returns a tuple with a single combined subscription (prefer it for cells that change together); useCell is the useState shape; usePublisher(node$) returns a stable publish function
• EngineProvider props: initFn(engine) (one-time setup: register nodes, seed with pubIn, attach bridges), initWith (initial cell values), updateFn/updateDeps (re-publish changed props; use singletonSub for callback props so re-renders replace instead of stack), engineId, engineRef, and diagnostics (attach before initFn)
• Remote hooks (useRemoteCellValue, useRemotePublisher, useRemoteCell, useRemoteCellValues, useRemoteEngineDiagnostics) reach another provider's engine via useEngineRef() or a string engineId — value hooks return undefined until that engine mounts; diagnostic observation waits and attaches automatically
• Components stay projection-only: read cells, publish actions. Logic lives in module-scope wiring, testable with a bare Engine and no React

Troubleshoot propagation with diagnostics

Use diagnostics when a value is missing, duplicated, unexpectedly suppressed, routed through the wrong engine, or throws during synchronous propagation:

1. Label the root, expected sink, and intermediate nodes with describeNode. Add narrow summarize functions only for application-specific values that the built-in bounded summary cannot explain.
2. Attach at the correct lifecycle: EngineProvider.diagnostics includes initFn; useEngineDiagnostics observes the nearest engine after commit; useRemoteEngineDiagnostics is for sibling tools and follows engine availability. Treat provider initialization records as at-least-once in development because React Strict Mode can evaluate initialization more than once.
3. Start with value capture disabled. Read roots and events in order. Inspect distinct-suppressed, no-candidate, prune events, and error.phase before enabling captureValues: 'summary'.
4. Compare engineInstanceId and engineLabel when state appears to come from the wrong instance. Use transactionId, origin, and parentCycle for synchronous nested or parent-child propagation. Async work starts a new transaction and needs application-level correlation.
5. Enable summaries only for the nodes needed to verify the hypothesis. Redact before exporting. Node summarizers receive live values and must not mutate them or cause side effects.
6. Remove the observer after reproduction. Core and React retain no history; build any diagnostic UI on a bounded external store, not EngineProvider state.

Read propagation diagnostics for the record model, safety rules, and evidence-to-cause table. Read React integration for provider, local-hook, remote-hook, Strict Mode, and SSR timing.

Data fetching (reactive-engine-query)

export const tasksQuery = Query<{ listId: string }, Task[]>({
  queryFn: async ({ listId }, signal) => {
    const res = await fetch(`/api/tasks?listId=${listId}`, { signal })
    if (!res.ok) throw new Error('Failed to fetch tasks')
    return res.json()
  },
  initialParams: { listId: '' },
})

Query returns nodes: data$ (cell with { type: 'pending' | 'success' | 'error', data, error, isLoading, isFetching, ... }), params$ (publish to refetch with new params), refetch$, invalidate$ (keep showing stale data, isFetching: true, re-execute), unload$ (abort and clear), enabled$. Queries auto-execute on activation and param changes; each execution aborts the previous via the signal. Options: retry (default 3, exponential backoff), refetchInterval, initialData, enabled.

Mutation({ mutationFn, onSuccess?, onError? }) returns data$, mutate$, reset$. Mutations never auto-execute — publish params to mutate$. Wire mutations to refetch queries in the graph, not in components:

e.link(
  e.pipe(
    e.merge(createTask.data$, deleteTask.data$),
    e.filter((r) => r.type === 'success'),
    e.map(() => undefined, false) // distinct=false so consecutive successes each fire
  ),
  tasksQuery.refetch$
)

Routing (reactive-engine-router)

export const user$ = Route('/users/{id:number}', UserPage) // params typed from the path
const root = Layout('/', ({ children }) => <main>{children}</main>)

<Router routes={[user$]} layouts={[root]} guards={[authGuard]} />

Route nodes emit typed params when matched, null otherwise. Navigate by publishing params to the route node (usePublisher(user$)({ id: 42 })). Pipe route params into query params to fetch on navigation. Guard(pattern, fn) runs on matching navigations — its context offers continue(), navigate(), redirect(); async guards render the route inside Suspense. Layouts match by URL prefix and nest by specificity; LayoutSlotPortal/LayoutSlot/LayoutSlotFill let pages fill layout regions. Router syncs with browser history by default.

Persistence (reactive-engine-storage)

linkCellToStorage(theme$, { storageType: 'localStorage', key: 'app-theme', debounceMs: 300 })

storageType: 'localStorage' | 'sessionStorage' | 'cookie' (cookie adds cookieOptions). Reads happen lazily, once per engine, when the engine first activates the cell; writes are debounced; localStorage links sync across tabs. The engine's id namespaces keys (my-app:app-theme) so multiple instances persist independently. SSR-safe (no-op without window).

Building a component or library on the engine

Follow the data-table architecture (full patterns):

• Module-scope node graph + one engine per component instance (via EngineProvider)
• Public API = action streams in (setX$, resetX$), state cells out (xState$, often DerivedCell); wiring stays internal
• Props flow into cells via pubIn in initFn/updateFn; callback props via singletonSub
• Optional features as separate modules (subpath exports) that attach wiring to core nodes — unused features cost nothing because inits are lazy
• External control via engineRef/engineId + remote hooks against the exported nodes
• Bridge imperative subsystems with subscribe-both-ways functions whose cleanup reaches engine.onDispose

Common mistakes

• Calling standalone pub/getValue/pubIn outside a subscription callback — use engine.* methods or React hooks.
• Mutating and republishing the same object — distinct is reference equality; nothing emits. Publish new references.
• Expecting a node to hold state by itself — values live in an engine; the same Cell in two engines has two values.
• Expecting Stream subscriptions to fire on subscribe with a current value — streams are stateless; only Cell has a value.
• Piping into a trigger through map without distinct: false — consecutive identical mapped values get filtered.
• Triggering a Mutation by changing params — mutations only run when params are published to mutate$.
• Plain sub in code that re-runs with React renders — subscriptions accumulate; use singletonSub.
• Creating cycles in the graph — cycles hang or loop; break one direction with an action stream or delayWithMicrotask.

References

Guides in references/, per package:

• references/core/ — core-concepts (node types, distinct, the e namespace), engine-and-lifecycle (activation, API flavors, child engines, disposal), transactions (cycles, topological execution, RxJS differences), operators-and-combinators (full reference), building-on-the-engine (architecture patterns from data-table), propagation-diagnostics (records, safety, troubleshooting), README
• references/react/ — react-integration (EngineProvider lifecycle, hooks, remote hooks, SSR), README
• references/query/ — queries-and-mutations (result states, lifecycle, wiring patterns), README
• references/router/ — routing (path syntax, navigation, layouts, guards), README
• references/storage/ — storage-links (timing, namespacing, cross-tab sync), README

The JSDoc on the exported symbols in @virtuoso.dev/reactive-engine-core is extensive — when in doubt about a signature, read the type definitions shipped with the package.