observed-overwrite-struct

State-based CRDT for fixed-key object structs with per-field overwrite tracking.

Compatibility

• Runtimes: Node >= 20; modern browsers; Bun; Deno; Cloudflare Workers; Edge Runtime.
• Module format: ESM + CommonJS.
• Required globals / APIs: EventTarget, CustomEvent, structuredClone.
• TypeScript: bundled types.

Goals

• Fixed-key replica shape defined by a default struct.
• One visible value per field at any time.
• Malformed ingress is ignored during hydration and merge instead of crashing the replica.
• read(), values(), entries(), snapshots, deltas, and change payloads are detached with structuredClone.
• Explicit acknowledge() and garbageCollect() APIs for overwrite-history compaction.
• Consistent behavior across Node, browsers, and worker/edge runtimes.

Installation

npm install @sovereignbase/observed-overwrite-struct
# or
pnpm add @sovereignbase/observed-overwrite-struct
# or
yarn add @sovereignbase/observed-overwrite-struct
# or
bun add @sovereignbase/observed-overwrite-struct
# or
deno add jsr:@sovereignbase/observed-overwrite-struct
# or
vlt install jsr:@sovereignbase/observed-overwrite-struct

Usage

Copy-paste example

import { OOStruct } from '@sovereignbase/observed-overwrite-struct'

type TodoStruct = {
  title: string
  count: number
  meta: { done: boolean }
  tags: string[]
}

const alice = OOStruct.create<TodoStruct>({
  title: '',
  count: 0,
  meta: { done: false },
  tags: [],
})
const bob = OOStruct.create<TodoStruct>({
  title: '',
  count: 0,
  meta: { done: false },
  tags: [],
})

alice.addEventListener('delta', (event) => bob.merge(event.detail))
alice.update('title', 'hello world')
alice.update('meta', { done: true })

console.log(bob.read('title')) // "hello world"
console.log(bob.read('meta')) // { done: true }

Hydrating from a snapshot

import {
  OOStruct,
  type OOStructSnapshot,
} from '@sovereignbase/observed-overwrite-struct'

type DraftStruct = {
  title: string
  count: number
}

const source = new OOStruct<DraftStruct>({
  title: '',
  count: 0,
})
let snapshot!: OOStructSnapshot<DraftStruct>

source.addEventListener(
  'snapshot',
  (event) => {
    snapshot = event.detail
  },
  { once: true }
)

source.update('title', 'draft')
source.snapshot()

const restored = OOStruct.create<DraftStruct>(
  {
    title: '',
    count: 0,
  },
  snapshot
)

console.log(restored.entries()) // [['title', 'draft'], ['count', 0]]

Event channels

import { OOStruct } from '@sovereignbase/observed-overwrite-struct'

const replica = new OOStruct({
  name: '',
  count: 0,
})

replica.addEventListener('delta', (event) => {
  console.log('delta', event.detail)
})

replica.addEventListener('change', (event) => {
  console.log('change', event.detail)
})

replica.addEventListener('ack', (event) => {
  console.log('ack', event.detail)
})

replica.addEventListener('snapshot', (event) => {
  console.log('snapshot', event.detail)
})

Acknowledgements and garbage collection

import {
  OOStruct,
  type OOStructAck,
} from '@sovereignbase/observed-overwrite-struct'

type CounterStruct = {
  title: string
  count: number
}

const left = new OOStruct<CounterStruct>({
  title: '',
  count: 0,
})
const right = new OOStruct<CounterStruct>({
  title: '',
  count: 0,
})

const frontiers: Array<OOStructAck<CounterStruct>> = []

left.addEventListener(
  'ack',
  (event) => {
    frontiers.push(event.detail)
  },
  { once: true }
)

right.addEventListener(
  'ack',
  (event) => {
    frontiers.push(event.detail)
  },
  { once: true }
)

left.acknowledge()
right.acknowledge()

left.garbageCollect(frontiers)
right.garbageCollect(frontiers)

Runtime behavior

Validation and errors

Local API misuse throws OOStructError with stable error codes:

• DEFAULTS_NOT_CLONEABLE
• VALUE_NOT_CLONEABLE
• VALUE_TYPE_MISMATCH

Hydration and merge are ingress-tolerant: malformed top-level payloads, unknown keys, malformed field entries, invalid UUIDs, invalid overwrite members, and mismatched runtime kinds are ignored instead of throwing.

Safety and copying semantics

• Constructor defaults must be structuredClone-compatible.
• read(), values(), and entries() return detached clones.
• delta, change, and snapshot event payloads are detached from live state.
• update() stores a cloned value, so later caller-side mutation does not mutate replica state through shared references.

Convergence and compaction

• The convergence guarantee is the resolved live struct state.
• Internal overwrite history may differ between replicas after acknowledgement-based garbage collection while the resolved live state still converges.
• garbageCollect() compacts overwritten identifiers that are below the smallest acknowledgement frontier for a key while preserving the active predecessor link.

Tests

• Suite: unit, integration, and end-to-end runtime tests.
• Node test runner: node --test for unit and integration suites.
• Coverage: c8 with 100% statements / branches / functions / lines on built dist/**/*.js.
• E2E runtimes: Node ESM, Node CJS, Bun ESM, Bun CJS, Deno ESM, Cloudflare Workers ESM, Edge Runtime ESM.
• Browser E2E: Chromium, Firefox, WebKit, mobile Chrome, mobile Safari via Playwright.
• Current status: npm run test passes on Node 22.14.0 (win32 x64).

Benchmarks

How it was run:

node benchmark/bench.js

Environment: Node 22.14.0 (win32 x64)

Benchmark	Result
constructor empty	44,359 ops/s (2254.3 ms)
constructor hydrate x64	19,610 ops/s (255.0 ms)
constructor hydrate x256	8,088 ops/s (247.3 ms)
constructor hydrate x1024	1,724 ops/s (290.0 ms)
create() empty	49,874 ops/s (2005.1 ms)
create() hydrate x256	6,886 ops/s (290.4 ms)
read primitive	846,289 ops/s (236.3 ms)
read object	298,983 ops/s (668.9 ms)
read array	278,710 ops/s (717.6 ms)
keys()	32,349,896 ops/s (6.2 ms)
values()	103,489 ops/s (966.3 ms)
entries()	110,300 ops/s (906.6 ms)
snapshot()	65,513 ops/s (305.3 ms)
acknowledge()	536,890 ops/s (93.1 ms)
update string	29,547 ops/s (1692.2 ms)
update number	30,591 ops/s (1634.5 ms)
update object	22,114 ops/s (2261.0 ms)
update array	24,763 ops/s (2019.1 ms)
delete(key)	8,352 ops/s (5986.8 ms)
delete() reset all	6,836 ops/s (2925.5 ms)
merge direct successor	32,541 ops/s (1536.5 ms)
merge stale conflict	30,995 ops/s (645.3 ms)
merge hydrate snapshot x256	5,748 ops/s (869.9 ms)
merge noop duplicate	7,576 ops/s (6600.1 ms)
garbageCollect() x512 history	3,111 ops/s (1607.0 ms)
add/remove listener roundtrip	49,005 ops/s (4081.2 ms)
update with listeners	25,120 ops/s (1194.3 ms)
merge with listeners	31,649 ops/s (631.9 ms)

Results vary by machine, runtime version, and payload shape.

License

Apache-2.0