> Part of the walkerOS documentation. Project overview and full index: # Fingerprint [Server](#)[ ](https://github.com/elbwalker/walkerOS/tree/main/packages/server/transformers/fingerprint) [Source code](https://github.com/elbwalker/walkerOS/tree/main/packages/server/transformers/fingerprint)[ ](https://www.npmjs.com/package/@walkeros/server-transformer-fingerprint) [Package](https://www.npmjs.com/package/@walkeros/server-transformer-fingerprint) Hashes configurable request fields (IP address, user agent, date, etc.) into a deterministic identifier and stores it on the event. No cookies, no PII stored. The same combination of inputs always produces the same hash, enabling server-side session continuity, cookie-free analytics, and cross-domain stitching without client-side IDs. ## Installation[​](#installation "Direct link to Installation") ``` npm install @walkeros/server-transformer-fingerprint ``` * Integrated * Bundled ``` import { startFlow } from '@walkeros/collector'; import { transformerFingerprint } from '@walkeros/server-transformer-fingerprint'; await startFlow({ transformers: { fingerprint: { code: transformerFingerprint, config: { settings: { fields: ['ingest.ip', 'ingest.userAgent'], output: 'user.hash', length: 16, }, }, }, }, }); ``` Add to your `flow.json`: ``` "transformers": { "fingerprint": { "package": "@walkeros/server-transformer-fingerprint", "config": { "settings": { "fields": ["ingest.ip", "ingest.userAgent"], "output": "user.hash", "length": 16 } } } } ``` [See bundled mode setup](https://www.walkeros.io/docs/getting-started/modes/bundled.md) | [CLI reference](https://www.walkeros.io/docs/apps/cli.md) ## Configuration[​](#configuration "Direct link to Configuration") This transformer uses the standard transformer config wrapper (consent, data, env, id, ...). For the shared fields see [transformer configuration](https://www.walkeros.io/docs/transformers.md#configuration). Package-specific fields live under `config.settings` and are listed below. ## Settings[​](#settings "Direct link to Settings") | Property | Type | Description | More | | --------- | ------------ | ----------------------------------------------------------------------------------------------------------- | ---- | | `fields*` | `Array` | Fields to include in hash (order matters). Each resolved via getMappingValue with source { event, ingest }. | | | `output` | `string` | Dot-notation path where hash is stored on the event. Default: "user.hash" | | | `length` | `integer` | Truncate hash to this length. Default: full 64-char SHA-256 hash | | \* Required fields ## Mapping[​](#mapping "Direct link to Mapping") This package does not define custom rule-level settings. For the standard rule fields (consent, condition, data, batch, name, policy) see [mapping](https://www.walkeros.io/docs/mapping.md). ## Examples ### IP anonymization Privacy-preserving fingerprint using key+fn pattern: fn truncates IP to /24 subnet before hashing, so 10.0.42.\* users share a hash. Config: fields: \[{ key: "ingest.ip", fn: ip => ip.replace(/\\.\d+$/, ".0") }, "ingest.userAgent"] Event ``` { "name": "page view", "data": { "domain": "www.example.com", "title": "Privacy Policy", "id": "/privacy" }, "id": "ev-1700000602", "trigger": "load", "entity": "page", "action": "view", "timestamp": 1700000602, "source": { "type": "express", "platform": "server" } } ``` Out ``` return { "event": { "name": "page view", "data": { "domain": "www.example.com", "title": "Privacy Policy", "id": "/privacy" }, "user": { "hash": "44d9154b9a9b3792" }, "id": "ev-1700000602", "trigger": "load", "entity": "page", "action": "view", "timestamp": 1700000602, "source": { "type": "express", "platform": "server" } } } ``` ### Server fingerprint Standard server fingerprint using ingest.ip and ingest.userAgent. Requires source config.ingest. Event ``` { "name": "page view", "data": { "domain": "www.example.com", "title": "Getting Started", "id": "/docs/getting-started" }, "id": "ev-1700000600", "trigger": "load", "entity": "page", "action": "view", "timestamp": 1700000600, "source": { "type": "express", "platform": "server" } } ``` Out ``` return { "event": { "name": "page view", "data": { "domain": "www.example.com", "title": "Getting Started", "id": "/docs/getting-started" }, "user": { "hash": "158f99cc06e33fd6" }, "id": "ev-1700000600", "trigger": "load", "entity": "page", "action": "view", "timestamp": 1700000600, "source": { "type": "express", "platform": "server" } } } ``` Fields are resolved from `{ event, ingest }` using walkerOS mapping. String values use dot notation (`"ingest.ip"`). Function values compute dynamically (`{ fn: () => new Date().getDate() }`). ## Daily rotation[​](#daily-rotation "Direct link to Daily rotation") Without rotation, the same IP + user agent always produce the same hash, indefinitely. To limit persistence, add a daily rotation field: ``` settings: { fields: [ 'ingest.ip', 'ingest.userAgent', { fn: () => new Date().toISOString().slice(0, 10) }, // "2024-01-15" ], output: 'user.hash', length: 16, } ``` The hash resets each day, limiting cross-day tracking while maintaining session continuity within a day. ## IP anonymization[​](#ip-anonymization "Direct link to IP anonymization") To avoid including the raw IP in the hash input, transform it before hashing: ``` settings: { fields: [ { key: 'ingest.ip', fn: (ip) => ip?.split('.').slice(0, 3).join('.') }, // drops last octet 'ingest.userAgent', ], output: 'user.hash', } ``` ## Result[​](#result "Direct link to Result") The hash is stored at the configured `output` path on the event: ``` // Input event (before fingerprint transformer) { name: 'page view', entity: 'page', action: 'view', ... } // Output event (after fingerprint transformer) { name: 'page view', entity: 'page', action: 'view', user: { hash: '158f99cc06e33fd6' }, ... } ``` If any field is missing (e.g., `ingest.ip` is undefined), it is treated as an empty string. The transformer never throws.