Understanding walkerOS Flow

Overview

walkerOS follows a Source → Collector → Destination(s) architecture for composable, modular event processing.

Core principle: Separation of concerns. Each component has one job. Components are composable and replaceable.

The Flow Pattern

[Source.before] → Sources → [Source.next] → Collector → [Dest.before] → Destinations → [Dest.next]
(Preprocessing)  (Capture)  (Pre-chain)   (Processing) (Post-chain)   (Delivery)     (Post-push)

Consent-exempt:                                                                        Post-consent:
- Decode                    - Validation   - Event creation - Validation   - Push       - Audit logging
- Validate format           - Enrichment   - Enrichment    - Enrichment   - Send       - Response parsing
- Authenticate              - Redaction    - Consent check - Routing      - Store      - Webhooks

Key Concepts

Composability

A Flow combines components. You can:

• Use multiple sources feeding one collector
• Route events to multiple destinations
• Swap components without changing others

The Flow Type

See packages/core/src/types/flow.ts for the canonical interface.

// Conceptual structure (see source for full type)
interface Flow {
  stores?: Record<string, Store>;
  sources?: Record<string, Source>;
  transformers?: Record<string, Transformer>;
  destinations?: Record<string, Destination>;
  collector?: Collector.InitConfig;
}

Universal Push Interface

All components communicate via push functions:

The elb() function is an alias for collector.push - used for component wiring.

startFlow Helper

See packages/collector/src/flow.ts for the startFlow function.

import { startFlow } from '@walkeros/collector';

const { collector, elb } = await startFlow({
  stores: {
    /* key-value storage, init first, destroy last */
  },
  sources: {
    /* ... */
  },
  transformers: {
    /* ... */
  },
  destinations: {
    /* ... */
  },
});

Ingest: Mutable Pipeline Context

walkerOS uses a two-layer data model:

• Event - strict schema, structured analytics data (name, data, context, etc.)
• Ingest - free-form mutable context that flows alongside events

Any step can read and write arbitrary keys on ingest. The runtime manages _meta:

• _meta.hops - increments per step (safety valve at 256)
• _meta.path - ordered list of step IDs visited (path[0] = source ID)

// In a transformer
push: async (event, context) => {
  context.ingest.botScore = 0.95; // Write freely
  context.ingest.geo = { country: 'DE' };
  console.log(context.ingest._meta.path); // ['express', 'validate', 'enrich']
  return { event };
};

Ingest is cloned per destination to prevent cross-contamination in parallel processing. After a destination push, the response is available at ingest._response.

Stores

Three pipeline components (Source / Transformer / Destination) plus Stores as passive infrastructure. Stores are not a fourth pipeline stage - they're key-value storage that other components consume via env. They have no push, no next, no before; they sit alongside the pipeline rather than inside it.

• Referenced via $store.storeId in env values (bundled mode) or passed directly as store instances (integrated mode)
• Init first, destroy last - stores are available before any source, transformer, or destination starts, and outlive them on shutdown
• No chains - stores don't participate in the event pipeline. Components access them through their env.
• Implementations: @walkeros/store-memory (sync, LRU), @walkeros/server-store-fs (async, filesystem), @walkeros/server-store-s3 (async, S3-compatible)

{
  "stores": {
    "data": { "package": "@walkeros/store-memory" }
  },
  "transformers": {
    "fingerprint": {
      "package": "@walkeros/server-transformer-fingerprint",
      "env": { "store": "$store.data" }
    }
  }
}

See walkeros-understanding-stores for the full store interface and lifecycle.

Separation of Concerns

Transformer Chains

Transformers run at two points in the pipeline, configured via next and before:

Pre-Collector Chain

Runs after source captures event, before collector processing:

Bundled mode (flow.json):

{
  "sources": {
    "browser": {
      "package": "@walkeros/web-source-browser",
      "next": "validate"
    }
  },
  "transformers": {
    "validate": {
      "package": "@walkeros/transformer-validator",
      "next": "enrich"
    },
    "enrich": {
      "package": "@walkeros/transformer-enricher"
    }
  }
}

Integrated mode (TypeScript):

sources: {
  browser: {
    code: sourceBrowser,
    next: 'validate'
  }
},
transformers: {
  validate: {
    code: transformerValidator,
    config: { next: 'enrich' }
  },
  enrich: {
    code: transformerEnrich
  }
}

Note: In flow.json, next is at the reference level. The CLI bundler automatically moves it into config.next for runtime - you don't need to handle this yourself.

Transformer before chain

Each transformer can have its own before chain that runs before its push function:

{
  "transformers": {
    "enrich": {
      "before": "lookup",
      "package": "@walkeros/transformer-enricher"
    },
    "lookup": {
      "package": "@walkeros/transformer-lookup"
    }
  }
}

Post-Collector Chain

Runs after collector enrichment, before destination receives event:

Bundled mode (flow.json):

{
  "destinations": {
    "gtag": {
      "package": "@walkeros/web-destination-gtag",
      "before": "redact"
    }
  },
  "transformers": {
    "redact": {
      "package": "@walkeros/transformer-redact"
    }
  }
}

Integrated mode (TypeScript):

destinations: {
  gtag: {
    code: destinationGtag,
    before: 'redact'
  }
},
transformers: {
  redact: {
    code: transformerRedact
  }
}

Post-push chain (destination.next)

Runs after destination push completes. The push response is available at context.ingest._response:

Bundled mode (flow.json):

{
  "destinations": {
    "api": {
      "package": "@walkeros/server-destination-api",
      "next": "auditLog"
    }
  },
  "transformers": {
    "auditLog": {
      "package": "@walkeros/transformer-audit"
    }
  }
}

Integrated mode (TypeScript):

destinations: {
  api: {
    code: destinationApi,
    next: 'auditLog'
  }
},
transformers: {
  auditLog: {
    code: transformerAudit
  }
}

Chain Resolution

• source.before → consent-exempt preprocessing chain
• source.next → starts pre-collector chain
• transformer.before → pre-transform enrichment chain
• transformer.next (flow.json) or transformer.config.next (runtime) → links transformers
• destination.before → starts post-collector chain per destination
• destination.next → post-push processing chain

Cross-Flow References ($flow)

When a single flow.json defines multiple flows, any flow can pull values from another flow's config block via $flow.<name>(.<path>)?. The most common case is wiring a web flow's API destination to a server flow's deployed URL, so the two stay in sync without duplication.

{
  "version": 4,
  "flows": {
    "server": {
      "config": {
        "platform": "server",
        "url": "https://collect.example.com"
      },
      "sources": {
        "http": { "package": "@walkeros/server-source-express" }
      }
    },
    "web": {
      "config": { "platform": "web" },
      "destinations": {
        "api": {
          "package": "@walkeros/web-destination-api",
          "config": { "settings": { "url": "$flow.server.url" } }
        }
      }
    }
  }
}

validate warns on unresolved $flow references (lenient), bundle errors out (strict), so production builds never ship with an empty cross-flow value.

Step Examples

Each step in a flow (source, transformer, destination) can ship step examples -- structured { in, out } pairs that define expected input/output behavior.

The Three Type Zones

Steps sit at boundaries between arbitrary formats and walkerOS events:

• Source: arbitrary in (HTTP request, DOM event) → walkerOS event out
• Transformer: walkerOS event in → walkerOS event out (or false)
• Destination: walkerOS event in → arbitrary out (vendor API call)

See using-step-examples for the full ASCII diagram and detailed explanation.

Example: Step with Examples in Flow Config

{
  "destinations": {
    "gtag": {
      "package": "@walkeros/web-destination-gtag",
      "config": { "measurementId": "G-XXXXXX" },
      "examples": {
        "purchase": {
          "in": {
            "name": "order complete",
            "data": { "id": "ORD-123", "total": 149.97 }
          },
          "out": [
            "event",
            "purchase",
            { "transaction_id": "ORD-123", "value": 149.97 }
          ]
        }
      }
    }
  }
}

Step examples enable it.each testing, CLI simulation with --example, and deep validation with --deep. See using-step-examples for the complete lifecycle.

Flow Graph Connection Rules

This section defines which components can connect to which, and how chains are resolved at runtime. Use it as the canonical reference for building flow graphs, validating configurations, and rendering UI visualizations.

Valid connection matrix

Pre-transformer chains (source.next)

• Entry: source.next: "transformerId" or source.next: ["t1", "t2"]
• Chaining: transformer.next: "nextId" walks forward; array stops walking
• Exit: chain ends, event reaches collector
• Multiple sources can reference the same transformer (fan-in)
• No next = source connects directly to collector

Post-transformer chains (destination.before)

• Entry: destination.before: "transformerId" or destination.before: ["t1", "t2"]
• Same transformer.next chain-walking logic as pre-chains
• Exit: chain ends, event reaches destination
• Multiple destinations can share the same transformer
• No before = collector connects directly to destination

Chain resolution algorithm (walkChain)

See packages/collector/src/transformer.ts for the implementation.

• String start: walks transformer.next links until chain ends
• Array start: uses array as-is (explicit chain, no walking)
• Array next inside chain: appends array elements and stops walking
• Circular references: detected via visited set, silently breaks loop
• Non-existent transformer ID: chain ends (no error, event proceeds without transformation)

Conditional routing (Route[])

The next and before properties support conditional routing via Route[] - an array of { match, next } objects evaluated against ingest data:

"next": [
  { "match": { "key": "ingest.path", "operator": "prefix", "value": "/api" }, "next": "api-handler" },
  { "match": "*", "next": "default" }
]

• Routes are evaluated in order - first match wins
• No match and no wildcard = event passes through unchanged
• Works on all chain positions: source.before, source.next, transformer.before, transformer.next, destination.before, and destination.next
• Routes are compiled to closures at init time for fast per-event evaluation
• See packages/core/src/route.ts for compileNext() and resolveNext()

Transformer sharing

A single transformer can appear in both pre-chains (source.next) and post-chains (destination.before). The same transformer pool is shared; role depends on which chain references it.

Deferred activation (require)

• source.config.require: ["consent"] - source deferred until "consent" event fires
• destination.config.require: ["user"] - destination deferred until "user" event fires
• Multiple requirements: all must be fulfilled (each fires independently)

Mapping and consent gating

• Source-level: source.config.mapping and source.config.consent - applied before pre-chain; blocks event entirely
• Destination-level: destination.config.mapping and destination.config.consent - applied after post-chain; skips only that destination, queues denied events

Canvas rendering rules (for UI graph visualization)

• Shared transformers (pre+post): duplicate visually with a link indicator; editing one updates the other
• Router fan-out: keep graph planar; trace edges individually
• Orphan transformers (not in any chain): render grey/muted; gain color when connected
• Diamond patterns (fan-in + fan-out): expected and valid
• Overlapping destination.before chains: intentional (e.g., shared validator for monitoring)

Related Skills

• walkeros-understanding-events - Event model
• walkeros-understanding-sources - Source interface
• walkeros-understanding-destinations - Destination interface

Package READMEs:

• packages/collector/README.md - Collector details

Source Files:

• packages/collector/src/flow.ts - startFlow implementation

Documentation:

• Website: Flow - Flow concept
• Website: Collector - Collector docs