Scripting

The script processor lets you transform every record with a Tengo script. Tengo is a small scripting language with Go-like syntax. This guide covers what a script can see and do, with working examples.

What the script sees

Your script gets one global variable: entity. It is a map with these fields:

Every field can be read and written. Two rules to remember:

• Fields the extractor did not set are undefined, not empty. Guard with entity.properties || {}.
• You can only write to the fields above. Setting a new top-level key like entity.foo fails the run.

Edges are not exposed to scripts. They pass through unchanged. A script also cannot drop a record — every record continues to the sinks.

Examples

Rename entities

processors:
  - name: script
    config:
      engine: tengo
      script: |
        entity.name = entity.name + " [" + entity.source + "]"

Add properties safely

Merge new keys without losing what the extractor set:

processors:
  - name: script
    config:
      engine: tengo
      script: |
        merge := func(m1, m2) {
          for k, v in m2 {
            m1[k] = v
          }
          return m1
        }
        entity.properties = merge(entity.properties || {}, {
          domain: "payments",
          reviewed: "true"
        })

Rewrite descriptions with text functions

Import Tengo's standard modules with import:

processors:
  - name: script
    config:
      engine: tengo
      script: |
        text := import("text")
        if !entity.description {
          entity.description = "No description provided for " + text.to_lower(entity.name)
        }

Available modules: math, text, times, rand, fmt, json, base64, hex, and enum. The os module is not available.

Enrich from an external service

The built-in http module can fetch data during processing. It supports GET requests only, with a 5-second timeout:

processors:
  - name: script
    config:
      engine: tengo
      script: |
        json := import("json")
        res := http.get("https://ownership-service.example.com/owners?urn=" + entity.urn)
        if res.code == 200 {
          owner := json.decode(res.body)
          entity.properties = entity.properties || {}
          entity.properties.owner_team = owner.team
        }

http.get takes an optional second argument with headers: http.get(url, {Authorization: "Bearer token"}). It returns a map with code (int) and body (string).

Error behavior

• The script compiles when the recipe starts. A syntax error fails the run before any extraction happens, so meteor lint catches it.
• A runtime error (for example dividing by zero, or writing an unknown field) fails the whole run. There is no per-record skip and no retry.
• Scripts have resource limits on allocations, so unbounded loops fail rather than hang.

Test scripts safely with a dry run:

meteor run recipe.yaml --dry-run --limit 10

This runs the extractor and your script on the first 10 records without sending anything to sinks.

Scripts in the HTTP sink

The HTTP sink also accepts a Tengo script, but with a different job: it builds the request payload. There the script sees the entity and calls sink(payload) to send custom JSON. See the sink's page for details.