Air Pipe Fundamentals

Category: Backend & APIs

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Reference config covering every core Air Pipe concept. Not a production pack — use it as a learning reference and to seed the RAG chatbot with authoritative Air Pipe syntax so it can answer "how do I do X?" questions.

All examples call public APIs (jsonplaceholder.typicode.com) — no database or managed variables required.

---

What's included

File	Purpose
config.yml	Reference config — one endpoint per concept area

---

Endpoints

Method	Path	Concept demonstrated
GET	/fundamentals/input/params	Query param access with a|params
POST	/fundamentals/input/body	Body access, hide_data_on_success
GET	/fundamentals/interpolation/demo	All interpolation forms
POST	/fundamentals/assertions/demo	All assertion types
POST	/fundamentals/run-conditions/demo	All run condition types
GET	/fundamentals/transforms/demo	rename, remove, keep, nested_transforms
GET	/fundamentals/transforms/filter	filter transform with query params
GET	/fundamentals/metrics/demo	counter and gauge metrics

---

Input sources

Air Pipe actions receive input from five sources. Set the source on an action with the input: key.

Source	When to use
a|body	Parsed JSON request body (POST/PUT/PATCH)
a|params	URL query parameters — ?key=value
a|headers	HTTP request headers (all keys are lowercased)
a|request	Full envelope: { method, path, headers, params, body }
a|raw_body	Raw unparsed request body string — required for HMAC verification

actions:
  - name: ReadBody
    input: a|body|        # capture the parsed JSON body

  - name: ReadParams
    input: a|params|      # capture ?key=value query string

  - name: ReadHeaders
    input: a|headers|     # capture request headers

  - name: ReadAll
    input: a|request|     # full request envelope

Access a specific field from a captured input:

# Simple field
a|ReadBody::title|

# Nested field (dot-path)
a|ReadBody::address.city|

---

Interpolation

Interpolation resolves values at runtime and injects them into strings, URLs, and SQL params.

# Managed variable (encrypted, never logged — set in the Air Pipe dashboard)
a|ap_var::OPENAI_API_KEY|

# Environment variable (self-hosted deployments only)
a|env::DATABASE_URL|

# Field from a previous action's result
a|ActionName::fieldName|

# Nested field via dot-path
a|ActionName::address.city|

# Wrap in JSON double-quotes (use inside JSON body strings)
a|double_quote(ActionName::title)|
a|double_quote(ap_var::MODEL_NAME)|

# Current Unix timestamp in seconds
a|timestamp|

# Current date as YYYY-MM-DD
a|timestamp:date|

# Raw unparsed request body (for HMAC verification)
a|raw_body|

IMPORTANT — missing fields do not resolve to null. When a field is absent from the source object, the interpolation token passes through as a literal string (e.g. "a|ReadBody::missingField|"). This causes:

• Silent wrong results for text comparisons (the literal string never matches a real value)
• Hard errors for typed casts (::int, ::timestamptz)

For optional SQL parameters, use the JSONB pattern instead of per-field interpolation:

# Pass the entire body as a single JSONB param
params:
  - a|ReadFilters|

# In SQL, missing keys return genuine NULL
WHERE ($1::jsonb->>'field' IS NULL OR column = $1::jsonb->>'field')

---

Assertions

Assertions validate an action's result. Failing an assertion marks the action as failed — downstream actions using run_when_succeeded will not run.

assert:
  http_code_on_error: 400      # HTTP status code on failure (default: 500)
  error_message: "Bad input"   # Response message on failure
  tests:
    # Presence
    - value: email
      is_not_null: true
      description: "Shown in error context"

    # Empty check
    - value: name
      is_not_empty: true

    # Regex
    - value: email
      regex: "^[^@]+@[^@]+\\.[^@]+$"

    # Equality
    - value: status
      is_equal_to: active
    - value: status
      is_not_equal_to: banned

    # Numeric
    - value: age
      is_greater_than: 0
    - value: score
      is_less_than: 100

    # Row/item count (database queries and arrays)
    - value: count()
      is_equal_to: 1
      error_message: "Expected exactly one result"

    # Null check (field must be absent or null)
    - value: deletedAt
      is_null: true

    # HMAC signature verification
    - value: signature_header
      is_valid_hmac:
        secret: a|ap_var::WEBHOOK_SECRET|
        body: "a|ParseSignature::timestamp|.a|raw_body|"
        algorithm: sha256

---

Run conditions

Run conditions control when an action executes relative to prior actions.

run_when_succeeded

Runs only if all listed actions succeeded.

- name: CreateRecord
  run_when_succeeded: [ValidateBody]

# With a custom error code if the dependency failed
- name: CreateRecord
  run_when_succeeded:
    actions: [ValidateBody]
    http_code_on_error: 400

run_when_failed

Runs as a fallback if the listed actions failed. Useful for alternative data sources or error recovery.

- name: FallbackUser
  run_when_failed: [FetchPrimaryUser]

run_when_finished

Runs after the listed actions complete, regardless of success or failure. Use for logging, cleanup, or guaranteed side-effects.

- name: LogAttempt
  run_when_finished: [FetchUser]

at_least

Runs if at least N of the listed actions succeeded.

- name: Proceed
  run_when_succeeded:
    at_least: 2
    actions: [StepA, StepB, StepC, StepD]

run_on_assertion

A conditional guard — the action only runs if the named tests pass. Can reference data from any prior action by name. Does not replace the run condition; it layers on top of it.

- name: HandleAdminAction
  run_when_succeeded: [FetchUser]
  run_on_assertion:
    tests:
      - action: FetchUser
        value: role
        is_equal_to: admin

previous keyword

Shorthand for the immediately preceding action in the list.

- name: Step1
  ...
- name: Step2
  run_when_succeeded: [previous]   # = run_when_succeeded: [Step1]

---

Transforms

post_transforms apply sequentially to an action's result after it completes. They are applied in order — each transform receives the output of the previous one.

extract_value

Extract a field or array element from the result.

post_transforms:
  - extract_value: body            # extract the 'body' field
  - extract_value: "[0]"           # extract first array element
  - extract_value: "choices[0].message.content"  # nested path (if supported)

extract_with_regex

Extract named capture groups from a string field.

post_transforms:
  - extract_with_regex:
      value: stripe-signature         # the field to parse
      expr: 't=(\d+).*?v1=([a-f0-9]+)'
      keys: [timestamp, v1_signature] # names for capture groups

filter

Filter an array by field conditions.

post_transforms:
  - filter:
      target: $
      conditions:
        $.title:
          contains: "search term"   # substring match
        $.status: "active"          # exact equality match

rename_attributes

Rename fields. Use $[*].old for all elements in an array, $.old for a single object.

post_transforms:
  - rename_attributes:
      $[*].name: fullName          # array
      $[*].email: contactEmail
      $.username: user             # object

remove_attributes

Delete specific fields.

post_transforms:
  - remove_attributes:
      - $[*].password
      - $[*].internalId
      - $.secretKey

keep_attributes

Remove everything except the listed fields.

post_transforms:
  - keep_attributes:
      - $[*].id
      - $[*].name
      - $[*].email

nested_transforms

Apply transforms inside a nested field.

post_transforms:
  - nested_transforms:
      $[*].address:
        - remove_attributes:
            - $.geo
            - $.zipcode

bcrypt

Hash a field in-place using bcrypt (for password storage).

post_transforms:
  - bcrypt:
      value: password
      cost: 12                     # bcrypt cost factor (default: 10)

---

Metrics

emit_metric publishes a Prometheus metric, visible at /metrics.

# Counter — increments by 1 on each emit
- name: TrackRequest
  hide_data_on_success: true
  emit_metric:
    name: app_requests_total
    type: counter
    labels:
      endpoint: users
      source: api

# Gauge — set to an explicit value on each emit
- name: TrackUserCount
  hide_data_on_success: true
  emit_metric:
    name: app_active_users
    type: gauge
    value: a|CountUsers::total|
    labels:
      region: us-east

---

hide_data_on_success and hide_data_on_error

Use these flags to prevent intermediate action results from appearing in the final API response or in logs.

- name: HashPassword
  hide_data_on_success: true   # result not returned in response or shown in logs
  hide_data_on_error: true     # error details (including input) not logged — use for secrets

- name: StoreEvent
  hide_data_on_success: true   # side-effect only — suppress from response

hide_data_on_error: true is important for actions that handle secrets or PII — it prevents sensitive input from appearing in error logs.

---

Database patterns

Basic query with params

- name: GetUser
  database: main
  query: |
    SELECT id, email, username FROM users WHERE id = $1;
  params:
    - a|ValidateBody::id|
  post_transforms:
    - extract_value: "[0]"

Assert row count

- name: GetUser
  database: main
  query: |
    SELECT id, email FROM users WHERE id = $1;
  params:
    - a|ValidateBody::id|
  assert:
    http_code_on_error: 404
    error_message: "User not found"
    tests:
      - value: count()
        is_equal_to: 1
  post_transforms:
    - extract_value: "[0]"

Idempotent insert

- name: StoreEvent
  database: main
  query: |
    INSERT INTO events (external_id, payload)
    VALUES ($1, $2::jsonb)
    ON CONFLICT (external_id) DO NOTHING
    RETURNING id;
  params:
    - a|ParseBody::id|
    - a|ParseBody|

Optional filter params (JSONB pattern)

# Pass the whole body as $1, use ->> to access fields
# Missing JSON keys return genuine SQL NULL — IS NULL checks work correctly
- name: SearchRecords
  database: main
  query: |
    SELECT id, name, status FROM records
    WHERE
      ($1::jsonb->>'status' IS NULL OR status = $1::jsonb->>'status')
      AND ($1::jsonb->>'name'   IS NULL OR name   ILIKE '%' || $1::jsonb->>'name' || '%')
    ORDER BY created_at DESC
    LIMIT 100;
  params:
    - a|ReadFilters|

---

Notes

• The [previous] keyword in run conditions is shorthand for the immediately preceding action in the action list.
• run_on_assertion can reference any prior action by name — it is not limited to the immediately preceding action.
• at_least: N in run_when_succeeded and run_when_failed sets the minimum number of listed actions that must satisfy the condition. Omitting it requires all listed actions to satisfy it.
• Transforms are applied in the order they are listed. Each transform receives the output of the previous one.
• count() in assertions counts rows from database queries or items in an array response.

Configuration

config.yml

name: AirPipeFundamentals
description: >
  Reference config covering every core Air Pipe concept: input sources,
  interpolation, assertions, run conditions, transforms, and metrics.
  Not a production pack — use it as a learning reference and to seed
  the RAG chatbot with authoritative Air Pipe syntax.

docs: true

# No managed variables required — all examples call public APIs or are self-contained.

interfaces:

  # ── Input sources ─────────────────────────────────────────────────────────
  #
  # Air Pipe actions receive input from five sources:
  #
  #   a|body|         — parsed JSON request body (POST/PUT/PATCH)
  #   a|params|       — URL query parameters (?key=value)
  #   a|headers|      — HTTP request headers (keys are lowercased)
  #   a|request|      — full request envelope: { method, path, headers, params, body }
  #   a|raw_body|     — raw request body as a string (needed for HMAC verification)
  #
  # Set the input on an action with: input: a|body|
  # Access a specific field with:    a|ActionName::fieldName|
  # Access a nested field with:      a|ActionName::outer.inner|

  # GET /fundamentals/input/params?userId=1&format=json
  # Demonstrates a|params| for query string access.
  fundamentals/input/params:
    output: http
    summary: Query param access
    description: Access URL query parameters with a|params|. GET /fundamentals/input/params?userId=1
    tags: [fundamentals, input]
    response_example:
      userId: "1"
      format: "json"

    actions:
      - name: ReadParams
        input: a|params|
        assert:
          http_code_on_error: 400
          tests:
            - value: userId
              is_not_null: true
              description: "userId query param is required"

      - name: FetchUser
        run_when_succeeded: [ReadParams]
        http:
          url: https://jsonplaceholder.typicode.com/users/a|ReadParams::userId|
          headers:
            content-type: application/json
        post_transforms:
          - extract_value: body

  # POST /fundamentals/input/body
  # Demonstrates a|body|, field access, and hide_data_on_success.
  fundamentals/input/body:
    output: http
    method: POST
    summary: Body + header access
    description: Access parsed JSON body and request headers from a single action.
    tags: [fundamentals, input]
    request_example:
      title: Hello
      userId: 1

    actions:
      # input: a|body| captures the parsed JSON body.
      # input: a|headers| would capture request headers.
      # input: a|request| captures the full envelope (method, path, headers, params, body).
      - name: ReadBody
        input: a|body|
        hide_data_on_success: true
        assert:
          http_code_on_error: 400
          tests:
            - value: title
              is_not_null: true
            - value: userId
              is_not_null: true

      - name: CreatePost
        run_when_succeeded:
          actions: [ReadBody]
          http_code_on_error: 400
        http:
          url: https://jsonplaceholder.typicode.com/posts
          method: POST
          headers:
            content-type: application/json
          body: |
            {
              "title":  a|double_quote(ReadBody::title)|,
              "body":   "Created via Air Pipe fundamentals",
              "userId": a|ReadBody::userId|
            }
        post_transforms:
          - extract_value: body

  # ── Interpolation ──────────────────────────────────────────────────────────
  #
  # Interpolation syntax: a|<source>|  or  a|<source>::<field>|
  #
  #   a|ap_var::NAME|            — managed variable (encrypted, never logged)
  #   a|env::NAME|               — environment variable (self-hosted only)
  #   a|ActionName::field|       — field from a previous action's result
  #   a|ActionName::outer.inner| — nested field via dot-path
  #   a|double_quote(X)|         — interpolate X and wrap in JSON double-quotes
  #                                Use inside JSON body strings to avoid injection.
  #   a|timestamp:s|             — current Unix timestamp (seconds)
  #   a|timestamp:date|          — current date as YYYY-MM-DD
  #   a|raw_body|                — raw unparsed request body string
  #
  # IMPORTANT: When a field is absent from the source, the interpolation token
  # is passed as a literal string — it does NOT resolve to null.
  # For optional parameters in SQL, use the JSONB pattern:
  #   $1::jsonb->>'field' IS NULL  (missing JSON keys return genuine SQL NULL)

  # GET /fundamentals/interpolation/demo
  # Shows all common interpolation forms in one place.
  fundamentals/interpolation/demo:
    output: http
    summary: Interpolation reference
    description: Demonstrates every interpolation form available in Air Pipe configs.
    tags: [fundamentals, interpolation]

    actions:
      - name: FetchPost
        http:
          url: https://jsonplaceholder.typicode.com/posts/1
        post_transforms:
          - extract_value: body

      - name: FetchUser
        run_when_succeeded: [FetchPost]
        http:
          # a|ActionName::field| — use a field from FetchPost's result
          url: https://jsonplaceholder.typicode.com/users/a|FetchPost::userId|
        post_transforms:
          - extract_value: body

      - name: BuildSummary
        run_when_succeeded: [FetchUser]
        http:
          url: https://jsonplaceholder.typicode.com/posts
          method: POST
          headers:
            content-type: application/json
          body: |
            {
              "postTitle":  a|double_quote(FetchPost::title)|,
              "authorName": a|double_quote(FetchUser::name)|,
              "authorEmail": a|double_quote(FetchUser::email)|,
              "generatedAt": a|timestamp:s|
            }
        post_transforms:
          - extract_value: body

  # ── Assertions ────────────────────────────────────────────────────────────
  #
  # Assertions validate action results. If any test fails the action fails,
  # and downstream actions using run_when_succeeded will not run.
  #
  # Available tests:
  #   is_not_null: true          — field must be present and non-null
  #   is_null: true              — field must be null or absent
  #   is_not_empty: true         — field must not be empty string/array
  #   is_equal_to: <value>       — strict equality
  #   is_not_equal_to: <value>
  #   is_greater_than: <number>  — numeric comparison
  #   is_less_than: <number>
  #   regex: "^pattern$"         — value must match the regex
  #   count()                    — number of rows/items in the result
  #   is_valid_hmac:             — HMAC signature verification
  #     secret: <value>
  #     body: <signed-string>
  #     algorithm: sha256
  #
  # Error control:
  #   http_code_on_error: 400    — HTTP status code returned on assertion failure
  #   error_message: "..."       — message returned on failure
  #   description: "..."         — field-level description shown in error context

  # POST /fundamentals/assertions/demo
  # Shows every assertion type with realistic examples.
  fundamentals/assertions/demo:
    output: http
    method: POST
    summary: Assertions reference
    description: Demonstrates every assertion type — field validation, regex, count, and HMAC.
    tags: [fundamentals, assertions]
    request_example:
      email: [email protected]
      age: 25
      role: admin

    actions:
      - name: ValidateFields
        input: a|body|
        hide_data_on_success: true
        assert:
          http_code_on_error: 400
          tests:
            # Null / presence checks
            - value: email
              is_not_null: true
              is_not_empty: true
              description: "Email address is required"

            # Regex — enforce email format
            - value: email
              regex: "^[^@]+@[^@]+\\.[^@]+$"
              description: "Must be a valid email address"

            # Numeric comparison
            - value: age
              is_not_null: true
              is_greater_than: 0
              description: "Age must be a positive number"

            # Equality
            - value: role
              is_equal_to: admin
              description: "Only admin role is accepted by this endpoint"

      - name: FetchUsers
        run_when_succeeded: [ValidateFields]
        http:
          url: https://jsonplaceholder.typicode.com/users
        post_transforms:
          - extract_value: body

      - name: AssertCount
        run_when_succeeded: [FetchUsers]
        input: a|FetchUsers|
        assert:
          http_code_on_error: 502
          error_message: "Expected exactly 10 users from upstream"
          tests:
            # count() checks the number of rows/items in the current result
            - value: count()
              is_equal_to: 10

  # ── Run conditions ────────────────────────────────────────────────────────
  #
  # Run conditions control when an action executes relative to prior actions.
  #
  #   run_when_succeeded: [ActionA, ActionB]
  #     — runs only if ALL listed actions succeeded.
  #       Use http_code_on_error to set the response code if this chain fails.
  #
  #   run_when_failed: [ActionA]
  #     — runs only if ALL listed actions failed. Useful for fallbacks.
  #
  #   run_when_finished: [ActionA]
  #     — runs after ActionA completes, regardless of success or failure.
  #
  #   run_when_succeeded:
  #     at_least: 2
  #     actions: [A, B, C, D]
  #     — runs if AT LEAST N of the listed actions succeeded.
  #
  #   run_on_assertion:
  #     — conditional guard: action only runs if the listed tests pass.
  #       Can reference data from any prior action by name.
  #
  # Special keyword: [previous]
  #   Shorthand for the immediately preceding action in the list.

  # POST /fundamentals/run-conditions/demo
  fundamentals/run-conditions/demo:
    output: http
    method: POST
    summary: Run conditions reference
    description: >
      Demonstrates all run condition types: run_when_succeeded, run_when_failed,
      run_when_finished, at_least, and run_on_assertion.
    tags: [fundamentals, workflow]
    request_example:
      userId: 1

    actions:
      - name: ValidateBody
        input: a|body|
        hide_data_on_success: true
        assert:
          tests:
            - value: userId
              is_not_null: true

      # run_when_succeeded — only runs if ValidateBody passed.
      - name: FetchUser
        run_when_succeeded:
          actions: [ValidateBody]
          http_code_on_error: 400
        http:
          url: https://jsonplaceholder.typicode.com/users/a|ValidateBody::userId|
        post_transforms:
          - extract_value: body

      # run_when_failed — runs as a fallback if FetchUser failed.
      - name: FallbackUser
        run_when_failed: [FetchUser]
        http:
          url: https://jsonplaceholder.typicode.com/users/1
        post_transforms:
          - extract_value: body

      # run_when_finished — runs after FetchUser regardless of success or failure.
      # Good for cleanup, logging, or guaranteed side-effects.
      - name: FetchUserPosts
        run_when_finished: [FetchUser]
        http:
          url: https://jsonplaceholder.typicode.com/posts?userId=a|ValidateBody::userId|
        post_transforms:
          - extract_value: body

      # run_on_assertion — conditional guard. Only runs if the named action's
      # field passes the test. Does not affect the parent run condition.
      - name: FetchAdminData
        run_when_succeeded: [FetchUser]
        run_on_assertion:
          tests:
            - action: FetchUser
              value: id
              is_equal_to: 1
        http:
          url: https://jsonplaceholder.typicode.com/users
        post_transforms:
          - extract_value: body

  # ── Transforms ────────────────────────────────────────────────────────────
  #
  # post_transforms apply sequentially to the action's result after it completes.
  #
  #   extract_value: "[0]"            — extract first array element
  #   extract_value: fieldName        — extract a field by name
  #   extract_value: "field.nested"   — extract a nested field
  #
  #   extract_with_regex:             — extract capture groups into named keys
  #     value: fieldName
  #     expr: 't=(\d+).*?v1=([a-f0-9]+)'
  #     keys: [timestamp, signature]
  #
  #   filter:                         — filter array by field conditions
  #     target: $
  #     conditions:
  #       $.fieldName:
  #         contains: "search term"   — substring match
  #       $.otherField: "exact match" — equality match
  #
  #   rename_attributes:              — rename keys (array or object)
  #     $[*].oldName: newName         — across all array elements
  #     $.oldName: newName            — on a single object
  #
  #   remove_attributes:              — delete keys from the result
  #     - $[*].sensitiveField
  #     - $.internalId
  #
  #   keep_attributes:                — remove everything EXCEPT listed keys
  #     - $[*].id
  #     - $[*].name
  #
  #   nested_transforms:              — apply transforms inside a nested field
  #     $[*].address:
  #       - remove_attributes:
  #           - $.geo
  #
  #   bcrypt:                         — hash a field in-place
  #     value: password
  #     cost: 12

  # GET /fundamentals/transforms/demo
  fundamentals/transforms/demo:
    output: http
    summary: Transforms reference
    description: >
      Demonstrates extract_value, filter, rename_attributes, remove_attributes,
      keep_attributes, and nested_transforms — applied to a real API response.
    tags: [fundamentals, transforms]
    response_example:
      - fullName: Leanne Graham
        contactEmail: [email protected]
        city: Gwenborough

    actions:
      - name: FetchUsers
        http:
          url: https://jsonplaceholder.typicode.com/users
        post_transforms:
          # 1. Extract the response body from the HTTP envelope
          - extract_value: body

          # 2. Rename: name → fullName, email → contactEmail across all elements
          - rename_attributes:
              $[*].name: fullName
              $[*].email: contactEmail

          # 3. Remove fields we don't want in the response
          - remove_attributes:
              - $[*].phone
              - $[*].website
              - $[*].company

          # 4. Clean up the nested address object — remove geo + suite
          - nested_transforms:
              $[*].address:
                - remove_attributes:
                    - $.geo
                    - $.suite
                    - $.zipcode

          # 5. Keep only the fields we want in the final output
          - keep_attributes:
              - $[*].fullName
              - $[*].contactEmail
              - $[*].address.city

  # GET /fundamentals/transforms/filter
  # Demonstrates the filter transform with query params as the search input.
  fundamentals/transforms/filter:
    output: http
    summary: Filter transform
    description: Filter an array by field value. GET /fundamentals/transforms/filter?title=sunt
    tags: [fundamentals, transforms]

    actions:
      - name: ReadParams
        input: a|params|
        assert:
          http_code_on_error: 400
          tests:
            - value: title
              is_not_null: true

      - name: FetchAndFilter
        run_when_succeeded: [ReadParams]
        http:
          url: https://jsonplaceholder.typicode.com/posts
        post_transforms:
          - extract_value: body
          - filter:
              target: $
              conditions:
                $.title:
                  contains: a|ReadParams::title|

  # ── Metrics ───────────────────────────────────────────────────────────────
  #
  # emit_metric publishes a Prometheus metric. Metrics are available at /metrics.
  #
  #   name: metric_name           — metric name (snake_case)
  #   type: counter               — counter (monotonically increasing)
  #   type: gauge                 — gauge (can go up or down)
  #   value: a|Action::field|     — for gauges, the current value
  #   labels:                     — optional key/value dimensions
  #     key: value
  #
  # Counters are auto-incremented on each emit.
  # Gauges require an explicit value.

  # GET /fundamentals/metrics/demo
  fundamentals/metrics/demo:
    output: http
    summary: Metrics reference
    description: Demonstrates emit_metric for both counter and gauge types.
    tags: [fundamentals, metrics]

    actions:
      - name: FetchUsers
        http:
          url: https://jsonplaceholder.typicode.com/users
        post_transforms:
          - extract_value: body

      # Counter — increments by 1 on each request.
      # Visible at /metrics as: app_users_listed_total{interface="fundamentals/metrics/demo"}
      - name: TrackListRequest
        run_when_succeeded: [FetchUsers]
        hide_data_on_success: true
        emit_metric:
          name: app_users_listed_total
          type: counter
          labels:
            source: api

      # Gauge — set to the current value on each request.
      # Useful for tracking sizes, counts, or any value that can decrease.
      - name: TrackUserCount
        run_when_succeeded: [FetchUsers]
        hide_data_on_success: true
        emit_metric:
          name: app_users_gauge
          type: gauge
          value: a|FetchUsers::count()|