pnp 0.9.0

Pull 'n' Push

Pull 'n' Push

Pulls data from sources and pushes it to sinks.

1. Installation
2. Getting started
3. Building Blocks
3.1. Pull
3.2. Push
3.3. Selector
3.4. Dependencies
3.5. Envelope (>= 0.7.1)
4. Plugins
4.1. Pull
4.1.1. pnp.plugins.pull.simple.Count
4.1.2. pnp.plugins.pull.sensor.DHT
4.1.3. pnp.plugins.pull.fs.FileSystemWatcher
4.1.4. pnp.plugins.pull.mqtt.MQTTPull
4.1.5. pnp.plugins.pull.simple.Repeat
4.1.6. pnp.plugins.pull.http.Server
4.1.7. pnp.plugins.pull.ZwayPoll
4.2. Push
4.2.1. pnp.plugins.push.simple.Echo
4.2.2. pnp.plugins.push.ml.FaceR
4.2.3. pnp.plugins.push.fs.FileDump
4.2.4. pnp.plugins.push.http.Call
4.2.5. pnp.plugins.push.timedb.InfluxPush
4.2.6. pnp.plugins.push.mqtt.MQTTPush

1. Installation

pip install pnp

Optional extras

• dht: Enables pnp.plugins.pull.sensor.DHT (temperature and humidity sensor). Only works on ARM-based systems (like raspberry, arduino)
• fswatcher: Enables pnp.plugins.pull.fs.FileSystemWatcher (Watch file system for created, modified, deleted, moved files)
• faceR: Enables pnp.plugins.push.ml.FaceR (Screen image files for known faces)

Installation with extras:

pip install pnp[fswatcher,faceR]
# In case of extra 'dht' you have to enable the option --process-dependency-links ...
# ... cause the required adafruit package is not on pypi.
pip install --process-dependency-links pnp[dht]

2. Getting started

Define pulls to suck/pull data from source systems. Define one push or multiple pushes per pull to transfer the pulled data anywhere else (you only need a plugin that knows how to handle the target). You can define your configurations in yaml or json. It is up to you. I prefer yaml...

- name: hello-world
  pull:
    plugin: pnp.plugins.pull.simple.Repeat
    args:
      wait: 1
      repeat: "Hello World"
  push:
    - plugin: pnp.plugins.push.simple.Echo

Copy this configuration and create the file helloworld.yaml. Run it:

pnp helloworld.yaml

This example yields the string 'Hello World' every second.

3. Building Blocks

Below the basic building blocks of pull 'n' push are explained in more detail

3.1. Pull

As stated before pulls fetch data from various source systems and/or apis. Please see the section plugins for already implemented pulls. To instantiate a pull by configuration file you only have to provide it's fully qualified name and the argument that should be passed.

- name: example
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: localhost
      port: 1883
      topic: test/#

The above snippet will create a pull that listens on the topic test/# on a mqtt broker. The output of the pull is a dictionary that contains the topic, levels and the actual payload.

# When the message 'Here i am' arrives on the topic 'test/foo/bar' then the output will look like that:
{'topic': 'test/foo/bar', 'levels': ['test', 'foo', 'bar'], 'payload': 'Here i am'}

3.2. Push

A pull passes its data to multiple pushes to transfer/transform the data. For example a push might save sensor data to influx or dump a file to the file system.

- name: example
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: localhost
      port: 1883
      topic: test/#
  push:
    - plugin: pnp.plugins.push.fs.FileDump
      args:
        directory: "/tmp"
        binary_mode: False
    - plugin: pnp.plugins.push.simple.Echo

The above snippet adds two pushes to the already known pull. The first push takes the incoming data and dumps it into the specified directory as a textfile. The second push just prints out the incoming data.

3.3. Selector

Sometimes the output of a pull needs to be transformed before the specified push can handle it. Selectors to the rescue. Given our input we decide to just dump the payload and print out the first level of the topic.

- name: example
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: localhost
      port: 1883
      topic: test/#
  push:
    - plugin: pnp.plugins.push.fs.FileDump
      selector: data.payload
      args:
        directory: "/tmp"
        binary_mode: False
    - plugin: pnp.plugins.push.simple.Echo
      selector: data.levels[0]

Easy as that. We can reference our incoming data via data or payload.

3.4. Dependencies

By default any pushes will execute in parallel (not completly true) when new incoming data is available. But now it would be nice if we could chain pushes together. So that the output if one push becomes the input of the next push. The good thing is: Yes we can.

Back to our example let's assume we want to print out the path to the created file dump after the dump is created.

- name: example
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: localhost
      port: 1883
      topic: test/#
  push:
    - plugin: pnp.plugins.push.fs.FileDump
      selector: data.payload
      args:
        directory: "/tmp"
        binary_mode: False
      deps:
        - plugin: pnp.plugins.push.simple.Echo
    - plugin: pnp.plugins.push.simple.Echo
      selector: data.levels[0]

As you can see we just add a dependant push to the previous one.

3.5. Envelope (>= 0.7.1)

Using envelopes it is possible to change the behaviour of pushes during runtime. Best examples are the pnp.plugins.push.fs.FileDump and pnp.plugins.push.mqtt.MQTTPush plugins, where you can override / set the actual file_name and extension of the file to dump resp. the topic where the message should be published.

Given the example ...

- name: envelope
  pull:
    plugin: pnp.plugins.pull.simple.Count
    args:
      wait: 1
  push:
    plugin: pnp.plugins.push.fs.FileDump
    selector: '{"file_name": str(data), "extension": ".cnt", "data": data}'
    args:
      directory: "/tmp/counter"
      file_name: "counter"  # Overridden by envelope
      extension: ".txt"  #  Overridden by envelope
      binary_mode: False  # text mode

... this push dumps multiple files (0.cnt, 1.cnt, 2.cnt, ...) for each pulled counter value, instead of dumping one file 'couter.txt' which is overridden each time a new counter is emitted.

How does this work: If the emitted or transformed payload (via selector) contains the key data or payload it is assumed that the actual payload is the data stored in this key and all other keys represent the so called envelope.

Remark: This feature might actually break your existing configurations if you use the plugin pnp.plugins.pull.mqtt.MQTTPull which will now emit an enveloped payload.

This snippet echoed a dictionary with the keys 'topic', 'levels' and 'payload' previously to version 0.7.2. It will now differentiate between the actual 'payload' (key 'payload' resp. 'data') and the envelope (other keys).

- name: subscriber
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: localhost
      topic: test/counter
  push:
    plugin: pnp.plugins.push.simple.Echo

If you want to "restore" the previous behaviour, you only have to wrap the whole payload into a dictionary inside the 'payload' or 'data' key via selector.

- name: subscriber
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: localhost
      topic: test/counter
  push:
    plugin: pnp.plugins.push.simple.Echo
    selector: "{'data': data}"

4. Plugins

4.1. Pull

4.1.1. pnp.plugins.pull.simple.Count

Emits every wait seconds a counting value which runs from from_cnt to to_cnt. If to_cnt is None the counter will count to infinity.

Arguments

wait (int): Wait the amount of seconds before emitting the next counter.
from_cnt (int): Starting value of the counter.
to_cnt (int, optional): End value of the counter. If not passed set to "infinity" (precise: int.max).

Result

Counter value (int).

Examples

- name: count
  pull:
    plugin: pnp.plugins.pull.simple.Count
    args:
      wait: 1
      from_cnt: 1
      to_cnt: 10
  push:
    plugin: pnp.plugins.push.simple.Echo

4.1.2. pnp.plugins.pull.sensor.DHT

Periodically polls a dht11 or dht22 (aka am2302) for temperature and humidity readings. Polling interval is controlled by interval.

Arguments

device (str, optional): The device to poll (one of dht22, dht11, am2302). Default is 'dht22'.
data_gpio (int, optional): The data gpio port where the device operates on. Default is 17.

Result

{
    "humidity": 65.4  # in %
    "temperature": 23.7  # in celsius
}

Examples

- name: dht
  pull:
    plugin: pnp.plugins.pull.sensor.DHT
    args:
      device: dht22  # Connect to a dht22
      data_gpio: 17  # DHT is connected to gpio port 17
      interval: 5m  # Polls the readings every 5 minutes
  push:
    - plugin: pnp.plugins.push.simple.Echo
      selector: payload.temperature  # Temperature reading
    - plugin: pnp.plugins.push.simple.Echo
      selector: payload.humidity  # Humidity reading

4.1.3. pnp.plugins.pull.fs.FileSystemWatcher

Watches the given directory for changes like created, moved, modified and deleted files. If ignore_directories is set to False, then directories will be reported as well.

Per default will recursively report any file that is touched, changed or deleted in the given path. The directory itself or subdirectories will be object to reporting too, if ignore_directories is set to False.

Arguments

path (str): The path to track for file / directory changes.
recursive (bool, optional): If set to True, any subfolders of the given path will be tracked too. Default is True.
patterns (str or list, optional): Any file pattern (e.g. .txt or [.txt, *.md]. If set to None no filter is applied. Default is None.
ignore_patterns (str or list, optional): Any patterns to ignore (specify like argument patterns). If set to None, nothing will be ignored. Default is None.
ignore_directories (str, optional): If set to True will send events for directories when file change. Default is False.
case_sensitive (bool, optional): If set to True, any pattern is case_sensitive, otherwise it is case insensitive. Default is False.
events (str or list, optional): The events to track. One or multiple of 'moved', 'deleted', 'created' and/or 'modified'. If set to None all events will be reported. Default is None.
load_file (bool, optional): If set to True the file contents will be loaded into the result. Default is False.
mode (str, optional): Open mode of the file (only necessary when load_file is True). Can be text, binary or auto (guessing). Default is auto.
base64 (bool, optional): If set to True the loaded file contents will be converted to base64 (only applicable when load_file is True). Argument mode will be automatically set to 'binary'. Default is False.
defer_modified (float, optional): If set greater than 0, it will defer the sending of modified events for that amount of time (seconds). There might be multiple flushes of a file before it is written completely to disk. Without defer_modified each flush will raise a modified event. Default is 0.5.

Result

Example of an emitted message

{
    'operation': 'modified',
    'source': '/tmp/abc.txt',
    'is_directory': False,
    'destination': None,  # Only non-None when operation = 'moved'
    'file': {  # Only present when load_file is True
        'file_name': 'abc.txt',
        'content': 'foo and bar',
        'read_mode': 'text',
        'base64': False
    }
}

Examples

- name: file_watcher
  pull:
    plugin: pnp.plugins.pull.fs.FileSystemWatcher
    args:
      path: "/tmp"
      ignore_directories: True
      events: [created, deleted, modified]
      load_file: False
  push:
    plugin: pnp.plugins.push.simple.Echo

4.1.4. pnp.plugins.pull.mqtt.MQTTPull

Pulls messages from the specified topic from the given mosquitto mqtt broker (identified by host and port).

Arguments

host (str): Host where the mosquitto broker is running.
port (int): Port where the mosquitto broker is listening.
topic (str): Topic to pull messages from. You can listen to multiple topics by using the #-wildcard (e.g. test/# will listen to all topics underneath test).

All arguments can be automatically injected via environment variables with MQTT prefix (e.g. MQTT_HOST).

Result

The emitted message will look like this:

{
    'topic': 'test/device/device1',
    'levels': ['test', 'device', 'device1']
    'payload': 'The actual event message'
}

Examples

- name: mqtt
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: localhost
      port: 1883
      topic: test/#
  push:
    plugin: pnp.plugins.push.simple.Echo

4.1.5. pnp.plugins.pull.simple.Repeat

Emits every wait seconds the same repeat.

Arguments

wait (int): Wait the amount of seconds before emitting the next repeat.
repeat (any): The object to emit.

Result

Emits the repeat-object as it is.

Examples

- name: repeat
  pull:
    plugin: pnp.plugins.pull.simple.Repeat
    args:
      repeat: "Hello World"  # Repeats 'Hello World'
      wait: 1  # Every second
  push:
    plugin: pnp.plugins.push.simple.Echo

4.1.6. pnp.plugins.pull.http.Server

Listens on the specified port for requests to any endpoint. Any data passed to the endpoint will be tried to be parsed to a dictionary (json). If this is not possible the data will be passed as is. See sections Result for specific payload and examples.

Remark: You will not able to make requests to the endpoint DELETE /_shutdown because it is used internally.

Arguments

port (int, optional): The port the rest server should listen to for requests. Default is 5000.
allowed_methods (str or list, optional): List of http methods that are allowed. Default is 'GET'.
server_impl (str, optional): Choose the implementation of the WSGI-Server (wraps the flask-app). Possible values are: [flask, gevent]. flask uses the internal Flask Development server. Not recommended for production use. gevent uses gevent. Default is gevent.

Result

curl -X GET 'http://localhost:5000/resource/endpoint?foo=bar&bar=baz' --data '{"baz": "bar"}'

{
    'endpoint': 'resource/endpoint,
    'method': 'GET',
    'query': {'foo': 'bar', 'bar': 'baz'},
    'data': {'baz': 'bar'},
    'is_json': True
}

curl -X GET 'http://localhost:5000/resource/endpoint' --data 'no json obviously'

{
    'endpoint': 'resource/endpoint,
    'method': 'GET',
    'query': {},
    'data': b'no json obviously',
    'is_json': False
}

Examples

- name: rest
  pull:
    plugin: pnp.plugins.pull.http.Server
    args:
      port: 5000
      allowed_methods: [GET, POST]
  push:
    plugin: pnp.plugins.push.simple.Echo

4.1.7. pnp.plugins.pull.ZwayPoll

Pulls the specified json content from the zway rest api. The content is specified by the url, e.g. http://<host>:8083/ZWaveAPI/Run/devices will pull all devices and serve the result as a json.

Specify the polling interval by setting the argument interval. User / password combination is required when your api is protected against guest access (by default it is).

Use multiple pushes and the related selectors to extract the required content like temperature readings (see the examples section for guidance).

Arguments

url (str): The url to poll periodically.
user (str): Authentication user name.
password (str): Authentication password.
interval (polling literal, optional): Polling interval (default: 1m).

All arguments (url, user and password) can be automatically injected via environment variables.

• ZWAY_URL
• ZWAY_USER
• ZWAY_PASSWORD

Result

Emits the content of the fetched url as it is.

Examples

#### Please make sure to adjust url and device ids
#### Username and Password are injected from environment variables:
####     export ZWAY_USER=admin
####     export ZWAY_PASSWORD=secret_one
- name: zway
  pull:
    plugin: pnp.plugins.pull.zway.ZwayPoll
    args:
      url: "http://smarthome:8083/ZWaveAPI/Run/devices"
      interval: 5s
  push:
    - plugin: pnp.plugins.push.simple.Echo
      # Temperature of fibaro motion sensor
      # You can access the returned json like you would inquire the zway-api
      selector: payload[19].instances[0].commandClasses[49].data[1].val.value
    - plugin: pnp.plugins.push.simple.Echo
      # Luminiscence of fibaro motion sensor
      selector: payload[19].instances[0].commandClasses[49].data[3].val.value

Appendix

Below are some common selector examples to fetch various metrics from various devices

Fibaro Motion Sensor

• Temperature payload[deviceid].instances[0].commandClasses[49].data[1].val.value
• Luminescence payload[deviceid].instances[0].commandClasses[49].data[3].val.value

fibaro Wallplug

• Meter payload[deviceid].instances[0].commandClasses[50].data[0].val.value

Thermostat (Danfoss / other should work as well)

• Setpoint payload[deviceid].instances[0].commandClasses[67].data[1].val.value

Battery operated devices

• Battery level payload[deviceid].instances[0].commandClasses[128].data.last.value

4.2. Push

4.2.1. pnp.plugins.push.simple.Echo

Simply log the passed payload to the default logging instance.

Arguments

None.

Result

Will return the payload as it is for easy chaining of dependencies.

Examples

- name: count
  pull:
    plugin: pnp.plugins.pull.simple.Count
    args:
      wait: 1
      from_cnt: 1
      to_cnt: 10
  push:
    plugin: pnp.plugins.push.simple.Echo

4.2.2. pnp.plugins.push.ml.FaceR

FaceR (short one for face recognition) tags known faces in images. Output is the image with all faces tagged whether with the known name or an unknown_label. Default for unknown ones is 'Unknown'.

Known faces can be ingested either by a directory of known faces (known_faces_dir) or by mapping of known_faces (dictionary: name -> [list of face files]).

The payload passed to the push method is expected to be a valid byte array that represents an image in memory.

Arguments

known_faces (dict<str, file_path as str>, optional): Mapping of a person's name to a list of images that contain the person's face. Default is None.
known_faces_dir (str, optional): A directory containing images with known persons (file_name -> person's name). Default is None.
unknown_label (str, optional): Tag label of unknown faces. Default is 'Unknown'.

You have to specify either known_faces or known_faces_dir. If both are unsupplied the push will fail.

Result

Will return a dictionary that contains the bytes of the tagged image (key tagged_image) and metadata (no_of_faces, known_faces)

{
    'tagged_image': <bytes of tagged image>
    'no_of_faces': 2
    'known_faces': ['obama']
}

Examples

- name: faceR
  pull:
    plugin: pnp.plugins.pull.fs.FileSystemWatcher
    args:
      path: "/tmp"
      recursive: True
      patterns: "*.jpg"
      ignore_directories: True
      case_sensitive: False
      events: [created]
      load_file: True
      mode: binary
      base64: False
  push:
    plugin: pnp.plugins.push.ml.FaceR
    args:
      known_faces_dir: /path/to/known/faces
      unknown_label: "don't know him"

4.2.3. pnp.plugins.push.fs.FileDump

This push dumps the given payload to a file to the specified directory. If argument file_name is None, a name will be generated based on the current datetime (%Y%m%d-%H%M%S). If file_name is not passed (or None) you should pass extension to specify the extension of the generated file name. Argument binary_mode controls whether the dump is binary (mode=wb) or text (mode=w).

Arguments

directory (str, optional): The target directory to store the dumps. Default is '.' (current directory).
file_name (str, optional): The name of the file to dump. If set to None a file name will be automatically generated. You can specify the file_name via the envelope, too. Envelope will override init file name. Default is None.
extension (str, optional): The extension to use when the file name is automatically generated. Can be overridden by envelope. Default is '.dump'.
binary_mode (bool, optional): If set to True the file will be written in binary mode ('wb'); otherwise in text mode ('w'). Default is False.

Result

Will return an absolute path to the file created.

Examples

- name: file_dump
  pull:
    plugin: pnp.plugins.pull.simple.Repeat
    args:
      repeat: "Hello World"
  push:
    plugin: pnp.plugins.push.fs.FileDump
    args:
      directory: "/tmp"
      file_name: null  # Auto-generated file (timestamp)
      extension: ".txt"  # Extension of auto-generated file
      binary_mode: False  # text mode
    deps:
      - plugin: pnp.plugins.push.simple.Echo

- name: file_dump
  pull:
    plugin: pnp.plugins.pull.simple.Repeat
    args:
      repeat: "Hello World"
  push:
    plugin: pnp.plugins.push.fs.FileDump
    # Override `file_name` and `extension` via envelope.
    # Instead of an auto generated file, the file '/tmp/hello-world.hello' will be dumped.
    selector: '{"payload": payload, "file_name": "hello-world", "extension": ".hello"}'
    args:
      directory: "/tmp"
      file_name: null  # Auto-generated file (timestamp)
      extension: ".txt"  # Extension of auto-generated file
      binary_mode: False  # text mode
    deps:
      - plugin: pnp.plugins.push.simple.Echo

4.2.4. pnp.plugins.push.http.Call

Makes a request to a http resource.

Arguments

url (str): Request url. Can be overridden via envelope.
method (str, optional): The http method to use for the request. Must be a valid http method (GET, POST, ...). Default is 'GET'. Can be overridden via envelope.
fail_on_error (bool, optional): If True the push will fail on a http status code <> 2xx. This leads to an error message recorded into the logs and no further execution of any dependencies. Default is False. Can be overridden by the envelope.
provide_response (bool, optional): If True the push will not return the payload as it is, but instead provide the response status_code, fetched url content and a flag if the url content is a json response. This is useful for other push instances in the dependency chain. Default is False.

Result

Will return the payload as it is for easy chaining of dependencies. If provide_response is True the push will return a dictionary that looks like this:

{
    "status_code": 200,
    "data": "fetched url content",
    "is_json": False
}

Please note that this structure will be interpreted as an envelope with the keys status_code and is_json along with the payload 'fetched url content' by other push instances in the dependency chain.

Examples

#### Simple example calling the built-in rest server
#### Oscillates between http method GET and POST. Depending on the fact if the counter is even or not.
- name: http_call
  pull:
    plugin: pnp.plugins.pull.simple.Count
    args:
      wait: 5
  push:
    plugin: pnp.plugins.push.http.Call
    selector: "dict(data=dict(counter=payload), method='POST' if int(payload) % 2 == 0 else 'GET')"
    args:
      url: http://localhost:5000/
- name: rest_server
  pull:
    plugin: pnp.plugins.pull.http.Server
    args:
      port: 5000
      allowed_methods:
        - GET
        - POST
  push:
    plugin: pnp.plugins.push.simple.Echo

#### Demonstrates the use of `provide_response` set to True.
#### Call will return a response object to dependent push instances.
- name: http_call
  pull:
    plugin: pnp.plugins.pull.simple.Count
    args:
      wait: 5
  push:
    plugin: pnp.plugins.push.http.Call
    args:
      url: http://localhost:5000/
      provide_response: True
    deps:
      plugin: pnp.plugins.push.simple.Echo
- name: rest_server
  pull:
    plugin: pnp.plugins.pull.http.Server
    args:
      port: 5000
      allowed_methods:
        - GET
  push:
    plugin: pnp.plugins.push.simple.Nop

4.2.5. pnp.plugins.push.timedb.InfluxPush

Pushes the given payload to an influx database using the line protocol. You have to specify host, port, user, password and the database.

The protocol is basically a string that will be augmented at push-time with data from the payload. E.g. {payload.metric},room={payload.location} value={payload.value} assumes that payload contains metric, location and value. See https://docs.influxdata.com/influxdb/v1.5/write_protocols/line_protocol_tutorial/

Arguments

host (str): The host where the influxdb is running.
port (int): The port where the influxdb service is listening on.
user (str): Username to use for authentication.
password (str): Related password.
database (str): The database to write to.
protocol (str): Line protocol template (augmented with payload-data).

All arguments can be automatically injected via environment variables with INFLUX prefix (e.g. INFLUX_HOST).

Result

For the ability to chain multiple pushes together the payload is simply returned as is.

Examples

- name: mqtt_pull
  pull:
    plugin: pnp.plugins.pull.mqtt.MQTTPull
    args:
      host: mqtt
      topic: home/#
  push:
    plugin: pnp.plugins.push.timedb.InfluxPush
    selector: "{'data': payload}"
    args:
      host: influxdb
      port: 8086
      user: root
      password: secret
      database: home
      protocol: "{payload.levels[2]},room={payload.levels[1]} {payload.levels[3]}={payload.payload}"

4.2.6. pnp.plugins.push.mqtt.MQTTPush

Will push the given payload to a mqtt broker (in this case mosquitto). The broker is specified by host and port. In addition a topic needs to be specified were the payload is pushed to (e.g. home/living/thermostat).

The payload will be pushed as it is. No transformation is applied. If you need to some transformations, use the selector.

Arguments

host (str): The host where the mosquitto broker is running.
port (int, optional): The port where the mosquitto broker is listening. Default is 1883.
topic (str, optional): The topic to subscribe to. If set to None the envelope of the payload has to contain a 'topic' key or the push will fail (default is None). If both exists the topic from the envelope will overrule the init one.
retain (bool, optional): If set to True will mark the message as retained. Default is False. See the mosquitto man page for further guidance https://mosquitto.org/man/mqtt-7.html.

Result

For chaining of pushes the payload is simply returned as is.

Examples

- name: mqtt
  pull:
    plugin: pnp.plugins.pull.simple.Count
  push:
    # Will push the counter to the 'home/counter/state' topic
    plugin: pnp.plugins.push.mqtt.MQTTPush
    args:
      host: localhost
      topic: home/counter/state
      port: 1883
      retain: True

- name: mqtt
  pull:
    plugin: pnp.plugins.pull.simple.Count
  push:
    plugin: pnp.plugins.push.mqtt.MQTTPush
    # Lets override the topic via envelope mechanism
    # Will publish even counts on topic 'even' and uneven counts on 'uneven'
    selector: "{'data': data, 'topic': 'even' if int(data) % 2 == 0 else 'uneven'}"
    args:
      host: localhost
      port: 1883