Cybus::Mapping

Transforming and routing MQTT data between topics.

The Cybus::Mapping resource describes how data is mapped from one MQTT topic or an array of topics (subscribe) to another MQTT topic (publish), with the possibility of adding rules to perform powerful transformations of the data directly within Connectware.

Mappings by default occur between topics of the internal broker, but are not limited to this. Mappings may also be established between different MQTT brokers.

Mapping Properties

Property	Type	Required	Default
mappings	object[]	Required
targetState	enum	Optional	"enabled"
agentName	string	Optional	"protocol-mapper"
inputBuffering	object	Optional

mappings

• Required

• Type: object[]

All items must be of type object with the following properties:

Property	Type	Required
subscribe	object	Required
publish	object	Required
rules	array	Optional

subscribe

The subscribe part of the Cybus::Mapping can subscribe either to a single endpoint or topic, or to an array of endpoints or topics. When subscribing to an array, the collect rule is particularly useful in the rules property of this mapping entry.

• Required

• Type: object or object[]

Property	Type	Required
endpoint	string	One of endpoint or topic
topic	string	One of endpoint or topic
qos	integer	Optional
connection	string	Optional
label	string	Optional

endpoint

Reference to a Cybus::Endpoint resource.

• Type: string

topic

An explicit topic name.

This topic will not automatically be prefixed.

• Type: string

qos

The quality of service for MQTT messages.

• Type: integer

• Allowed values:

  • 0

  • 1

  • 2

• Default: 0

connection

Reference to a Cybus::Connection resource with protocol Mqtt only. When set, the mapping subscribes via that MQTT connection instead of the internal broker.

• Type: string

Only MQTT protocol connections are supported here. Using any other protocol will fail.

Example

resources:
  externalMqtt:
    type: Cybus::Connection
    properties:
      protocol: Mqtt
      connection:
        host: broker.example.org
        port: 1883

  mapFromExternal:
    type: Cybus::Mapping
    properties:
      mappings:
        - subscribe:
            topic: 'shopfloor/in/#topic'
            connection: !ref externalMqtt
          publish:
            topic: 'internal/out/$topic' # published on internal broker

label

An optional label used as the key of the output object built when the collect rule is used in combination with an array of subscriptions. If no label is provided, the topic is used instead.

Labels can be dynamically constructed when wildcards are used in the topics. To construct dynamic labels, the name of a wildcard enclosed in curly braces must be used to specify that the final value of the label will be replaced with the value of the wildcard.

When using dynamic labels, the label must be enclosed in double quotes.

Example:

testMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          - topic: factory/+machineName/+sensorName
            label: 'Machine: {machineName} Sensor: {sensorName}'
        publish:
          topic: 'factory/out'
        rules:
          - collect: {}

In this case, a message arriving at /factory/Robot001/Temperature will yield a label Machine: Robot001 Sensor: Temperature, and a message arriving at /factory/Robot001/Current will yield a label Machine: Robot001 Sensor: Current.

The wildcards used in the topic can be placed in any order in the label template, as the matching to wildcards is done by name. For example, another option for the previous example could be a label configured as “Sensor: {sensorName} Machine: {machineName}”, which will yield a label “Sensor: Temperature Machine: Robot001”.

Dynamic labels make it easy to write mappings that use the collect rule in a very concise manner.

• Type: string

publish

• Required

• Type: object

Property	Type	Required
endpoint	string	One of endpoint or topic
topic	string	One of endpoint or topic
qos	integer	Optional
retain	boolean	Optional
connection	string	Optional

endpoint

Reference to a Cybus::Endpoint resource.

• Type: string

topic

An explicit topic name.

This topic will not automatically be prefixed.

• Type: string

qos

The quality of service for MQTT messages.

• Type: integer

• Allowed values:

  • 0

  • 1

  • 2

• Default: 0

Note: This particular qos setting is applied only for this particular subscribe (or publish) MQTT connection of this particular mapping, i.e., from the broker to the protocol-mapper instance of this mapping or the other way around. If your intention is to set up an end-to-end data connection with qos level 1 or 2, every single MQTT connection involved must have this specific qos level set.

retain

Whether the last message should be retained (last-value-cached) on the internal or external broker (default: false).

• Type: boolean

• Default: false

connection

Reference to a Cybus::Connection resource with protocol Mqtt only. When set, the mapping publishes via that MQTT connection instead of the internal broker.

• Type: string

Only MQTT protocol connections are supported here. Using any other protocol will fail.

Example

resources:
  externalMqttOut:
    type: Cybus::Connection
    properties:
      protocol: Mqtt
      connection:
        host: mqtt.acme.cloud
        port: 1883
        # username: myUser
        # password: myPass

  mapToExternal:
    type: Cybus::Mapping
    properties:
      mappings:
        - subscribe:
            topic: 'internal/in/#topic' # read from internal broker
          publish:
            topic: 'cloud/$topic'
            connection: !ref externalMqttOut
            qos: 1
            retain: false

rules

• Optional

• Type: array of Rule Engine

This modifies your payload/topic before publishing it.

targetState

• Optional

• Type: enum

• Default: "enabled"

• The value of this property must be one of:

  • enabled

  • disabled

agentName

• Optional

• Type: string

• Default: "protocol-mapper"

If this property is set to any (non-default) value, it is used as the agent name on which this particular Mapping resource should run. This is useful for load sharing if Mapping instances with a lot of processing rules need to be distributed across a larger number of Agent instances. However, the specified agent name must match exactly the actual agent name; otherwise, an error is thrown upon enabling this resource.

Also, keep in mind that each Mapping resource subscribes to the central MQTT broker for the subscribe side and publishes the results to the central MQTT broker. Running this on a distributed agent causes an extra two MQTT message transmissions for each mapping.

inputBuffering

Each mapping’s subscription receives input data. This input data at the subscription can optionally be managed through an individual input buffer (also called input queue) to establish fine-grained control for high data rate behavior. By default, this input buffering is disabled, and instead, all input data is handled on the global event queue, which works fine as long as there is no risk of out-of-memory exceptions due to unexpected slow data processing or forwarding.

When enabling the individual input buffer, the buffer properties determine the behavior in situations when the input buffer is filling up. The buffer is filling up when the message arrival rate is higher than the processing or forwarding (publishing) data rate. If this situation happens for longer durations, the input buffer will reach its configured capacity limits, and arriving messages will be dropped to prevent an uncontrollable out-of-memory exception. This is a fundamental and unavoidable property of distributed systems due to finite resources. However, the actual behavior of the input buffer can be adapted to the application scenario by setting the properties in the inputBuffering section (optional).

Supported properties are (all optional):

• enabled (type: boolean, default: false): Enable or disable input buffering.

• maxInputBufferSize (type: integer, default: 5000): Maximum number of input messages that are queued in the input buffer. Exceeding messages will be discarded. Adjust this to a higher value if you are handling bursty traffic.

• maxConcurrentMessages (type: integer, default: 2): Maximum number of concurrently processed messages as long as the input buffer queue is non-empty.

• waitingTimeOnEmptyQueue (type: integer, default: 10): Waiting time in milliseconds after the input buffer queue runs empty and before checking again for newly queued input messages. Regardless of this value, on a non-empty input buffer queue, all messages will be processed without waiting time in between until the queue is empty again.

Mapping Concept

It is possible to map data from one topic to another, from endpoints to topics, from topics to endpoints, or from endpoints to endpoints directly.

An example is shown in the following code snippet, where the topic test/topic/from is mapped to the other topic test/topic/to, and data from the endpoint testEndpoint1 is mapped to the topic test/endpoint/to/topic.

testMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          topic: test/topic/from
        publish:
          topic: test/topic/to
      - subscribe:
          endpoint: !ref testEndpoint1
        publish:
          topic: 'test/endpoint/to/topic'

It is also possible to map data from several topics and endpoints to a single topic.

When an array of subscribe objects is provided in a mapping, the data received in the subscribed topics is published to the target publish topic.

testMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          - topic: test/topic/from
            label: test_topic_subs
          - endpoint: !ref testEndpoint1
            label: test_endpoint_1
          - endpoint: no/labels/are/ok/too
        publish:
          topic: 'test/endpoint/to/topic'
      - subscribe:
          endpoint: regular/subscribe
        publish:
          topic: output/topic

Additionally, if the collect rule is used, all the values from the subscribed topics and endpoints are stored in a Last Value Cache and published to the target topic as an object using the labels, or the topics if no labels were provided, as keys for the object.

testMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          - topic: test/topic/from
            label: test_topic_subs
          - endpoint: !ref testEndpoint1
            label: test_endpoint_1
          - endpoint: no/labels/are/ok/too
        publish:
          topic: 'test/endpoint/to/topic'
        rules:
          - collect: {}

Wildcards

The Cybus:Mapping resource supports wildcards in both subscribe and publish mode. In most use cases, it is desirable to have the ability to reference the actual topic value at the wildcard position either in the target topic or in the payload. Both are possible. To use this, use a named wildcard + and add a variable name after it. The content of the wildcard variable is then made available in the $context.vars object in the Transform, Filter, and SetContextVars rules of a Rule Engine. The following code example shows some combinations.

This example will swap the last two topic components:

wildcardMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          topic: source/+a/+b
        publish:
          topic: target/$b/$a

This example will store some value from the message payload using the setContextVars rule, then use that value as the output topic. The expected input message must be a JSON object with at least the property value, e.g., {"value": "abc"}, and it might contain further values that are forwarded unchanged in this example.

wildcardMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          topic: source/+x
        rules:
          - setContextVars:
              vars:
                fromPayload: 'value'
        publish:
          topic: target/$x/$fromPayload

You may even map entire sub-topic trees using the # wildcard, as shown here:

wildcardMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          topic: in/#topic
        publish:
          topic: out/$topic

See below some examples of how messages arriving on in are mapped to out:

• in/hall2 -> out/hall2

• in/hall2/machine1 -> out/hall2/machine1

Note

Since in MQTT the named ‘#’ wildcard includes the parent level while matching, a message published directly to in would be mapped to out in this case.

Duplicated Messages Due to Wildcards

Duplicated messages are typically observed either in the Data Explorer or in target applications that receive the same message twice. This arises in configurations where mappings use wildcards (# or +). If you encounter duplicated messages, check for the use of wildcards first.

For example, consider the following mapping:

mapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          - topic: test/from/topic1
          - topic: test/from/#
        publish:
          topic: test/to

The overlapping subscriptions do not need to be defined within the same mapping. Whenever an MQTT client creates overlapping subscriptions, duplication will occur. Since the protocol-mapper uses the same MQTT client by default for all resources across services, overlapping can also occur across multiple services. This means that using wildcards can be risky, as future subscriptions may unintentionally overlap.

The MQTT broker implementation used in Connectware (VerneMQ) delivers messages from overlapping subscriptions twice to each subscription. Since the MQTT Specification (3.1.1) does not define the exact behavior for this scenario, it is not necessarily considered a bug. Other brokers, such as Mosquitto, behave differently and do not deliver duplicated messages in the same situation.

• To prevent duplicates, create a dedicated MQTT connection for wildcard subscriptions as shown below:

resources:
  mqttConnection:
    type: Cybus::Connection
    properties:
      protocol: Mqtt
      connection:
        host: !ref Cybus::MqttHost
        port: !ref Cybus::MqttPort
        username: !ref Cybus::MqttUser
        password: !ref Cybus::MqttPassword

  mapping:
    type: Cybus::Mapping
    properties:
      mappings:
        - subscribe:
            - topic: services/+serviceId/connections/+connection
              connection: !ref mqttConnection
          publish:
            topic: !sub '${Cybus::ServiceId}/connections/state'

  defaultRole:
    type: Cybus::Role
    properties:
      permissions:
        - resource: services/#
          operation: read
          context: mqtt

All other mappings can then be used as usual without explicitly defining a connection and will not be affected by message duplication.

You may need to extend the default user permissions (defaultRole) with the topics that should be subscribed to. By default, access is restricted to the service’s MQTT root.

Examples

Simple mapping from endpoint to custom topic:

myMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          endpoint: !ref myEndpoint
        publish:
          topic: !sub '${Cybus::MqttRoot}/topic-in-my-container'

Relaying messages from internal broker to external one:

myMapping:
  type: Cybus::Mapping
  properties:
    mappings:
      - subscribe:
          topic: 'internal-scope/#topic'
        publish:
          topic: external-scope/$topic
          connection: !ref myMqttConnection