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Application Service Triggers

Introduction

Triggers determine how the App Functions Pipeline begins execution. The trigger is determined by the [Trigger] configuration section in the configuration.toml file.

Edgex 2.0

For Edgex 2.0 the [Binding] configuration section has been renamed to [Trigger]. The [MessageBus] section has been renamed to EdgexMessageBus and moved under the [Trigger] section. The [MqttBroker] section has been renamed to ExternalMqtt and moved under the [Trigger] section.

There are 4 types of Triggers supported in the App Functions SDK which are discussed in this document

  1. EdgeX Message Bus - Default Trigger for most use cases as this is how the App Services receive Events from EdgeX Core Data and/or Devices Services
  2. External MQTT - Useful when receiving commands from an external/Cloud MQTT broker.
  3. HTTP - Useful during development and testing of custom functions.
  4. Custom - Allows custom Application Services to implement their own Custom Trigger

EdgeX MessageBus Trigger

An EdgeX MessageBus trigger will execute the pipeline every time data is received from the configured Edgex MessageBus SubscribeTopics. The EdgeX MessageBus is the central message bus internal to EdgeX and has a specific message envelope that wraps all data published to this message bus.

There currently are three implementations of the EdgeX MessageBus available to be used. These are Redis Pub/Sub(default), MQTT and ZeroMQ(ZMQ). The implementation type is selected via the [Trigger.EdgexMessageBus] configuration described below.

Type Configuration

Edgex 2.0

For EdgeX 2.0 the SubscribeTopic has been renamed to SubscribeTopics and moved under the EdgexMessageBus SubscribeHost section. The PublishTopic has also been moved under the EdgexMessageBus PublishHost section. Also the legacy type of messagebus has been removed.

Here's an example:

[Trigger]
Type="edgex-messagebus"
The Type= is set to edgex-messagebus trigger type. The Context function ctx.SetResponseData([]byte outputData) stores the data to send back to the EdgeX MessageBus on the topic specified by the PublishHost PublishTopic= setting.

MessageBus Connection Configuration

The other piece of configuration required are the connection settings: 

[Trigger.EdgexMessageBus]
Type = "redis" # message bus type (i.e "redis`, `mqtt` or `zero` for ZeroMQ)
    [Trigger.EdgexMessageBus.SubscribeHost]
        Host = "localhost"
        Port = 6379
        Protocol = "redis"
        SubscribeTopics="edgex/events/#"
    [Trigger.EdgexMessageBus.PublishHost]
        Host = "localhost"
        Port = 6379
        Protocol = "redis"
        PublishTopic="" # optional if publishing response back to the MessageBus

Edgex 2.0

For Edgex 2.0 the PublishTopic can now have placeholders. See Publish Topic Placeholders section below for more details

As stated above there are three EdgeX MessageBus implementations you can choose from. These type values are as follows:

redis - for Redis Pub/Sub (Requires Redis running and Core Data and/or Device Services configure to use Redis Pub/Sub)
mqtt  - for MQTT (Requires a MQTT Broker running and Core Data and/or Device Services configure to use MQTT)
zero  - for ZeroMQ (No Broker/Service required. Core Data must be configured to use Zero and Device service configure to use REST to Core Data)

Edgex 2.0

For Edgex 2.0 Redis is now the default EdgeX MessageBus implementation used. Also, the Redis implementation changed from Redis streams to Redis Pub/Sub, thus the type value changed from redisstreams to redis

Important

When using ZMQ for the message bus, the Publish Host MUST be different for each publisher to since the they will bind to the specific port. 5563 for example cannot be used to publish since EdgeX Core Data has bound to that port. Similarly, you cannot have two separate instances of the app functions SDK running and publishing to the same port. This is why once Device services started publishing the the EdgeX MessageBus the default was changed to Redis Pub/Sub

Note

When using MQTT for the message bus, there is additional configuration required for specifying the MQTT specific options.

Example Using MQTT

Here is example EdgexMessageBus configuration when using MQTT as the message bus:

[Trigger.EdgexMessageBus]
Type = "mqtt"
    [Trigger.EdgexMessageBus.SubscribeHost]
    Host = "localhost"
    Port = 1883
    Protocol = "tcp"
    SubscribeTopics="edgex/events/#"
    [Trigger.EdgexMessageBus.PublishHost]
    Host = "localhost"
    Port = 1883
    Protocol = "tcp"        
    PublishTopic="" # optional if publishing response back to the MessageBus
    [Trigger.EdgexMessageBus.Optional]
    # MQTT Specific options
    ClientId ="new-app-service"
    Qos            = "0" # Quality of Service values are 0 (At most once), 1 (At least once) or 2 (Exactly once)
    KeepAlive      = "10" # Seconds (must be 2 or greater)
    Retained       = "false"
    AutoReconnect  = "true"
    ConnectTimeout = "30" # Seconds
    SkipCertVerify = "false"
    authmode = "none"  # change to "usernamepassword", "clientcert", or "cacert" for secure MQTT messagebus.
    secretname = "mqtt-bus"

EdgeX 2.0

New for EdgeX 2.0 is the Secure MessageBus when use the Redis Pub/Sub implementation. See the Secure MessageBus documentation for more details.

EdgeX 2.0

Also new for EdgeX 2.0 is the MQTT MessageBus implementation now supports retrieving secrets from the Secret Store for secure MQTT connection, but there is not any facility yet to generate the credentials on first startup and distribute them to all services, as is done with Redis Pub/sub. This MQTT credentials generation and distribution is a future enhancement for EdgeX security services.

Filter By Topics

EdgeX 2.0

New for EdgeX 2.0

App services now have the capability to filter by EdgeX MessageBus topics rather then using Filter functions in the functions pipeline. Filtering by topic is more efficient since the App Service never receives the data off the MessageBus. Core Data and/or Device Services now publish to multi-level topics that include the profilename, devicename and sourcename . Sources are the commandname or resourcename that generated the Event. The publish topics now look like this:

# From Core Data
edgex/events/core/<profile-name>/<device-name>/<source-name>

# From Device Services
edgex/events/device/<profile-name>/<device-name>/<source-name>

This with App Services capability to have multiple subscriptions allows for multiple filters by subscriptions. The SubscribeTopics setting takes a comma separated list of subscribe topics.

Here are a few examples of how to configure the SubscribeTopics setting under the Trigger.EdgexMessageBus.SubscribeHost section to filter by subscriptions using the profile, device and source names from the SNMP Device Service file here:

  • Filter for all Events 
SubscribeTopics="edgex/events/#"
  • Filter for Events only from a single class of devices (device profile defines a class of device)
SubscribeTopics="edgex/events/#/trendnet/#"
  • Filter for Events only from a single actual device
SubscribeTopics="edgex/events/#/#/trendnet01/#"
  • Filter for Events from two specific actual devices
SubscribeTopics="edgex/events/#/#/trendnet01/#, edgex/events/#/#/trendnet02/#"
  • Filter for Events from two specific sources. 
SubscribeTopics="edgex/events/#/#/#/Uptime, edgex/events/#/#/#/MacAddress"

Note

The above examples are for when Redis is used as the EdgeX MessageBus implementation, which is now the default. The Redis implementation uses the # wildcard character for multi-level and single level. The implementation actually converts all #'s to the *'s. The *is the actual wildcard character used by Redis Pub/Sub. In the first example (multi-level) the # is used at the end in the location for where Core Data's and Device Service's publish topics differ. This location will be core when coming from Core Data or device when coming from a Device Service. The additional use of # within the topic, not at the end, (single-level) allows for any Profile, Device or Source when specifying one of the others.

Note

For the MQTT implementation of the EdgeX MessageBus, the # is also used for the multi-level wildcard, but the single-level wildcard is the + character. So the first and last examples above would be as follows for when using the MQTT implementation

SubscribeTopics="edgex/events/#"
SubscribeTopics="edgex/events/+/trendnet/#"
SubscribeTopics="edgex/events/+/+/trendnet01/#"
SubscribeTopics="edgex/events/+/+/trendnet01/#, edgex/events/+/+/trendnet02/#"
SubscribeTopics="edgex/events/+/+/+/Uptime, edgex/events/+/+/+/MacAddress"

External MQTT Trigger

An External MQTT trigger will execute the pipeline every time data is received from an external MQTT broker on the configured SubscribeTopics.

Note

The data received from the external MQTT broker is not wrapped with any metadata known to EdgeX. The data is handled as JSON or CBOR. The data is assumed to be JSON unless the first byte in the data is not a { or a [, in which case it is then assumed to be CBOR.

Note

The data received, encoded as JSON or CBOR, must match the TargetType defined by your application service. The default TargetType is an Edgex Event. See TargetType for more details.

Type Configuration

Here's an example: 

[Trigger]
Type="external-mqtt"

Edgex 2.0

For EdgeX 2.0 the SubscribeTopic has been renamed to SubscribeTopics and moved under the ExternalMqtt section. The PublishTopic has also been moved under the ExternalMqtt section.

The Type= is set to external-mqtt. To receive data from the external MQTT Broker you must set your SubscribeTopics= to the appropriate topic(s) that the external publisher is using. You may also designate a PublishTopic= if you wish to publish data back to the external MQTT Broker. The Context function ctx.SetResponseData([]byte outputData) stores the data to send back to the external MQTT Broker on the topic specified by the PublishTopic= setting.

External MQTT Broker Configuration

The other piece of configuration required are the MQTT Broker connection settings: 

[Trigger.ExternalMqtt]
    Url = "tcp://localhost:1883" #  fully qualified URL to connect to the MQTT broker
    SubscribeTopics="SomeTopics"
    PublishTopic="" # optional if publishing response back to the the External MQTT Broker
    ClientId = "AppService" 
    ConnectTimeout = "5s" # 5 seconds
    AutoReconnect = true
    KeepAlive = 10 # Seconds (must be 2 or greater)
    QoS = 0 # Quality of Service 0 (At most once), 1 (At least once) or 2 (Exactly once)
    Retain = true
    SkipCertVerify = false
    SecretPath = "mqtt-trigger" 
    AuthMode = "none" # Options are "none", "cacert" , "usernamepassword", "clientcert".

Edgex 2.0

For Edgex 2.0 the PublishTopic can have placeholders. See Publish Topic Placeholders section below for more details

HTTP Trigger

Designating an HTTP trigger will allow the pipeline to be triggered by a RESTful POST call to http://[host]:[port]/api/v2/trigger/.

Type Configuration

Here's an example:

[Trigger]
Type="http"

The Type= is set to http. This will will enable listening to the api/v2/trigger/ endpoint. No other configuration is required. The Context function ctx.SetResponseData([]byte outputData) stores the data to send back as the response to the requestor that originally triggered the HTTP Request.

Note

The HTTP trigger uses the content-type from the HTTP Header to determine if the data is JSON or CBOR encoded and the optional X-Correlation-ID to set the correlation ID for the request.

Note

The data received, encoded as JSON or CBOR, must match the TargetType defined by your application service. The default TargetType is an Edgex Event. See TargetType for more details.

Custom Triggers

Edgex 2.0

New for EdgeX 2.0

It is also possible to define your own trigger and register a factory function for it with the SDK. You can then configure the trigger by registering a factory function to build it along with a name to use in the config file. These triggers can be registered with:

service.RegisterCustomTriggerFactory("my-trigger-name", myFactoryFunc)

Note

You can NOT override trigger names built into the SDK ( "edgex-messagebus", "external-mqtt", or "http") for a custom trigger.

The trigger factory function is bound to an instance of a trigger configuration struct that is provided by the SDK:

type TriggerConfig struct {
    Logger           logger.LoggingClient
    ContextBuilder   TriggerContextBuilder
    // Deprecated: use MessageReceived
    MessageProcessor TriggerMessageProcessor
    MessageReceived  TriggerMessageHandler
    ConfigLoader     TriggerConfigLoader
}

This type carries a pointer to the internal edgex logger, along with three functions:

  • ContextBuilder builds an interfaces.AppFunctionContext from a message envelope you construct.
  • MessageProcessor (DEPRECATED) exposes a function that sends your message envelope and context built above into the default function pipeline.
  • MessageReceived exposes a function that sends your message envelope and context to any pipelines configured in the EdgeX service. It also takes a function that will be run to process the response for each successful pipeline.

Note

The context passed in to Received will be cloned for each pipeline configured to run. If a nil context is passed a new one will be initialized from the message.

  • ConfigLoader exposes a function that loads your custom config struct. By default this is done from the primary EdgeX configuration pipeline, and only loads root-level elements.

If you need to override these functions it can be done in the factory function registered with the service.

The custom trigger constructed here will then need to implement the trigger interface so that the SDK can invoke it:

type Trigger interface {
    Initialize(wg *sync.WaitGroup, ctx context.Context, background <-chan BackgroundMessage) (bootstrap.Deferred, error)
}

type BackgroundMessage interface {
    Message() types.MessageEnvelope
    Topic() string
}

This leaves a lot of flexibility for how you want the trigger to behave (for example you could write a trigger to watch for file changes, or run on a timer). Below is a sample implementation of a trigger that reads lines from os.Stdin and pass the captured string through the edgex function pipeline. In this case the target type for the service is set to &[]byte{}.

type stdinTrigger struct{
    tc appsdk.TriggerConfig
}

func (t *stdinTrigger) Initialize(wg *sync.WaitGroup, ctx context.Context, _ <-chan interfaces.BackgroundMessage) (bootstrap.Deferred, error) {
    msgs := make(chan []byte)

    receiveMessage := true

    responseHandler := func(ctx AppFunctionContext, pipeline *FunctionPipeline) {
        // do stuff
    }

    go func() {
        fmt.Print("> ")
        rdr := bufio.NewReader(os.Stdin)
        for receiveMessage {
            s, err := rdr.ReadString('\n')
            s = strings.TrimRight(s, "\n")

            if err != nil {
                t.tc.Logger.Error(err.Error())
                continue
            }

            msgs <- []byte(s)
        }
    }()

    go func() {
        for receiveMessage {
            select {
            case <-ctx.Done():
                receiveMessage = false

            case m := <-msgs:
                go func() {
                    env := types.MessageEnvelope{
                        Payload: m,
                    }

                    ctx := t.tc.ContextBuilder(env)

                    err := t.tc.MessageReceived(ctx, env, responseHandler)

                    if err != nil {
                        t.tc.Logger.Error(err.Error())
                    }
                }()
            }
        }
    }()

    return cancel, nil
}

This trigger can then be registered by calling:

appService.RegisterCustomTriggerFactory("custom-stdin", func(config appsdk.TriggerConfig) (appsdk.Trigger, error) {
    return &stdinTrigger{
        tc: config,
    }, nil
})

Type Configuration

Here's an example:

[Trigger]
Type="custom-stdin"

Now the custom trigger is configured to be used rather than one of the built-in triggers.

A complete working example can be found here

Publish Topic Placeholders

Edgex 2.0

New for EdgeX 2.0

Both the EdgeX MessageBusand the External MQTT triggers support the new Publish Topic Placeholders capability. The configured PublishTopic for either of these triggers can contain placeholders for runtime replacements. The placeholders are replaced with values from the new Context Storage whose key match the placeholder name. Function pipelines can add values to the Context Storage which can then be used as replacement values in the publish topic. If an EdgeX Event is received by the configured trigger the Event's profilename, devicename and sourcename as well as the will be seeded into the Context Storage. See the Context Storage documentation for more details.

The Publish Topic Placeholders format is a simple {<key-name>} that can appear anywhere in the topic multiple times. An error will occur if a specified placeholder does not exist in the Context Storage.

Example

PublishTopic = "data/{profilename}/{devicename}/{custom}"

Received Topic

Edgex 2.0

New for EdgeX 2.0

The topic the data was received on for EdgeX MessageBus and the External MQTT triggers is now stored in the new Context Storage with the key receivedtopic. This makes it available to pipeline functions via the Context Storage .