Posting ExternalEvents

You can post an ExternalEvent using the PutExternalEvent rpc call.

PutExternalEventRequest

You need to create a PutExternalEventRequest to do so. The protobuf definition is:

message PutExternalEventRequest {
  string wf_run_id = 1;
  string external_event_def_name = 2;
  optional string guid = 3;
  VariableValue content = 5;
  optional int32 thread_run_number = 6;
  optional int32 node_run_position = 7;
}

Required Parameters

The three required values are:

1. wfRunId is the ID of the WfRun that the ExternalEvent is sent to.
2. externalEventDefName is the name of the ExternalEventDef to send.
3. content is the actual payload of the ExternalEvent.

Idempotence

The guid parameter is optional, but it is highly recommended to set it for idempotence. As per the ExternalEvent Documentation, an ExternalEvent has a composite ID:

1. wf_run_id
2. external_event_def_name
3. guid

If no guid is provided, the server generates a random one for you. If you provide a guid and an ExternalEvent has already been posted with that guid, your call has no effect. This is a good strategy for enabling idempotent retries.

Targeting Specific Threads and Nodes

The thread_run_number parameter can be used to send the ExternalEvent to a specific ThreadRun.

This feature is rarely used; only for advanced use-cases.

Making the Request

The last thing we need to do to send the request is to set the content VariableValue.

Java

In Java, we can use LHLibUtil#objToVarVal() to convert an aribitrary Java value or object into a VariableValue. You can post an event as follows:

LittleHorseBlockingStub client = ...;

String idempotencyKey = "asdf13hoasdifoje"; // optional

client.PutExternalEvent(PutExternalEventRequest.newBuilder()
        .setExternalEventDefId(ExternalEventDefId.newBuilder().setName("my-external-event-def"))
        .setWfRunId(WfRunId.newBuilder().setId("asdf-1234"))
        .setContent(LHLibUtil.objToVarVal(someObject))
        .setGuid(idempotencyKey) // optional
        .build());

Go

In Go, you can use the littlehorse.InterfaceToVarVal() method to create the content parameter.

idempotencyKey := "asdfo2htoijsvlk" // optional

eventContent, _ := littlehorse.InterfaceToVarVal("some-interface")
result, _ := (*client).PutExternalEvent(context.Background(), &lhproto.PutExternalEventRequest{
	WfRunId:            &lhproto.WfRunId{Id: "some-wfrun-id"},
	ExternalEventDefId: &lhproto.ExternalEventDefId{Name: "some-external-event-def"},
	Content:            eventContent,
	Guid:               &idempotencyKey, // optional
})

Python

In Python, you can use the littlehorse.to_variable_value() method to create the content parameter.

import littlehorse
from littlehorse.config import LHConfig
from littlehorse.model import *

config = LHConfig()
client = config.stub()

idempotency_key = "asfjok23jtoadfjl" # optional

client.PutExternalEvent(
    PutExternalEventRequest(
        wf_run_id=WfRunId(id="asdf-1234"),
        external_event_def_id=ExternalEventDefId(name="my-external-event-def"),
        content=littlehorse.to_variable_value("my value"),
        guid=idempotency_key, # optional
    )
)