Service task output

[linonetwo]

Hi, do you know how to pipe one service task's output as next's input? Just like AI workflow like defy, coze.

Is previous service task's output variable available in next task's input expression? And how to gather input from multiple "previous node"?

[ralphhanna]

I am not familiar with AI workflow, can you please explain?

Currently this is done through scripting and use of instance data please see https://bpmnserver.github.io/docs/data/#inputoutput-using-scripts

[linonetwo]

For example in this bpmn

import { Execution, IServiceProvider } from 'bpmn-server';

export const twServiceProvider: IServiceProvider = {
  'log-console'(input: Record<string, unknown>, _execution: Execution) {
    console.log('log-console service called:', input);
    return {
      a: 'b',
    }
  },
  a: {
    b: (input: Record<string, unknown>, execution: Execution) => {
      console.log(`b is called with ${JSON.stringify(input)} and last output is ${JSON.stringify(execution.item.output)}`);
    },
  },
};

first service task called the log-console and it returns { a: 'b'}, I want to get it in second service task called a.b.

I find the return value is placed in item.output, but I can't get previous item's output. (I think item means service task node?)

bpmn-server/src/elements/Tasks.ts

Lines 84 to 90 in 682d4cf

	ret = await method(item.input, item.context);
	} else {
	ret = await appDelegate['serviceCalled'](item.input, item.context, item);
	}
	item.log("service returned " + ret);
	item.output = ret;

I want to achieve data-flow https://docs.camunda.io/docs/components/modeler/bpmn/data-flow/ , where second worker can get result data from previous workers (one or many).

About AI workflow, one service task might call OpenAi API, and get LLM result test, it put the text in result. And second service task will call OpenAi API with previous result text again to shorten the result. This kind of chainned AI is called Agent these days.

[linonetwo]

This works execution.item.token.lastItem.output

export const twServiceProvider: IServiceProvider = {
  'log-console'(input: Record<string, unknown>, _execution: Execution) {
    console.log('log-console service called:', input);
    return {
      a: 'b',
    }
  },
  a: {
    b: (input: Record<string, unknown>, execution: Execution) => {
      console.log(`b is called with ${JSON.stringify(input)} and last output is ${JSON.stringify(execution.item.token.lastItem.output)}`);
    },
  },
};

log

log-console service called: { str: 'The text' }
b is called with {} and last output is {"a":"b"}

[linonetwo]

Another question is what does the output on the panel mean? I didn't see where it is used in the ScriptTask.run

So does the input in the panel.

And exclusive gateway's expression, where does the variable comes from?

https://docs.camunda.io/docs/components/concepts/workflow-patterns/#conditions-ifthen

I simply want to create something like defi and coze.

[ralphhanna]

I really like to know what does defi and coze do??

in bpmn-server, output variable is what is returned from the activity into item.output

[linonetwo]

I think they works exactly like noflo, see https://github.com/noflo or https://app.flowhub.io/ . You can define node to send API request or do DB search, and use a node to merge the result, and pass to AI API to generate summary, and return to user.

Sadly noflo is dead, doesn't live as long as BPMN.

returned from the activity into item.output

I'm sure about this, it is what I return in the code. But not what I defined on the properties panel. So the output definition on the properties panel is truely not being used in bpmn-server?

[ralphhanna]

There are two problems with the above solution execution.item.token.lastItem.output

1. the lastItem may not be in the same token
2. input and output variables are not persistent, so if there is a wait state the value of output is not preserved

So a more sure solution is using instance.data to hold transient data

[linonetwo]

I see, camunda works in this way, it put everything on a global context

[ralphhanna]

In addition to above, Release 2.2.10, stores item.output so you can search back for the item and get its output.

Thanks