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Workflows

A WfSpec diagram featuring a directed graph of nodes with a conditional that branches based on whether a user approved an IT Request or not.
Screenshot of a WfSpec in LittleHorse Dashboard

Concepts​

At its core, LittleHorse is a workflow engine. Workflows consist of two LittleHorse API Resources:

  1. WfSpecs, or "Workflow Specifications", which are a blueprint for a WfRun. A WfSpec consists of a series of steps (most often, Tasks) and defines the control flow for your worklfow runs.
  2. WfRuns, or "Workflow Runs", which are running instances of a WfSpec.

Before you can run a workflow (i.e. create a WfRun), you must first define the WfSpec.

The WfSpec​

In LittleHorse, the WfSpec object is a Metadata Object defining the blueprint for a WfRun, which is a running instance of a workflow.

A WfSpec (Workflow Specification) is a blueprint that defines the control flow of your WfRuns (Workflow Run). Before you can run a WfRun, you must first register a WfSpec in LittleHorse (for an example of how to do that, see here).

Under the hood, a WfSpec is a directed graph consisting of Nodes and Edges, where a Node defines a "step" of the workflow process, and an Edge tells the workflow what Node to go to next. You can see the control flow specified by this directed graph in the LittleHorse dashboard.

The WfRun​

A WfRun is a LittleHorse API Object that represents a running instance of a WfSpec. Each WfRun is uniquely identified by a WfRunId, which you can pass to the LittleHorse Kernel when running the WfSpec or which can be autogenerated for you by the LittleHorse Kernel.

The WfRun follows the logic defined in the WfSpec and results in TaskRuns, UserTaskRuns, and other LittleHorse API Objects being created throughout its lifecycle.

In Practice​

tip

If you want to follow along, all of the code snippets in this section can be copy-n-pasted and executed as standalone files. However, to do this, you'll need access to a LittleHorse Kernel and lhctl. You can do that via the instructions in our installation docs or by running the following commands:

  • brew install littlehorse-enterprises/lh/lhctl
  • docker run --rm -d --name littlehorse --pull always -p 2023:2023 -p 8080:8080 ghcr.io/littlehorse-enterprises/littlehorse/lh-standalone:latest

To run a workflow, you need to do the following:

  1. Register any TaskDefs, ExternalEventDefs, UserTaskDefs, or WorkflowEventDefs used by your WfSpec.
  2. Define and register your WfSpec.
  3. Run the WfSpec (thereby creating a WfRun).

In this example we will run a simple "hello world" workflow which simply executes one task, the greet task.

Register the TaskDef​

Let's create a greet Task Worker. This "hello-world" task accepts a name (of type String) and returns another String which is a customized greeting.

package io.littlehorse.quickstart;

import io.littlehorse.sdk.common.config.LHConfig;
import io.littlehorse.sdk.worker.LHTaskMethod;
import io.littlehorse.sdk.worker.LHTaskWorker;

class Greeter {
    @LHTaskMethod("greet")
    public String greeting(String name) {
        String result = "Hello " + name + "!";
        System.out.println(result);
        return result;
    }
}

public class Main {

    public static void main(String[] args) {
        LHConfig config = new LHConfig();

        Greeter greeter = new Greeter();

        // Create a Task Worker
        LHTaskWorker worker = new LHTaskWorker(greeter, "greet", config);
        Runtime.getRuntime().addShutdownHook(new Thread(worker::close));

        // Register the TaskDef
        worker.registerTaskDef();

        // Start the Worker
        worker.start();
    }
}

Register the WfSpec​

Now that the Task Worker is running, we can create a WfSpec using the LittleHorse workflow DSL. As a simple "getting started" workflow, we will build a simple "hello-world" WfSpec that takes in a String (name) and passes it into our greet task. If you continue reading the Concepts section, we will get to more advanced and more useful workflow logic; however, this example will help you get started.

In Java, we use the Workflow.newWorkflow() method to create a Workflow object. The newWorkflow() method accepts a lambda function that defines the actual WfSpec logic. Then, we can use the Workflow object to register the WfSpec to the LittleHorse Kernel.

package io.littlehorse.quickstart;

import io.littlehorse.sdk.common.config.LHConfig;
import io.littlehorse.sdk.wfsdk.WfRunVariable;
import io.littlehorse.sdk.wfsdk.Workflow;

public class Main {
    public static final String WF_NAME = "quickstart";

    public static void main(String[] args) {
        LHConfig config = new LHConfig();

        Workflow workflowGenerator = Workflow.newWorkflow(WF_NAME, wf -> {
            WfRunVariable name = wf.declareStr("name").searchable().required();
            wf.execute("greet", name);
        });

        workflowGenerator.registerWfSpec(config.getBlockingStub());
    }
}

After running the above code, you should be able to see your WfSpec in the LittleHorse Dashboard:

A screen capture of the 'quickstart' WfSpec in the LittleHorse Dashboard. There are three nodes: 1) an entrypoint, 2) the greet task, 3) the exit node.
The `quickstart` WfSpec in the dashboard.
tip

For more information about how to develop complex WfSpecs, continue reading in the Concepts section or skip ahead to the WfSpec Development Guide.

Run the WfSpec​

You can run the WfSpec and create a WfRun by using the LittleHorse dashboard. Navigate to the WfSpec page, click Execute, and pass in a value for the name variable.

A screen capture of the 'Execute WfRun' widget in the LittleHorse dashboard, in which the user provides a value for the `name` variable..
Running our Workflow from the Dashboard

Alternatively, you can run the WfSpec by using lhctl and provide the name variable as follows:

lhctl run quickstart name Obi-Wan

Click around the dashboard, click on the nodes, and see what happened!

Further Resources​

This workflow barely scratches the surface of what LittleHorse is capable of. To dig further, check out the following: