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Tasks

Tasks are used to call Java code that implements functionality that is too complex to express with the Concord DSL and EL in YAML directly. They are plugins of Concord.

Using Tasks

Tasks allow you to call Java methods implemented in one of the projects. In order to be able to use a task a URL to the JAR containing the implementation has to be added as a dependency. Typically the JAR is published to a repository manager and a URL pointing to the JAR in the repository is used.

You can invoke a task via an expression or with the task step type.

Following are a number of examples:

configuration:
  dependencies:
    - "http://repo.example.com/myConcordTask.jar"
flows:
  default:
    # invoking via usage of an expression and the call method
    - ${myTask.call("hello")}

    # calling a method with a single argument
    - myTask: hello

    # calling a method with a single argument
    # the value will be a result of expression evaluation
    - myTask: ${myMessage}

    # calling a method with two arguments
    # same as ${myTask.call("warn", "hello")}
    - myTask: ["warn", "hello"]

    # calling a method with a single argument
    # the value will be converted into Map<String, Object>
    - myTask: { "urgency": "high", message: "hello" }

    # multiline strings and string interpolation is also supported
    - myTask: |
        those line breaks will be
        preserved. Here will be a ${result} of EL evaluation.

If a task implements the #execute(Context) method, some additional features like in/out variables mapping can be used:

flows:
  default:
    # calling a task with in/out variables mapping
    - task: myTask
      in:
        taskVar: ${processVar}
        anotherTaskVar: "a literal value"
      out:
        processVar: ${taskVar}
      error:
        - log: something bad happened

Creating Tasks

Tasks must implement com.walmartlabs.concord.sdk.Task Java interface.

The Task interface is provided by the concord-sdk module:

<dependency>
  <groupId>com.walmartlabs.concord</groupId>
  <artifactId>concord-sdk</artifactId>
  <version>1.35.1</version>
  <scope>provided</scope>
</dependency>

Some dependencies are provided by the runtime. It is recommended to mark them as provided in the POM file:

  • com.fasterxml.jackson.core/*
  • javax.inject/javax.inject
  • org.slf4j/slf4j-api

Here’s an example of a simple task:

import com.walmartlabs.concord.sdk.Task;
import javax.inject.Named;

@Named("myTask")
public class MyTask implements Task {

    public void sayHello(String name) {
        System.out.println("Hello, " + name + "!");
    }

    public int sum(int a, int b) {
        return a + b;
    }
}

This task can be called using an expression in short or long form:

flows:
  default:
  - ${myTask.sayHello("world")}         # short form

  - expr: ${myTask.sum(1, 2)}           # full form
    out: mySum
    error:
    - log: "Wham! ${lastError.message}"

If a task implements Task#execute method, it can be started using task step type:

import com.walmartlabs.concord.sdk.Task;
import com.walmartlabs.concord.sdk.Context;
import javax.inject.Named;

@Named("myTask")
public class MyTask implements Task {

    @Override
    public void execute(Context ctx) throws Exception {
        System.out.println("Hello, " + ctx.getVariable("name"));
        ctx.setVariable("success", true);
    }
}
flows:
  default:
  - task: myTask
    in:
      name: world
    out:
      success: callSuccess
    error:
      - log: "Something bad happened: ${lastError}"

This form allows use of in and out variables and error-handling blocks.

The task syntax is recommended for most use cases, especially when dealing with multiple input parameters.

If a task contains method call with one or more arguments, it can be called using the short form:

import com.walmartlabs.concord.common.Task;
import javax.inject.Named;

@Named("myTask")
public class MyTask implements Task {

    public void call(String name, String place) {
        System.out.println("Hello, " + name + ". Welcome to " + place);
    }
}
flows:
  default:
  - myTask: ["user", "Concord"]   # using an inline YAML array

  - myTask:                       # using a regular YAML array
    - "user"
    - "Concord"

Context variables can be automatically injected into task fields or method arguments:

import com.walmartlabs.concord.common.Task;
import com.walmartlabs.concord.common.InjectVariable;
import com.walmartlabs.concord.sdk.Context;
import javax.inject.Named;

@Named("myTask")
public class MyTask implements Task {

    @InjectVariable("context")
    private Context ctx;

    public void sayHello(@InjectVariable("greeting") String greeting, String name) {
        String s = String.format(greeting, name);
        System.out.println(s);

        ctx.setVariable("success", true);
    }
}
flows:
  default:
  - ${myTask.sayHello("Concord")}

configuration:
  arguments:
    greeting: "Hello, %s!"

Using External Artifacts

The runtime provides a way for tasks to download and cache external artifacts:

import com.walmartlabs.concord.sdk.DependencyManager;

@Named("myTask")
public class MyTask implements Task {
    
    private final DependencyManager dependencyManager;
    
    @Inject
    public MyTask(DependencyManager dependencyManager) {
        this.dependencyManager = dependencyManager;
    }
    
    @Override
    public void execute(Context ctx) throws Exception {
        URI uri = ...
        Path p = dependencyManager.resolve(uri);
        // ...do something with the returned path
    }
}

The DependencyManager is an @Inject-able service that takes care of resolving, downloading and caching URLs. It supports all URL types as the regular dependencies section in Concord YAML files - http(s), mvn, etc.

Typically, cached copies are persistent between process executions (depends on the Concord’s environment configuration).

The tasks shouldn’t expect the returning path to be writable (i.e. read-only access).

DependencyManager shouldn’t be used as a way to download deployment artifacts. It’s not a replacement for Ansible or any other deployment tool.

Best Practices

Here are some of the best practices when creating a new plugin with one or multiple tasks.

Environment Defaults

Instead of hard coding parameters like endpoint URLs, credentials and other environment-specific values, use injectable defaults:

@Named("myTask")
public class MyTask implements Task {

    @Override
    public void execute(Context ctx) throws Exception {
        Map<String, Object> defaults = ctx.getVariable("myTaskDefaults");

        String value = (String) ctx.getVariable("myVar");
        if (value == null) {
            // fallback to the default value
            value = (String) defaults.get("myVar");
        }
        System.out.println("Got " + value);
    }
}

The environment-specific defaults are provided using the Default Process Variables file.

The task’s default can also be injected using @InjectVariable annotation - check out the GitHub task as the example.

Full Syntax vs Expressions

There are two ways how the task can be invoked: the task syntax and using expressions. Consider the task syntax for tasks with multiple parameters and expressions for tasks that return data and should be used inline:

# use the `task` syntax when you need to pass multiple parameters and/or complex data structures
- task: myTask
  in:
    param1: 123
    param2: "abc"
    nestedParams:
      x: true
      y: false
      
# use expressions for tasks returning data
- log: "${myTask.getAListOfThings()}"

Task Output and Error Handling

Consider storing the task’s results in a result variable of the following structure:

Successful execution:

result:
  ok: true
  data: "the task's output"  

Failed execution:

result:
  ok: false  
  errorCode: 404
  error: "Not found"  

The ok parameter allows users to quickly test whether the execution was successful or not:

- task: myTask

- if: ${!result.ok}
  then:
    - throw: "Something went wrong: ${result.error}"

By default the task should throw an exception in case of any execution errors or invalid input parameters. Consider adding the ignoreErrors parameter to catch all execution errors, but not the invalid arguments errors. Store the appropriate error message and/or the error code in the result variable:

Throw an exception:

- task: myTask
  in:
    url: "https://httpstat.us/404"

Save the error in the result variable:

- task: myTask
  in:
    url: "https://httpstat.us/404"
    ignoreErrors: true

- log: "${result.errorCode}"

Use the standard JRE classes in the task’s results. Custom types can cause serialization issues when the process suspends, e.g. on a form call. If you need to return some complex data structure, consider converting it to regular Java collections. The runtime provides Jackson as the default JSON/YAML library which can also be used to convert arbitrary data classes into regular Map’s and List’s:

import com.fasterxml.jackson.databind.ObjectMapper;

@Named("myTask")
public class MyTask implements Task {

    @Override
    public void execute(Context ctx) throws Exception {
        MyResult result = new MyResult();
        ObjectMapper om = new ObjectMapper();
        ctx.setVariable("result", om.convertValue(result, Map.class));
    }

    public static class MyResult implements Serializable {
        boolean ok;
        String data;
    }
}

Unit Tests

Consider using unit tests to quickly test the task without publishing SNAPSHOT versions. Use a library like Mockito to replace the dependencies in your task with “mocks”:

@Test
public void test() throws Exception {
    SomeService someService = mock(SomeService.class);

    Map<String, Object> params = new HashMap();
    params.put("url", "https://httpstat.us/404");
    Context ctx = new MockContext(params);

    MyTask t = new MyTask(someService);
    t.execute(ctx);

    assertNotNull(ctx.getVariable("result"));
}

Integration Tests

It is possible to test a task using a running Concord instance without publishing the task’s JAR. Concord automatically adds lib/*.jar files from the payload archive to the process’ classpath. This mechanism can be used to upload local JAR files and, consequently, to test locally-built JARs. Check out the custom_task example. It uses Maven to collect all compile dependencies of the task and creates a payload archive with the dependencies and the task’s JAR.

Note: It is important to use provided scope for the dependencies that are already included in the runtime. See Creating Tasks section for the list of provided dependencies.