Edit this page on GitHub

Home > docs > processes v1 > Flows

Flows

Concord flows consist of series of steps executing various actions: calling plugins (also known as “tasks”), performing data validation, creating forms and other steps.

The flows section should contain at least one flow definition:

flows:
  default:
    ...

  anotherFlow:
    ...

Each flow must have a unique name and at least one step.

Steps

Each flow is a list of steps:

flows:
  default:
    - log: "Hello!"

    - if: "${1 > 2}"
      then:
        - log: "How is this possible?"

    - log: "Bye!"

Flows can contain any number of steps and call each other. See below for the description of available steps and syntax constructs.

Expressions

Expressions must be valid Java Expresssion Language EL 3.0 syntax and can be simple evaluations or perform actions by invoking more complex code.

Short form:

flows:
  default:
    # calling a method
    - ${myBean.someMethod()}

    # calling a method with an argument
    - ${myBean.someMethod(myContextArg)}

    # literal values
    - ${1 + 2}

    # EL 3.0 extensions:
    - ${[1, 2, 3].stream().map(x -> x + 1).toList()}

Full form:

flows:
  default:
    - expr: ${myBean.someMethod()}
      out: myVar
      error:
        - ${log.error("something bad happened")}

Full form can optionally contain additional declarations:

  • out field: contains the name of a variable, in which a result of the expression will be stored;
  • error block: to handle any exceptions thrown by the evaluation. Exceptions are wrapped in BpmnError type.

Literal values, for example arguments or form field values, can contain expressions:

flows:
  default:
    - myTask: ["red", "green", "${colors.blue}"]
    - myTask: { nested: { literals: "${myOtherTask.doSomething()}"} }

Classes from the package java.lang can be accessed via EL syntax:

    - log: "Process running on ${System.getProperty('os.name')}"

Conditional Expressions

flows:
  default:
    - if: ${myVar > 0}
      then:                           # (1)
        - log: it's clearly non-zero
      else:                           # (2)
        - log: zero or less

    - ${myBean.acceptValue(myVar)}    # (3)

In this example, after then (1) or else (2) block are completed, the execution continues with the next step in the flow (3).

“And”, “or” and “not” operations are supported as well:

flows:
  default:
    - if: ${true && true}
      then:
      - log: "Right-o"
    - if: ${true || false}
      then:
      - log: "Yep!"
    - if: ${!false}
      then:
      - log: "Correct!"

To compare a value (or the result of an expression) with multiple values, use the switch block:

flows:
  default:
    - switch: ${myVar}
      red:
        - log: "It's red!"
      green:
        - log: "It's definitely green"
      default:
        - log: "I don't know what it is"

    - log: "Moving along..."

In this example, branch labels red and green are the compared values and default is the block which will be executed if no other value fits.

Expressions can be used as branch values:

flows:
  default:
    - switch: ${myVar}
      ${aKnownValue}:
        - log: "Yes, I recognize this"
      default:
        - log: "Nope"

Return Command

The return command can be used to stop the execution of the current (sub) flow:

flows:
  default:
    - if: ${myVar > 0}
      then:
        - log: moving along
      else:
        - return

The return command can be used to stop the current process if called from an entry point.

Exit Command

The exit command can be used to stop the execution of the current process:

flows:
  default:
    - if: ${myVar > 0}
      then:
        - exit
    - log: "message"

Groups of Steps

Several steps can be grouped into one block. This allows try-catch-like semantics:

flows:
  default:
    - log: a step before the group

    - try:
      - log: "a step inside the group"
      - ${myBean.somethingDangerous()}
      error:
        - log: "well, that didn't work"

Calling Other Flows

Flows, defined in the same YAML document, can be called by their names or using the call step:

flows:
  default:
  - log: hello

  # short form: call another flow by its name
  - mySubFlow

  # full form: use `call` step
  - call: anotherFlow
    # (optional) additional call parameters
    in:
      msg: "Hello!"

  - log: bye

  mySubFlow:
  - log: "a message from the sub flow"

  anotherFlow:
  - log: "message from another flow: ${msg}"

Loops

Concord flows can iterate through a collection of items in a loop using the call step and the withItems collection of values:

  - call: myFlow
    withItems:
    - "first element"
    - "second element"
    - 3
    - false

  # withItems can also be used with tasks
  - task: myTask
    in:
      myVar: ${item}
    withItems:
    - "first element"
    - "second element"

The collection of items to iterate over can be provided by an expression:

configuration:
  arguments:
    myItems:
    - 100500
    - false
    - "a string value"

flows:
  default:
  - call: myFlow
    withItems: ${myItems}

The items are referenced in the invoked flow with the ${item} expression:

  myFlow:
  - log: "We got ${item}"

Maps (dicts, in Python terms) can also be used:

flows:
  default:
    - call: log
      in:
        msg: "${item.key} - ${item.value}"
      withItems:
        a: "Hello"
        b: "world"

In the example above withItems iterates over the keys of the object. Each ${item} provides key and value attributes.

Lists of nested objects can be used in loops as well:

flows:
  default:
  - call: deployToClouds
    withItems:
    - name: cloud1
      fqdn: cloud1.myapp.example.com
    - name: cloud2
      fqdn: cloud2.myapp.example.com

  deployToClouds:
  - log: "Starting deployment to ${item.name}"
  - log: "Using fqdn ${item.fqdn}"

Error Handling

The full form syntax allows using input variables (call arguments) and supports error handling.

Task and expression errors are normal Java exceptions, which can be "caught" and handled using a special syntax.

Expressions, tasks, groups of steps and flow calls can have an optional error block, which will be executed if an exception occurs:

flows:
  default:
  # handling errors in an expression
  - expr: ${myTask.somethingDangerous()}
    error:
    - log: "Gotcha! ${lastError}"

  # handling errors in tasks
  - task: myTask
    error:
    - log: "Fail!"

  # handling errors in groups of steps
  - try:
    - ${myTask.doSomethingSafe()}
    - ${myTask.doSomethingDangerous()}
    error:
    - log: "Here we go again"

  # handling errors in flow calls
  - call: myOtherFlow
    error:
    - log: "That failed too"

The ${lastError} variable contains the last caught java.lang.Exception object.

If an error was caught, the execution will continue from the next step:

flows:
  default:
  - try:
      - throw: "Catch that!"
    error:
    - log: "A"

  - log: "B"

An execution logs A and then B.

When a process is cancelled (killed) by a user, a special flow onCancel is executed:

flows:
  default:
  - log: "Doing some work..."
  - ${sleep.ms(60000)}

  onCancel:
  - log: "Pack your bags, boys. Show's cancelled"

Similarly, onFailure flow is executed if a process crashes:

flows:
  default:
  - log: "Brace yourselves, we're going to crash!"
  - throw: "Crash!"

  onFailure:
  - log: "Yep, we just did"

In both cases, the server starts a child process with a copy of the original process state and uses onCancel or onFailure as an entry point.

Note: onCancel and onFailure handlers receive the last known state of the parent process’ variables. This means that changes in the process state are visible to the child processes:

flows:
  default:
  # let's change something in the process state...
  - set:
      myVar: "xyz"

  # will print "The default flow got xyz"
  - log: "The default flow got ${myVar}"

  # ...and then crash the process
  - throw: "Boom!"

  onFailure:
  # will log "I've got xyz"
  - log: "I've got ${myVar}"

configuration:
  arguments:
    # original value
    myVar: "abc"

In addition, onFailure flow receives lastError variable which contains the parent process’ last (unhandled) error:

flows:
  default:
  - throw: "Kablam!"
        
  onFailure:
  - log: "${lastError.cause}"

Nested data is also supported:

flows:
  default:
  - throw:
      myCause: "I wanted to"
      whoToBlame:
        mainCulpit: "${currentUser.username}"
        
  onFailure:
  - log: "The parent process failed because ${lastError.cause.payload.myCause}."
  - log: "And ${lastError.cause.payload.whoToBlame.mainCulpit} is responsible for it!"

If an onCancel or onFailure flow fails, it is automatically retried up to three times.

Retry

The retry attribute is used to restart the task/flow automatically in case of errors or failures. Users can define the number of times the task/flow can be re-tried and a delay for each retry.

  • delay - the time span after which it retries. The delay time is always in seconds, default value is 5;
  • in - additional parameters for the retry
  • times - the number of times a task/flow can be retried;

For example the below section executes the myTask using the provided in parameters. In case of errors, the task retries up to 3 times with 3 seconds delay each. Additional parameters for the retry are supplied in the in block.

- task: myTask
  in:
    ...
  retry:
    in:
      ...additional parameters...
    times: 3
    delay: 3

Retry flow call:

- call: myFlow
  in:
    ...
  retry:
    in:
      ...additional parameters...
    times: 3
    delay: 3

The default in and retry variables with the same values are overwritten.

In the example below the value of someVar is overwritten to 321 in the retry block..

- task: myTask
  in:
    someVar:
      nestedValue: 123
  retry:
    in:
      someVar:
        nestedValue: 321
        newValue: "hello"

The retry block also supports expressions:

configuration:
  arguments:
    retryTimes: 3
    retryDelay: 2

flows:
  default:
    - task: myTask
      retry:
        times: "${retryTimes}"
        delay: "${retryDelay}"

Throwing Errors

The throw step can be used to throw a new RuntimeException wit hthe supplied message anywhere in a flow including in error sections and in conditional expressions such as if-then or switch-case.

flows:
  default:
  - try:
    - log: "Do something dangerous here"
    error:
    - throw: "oh, something went wrong."

Alternatively a caught exception can be thrown again using the lastError variable:

flows:
  default:
  - try:
    - log: "Do something dangerous here"
    error:
    - throw: ${lastError}

Setting Variables

The set step can be used to set variables in the current process context:

flows:
  default:
  - set:
      a: "a-value"
      b: 3
  - log: ${a}
  - log: ${b}

Nested data can be updated using the . syntax:

configuration:
  arguments:
    myComplexData:
      nestedValue: "Hello"

flows:
  default:
  - set:
      myComplexData.nestedValue: "Bye"

  # will print "Bye, Concord"
  - log: "${myComplexData.nestedValue}, Concord"

A number of variables are automatically set in each process and available for usage.

Checkpoints

A checkpoint is a point defined within a flow at which the process state is persisted in Concord. This process state can subsequently be restored and process execution can continue. A flow can contain multiple checkpoints.

The REST API can be used for listing and restoring checkpoints. Alternatively you can restore a checkpoint to continue processing directly from the Concord Console.

The checkpoint step can be used to create a named checkpoint:

flows:
  default:
  - log: "Starting the process..."
  - checkpoint: "first"
  - log: "Continuing the process..."
  - checkpoint: "second"
  - log: "Done!"

The example above creates two checkpoints: first and second. These checkpoints can be used to restart the process from the point after the checkpoint’s step. For example, if the process is restored using first checkpoint, all steps starting from Continuing the process... message and further are executed.

Checkpoint names can contain expressions:

configuration:
  arguments:
    checkpointSuffix: "checkpoint"

flows:
  default:
  - log: "Before the checkpoint"
  - checkpoint: "first_${checkpointSuffix}"
  - log: "After the checkpoint"

Checkpoint names must start with a (latin) letter or a digit, can contain whitespace, underscores _, @, dots ., minus signs - and tildes ~. The length must be between 2 and 128 characters. Here’s the regular expression used for validation:

^[0-9a-zA-Z][[email protected]\\-~ ]{1,128}$

Only process initiators, administrators and users with WRITER access level to the process’ project can restore checkpoints with the API or the user console.

After restoring a checkpoint, its name can be accessed using the resumeEventName variable.

Note: files created during the process’ execution are not saved during the checkpoint creation.