Execute workflow steps in parallel

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Parallel steps can reduce the total execution time for a workflow by performing multiple blocking calls at the same time.

Blocking calls such as sleep, HTTP calls, and callbacks can take time, from milliseconds to days. Parallel steps are intended to assist with such concurrent long-running operations. If a workflow must perform multiple blocking calls that are independent of each other, using parallel branches can reduce the total execution time by starting the calls at the same time, and waiting for all of them to complete.

For example, if your workflow must retrieve customer data from several independent systems before continuing, parallel branches allow for concurrent API requests. If there are five systems and each takes two seconds to respond, performing the steps sequentially in a workflow could take at least 10 seconds; performing them in parallel could take as little as two.

Create a parallel step

Create a parallel step to define a part of your workflow where two or more steps can execute concurrently.

YAML

  - PARALLEL_STEP_NAME:
      parallel:
          exception_policy: POLICY
          shared: [VARIABLE_A, VARIABLE_B, ...]
          BRANCHES_OR_FOR:
             ...

JSON

  [
    {
      "PARALLEL_STEP_NAME": {
        "parallel": {
          "exception_policy": "POLICY",
          "shared": [
            "VARIABLE_A",
            "VARIABLE_B",
            ...
          ],
          "BRANCHES_OR_FOR":
          ...
        }
      }
    }
  ]

Replace the following:

  • PARALLEL_STEP_NAME: the name of the parallel step.
  • POLICY (optional): determines the action other branches will take when an unhandled exception occurs. The default policy, continueAll, results in no further action, and all other branches will attempt to run. Note that continueAll is the only policy currently supported.
  • VARIABLE_A, VARIABLE_B, and so on: a list of writable variables with parent scope that allow assignments within the parallel step. For more information, see Shared variables.
  • BRANCHES_OR_FOR: use either branches or for to indicate one of the following:
    • Branches that can run concurrently.
    • A loop where iterations can run concurrently.

Note the following:

  • Parallel branches and iterations can run in any order, and might run in a different order with each execution.
  • Parallel steps can include other, nested parallel steps up to the depth limit. See Quotas and limits.
  • For more details, see the syntax reference page for parallel steps.

Replace experimental function with parallel step

If you are using experimental.executions.map to support parallel work, you can migrate your workflow to use parallel steps instead, executing ordinary for loops in parallel. For examples, see Replace experimental function with parallel step.

Samples

These samples demonstrate the syntax.

Perform operations in parallel (using branches)

If your workflow has multiple and different sets of steps that can be executed at the same time, placing them in parallel branches can decrease the total time needed to complete those steps.

In the following example, a user ID is passed as an argument to the workflow and data is retrieved in parallel from two different services. Shared variables allow values to be written to in the branches, and read after the branches complete:

YAML

main:
  params: [input]
  steps:
    - init:
        assign:
          - userProfile: {}
          - recentItems: []
    - enrichUserData:
        parallel:
          shared: [userProfile, recentItems]  # userProfile and recentItems are shared to make them writable in the branches
          branches:
            - getUserProfileBranch:
                steps:
                  - getUserProfile:
                      call: http.get
                      args:
                        url: '${"https://example.com/users/" + input.userId}'
                      result: userProfile
            - getRecentItemsBranch:
                steps:
                  - getRecentItems:
                      try:
                        call: http.get
                        args:
                          url: '${"https://example.com/items?userId=" + input.userId}'
                        result: recentItems
                      except:
                        as: e
                        steps:
                          - ignoreError:
                              assign:  # continue with an empty list if this call fails
                                - recentItems: []

JSON

{
  "main": {
    "params": [
      "input"
    ],
    "steps": [
      {
        "init": {
          "assign": [
            {
              "userProfile": {}
            },
            {
              "recentItems": []
            }
          ]
        }
      },
      {
        "enrichUserData": {
          "parallel": {
            "shared": [
              "userProfile",
              "recentItems"
            ],
            "branches": [
              {
                "getUserProfileBranch": {
                  "steps": [
                    {
                      "getUserProfile": {
                        "call": "http.get",
                        "args": {
                          "url": "${\"https://example.com/users/\" + input.userId}"
                        },
                        "result": "userProfile"
                      }
                    }
                  ]
                }
              },
              {
                "getRecentItemsBranch": {
                  "steps": [
                    {
                      "getRecentItems": {
                        "try": {
                          "call": "http.get",
                          "args": {
                            "url": "${\"https://example.com/items?userId=\" + input.userId}"
                          },
                          "result": "recentItems"
                        },
                        "except": {
                          "as": "e",
                          "steps": [
                            {
                              "ignoreError": {
                                "assign": [
                                  {
                                    "recentItems": []
                                  }
                                ]
                              }
                            }
                          ]
                        }
                      }
                    }
                  ]
                }
              }
            ]
          }
        }
      }
    ]
  }
}

Process items in parallel (using a parallel loop)

If you need to perform the same action for each item in a list, you can complete the execution more quickly by using a parallel loop. A parallel loop allows multiple loop iterations to be performed in parallel. Note that, unlike regular for loops, iterations can be performed in any order.

In the following example, a set of user notifications are processed in a parallel for loop:

YAML

main:
  params: [input]
  steps:
    - sendNotifications:
        parallel:
          for:
            value: notification
            in: ${input.notifications}
            steps:
              - notify:
                  call: http.post
                  args:
                    url: https://example.com/sendNotification
                    body:
                      notification: ${notification}

JSON

{
  "main": {
    "params": [
      "input"
    ],
    "steps": [
      {
        "sendNotifications": {
          "parallel": {
            "for": {
              "value": "notification",
              "in": "${input.notifications}",
              "steps": [
                {
                  "notify": {
                    "call": "http.post",
                    "args": {
                      "url": "https://example.com/sendNotification",
                      "body": {
                        "notification": "${notification}"
                      }
                    }
                  }
                }
              ]
            }
          }
        }
      }
    ]
  }
}

Aggregate data (using a parallel loop)

You can process a set of items while collecting data from the operations performed on each item. For example, you might want to track the IDs of created items, or maintain a list of items with errors.

In the following example, 10 separate queries to a public BigQuery dataset each return the number of words in a document, or set of documents. A shared variable allows the count of the words to accumulate and be read after all the iterations complete. After calculating the number of words across all the documents, the workflow returns the total.

YAML

main:
  params: [input]
  steps:
    - init:
        assign:
          - numWords: 0
          - corpuses:
              - sonnets
              - various
              - 1kinghenryvi
              - 2kinghenryvi
              - 3kinghenryvi
              - comedyoferrors
              - kingrichardiii
              - titusandronicus
              - tamingoftheshrew
              - loveslabourslost
    - runQueries:
        parallel:  # numWords is shared so it can be written within the parallel loop
          shared: [numWords]
          for:
            value: corpus
            in: ${corpuses}
            steps:
              - runQuery:
                  call: googleapis.bigquery.v2.jobs.query
                  args:
                    projectId: ${sys.get_env("GOOGLE_CLOUD_PROJECT_ID")}
                    body:
                      useLegacySql: false
                      query: ${
                        "SELECT COUNT(DISTINCT word) FROM `bigquery-public-data.samples.shakespeare` " +
                        " WHERE corpus='" + corpus + "' "
                        }
                  result: query
              - add:
                  assign:
                    - numWords: ${numWords + int(query.rows[0].f[0].v)}  # first result is the count
    - done:
        return: ${numWords}

JSON

{
  "main": {
    "params": [
      "input"
    ],
    "steps": [
      {
        "init": {
          "assign": [
            {
              "numWords": 0
            },
            {
              "corpuses": [
                "sonnets",
                "various",
                "1kinghenryvi",
                "2kinghenryvi",
                "3kinghenryvi",
                "comedyoferrors",
                "kingrichardiii",
                "titusandronicus",
                "tamingoftheshrew",
                "loveslabourslost"
              ]
            }
          ]
        }
      },
      {
        "runQueries": {
          "parallel": {
            "shared": [
              "numWords"
            ],
            "for": {
              "value": "corpus",
              "in": "${corpuses}",
              "steps": [
                {
                  "runQuery": {
                    "call": "googleapis.bigquery.v2.jobs.query",
                    "args": {
                      "projectId": "${sys.get_env(\"GOOGLE_CLOUD_PROJECT_ID\")}",
                      "body": {
                        "useLegacySql": false,
                        "query": "${\n\"SELECT COUNT(DISTINCT word) FROM `bigquery-public-data.samples.shakespeare` \" +\n\" WHERE corpus='\" + corpus + \"' \"\n}"
                      }
                    },
                    "result": "query"
                  }
                },
                {
                  "add": {
                    "assign": [
                      {
                        "numWords": "${numWords + int(query.rows[0].f[0].v)}"
                      }
                    ]
                  }
                }
              ]
            }
          }
        }
      },
      {
        "done": {
          "return": "${numWords}"
        }
      }
    ]
  }
}

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