1   Note

The current AstroGrid workflow system has a certain number of limitations which compromise use. A re-engineered system is currently being completed which will be released in 2007 Q3. Thus users may prefer to wait for this future release.

2   Introduction

A workflow aims to accomplish a complex piece of work. In Astrogrid terms this would be an astronomical investigation. If the work could be done in one simple step (eg: by executing a single program from the command line), then it would hardly be a workflow. A prime dimension of a workflow is therefore that the objective will take more than one step to accomplish - it has a degree of complexity.

A simple example of a workflow might consist of the following steps:

1. Make a query against against one catalog (say a cone search of one area of the sky).
2. Make a similar query against another catalog.
3. Merge the contents.
4. Analyse the resulting data.

Each step is usually the invocation of a separate computer program (a tool), each tool requiring its own inputs and outputs. Some inputs to a step will be astronomical data, and some inputs will be control information for the selected tool. Outputs will normally be processed data destined as a final result, or as intermediate data to be used as inputs for subsequent steps within the workflow. For Astrogrid, the final result of executing a workflow is usually a file held in MySpace. From that viewpoint, Astrogrid workflow is itself an intermediate tool, since we would envisage the final results file being subsequently loaded into another tool, for example, for visualization, or even for input into a different workflow.

3   Workflow Builder

It is important to note that workflows are a set of tasks executed sequentially or in parallel. You can go to the Task Launcher and execute each task individually, but then you have to wait for one task to finish to launch the other (that is if the inputs of one depends on the output of another). Workflows provide a convenient solution when one wants to execute several tasks. Workflows are built using the Workflow Builder.

The image above shows a screenshot of the workflow builder. The left panel...

Step

this is the element which executes a task. After inserting a Step you will be asked for the task the execute in the same was as in the Task Launcher.

Flow

dunno, ask Phil

Sequence

ditto

Script

A piece of programming, at the moment in Groovy.

If

Execute block only if argument is true

Else

Execute block if argument is false

Scope

Defines a block in which variables defined are only valid within and not accessible outside

Set

Sets the value of a variable

Unset

Unsets the value of a variable

For Loop

Includes a set of steps executed in a loop

While Loop

A loop executed while the argument is true

Parallel Loop

As the for loop except that the block is executed at the same time.

The figure bellow shows a built workflow that takes as input a VOTable and searches several cone search services to get lists of objects around each object in the input table and then cross match them.