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5.6. QAD GRID WORKFLOWS 143
Services
Each registered service (see section 5.5.1) can be used in a workflow. During
the service registration process the needed input parameters and the output
format are defined. When a service is included in a workflow for each of these
input parameters a source must be specified. This can be a fixed parameter
(e.g. a numeric value for an algorithm), the result of a database query, such
as the result of a preceding workflow step or a user defined parameter that
will be fixed at execution time (see below).
In addition to these Grid services there exist further system services that
require client/server interaction. For example, services that copy files from a
server to a client.
Programming Language Elements
A QAD Grid workflow can contain so-called programming language elements.
These are constructs known from imperative programming languages. The
QAD Grid system knows the following constructs:
Variables: The QAD Grid workflow system provides two kinds of variables:
(a) workflow variables that are initialized and modified during the workflow
design phase and (b) user defined variables that are initialized by
the user at the time of execution and can be modified during workflow
execution. Usually at least one user defined variable is used within a
workflow that defined the dataset that is used.
Assignment statements: Each variable can be assigned a value. Internally
the value is represented as a string object and converted to numbers if
required during workflow execution.
Evaluation of expressions: Simple expressions (such as basic arithmetics
or relational operators) can be evaluated at runtime. For example a
variable “VAR1” can be assigned a value “VAR2 * 5” that would be
first evaluated to five times the current value of the variable “VAR2”
and then assigned to “VAR1”.
Unconditional branching statements: This is commonly known as jump
or goto. This item has a parameter that contains the target item the
workflow execution process will jump to. Only special items (tags) can
be used as jump targets.
Conditional branching statements: When the workflow execution process
hits this item type it first evaluates the condition parameter which must
evaluate to “false” or “true”. For example a valid condition might be
“VAR1 ≥ 5”. For each outcome (“true or false”) a jump target can be
defined where the process will branch to. If none is given the process
will continue with the next item.
There are also higher conditional branching elements available for the
user, such as readily implemented checks whether all jobs of a particular
kind of this particular workflow are finished or whether a task finished
with an error. This gets very handy for example if during pre-processing
a 2D spectrum is to be split up into its 1D components. Then, this item
can be used to check whether all of this splitting jobs have finished so
the next workflow stage can be started.