How can I do looping in a workflow?

Taverna does not support looping in the sense that there are no cycles within a workflow graph. Taverna in its present form supports iteration over a set of independent jobs (and combinators over this iteration so allowing 'all against all' and similar strategies). Taverna's capabilities correspond to a 'for' loop with no side effects in an imperative language such as Java. It is possible to implement side effecting loops through external stores and nested workflows but this is not recommended - if you really need this then it's possible. Recursive invocation similarly can be hacked together but it's not 'clean'

Taverna 2 (under development) supports a much richer set of recursive and iterative invocation semantics, so parameter sweeps become possible (as an example).