# Building Pipelines

### Dependencies

A Disdat PipeTask consists of two functions: `pipe_requires` and `pipe_run`. If you know Luigi, these are analagous to `requires` and `run`. Let's first describe what they do, and then we can describe how they are different from those analogs.

* `pipe_requires`: Here you declare the tasks that must run before this task. To do so you write statements like: `self.add_dependency("input_gzs", GetDataGz, {})`. This says that the current task needs a `GetDataGz` instance to run with no parameters. It also says that Disdat should setup the output of that task as a named parameter to `pipe_run` called `'input_gzs'`.

  Users may optionally name the output bundle in this function with `set_bundle_name(<your name>)`.
* `pipe_run`: Here you perform the task's main work. This function gets a set of named parameters.
  * `pipeline_input`:  This contains the *input* bundle data.
  * :  Disdat prepares named parameters for each upstream dependency.  The variable will be they same type as the upstream task returned.