Learn about the concept of a Datum in Pachyderm.

December 5, 2022


TLDR: Datums define what input data is seen by your code. It can be all data at once, each directory independently, individual files one by one, or combined data from multiple inputs together.

Definition #

A datum is the smallest indivisible unit of computation within a job. A job can have one, many or no datums. Each datum is processed independently with a single execution of the user code on one of the pipeline worker pods. The files output by all of the datums are then combined together to create the final output commit.

Zero-Datum Jobs #

A “zero-datum” job is a job that is successfully executed but has no matching files to transform with the provided user code.

Data distribution #

Think of datums as a way to divide your input data and distribute processing workloads. They are instrumental in optimizing your pipeline performance.

You define how your data is spread among workers by specifying pipeline inputs for your pipeline in its pipeline specification file.

Based on this specification file, the data in the input of your pipeline is turned into datums, each of which can contain 1-to-many files. Pachyderm provides a wide variety of ways to define the granularity of each datum.

For example, you can configure a whole branch of an input repository to be one datum, each top-level filesystem object of a given branch to be a separate datum, specific paths on a given branch can be datums, etc… You can also create a combination of the above by aggregating multiple input.

Pipeline Inputs #

This section details the tools at your disposal to “break down” your data and fit your specific use case.

PFS Input and Glob Pattern #

The most primitive input of a pipeline is a PFS input, defined, at a minimum, by:

  • a repo containing the data you want your pipeline to consider
  • a branch to watch for commits
  • and a glob pattern to determine how the input data is partitioned.

A pipeline input can have one or multiple PFS inputs. In the latter case, Pachyderm provides a variety of options to aggregate several PFS inputs together.