PIPELINE
Shuffled
Partition 0
{type: impression, key: 186bd591-9442-48f0, publisher: foo, …}
{type: impression, key: 1079026c-7151-4871, publisher: baz, …}
{type: click, key: 186bd591-9442-48f0}
…
Partition 1
{type: impression, key: 9b5e2cd2-a8ac-4232, publisher: qux, …}
…