[StevePerkins]

I don't have experience with Samza or Apex, but as for the first three:

1. Flink - Focused on stateful stream processing.

2. Spark - Focused on batch processing. Can be used for continuous streams, but approaches them as "micro-batches".

3. Kafka - A message queue system (for all practical purposes). Has an optional stream processing add-on for basic needs.

Separate use cases and strengths aside, it's worth calling out that all of these products are primarily backed by completely different companies. Apache is a consortium made of many companies, and serves as common branding for "community editions" of their "enterprise edition" products. There can quite a lot of overlap between sponsored products in this consortium.

[lern_too_spel]

Spark supports both microbatch and continuous stream processing.

Apache Software Foundation is not a consortium made of many companies but a single non-profit that provides organizational support for open source projects, some of which have contributors employed as such by other companies and some of which have only volunteer contributors.

[nivertech]

4. Apache BEAM (same model as Google Cloud Dataflow)