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Design: Known Incompatibilities

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Xavier R. Guérin edited this page Feb 19, 2021 · 1 revision

Known Incompatibilities

One of our main design goals is full compatibility with Streams 4.3 applications. We want .sabs from Streams 4.3 applications to be able to run unchanged. However, we have discovered some incompatibilities that are either unavoidable, or supporting them would have hampered the other goals, such as fully native Kubernetes integration and usability.

  1. No JMX. The Consistent Region implementation does not rely on JMX, as that would introduced a communication and management layer outside of Kubernetes itself. Platform communication in general has also moved to more Cloud-native methods. User-applications in Streams 4.3 could use JMX for their own needs. But, we are not aware of any user applications which actually made use of this feature. Since Streams itself no longer needs JMX, it is not a widely used feature, and continuing to support it would impose a significant burden on the new Streams platform, we will not support it.

  2. Host colocation and exlocation tokens. The hostColocation and hostExlocation config options in SPL allow arbitrary rstring values as the coordinating identifier. A typical example would be "sources", but a string with spaces is also valid in Streams 4.3, such as "all of the sources". As part of the Host Constraints implementation, all of the colocation and exlocation identifiers are becoming values in pod labels. However, Kubernetes values in labels are restricted to 63 characters and do not allow spaces. The key in the label already uses the SHA-1 of the identifier. We could have made the value the SHA-1 of the identifier as well, which would have allowed us to fully support the valid identifiers from Streams 4.3. But, that would have removed the usefulness of human-readable labels, and the label itself would no longer has an obvious connection to what was written in the application. In practice, we think most identifiers will be under 63 characters and will not use spaces, so this incompatibility should not impact many user applications.

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