Details
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Type:
Story
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Status: Done
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Resolution: Done
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Fix Version/s: None
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Component/s: None
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Labels:
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Story Points:2
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Epic Link:
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Sprint:TSSW Sprint - Apr 11 - Apr 25
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Team:Telescope and Site
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Urgent?:No
Description
It turns out we can easily support schema evolution in Kafka/Avro by specifying a default value for each field in the schema created by the SAL/Kafka producers.
This is a simple change. The main question is whether the default float value should be 0 (like DDS) or NaN. I doubt it makes much difference, because by the time the SAL/Kafka producer sees the SAL data, all fields that are in the current schema have been filled in. Thus this default only affects fields that we remove from the XML. I am inclined to use 0 because it appears NaN support in Avro is somewhat iffy and it matches OpenSplice.
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I agree that nan would be nice in principle, but I am strongly inclined to use 0 because that is what Kafka and DDS both use. I fear it will result in hard-to-diagnose errors if we don't match the standard messaging systems. Using 0 means that fields we no longer write are treated the same as fields that we don't specify.
To test this I ran kafka-aggregator and did the following (this is based on the testing Angelo did in DM-30082, but simplified):
First I the changed the code so that private_efdStamp and private_kafkaStamp had no default value. Then:
- Run a producer for the Test SAL component to register the standard schema.
- Modify this code to remove private_kafkaStamp. The new schema was rejected because private_kafkaStamp had no default value.
- Undo that change and add a garbage field with no default. The new schema was rejected.
Fix the code to specify default=0 for private_efdStamp and private_kafkaStamp (the two fields salkafka adds to each topic). Restart the Kafka servers. Run the following tests:
- Run with standard schema.
- Delete the private_efdStamp field and run. The new schema was accepted.
- Add a garbage field with a default value. The new schema was accepted.
I also tried to read the lsst.sal.Test.logevent_heartbeat topic in all cases, but no data arrived, even with the develop branch, so I was clearly doing something wrong. However, I feel the tests above are fairly convincing.
Pull request: https://github.com/lsst-ts/ts_salkafka/pull/25
Adding a default value for fields in the Avro schema definitely helps with schema evolution. Ideally, the default value should be NaN as proposed in SQR-053, but the DDS version we use does not support the @default annotation. The convention is to use 0 in DDS. Note that using 0 as the default value in the Avro messages will write 0's all the way to InfluxDB and Pandas when the data is retrieved (instead of NaN). I don’t think we can do better at this moment and the advantage I see is that we are using the same convention in both DDS and Kafka.
This is related to RFC-763 which we might still consider once our DDS version support the @default annotation.
Merged to develop. I'll hold off release for now since cycle 25 is frozen.
For the default values, I would recommend using NaN for float and arrays of float for the reasons discussed in https://sqr-053.lsst.io/#recommendations. 0 is not a good representation for a missing value, NaN works better with InfluxDB and Parquet and correctly represents missing values when you return a Pandas data frame.