kfaraz commented on pull request #11848:
URL: https://github.com/apache/druid/pull/11848#issuecomment-960545965
> 1. If someone updates to the new version, but doesn't use the new
feature (i.e. they keep submitting ingest specs with `single_dim` shard specs),
will that lead to segments getting written with `single_dim` or `multi_dim`
shard specs? Hopefully it's `single_dim`, since that means people can safely
roll back without losing the ability to load their existing segments.
Thanks for pointing this out, @gianm . I had originally intended to add this
logic to ensure backward compatibility but I had missed it.
I have added it now by making `SingleDimensionShardSpec` extend
`MultiDimensionShardSpec`.
Thus, while converting a `BuildingMultiDimensionShardSpec`, a
`SingleDimensionShardSpec` is created if there is only one dimension, otherwise
a `MultiDimensionShardSpec` is created.
```java
@Override
public MultiDimensionShardSpec convert(int numCorePartitions)
{
return dimensions != null && dimensions.size() == 1
? new SingleDimensionShardSpec(...)
: new MultiDimensionShardSpec(...);
}
```
The only drawback is that if someone tried to partition their data using
type `multi_dim` with a single dimension specified in the `partitionDimensions`
field, the shard specs created would still have type `single_dim`. But I guess
that is to be expected.
> 2. If someone is in the middle of a rolling update to the new version,
and has some servers updated but not others, will that be safe? Or will some
servers write out `multi_dim` shard specs or other `multi_dim` things that
other servers on the older version cannot read?
The above mentioned approach will take care of this as well, because servers
will __output `multi_dim` shard specs__ only if a user explicitly chooses
`multi_dim` partitioning __with more than one dimensions__.
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