The Kafka table engine allows global configuration and per-Kafka-topic
configuration. The latter uses syntax <kafka_TOPIC>, e.g. for topic
"football":
<kafka_football>
<retry_backoff_ms>250</retry_backoff_ms>
<fetch_min_bytes>100000</fetch_min_bytes>
</kafka_football>
Some users had to find out the hard way that such configuration doesn't
take effect if the topic name contains a period, e.g. "sports.football".
The reason is that ClickHouse configuration framework already uses
periods as level separators to descend the configuration hierarchy.
(Besides that, per-topic configuration at the same level as global
configuration could be considered ugly.)
Note that Kafka topics may contain characters "a-zA-Z0-9._-" (*) and
a tree-like topic organization using periods is quite common in
practice.
This PR deprecates the existing per-topic configuration syntax (but
continues to support it for backward compat) and introduces a new
per-topic configuration syntax below the global Kafka configuration of
the form:
<kafka>
<topic name="football">
<retry_backoff_ms>250</retry_backoff_ms>
<fetch_min_bytes>100000</fetch_min_bytes>
</topic>
</kafka>
The period restriction doesn't apply to XML attributes, so <topic
name="sports.football"> will work. Also, everything Kafka-related is
below <kafka>.
Considered but rejected alternatives:
- Extending Poco ConfigurationView with custom separators (e.g."/"
instead of "."). Won't work easily because ConfigurationView only
builds a path but defers descending the configuration tree to the
normal configuration classes.
- Reloading the configuration file in StorageKafka (instead of reading
the loaded file) but with a custom separator. This mode is supported
by XML configuration. Too ugly and error-prone since the true
configuration is composed from multiple configuration files.
(*) https://stackoverflow.com/a/37067544
