C0urante commented on a change in pull request #11773:
URL: https://github.com/apache/kafka/pull/11773#discussion_r818829273
##########
File path:
connect/api/src/main/java/org/apache/kafka/connect/source/SourceConnector.java
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@@ -28,4 +30,46 @@
protected SourceConnectorContext context() {
return (SourceConnectorContext) context;
}
+
+ /**
+ * Signals whether the connector supports exactly-once delivery guarantees
with a proposed configuration.
+ * Developers can assume that worker-level exactly-once support is enabled
when this method is invoked.
+ *
+ * <p>For backwards compatibility, the default implementation will return
{@code null}, but connector developers are
+ * strongly encouraged to override this method to return a non-null value
such as
+ * {@link ExactlyOnceSupport#SUPPORTED SUPPORTED} or {@link
ExactlyOnceSupport#UNSUPPORTED UNSUPPORTED}.
+ *
+ * <p>Similar to {@link #validate(Map) validate}, this method may be
called by the runtime before the
+ * {@link #start(Map) start} method is invoked when the connector will be
run with exactly-once support.
Review comment:
Hmmmm... I think this is valid and could be useful but there are a few
complications.
If we want to guarantee that the config has been validated first (via
`Connector::validate` and possibly with other checks like making sure that the
value for `transaction.boundary` is valid), should we then only invoke this
method if there are no errors in the connector config?
This might make life easier for connector authors who want to be able to
instantiate an `AbstractConfig` subclass for their connector and then make
decisions about its exactly-once viability based on the already-parsed values
from that config class instead of, like you say, having to duplicate that
validation logic in this method.
On the other hand, it'll prevent issues with exactly-once support from
surfacing the first time around and will require a follow-up validation attempt
to discover any, even if the problems with the connector config don't impact
its ability to provide exactly-once guarantees.
Thinking about just this specific use case (instantiating an
`AbstractConfig` subclass inside this method), I think it'd be best to do two
things:
1. Add a `catch` clause specifically for `ConfigException` instances around
the calls to `Connector::exactlyOnceSupport` and
`Connector::canDefineTransactionBoundaries` that gets translated into a
specific error message stating that the connector config appears to be invalid
and exactly-once support for the connector cannot be determined (instead of the
[existing "An unexpected error occurred..."
message](https://github.com/C0urante/kafka/blob/c687a12af3ec6838140bedf17efae4e9e5c19df1/connect/runtime/src/main/java/org/apache/kafka/connect/runtime/distributed/DistributedHerder.java#L907)).
2. Add a `try`/`catch` block around the [parsing logic for
ConnectorTransactionBoundaries](https://github.com/C0urante/kafka/blob/69d63eda9c96bc76ce29b07c43f0e92c776fa5fb/connect/api/src/main/java/org/apache/kafka/connect/source/SourceTask.java#L67)
that translates an `IllegalArgumentException` into a `ConfigException`, so
that connector authors can freely invoke
`ConnectorTransactionBoundaries::fromProperty` on their connector config inside
`SourceConnector::exactlyOnceSupport` and, if there's an issue, it'll be
translated into a helpful error message to the user. And if the connector
transaction boundary style isn't relevant at all to its exactly-once support,
we won't double-ding the connector config with two error messages related to
the `transaction.boundary` property.
Do you think that this is a reasonable way to handle that specific connector
development style? And are there other development styles that we should aim to
accommodate?
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