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https://issues.apache.org/jira/browse/KAFKA-19758?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18028298#comment-18028298
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Mario Fiore Vitale edited comment on KAFKA-19758 at 10/8/25 7:58 AM:
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?? I don't think communication via static fields is good practice, and should
be avoided if possible. I personally have been on the receiving end of bugs
caused by static fields within a single plugin; I also think I personally would
not like to debug static fields shared across plugins.
I agree that this is not the best approach, but the problem here is to
understand what is possible and what is not.
AFAIK, until KC 4.0, this was possible due to plugin isolation, as the same
classloader was used to load the connector and its dependencies.
Even I think that plugin versioning is compatible also with the isolation (and
this seems to be the goal of
[KIP-891|[https://cwiki.apache.org/confluence/display/KAFKA/KIP-891%3A+Running+multiple+versions+of+Connector+plugins])]),
if this is something that is not true anymore, I would have expected it to be
changed in a major release and not in a minor one.
??In deployments which make use of the plugin.version configs, changing the
pinned version will necessarily change which instance of a static variable is
used. So even if the isolation is bug-free, it is trivial for a user or
automated tooling to foot-gun a plugin which uses static fields by changing the
plugin.version??
Can you better clarify this point?
??IMHO we should investigate the backwards-compatibility problem and try and
make a minimal fix to preserve the old behavior when plugin.version is unset??
Another thing to say here is that the
`DelegatingClassLoader.findPluginLoader()` is not only used for transformations
and predicates but also for loading the connectors themselves. So to me, this
also seems to break the versioning of the connectors.
??And the affected plugins should eliminate their usage of static variables in
order to be well behaved when the plugin.version is set??
Honestly, I'm not getting the point here. I expect that even with a correct
management of `plugin.version`, the isolation must be preserved. What happens
in the following situation where `plugin.path=opt/plugins/`:
{noformat}
opt/
plugins/
blue/
foo-connector-1.8/
foo-connector-1.8.jar
foo-dependencies-1.0.jar
foo-connector-1.9/
foo-connector-1.9.jar
foo-dependencies-1.1.jar
{noformat}
And I register a connector `foo-connector` with version 1.8 and SMT (suppose it
is in the foo-dependencies) with version 1.1? Is this possible? In the same
way, this breaks the isolation. Isn't it?
was (Author: JIRAUSER303467):
?? I don't think communication via static fields is good practice, and should
be avoided if possible. I personally have been on the receiving end of bugs
caused by static fields within a single plugin; I also think I personally would
not like to debug static fields shared across plugins.
I agree that this is not the best approach, but the problem here is to
understand what is possible and what is not.
AFAIK, until KC 4.0, this was possible due to plugin isolation, as the same
classloader was used to load the connector and its dependencies.
Even I think that plugin versioning is compatible also with the isolation (and
this seems to be the goal of
[KIP-891|[https://cwiki.apache.org/confluence/display/KAFKA/KIP-891%3A+Running+multiple+versions+of+Connector+plugins]),]]),
if this is something that is not true anymore, I would have expected it to be
changed in a major release and not in a minor one.
??In deployments which make use of the plugin.version configs, changing the
pinned version will necessarily change which instance of a static variable is
used. So even if the isolation is bug-free, it is trivial for a user or
automated tooling to foot-gun a plugin which uses static fields by changing the
plugin.version??
Can you better clarify this point?
??IMHO we should investigate the backwards-compatibility problem and try and
make a minimal fix to preserve the old behavior when plugin.version is unset??
Another thing to say here is that the
`DelegatingClassLoader.findPluginLoader()` is not only used for transformations
and predicates but also for loading the connectors themselves. So to me, this
also seems to break the versioning of the connectors.
??And the affected plugins should eliminate their usage of static variables in
order to be well behaved when the plugin.version is set??
Honestly, I'm not getting the point here. I expect that even with a correct
management of `plugin.version`, the isolation must be preserved. What happens
in the following situation where `plugin.path=opt/plugins/`:
{noformat}
opt/
plugins/
blue/
foo-connector-1.8/
foo-connector-1.8.jar
foo-dependencies-1.0.jar
foo-connector-1.9/
foo-connector-1.9.jar
foo-dependencies-1.1.jar
{noformat}
And I register a connector `foo-connector` with version 1.8 and SMT (suppose it
is in the foo-dependencies) with version 1.1? Is this possible? In the same
way, this breaks the isolation. Isn't it?
> Weird behavior on Kafka Connect 4.1 class loading
> -------------------------------------------------
>
> Key: KAFKA-19758
> URL: https://issues.apache.org/jira/browse/KAFKA-19758
> Project: Kafka
> Issue Type: Bug
> Components: connect
> Affects Versions: 4.1.0
> Reporter: Mario Fiore Vitale
> Assignee: Mickael Maison
> Priority: Blocker
> Attachments: connect-service.log
>
>
> I have the
> [DebeziumOpenLineageEmitter|https://github.com/debezium/debezium/blob/main/debezium-openlineage/debezium-openlineage-api/src/main/java/io/debezium/openlineage/DebeziumOpenLineageEmitter.java]
> class in the *debezium-openlineage-api* that internally has a static map to
> maintain the registered emitter, the key of this map is
> "connectoLogicalName-taskid"
> Then there is the [OpenLineage
> SMT|https://github.com/debezium/debezium/blob/main/debezium-core/src/main/java/io/debezium/transforms/openlineage/OpenLineage.java],
> which is part of the *debezium-core.* In this SMT, I simply pass the same
> context to instantiate the same emitter via the connector.
> Now I'm running the following image
> {code:java}
> FROM quay.io/debezium/connect:3.3.0.Final
> ENV MAVEN_REPO="https://repo1.maven.org/maven2";
> ENV GROUP_ID="io/debezium"
> ENV DEBEZIUM_VERSION="3.3.0.Final"
> ENV ARTIFACT_ID="debezium-openlineage-core"
> ENV CLASSIFIER="-libs"
> COPY log4j.properties /kafka/config/log4j.properties
> Add OpenLineage
> RUN mkdir -p /tmp/openlineage-libs && \
> curl
> "$MAVEN_REPO/$GROUP_ID/$ARTIFACT_ID/$DEBEZIUM_VERSION/$ARTIFACT_ID-${DEBEZIUM_VERSION}${CLASSIFIER}.tar.gz"
> -o /tmp/debezium-openlineage-core-libs.tar.gz && \
> tar -xzvf /tmp/debezium-openlineage-core-libs.tar.gz -C
> /tmp/openlineage-libs --strip-components=1
> RUN cp -r /tmp/openlineage-libs/* /kafka/connect/debezium-connector-postgres/
> RUN cp -r /tmp/openlineage-libs/* /kafka/connect/debezium-connector-mongodb/
> ADD openlineage.yml /kafka/ {code}
> So is practically debezium connect image with just openlineage jars copied
> into postgres and mongodb connector folders.
> When I register the PostgreSQL connector
> {code:java}
> {
> "name": "inventory-connector-postgres",
> "config": {
> "connector.class": "io.debezium.connector.postgresql.PostgresConnector",
> "tasks.max": "1",
> "database.hostname": "postgres",
> "database.port": "5432",
> "database.user": "postgres",
> "database.password": "postgres",
> "database.server.id": "184054",
> "database.dbname": "postgres",
> "topic.prefix": "inventory",
> "snapshot.mode": "initial",
> "schema.history.internal.kafka.bootstrap.servers": "kafka:9092",
> "schema.history.internal.kafka.topic": "schema-changes.inventory",
> "slot.name": "postgres",
> "openlineage.integration.enabled": "true",
> "openlineage.integration.config.file.path": "/kafka/openlineage.yml",
> "openlineage.integration.job.description": "This connector does cdc for
> products",
> "openlineage.integration.tags": "env=prod,team=cdc",
> "openlineage.integration.owners": "Mario=maintainer,John Doe=Data
> scientist,IronMan=superero",
> "transforms": "openlineage",
> "transforms.openlineage.type":
> "io.debezium.transforms.openlineage.OpenLineage"
> }
> } {code}
>
> I get the following error
> {code:java}
> 2025-10-03T14:22:09,761 ERROR ||
> WorkerSourceTask{id=inventory-connector-postgres-0} Task threw an uncaught
> and unrecoverable exception. Task is being killed and will not recover until
> manually restarted [org.apache.kafka.connect.runtime.WorkerTask]
> org.apache.kafka.connect.errors.ConnectException: Tolerance exceeded in error
> handler
> at
> org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execAndHandleError(RetryWithToleranceOperator.java:260)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execute(RetryWithToleranceOperator.java:180)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.TransformationChain.apply(TransformationChain.java:58)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.AbstractWorkerSourceTask.sendRecords(AbstractWorkerSourceTask.java:415)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.AbstractWorkerSourceTask.execute(AbstractWorkerSourceTask.java:376)
> ~[connect-runtime-4.1.0.jar:?]
> at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:243)
> ~[connect-runtime-4.1.0.jar:?]
> at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:298)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.AbstractWorkerSourceTask.run(AbstractWorkerSourceTask.java:83)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.isolation.Plugins.lambda$withClassLoader$1(Plugins.java:254)
> ~[connect-runtime-4.1.0.jar:?]
> at
> java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:572)
> ~[?:?]
> at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
> ~[?:?]
> at
> java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
> ~[?:?]
> at
> java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
> ~[?:?]
> at java.base/java.lang.Thread.run(Thread.java:1583) [?:?]
> Caused by: java.lang.IllegalStateException: DebeziumOpenLineageEmitter not
> initialized for connector ConnectorContext[connectorLogicalName=inventory,
> connectorName=postgresql, taskId=0, version=null, config=null]. Call init()
> first.
> at
> io.debezium.openlineage.DebeziumOpenLineageEmitter.getEmitter(DebeziumOpenLineageEmitter.java:176)
> ~[debezium-openlineage-api-3.3.0.Final.jar:3.3.0.Final]
> at
> io.debezium.openlineage.DebeziumOpenLineageEmitter.emit(DebeziumOpenLineageEmitter.java:153)
> ~[debezium-openlineage-api-3.3.0.Final.jar:3.3.0.Final]
> at
> io.debezium.transforms.openlineage.OpenLineage.apply(OpenLineage.java:74)
> ~[debezium-core-3.3.0.Final.jar:3.3.0.Final]
> at
> org.apache.kafka.connect.runtime.TransformationStage.apply(TransformationStage.java:95)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.TransformationChain.lambda$apply$0(TransformationChain.java:58)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execAndRetry(RetryWithToleranceOperator.java:208)
> ~[connect-runtime-4.1.0.jar:?]
> at
> org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execAndHandleError(RetryWithToleranceOperator.java:244)
> ~[connect-runtime-4.1.0.jar:?]
> ... 13 more {code}
> Full logs [^connect-service.log]
>
> This is evidence that the emitters map is not shared between the connector
> and the SMT.
> The situation becomes weirder if I remove all connectors from the image
> except PostgreSQL and MongoDB.
> In that case, the PostgreSQL connector works perfectly.
> The plugins are in the folder */kafka/connect* (that is, the only
> `plugin.path` configured folder), each under a dedicated folder with their
> dependencies.
> I then started to add more connectors, and it continued to work until I added
> the SQL Server connector.
> To summarize, the problem arises when I put one or all of [sqlserver,
> spanner,vitess].
>
> The commonality for these connectors seems to be that they support
> multi-task. The others don't.
> Am I correct that Kafka Connect guarantees that each connector is loaded with
> an isolated class loader with its dependencies so that the static emitters
> should be shared between the Connector and the SMT?
> To add more, if I run the image from 3.2.0.Final (so Kafka 4.0.0) with all
> connectors, it works fine.
> I did other tests, and things are more and more weird. All tests were done
> with *{{plugin.path=/kafka/connect}}* and *KC 4.1*
> My original tests were with this directory structure
>
> {code:java}
> /kafka/connect
> |___ debezium-connector-postgres
> |___ debezium-connector-mongodb
> |___ debezium-connector-sqlserver{code}
>
> In this case, each connector should be isolated from each others (having a
> dedicated class loader). In that case, the sharing between the connector and
> SMT does not work for KC 4.0
> Then I tried with
>
> {code:java}
> /kafka/connect
> |___ debezium-connectors
> |___ debezium-connector-postgres
> |___ debezium-connector-mongodb
> |___ debezium-connector-sqlserver{code}
>
> So all connectors are not isolated and share the same class loader. In this
> case, no issue. And I'll say that this is expected.
> Then I tried with
>
> {code:java}
> /kafka/connect
> |___ debezium-connectors
> | |___ debezium-connector-postgres
> | |___ debezium-connector-mongodb
> |___ debezium-connector-sqlserver{code}
>
> where *{{postgres}}* and *{{mongodb}}* are not isolated (same classloader)
> and *{{sqlserver}}* is isolated (different classloader), and in this case, it
> still works. I expected this to fail as with the first setup.
> The SMT is in the *debezium-core* jar that and each connector has its own copy
> So in each connector folder, there are:
> {code:java}
> debezium-api-3.3.0.Final.jar
> debezium-common-3.3.0.Final.jar
> debezium-connector-[connectorName]-3.3.0.Final.jar
> debezium-core-3.3.0.Final.jar
> debezium-openlineage-api-3.3.0.Final.jar{code}
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