kowshik commented on a change in pull request #9001:
URL: https://github.com/apache/kafka/pull/9001#discussion_r463915553
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File path: core/src/main/scala/kafka/controller/KafkaController.scala
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@@ -266,6 +275,178 @@ class KafkaController(val config: KafkaConfig,
}
}
+ private def createFeatureZNode(newNode: FeatureZNode): Int = {
+ info(s"Creating FeatureZNode at path: ${FeatureZNode.path} with contents:
$newNode")
+ zkClient.createFeatureZNode(newNode)
+ val (_, newVersion) = zkClient.getDataAndVersion(FeatureZNode.path)
+ newVersion
+ }
+
+ private def updateFeatureZNode(updatedNode: FeatureZNode): Int = {
+ info(s"Updating FeatureZNode at path: ${FeatureZNode.path} with contents:
$updatedNode")
+ zkClient.updateFeatureZNode(updatedNode)
+ }
+
+ /**
+ * This method enables the feature versioning system (KIP-584).
+ *
+ * Development in Kafka (from a high level) is organized into features. Each
feature is tracked by
+ * a name and a range of version numbers. A feature can be of two types:
+ *
+ * 1. Supported feature:
+ * A supported feature is represented by a name (String) and a range of
versions (defined by a
+ * {@link SupportedVersionRange}). It refers to a feature that a particular
broker advertises
+ * support for. Each broker advertises the version ranges of its own
supported features in its
+ * own BrokerIdZNode. The contents of the advertisement are specific to the
particular broker and
+ * do not represent any guarantee of a cluster-wide availability of the
feature for any particular
+ * range of versions.
+ *
+ * 2. Finalized feature:
+ * A finalized feature is represented by a name (String) and a range of
version levels (defined
+ * by a {@link FinalizedVersionRange}). Whenever the feature versioning
system (KIP-584) is
+ * enabled, the finalized features are stored in the cluster-wide common
FeatureZNode.
+ * In comparison to a supported feature, the key difference is that a
finalized feature exists
+ * in ZK only when it is guaranteed to be supported by any random broker in
the cluster for a
+ * specified range of version levels. Also, the controller is the only
entity modifying the
+ * information about finalized features.
+ *
+ * This method sets up the FeatureZNode with enabled status, which means
that the finalized
+ * features stored in the FeatureZNode are active. The enabled status should
be written by the
+ * controller to the FeatureZNode only when the broker IBP config is greater
than or equal to
+ * KAFKA_2_7_IV0.
+ *
+ * There are multiple cases handled here:
+ *
+ * 1. New cluster bootstrap:
+ * A new Kafka cluster (i.e. it is deployed first time) is almost always
started with IBP config
+ * setting greater than or equal to KAFKA_2_7_IV0. We would like to start
the cluster with all
+ * the possible supported features finalized immediately. Assuming this
is the case, the
+ * controller will start up and notice that the FeatureZNode is absent in
the new cluster,
+ * it will then create a FeatureZNode (with enabled status) containing
the entire list of
+ * default supported features as its finalized features.
+ *
+ * 2. Broker binary upgraded, but IBP config set to lower than KAFKA_2_7_IV0:
+ * Imagine there is an existing Kafka cluster with IBP config less than
KAFKA_2_7_IV0, and the
+ * broker binary has been upgraded to a newer version that supports the
feature versioning
+ * system (KIP-584). This means the user is upgrading from an earlier
version of the broker
+ * binary. In this case, we want to start with no finalized features and
allow the user to
+ * finalize them whenever they are ready i.e. in the future whenever the
user sets IBP config
+ * to be greater than or equal to KAFKA_2_7_IV0, then the user could
start finalizing the
+ * features. This process ensures we do not enable all the possible
features immediately after
+ * an upgrade, which could be harmful to Kafka.
+ * This is how we handle such a case:
+ * - Before the IBP config upgrade (i.e. IBP config set to less than
KAFKA_2_7_IV0), the
+ * controller will start up and check if the FeatureZNode is absent.
If absent, it will
+ * react by creating a FeatureZNode with disabled status and empty
finalized features.
+ * Otherwise, if a node already exists in enabled status then the
controller will just
+ * flip the status to disabled and clear the finalized features.
+ * - After the IBP config upgrade (i.e. IBP config set to greater than
or equal to
+ * KAFKA_2_7_IV0), when the controller starts up it will check if the
FeatureZNode exists
+ * and whether it is disabled. In such a case, it won’t upgrade all
features immediately.
+ * Instead it will just switch the FeatureZNode status to enabled
status. This lets the
+ * user finalize the features later.
+ *
+ * 3. Broker binary upgraded, with existing cluster IBP config >=
KAFKA_2_7_IV0:
+ * Imagine an existing Kafka cluster with IBP config >= KAFKA_2_7_IV0,
and the broker binary
+ * has just been upgraded to a newer version (that supports IBP config
KAFKA_2_7_IV0 and higher).
+ * The controller will start up and find that a FeatureZNode is already
present with enabled
+ * status and existing finalized features. In such a case, the controller
needs to scan the
+ * existing finalized features and mutate them for the purpose of version
level deprecation
+ * (if needed).
+ * This is how we handle this case: If an existing finalized feature is
present in the default
+ * finalized features, then, its existing minimum version level is
updated to the default
+ * minimum version level maintained in the BrokerFeatures object. The
goal of this mutation is
+ * to permanently deprecate one or more feature version levels. The range
of feature version
+ * levels deprecated are from the closed range:
[existing_min_version_level, default_min_version_level].
+ * NOTE: Deprecating a feature version level is an incompatible change,
which requires a major
+ * release of Kafka. In such a release, the minimum version level
maintained within the
+ * BrokerFeatures class is updated suitably to record the deprecation of
the feature.
+ *
+ * 4. Broker downgrade:
+ * Imagine that a Kafka cluster exists already and the IBP config is
greater than or equal to
+ * KAFKA_2_7_IV0. Then, the user decided to downgrade the cluster by
setting IBP config to a
+ * value less than KAFKA_2_7_IV0. This means the user is also disabling
the feature versioning
+ * system (KIP-584). In this case, when the controller starts up with the
lower IBP config, it
+ * will switch the FeatureZNode status to disabled with empty features.
+ */
+ private def enableFeatureVersioning(): Unit = {
+ val defaultFinalizedFeatures = brokerFeatures.getDefaultFinalizedFeatures
+ val (mayBeFeatureZNodeBytes, version) =
zkClient.getDataAndVersion(FeatureZNode.path)
+ if (version == ZkVersion.UnknownVersion) {
+ val newVersion = createFeatureZNode(new
FeatureZNode(FeatureZNodeStatus.Enabled, defaultFinalizedFeatures))
+ featureCache.waitUntilEpochOrThrow(newVersion,
config.zkConnectionTimeoutMs)
+ } else {
+ val existingFeatureZNode =
FeatureZNode.decode(mayBeFeatureZNodeBytes.get)
+ var newFeatures: Features[FinalizedVersionRange] =
Features.emptyFinalizedFeatures()
+ if (existingFeatureZNode.status.equals(FeatureZNodeStatus.Enabled)) {
+ newFeatures =
Features.finalizedFeatures(existingFeatureZNode.features.features().asScala.map
{
+ case (featureName, existingVersionRange) =>
+ val brokerDefaultVersionRange =
defaultFinalizedFeatures.get(featureName)
+ if (brokerDefaultVersionRange == null) {
+ warn(s"Existing finalized feature: $featureName with
$existingVersionRange"
+ + s" is absent in default finalized $defaultFinalizedFeatures")
+ (featureName, existingVersionRange)
+ } else if (brokerDefaultVersionRange.max() >=
existingVersionRange.max() &&
+ brokerDefaultVersionRange.min() <=
existingVersionRange.max()) {
+ // Through this change, we deprecate all version levels in the
closed range:
+ // [existingVersionRange.min(), brokerDefaultVersionRange.min()
- 1]
+ (featureName, new
FinalizedVersionRange(brokerDefaultVersionRange.min(),
existingVersionRange.max()))
+ } else {
+ // If the existing version levels fall completely outside the
+ // range of the default finalized version levels (i.e. no
intersection), or, if the
+ // existing version levels are ineligible for a modification
since they are
+ // incompatible with default finalized version levels, then we
skip the update.
+ warn(s"Can not update minimum version level in finalized
feature: $featureName,"
+ + s" since the existing $existingVersionRange is not eligible
for a change"
+ + s" based on the default $brokerDefaultVersionRange.")
+ (featureName, existingVersionRange)
+ }
+ }.asJava)
+ }
+ val newFeatureZNode = new FeatureZNode(FeatureZNodeStatus.Enabled,
newFeatures)
+ if (!newFeatureZNode.equals(existingFeatureZNode)) {
+ val newVersion = updateFeatureZNode(newFeatureZNode)
+ featureCache.waitUntilEpochOrThrow(newVersion,
config.zkConnectionTimeoutMs)
+ }
+ }
+ }
+
+ /**
+ * Disables the feature versioning system (KIP-584).
+ *
+ * Sets up the FeatureZNode with disabled status. This status means the
feature versioning system
+ * (KIP-584) is disabled, and, the finalized features stored in the
FeatureZNode are not relevant.
+ * This status should be written by the controller to the FeatureZNode only
when the broker
+ * IBP config is less than KAFKA_2_7_IV0.
+ *
+ * NOTE:
+ * 1. When this method returns, existing finalized features (if any) will be
cleared from the
+ * FeatureZNode.
+ * 2. This method, unlike enableFeatureVersioning() need not wait for the
FinalizedFeatureCache
+ * to be updated, because, such updates to the cache (via
FinalizedFeatureChangeListener)
+ * are disabled when IBP config is < than KAFKA_2_7_IV0.
+ */
+ private def disableFeatureVersioning(): Unit = {
+ val newNode = FeatureZNode(FeatureZNodeStatus.Disabled,
Features.emptyFinalizedFeatures())
+ val (mayBeFeatureZNodeBytes, version) =
zkClient.getDataAndVersion(FeatureZNode.path)
+ if (version == ZkVersion.UnknownVersion) {
+ createFeatureZNode(newNode)
Review comment:
No, that is not required. Please refer to the documentation under `NOTE`
for this method where I have explained why.
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