A ZooKeeper broker config
What converts, what has no KRaft equivalent, and why this is a migration not a conversion
zookeeper.connect=zk:2181 broker.id=1 log.dirs=/var/lib/kafka
Kafka ZooKeeper to KRaft Config Converter
Turn a ZooKeeper-mode server.properties into a KRaft one. Every setting is sorted into carries across, renamed, or no equivalent at all, and nothing is dropped silently: the ones with no counterpart are commented with the reason.
Three controllers tolerate one failure; an even number tolerates the same as the odd number below it. Combined mode is for development, where it saves running separate processes.
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Results
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No formatting problems, and nothing the rules object to. Worth remembering what that covers: this reads the file you pasted, not the account or cluster it will be applied to.
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Examples
Real input you can load into the tool above. Each one shows a different thing going wrong, because that is what the tool is for.
Controller quorum
The settings a KRaft cluster needs that a ZooKeeper one never had
broker.id=1 log.dirs=/var/lib/kafka num.partitions=6
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Treating the config conversion as the migration
Converting settings is the easy part. The metadata itself has to move, which is a documented multi-step procedure with a dual-write phase.
Instead:Follow the migration procedure. This page only shows the config side.
Forgetting to format the storage directory
KRaft needs kafka-storage.sh format with a cluster id before the first start, which ZooKeeper never required.
Instead:Format before starting, and keep the cluster id.
Assuming every setting has an equivalent
Several ZooKeeper-era settings have no KRaft counterpart and are simply gone.
Instead:Read the ones flagged as having no equivalent rather than translating them.
Converting the config is the easy half
Most settings carry across untouched. The ones that do not divide into renames, which are harmless, and settings with no KRaft equivalent at all, which is where a migration goes wrong.
inter.broker.protocol.version stops doing anything
In KRaft the cluster-wide protocol level is a feature called metadata.version, raised with kafka-features rather than by editing every broker's config. Left in a KRaft server.properties it is simply ignored. That matters because pinning it is what gives a rolling upgrade its rollback window: a config carried across looks pinned and is not.
Log directories have to be formatted before first start
KRaft keeps cluster metadata in the log directory, so kafka-storage format runs once per node with the same cluster id across the whole cluster. A node started against an unformatted directory refuses to start; one formatted with a different cluster id refuses to join. Generate the id once and reuse it everywhere.
KAFKA_CLUSTER_ID=$(bin/kafka-storage.sh random-uuid)
# on every node, with the SAME id
bin/kafka-storage.sh format \
--cluster-id $KAFKA_CLUSTER_ID \
--config config/server.propertiesAn even number of controllers buys nothing
The controller quorum needs a majority, so tolerance is (n-1)/2 rounded down: one controller tolerates no failures, three tolerate one, five tolerate two. Two controllers tolerate no failures and cost twice as much as one, because losing either loses the majority. Three is the answer for almost every cluster.
Combined mode is for laptops
process.roles=broker,controller puts the quorum on the same JVMs serving client traffic, so a broker under load is a controller under load and losing a node loses both roles. It is genuinely fine for development and Kafka's own documentation recommends dedicated controllers for production.
This is not a migration tool
It produces the config a KRaft node needs, which is what you want for a new cluster or as the target state for a real migration. Moving an existing cluster's topics, ACLs and consumer offsets out of ZooKeeper is a separate operation that runs on a 3.x release and must complete before the 4.0 upgrade, because 4.0 has no ZooKeeper support left to migrate from. The version compatibility reference on this site has the ordering.
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