Kafka, live

Brokers

• Power light. Green: running. Red: stopped.
• Activity light. Blinks while records go in or out.
• The KRaft controller. It decides which replica leads each partition.
• In and Out: network bytes per second. Disk: bytes in its log files. Cache: bytes written but not yet on disk.
• Dashed border: a slow broker. Its followers fall behind.
• Grey stripes: a stopped broker.
• LLeader replica: takes writes and serves reads
• FFollower replica: copies the leader
• ×Out of the ISR: too far behind to count
• 12Number on the right: the log end offset, the next offset this replica will write.

A partition’s log

• A record; the colour is its key
• No key
• Striped: only in the page cache, lost if the power goes
• Faded: above the high watermark, not yet on every ISR replica, so consumers can’t read it
• Red inner border: a tombstone (null value) that deletes its key in a compacted topic
• Grey hatching: deleted by retention or compaction
• Dotted: an offset this replica doesn’t have yet
• Red outline: differs from the leader at the same offset
• High watermark: consumers read only below this line
• A follower’s own idea of the high watermark, which lags the leader’s
• Blue mark (in Inspect): where a consumer group will read next
• Gold frame: the partition you picked. The Inspect panel below shows it.

Producers and consumers

• Envelope: a producer. acks says how many replicas must have a record before it counts as written.
• Dashed producer: records are waiting in it because no leader can be reached.
• Face: one consumer in a group. Lag is how many records it hasn’t read yet.
• Group: its state (Stable, rebalancing…), assignor and generation, which goes up at every rebalance.

What moves

• Solid square: a record on its way from a producer to the leader (colour = key)
• Hollow square: a follower copying records from the leader
• Round dot: records fetched by a consumer
• Crossed square: a request that failed