Kafka Fetch Request Wire Format v5

What is Fetch?

Consumers and follower replicas both issue this continuously for log replication and consumption. When a consumer's fetch stalls with empty responses despite records existing, check isolation.level: read_committed consumers are blocked by open transactions until EndTxn.

Wire Diagram

Request Header

message_size int32 · 4B

api_key int16 · 2B

api_version int16 · 2B

correlation_id int32 · 4B

client_id string

FetchRequest v5

ReplicaId int32 · 4B

MaxWaitMs int32 · 4B

MinBytes int32 · 4B

MaxBytes int32 · 4B

IsolationLevel int8 · 1B

Topics array

Topic string

Partitions array

Partition int32 · 4B

FetchOffset int64 · 8B

LogStartOffset int64 · 8B

PartitionMaxBytes int32 · 4B

Schema & Example

Schema
{
  "ReplicaId": int32,
  "MaxWaitMs": int32,
  "MinBytes": int32,
  "MaxBytes": int32,
  "IsolationLevel": int8,
  "Topics": [{
      "Topic": string,
      "Partitions": [{
          "Partition": int32,
          "FetchOffset": int64,
          "LogStartOffset": int64,
          "PartitionMaxBytes": int32
      }]
  }]
}

Example
{
  "ReplicaId": -1,
  "MaxWaitMs": 500,
  "MinBytes": 65536,
  "MaxBytes": 65536,
  "IsolationLevel": 1,
  "Topics": [{
      "Topic": "orders",
      "Partitions": [{
          "Partition": 0,
          "FetchOffset": 150382,
          "LogStartOffset": 284729,
          "PartitionMaxBytes": 65536
      }]
  }]
}

Kafka Fetch Request Wire Format v5 — Binary Protocol Layout

What is Fetch?

Wire Diagram

Schema & Example