What are the primitive types in Avro?

Avro defines eight primitives: null, boolean, int (32-bit), long (64-bit), float, double, bytes and string. int and long are variable-length zig-zag encoded, and bytes/string are length-prefixed.

How do you make a field nullable in Avro?

You use a union with null, like ["null", "string"]. By convention null is listed first when the default is null. The union order also determines which branch the default value applies to.

What is an Avro logical type?

A logical type annotates a primitive or complex type with extra meaning, such as decimal on bytes/fixed, date on int, or timestamp-millis on long. Readers that do not understand the logical type fall back to the underlying base type.

What is the difference between an Avro enum and a union?

An enum is a single type whose value is one of a fixed set of named symbols. A union lets a field hold a value of any one of several different types, including null. They solve different problems.

How is an Avro decimal stored?

A decimal is a logical type backed by bytes or fixed, with precision and scale attributes. The bytes contain the unscaled two's-complement integer value; the scale attribute says where the decimal point goes.

Apache Avro Schema Types

Apache Avro schema types

Apache Avro is a compact, schema-driven serialization format used heavily in Kafka and Hadoop ecosystems. An Avro schema is itself JSON, declaring one of three families of types: primitives, complex types, and logical types layered on top of those. This reference lists each type with its JSON schema representation so you can build correct .avsc files quickly.

How it works

A schema is a JSON value: a string naming a primitive ("string"), an array forming a union (["null", "long"]), or an object describing a complex type with a "type" field.

The eight primitives are null, boolean, int, long, float, double, bytes and string. Integers use variable-length ZigZag encoding so small numbers are tiny.

The complex types add structure:

record — named fields, each a nested schema.
enum — a fixed set of named symbols.
array — a list of items of one type.
map — string keys to values of one type.
union — a value of exactly one of several listed schemas.
fixed — a fixed number of bytes.

Logical types annotate a base type with semantic meaning via a logicalType attribute. For example decimal rides on bytes or fixed with precision and scale; date rides on int as days since the Unix epoch; timestamp-millis rides on long. A reader that does not recognize the logical type still reads the base value.

Tips and example

A nullable timestamp field inside a record:

{
  "type": "record",
  "name": "Event",
  "fields": [
    {
      "name": "createdAt",
      "type": ["null", { "type": "long", "logicalType": "timestamp-millis" }],
      "default": null
    }
  ]
}

When making fields optional, put "null" first in the union and set "default": null — that lets new readers tolerate old data and aids schema evolution. Filter the table below by kind to find the exact JSON shape for any Avro type.

Apache Avro Schema Types

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Apache Avro schema types

How it works

Tips and example