Apache Avro™ 1.7.4 IDL

This document defines Avro IDL, a higher-level language for authoring Avro schemata. Before reading this document, you should have familiarity with the concepts of schemata and protocols, as well as the various primitive and complex types available in Avro.

Purpose

The aim of the Avro IDL language is to enable developers to author schemata in a way that feels more similar to common programming languages like Java, C++, or Python. Additionally, the Avro IDL language may feel more familiar for those users who have previously used the interface description languages (IDLs) in other frameworks like Thrift, Protocol Buffers, or CORBA.

Usage

Each Avro IDL file defines a single Avro Protocol, and thus generates as its output a JSON-format Avro Protocol file with extension .avpr.

To convert a .avdl file into a .avpr file, it may be processed by the idl tool. For example:

$ java -jar avroj-tools.jar idl src/test/idl/input/namespaces.avdl /tmp/namespaces.avpr
$ head /tmp/namespaces.avpr
{
  "protocol" : "TestNamespace",
  "namespace" : "avro.test.protocol",
        

The idl tool can also process input to and from stdin and stdout. See idl --help for full usage information.

A Maven plugin is also provided to compile .avdl files. To use it, add something like the following to your pom.xml:

<build>
  <plugins>
    <plugin>
      <groupId>org.apache.avro</groupId>
      <artifactId>avro-maven-plugin</artifactId>
      <executions>
        <execution>
          <goals>
            <goal>idl-protocol</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

An Avro IDL file consists of exactly one protocol definition. The minimal protocol is defined by the following code:

protocol MyProtocol {
}
      

This is equivalent to (and generates) the following JSON protocol definition:

{
"protocol" : "MyProtocol",
  "types" : [ ],
  "messages" : {
  }
}
      

The namespace of the protocol may be changed using the @namespace annotation:

@namespace("mynamespace")
protocol MyProtocol {
}
      

This notation is used throughout Avro IDL as a way of specifying properties for the annotated element, as will be described later in this document.

Protocols in Avro IDL can contain the following items:

• Imports of external protocol and schema files.
• Definitions of named schemata, including records, errors, enums, and fixeds.
• Definitions of RPC messages

Files may be imported in one of three formats:

• An IDL file may be imported with a statement like:

  import idl "foo.avdl";

• A JSON protocol file may be imported with a statement like:

  import protocol "foo.avpr";

• A JSON schema file may be imported with a statement like:

  import schema "foo.avsc";

Messages and types in the imported file are added to this file's protocol.

Imported file names are resolved relative to the current IDL file.

Enums are defined in Avro IDL using a syntax similar to C or Java:

enum Suit {
  SPADES, DIAMONDS, CLUBS, HEARTS
}
      

Note that, unlike the JSON format, anonymous enums cannot be defined.

Fixed fields are defined using the following syntax:

fixed MD5(16);
      

This example defines a fixed-length type called MD5 which contains 16 bytes.

Records are defined in Avro IDL using a syntax similar to a struct definition in C:

record Employee {
  string name;
  boolean active = true;
  long salary;
}
      

The above example defines a record with the name “Employee” with three fields.

To define an error, simply use the keyword error instead of record. For example:

error Kaboom {
  string explanation;
  int result_code = -1;
}
      

Each field in a record or error consists of a type and a name, optional property annotations and an optional default value.

A type reference in Avro IDL must be one of:

• A primitive type
• A named schema defined prior to this usage in the same Protocol
• A complex type (array, map, or union)

Primitive Types

The primitive types supported by Avro IDL are the same as those supported by Avro's JSON format. This list includes int, long, string, boolean, float, double, null, and bytes.

References to Named Schemata

If a named schema has already been defined in the same Avro IDL file, it may be referenced by name as if it were a primitive type:

record Card {
  Suit suit; // refers to the enum Card defined above
  int number;
}
        

Default Values

Default values for fields may be optionally specified by using an equals sign after the field name followed by a JSON expression indicating the default value. This JSON is interpreted as described in the spec.

Complex Types

Arrays

Array types are written in a manner that will seem familiar to C++ or Java programmers. An array of any type t is denoted array<t>. For example, an array of strings is denoted array<string>, and a multidimensional array of Foo records would be array<array<Foo>>.

Maps

Map types are written similarly to array types. An array that contains values of type t is written map<t>. As in the JSON schema format, all maps contain string-type keys.

Unions

Union types are denoted as union { typeA, typeB, typeC, ... }. For example, this record contains a string field that is optional (unioned with null):

record RecordWithUnion {
  union { null, string } optionalString;
}
          

Note that the same restrictions apply to Avro IDL unions as apply to unions defined in the JSON format; namely, a record may not contain multiple elements of the same type.

The syntax to define an RPC message within a Avro IDL protocol is similar to the syntax for a method declaration within a C header file or a Java interface. To define an RPC message add which takes two arguments named foo and bar, returning an int, simply include the following definition within the protocol:

int add(int foo, int bar = 0);
      

Message arguments, like record fields, may specify default values.

To define a message with no response, you may use the alias void, equivalent to the Avro null type:

void logMessage(string message);
      

If you have previously defined an error type within the same protocol, you may declare that a message can throw this error using the syntax:

void goKaboom() throws Kaboom;
      

To define a one-way message, use the keyword oneway after the parameter list, for example:

void fireAndForget(string message) oneway;
      

Comments

All Java-style comments are supported within a Avro IDL file. Any text following // on a line is ignored, as is any text between /* and */, possibly spanning multiple lines.

Comments that begin with /** are used as the documentation string for the type or field definition that follows the comment.

Escaping Identifiers

Occasionally, one will need to use a reserved language keyword as an identifier. In order to do so, backticks (`) may be used to escape the identifier. For example, to define a message with the literal name error, you may write:

void `error`();
        

This syntax is allowed anywhere an identifier is expected.

Annotations for Ordering and Namespaces

Java-style annotations may be used to add additional properties to types and fields throughout Avro IDL.

For example, to specify the sort order of a field within a record, one may use the @order annotation before the field name as follows:

record MyRecord {
  string @order("ascending") myAscendingSortField;
  string @order("descending")  myDescendingField;
  string @order("ignore") myIgnoredField;
}
        

A field's type may also be preceded by annotations, e.g.:

record MyRecord {
  @java_class("java.util.ArrayList") array<string> myStrings;
}
        

Similarly, a @namespace annotation may be used to modify the namespace when defining a named schema. For example:

@namespace("org.apache.avro.firstNamespace")
protocol MyProto {
  @namespace("org.apache.avro.someOtherNamespace")
  record Foo {}

  record Bar {}
}
        

will define a protocol in the firstNamespace namespace. The record Foo will be defined in someOtherNamespace and Bar will be defined in firstNamespace as it inherits its default from its container.

Type and field aliases are specified with the @aliases annotation as follows:

@aliases(["org.old.OldRecord", "org.ancient.AncientRecord"])
record MyRecord {
  string @aliases(["oldField", "ancientField"]) myNewField;
}
        

Some annotations like those listed above are handled specially. All other annotations are added as properties to the protocol, message, schema or field.

The following is a complete example of a Avro IDL file that shows most of the above features:

/**
 * An example protocol in Avro IDL
 */
@namespace("org.apache.avro.test")
protocol Simple {

  @aliases(["org.foo.KindOf"])
  enum Kind {
    FOO,
    BAR, // the bar enum value
    BAZ
  }

  fixed MD5(16);

  record TestRecord {
    @order("ignore")
    string name;

    @order("descending")
    Kind kind;

    MD5 hash;

    union { MD5, null} @aliases(["hash"]) nullableHash;

    array<long> arrayOfLongs;
  }

  error TestError {
    string message;
  }

  string hello(string greeting);
  TestRecord echo(TestRecord `record`);
  int add(int arg1, int arg2);
  bytes echoBytes(bytes data);
  void `error`() throws TestError;
  void ping() oneway;
}
      

Additional examples may be found in the Avro source tree under the src/test/idl/input directory.