oracle.kv.avro

Interface SpecificAvroBinding<T extends SpecificRecord>

Type Parameters:

T - is the SpecificRecord type of the deserialized object that is passed to toValue and returned by toObject. When using a single schema binding, T is the generated Avro specific class, which implements SpecificRecord. When using a multiple schema binding, T is the SpecificRecord interface itself.

All Superinterfaces:

AvroBinding<T>, ValueBinding<T>

public interface SpecificAvroBinding<T extends SpecificRecord>
extends AvroBinding<T>

The SpecificAvroBinding interface has the same methods as AvroBinding, but represents values as instances of a generated Avro specific class which implements SpecificRecord. A single schema binding is created using AvroCatalog.getSpecificBinding(java.lang.Class<T>), and a multiple schema binding is created using AvroCatalog.getSpecificMultiBinding().

The trade-offs in using a SpecificAvroBinding, compared to other types of bindings, are:

• Avro specific classes provide type safety and ease of use. Properties are accessed through getter/setter methods with parameters and return values of the correct type, as defined in the schema.

• When using Avro specific classes, the client application does not need to be aware of the Avro schema at runtime. The generated class has an internal reference to its associated schema, and this schema is used by the binding, so the application does not need to explicitly supply a schema. Schemas are supplied at build time, when the source code for the specific class is generated.

• A potential disadvantage to using Avro specific classes is that the set of specific classes used in the application is fixed at build time. Applications that wish to treat values generically, or dynamically based on the schema, should use a generic or JSON binding instead.

See AvroCatalog for general information on Avro bindings and schemas. The schemas used in the examples below are described in the AvroCatalog javadoc.

When using a SpecificAvroBinding, an Avro specific Java class, which implements the Avro SpecificRecord interface, is used to represent values. An Avro specific class is a POJO (Plain Old Java Object) having properties (getter and setter methods) for each field in its associated Avro schema. The source code for an Avro specific class is generated from an Avro schema using the Avro compiler tools; see the Avro example in KVHOME/examples/avro for details.

For example, the generated Avro specific classes for the example schemas are:

  package com.example;  

  import org.apache.avro.Schema;
  import org.apache.avro.specific.SpecificRecord;

  public class FullName implements SpecificRecord {
    public FullName() {}
    public Schema getSchema() { ... }
    public String getFirst() { ... }
    public void setFirst(String value) { ... }
    public String getLast() { ... }
    public void setLast(String value) { ... }
  }

  public class MemberInfo implements SpecificRecord {
    public MemberInfo() {}
    public Schema getSchema() { ... }
    public FullName getName() { ... }
    public void setName(FullName value) { ... }
    public Integer getAge() { ... }
    public void setAge(Integer value) { ... }
  }

In addition to the methods shown, useful Object.toString(), Object.equals(java.lang.Object) and Object.hashCode() methods are generated. Avro also generates internal methods for performing serialization and deserialization, and an internal static field that holds the schema used to compile the class. Note that the full Avro schema name and full Java class name are always equal.

The following code fragment demonstrates writing and reading a value using a specific single schema binding. Note that no casting is needed when a single schema binding is used.

 AvroBinding<MemberInfo> binding =
     avroCatalog.getSpecificBinding(MemberInfo.class);

 // Create object
 FullName name = new FullName();
 name.setFirst(...);
 name.setLast(...);
 MemberInfo object = new MemberInfo();
 object.setName(name);
 object.setAge(...);

 // Serialize and store
 Value value = binding.toValue(object);
 kvStore.put(key, value);

 // Sometime later, retrieve and deserialize
 ValueVersion vv = kvStore.get(key);
 MemberInfo object = binding.toObject(vv.getValue());

 // Use object
 FullName name = object.getName();
 int age = object.getAge();
 ...

The following code fragment demonstrates reading values with different schemas (different specific classes) using a multiple schema binding. Note that schema name could also be used to distinguish between classes, instead of using instanceof.

 SpecificAvroBinding<SpecificRecord> binding =
     avroCatalog.getSpecificMultiBinding();

 Iterator<KeyValueVersion> iter = kvStore.multiGetIterator(...);
 for (KeyValueVersion kvv : iter) {
     SpecificRecord object = binding.toObject(kvv.getValue());
     if (object instanceof MemberInfo) {
         MemberInfo member = (MemberInfo) object;
         ...
     } else if (object instanceof OtherSpecificClass) {
         ...
     } else {
         ...
     }
 }

Since:

2.0

Method Summary

Methods

Modifier and Type	Method and Description
T	toObject(Value value) After doing a read operation using a KVStore method, the user calls toObject with the Value obtained from the read operation.
Value	toValue(T object) Before doing a write operation, the user calls toValue passing an object she wishes to store.

Method Detail

toObject

T toObject(Value value)
                                  throws SchemaNotAllowedException,
                                         IllegalArgumentException

After doing a read operation using a KVStore method, the user calls toObject with the Value obtained from the read operation.

If necessary, this method automatically performs schema evolution, as described in AvroCatalog. In the context of schema evolution, the writer schema is the one associated internally with the value parameter (this association was normally made earlier when the value was stored), and the reader schema is the one associated with the Avro specific class of the caller. The schema of an Avro specific class is the one from which the class was generated using the Avro compiler tools.

In other words, this method transforms the serialized data in the value parameter to conform to the schema of the Avro specific class that is returned.

Specified by:

toObject in interface AvroBinding<T extends SpecificRecord>

Specified by:

toObject in interface ValueBinding<T extends SpecificRecord>

Parameters:

value - the Value obtained from a KVStore read operation method.. The byte array of the Value is serialized Avro data, packaged in an internal format that includes a reference to the Avro schema

Returns:

the deserialized SpecificRecord instance.

Throws:

SchemaNotAllowedException - if the schema associated with the value parameter is not allowed with this binding.

IllegalArgumentException - if the value format is not Value.Format.AVRO, the schema identifier embedded in the value parameter is invalid, or the serialized data cannot be parsed.

toValue

Value toValue(T object)
              throws SchemaNotAllowedException,
                     UndefinedSchemaException,
                     IllegalArgumentException

Before doing a write operation, the user calls toValue passing an object she wishes to store. The resulting Value is then passed to the write operation method in KVStore.

In the context of schema evolution, as described in AvroCatalog, the returned value is serialized according to the writer schema. The writer schema is the one associated with the Avro specific class of the object parameter. The schema of an Avro specific class is the one from which the class was generated using the Avro compiler tools.

In other words, this method returns serialized data that conforms to the schema of the Avro specific class.

Specified by:

toValue in interface AvroBinding<T extends SpecificRecord>

Specified by:

toValue in interface ValueBinding<T extends SpecificRecord>

Parameters:

object - the SpecificRecord instance the user wishes to store, or at least serialize.

Returns:

the serialized object.. The byte array of the Value is serialized Avro data, packaged in an internal format that includes a reference to the Avro schema

Throws:

SchemaNotAllowedException - if the object parameter's class is not the class passed to AvroCatalog.getSpecificBinding(java.lang.Class<T>); this can only happen if generic type warnings are ignored or suppressed. This exception is never thrown by a specific multi-binding.

UndefinedSchemaException - if the schema associated with the object parameter has not been defined using the NoSQL Database administration interface. Note that when the allowed schemas for a binding are specified (and validate) at the time the binding is created, this exception is extremely unlikely and is only possible if a schema is mistakenly disabled after the binding is created.

IllegalArgumentException - if the object parameter is invalid according to its schema, and cannot be serialized.