public interface GenericAvroBinding extends AvroBinding<GenericRecord>
GenericAvroBinding
interface has the same methods as AvroBinding
, but represents values as instances of GenericRecord
.
A single schema binding is created using AvroCatalog.getGenericBinding(org.apache.avro.Schema)
, and a multiple schema binding is created
using AvroCatalog.getGenericMultiBinding(java.util.Map<java.lang.String, org.apache.avro.Schema>)
.
The trade-offs in using a GenericAvroBinding
, compared to other
types of bindings, are:
GenericRecord
is part of the Avro API, it conveniently
represents all data types defined by Avro. This is in contrast to a
JsonAvroBinding
.
GenericRecord
does not provide type safety because field
values must be cast to the type defined in the schema. It can also be
error prone, because fields are accessed by string name.
Schema
objects at runtime. In other words, the application must maintain a set
of known schemas for use at runtime.
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 GenericAvroBinding
, a GenericRecord
is used to
represent values. A GenericRecord
represents an Avro object roughly
as a map of string field names to field values, as follows:
public interface GenericRecord { public Schema getSchema(); public Object get(String key); public void put(String key, Object value); }Note that field values in this API are type Object and must be cast to the appropriate type, as defined by the schema. The mapping from Avro schema data types to Java data types is described at the bottom of the
org.apache.avro.generic
package description.
The following code fragment demonstrates writing and reading a value using a generic single schema binding.
Schema.Parser parser = new Schema.Parser(); Schema nameSchema = parser.parse(nameSchemaText); Schema memberSchema = parser.parse(memberSchemaText); GenericAvroBinding binding = avroCatalog.getGenericBinding(memberSchema); // Create object GenericRecord name = new GenericData.Record(nameSchema) name.put("first", ...); name.put("last", ...); GenericRecord object = new GenericData.Record(memberSchema) object.put("name", name); object.put("age", new Integer(...)); // Serialize and store Value value = binding.toValue(object); kvStore.put(key, value); // Sometime later, retrieve and deserialize ValueVersion vv = kvStore.get(key); GenericRecord object = binding.toObject(vv.getValue()); // Use object GenericRecord name = (GenericRecord) object.get("name"); Integer age = (Integer) object.get("age"); ...
The following code fragment demonstrates reading values with different schemas using a generic multiple schema binding.
Schema.Parser parser = new Schema.Parser(); Schema nameSchema = parser.parse(nameSchemaText); Schema memberSchema = parser.parse(memberSchemaText); Schema anotherSchema = parser.parse(anotherSchemaText); Map<String, Schema> schemas = new HashMap<String, Schema>() schemas.put(memberSchema.getFullName(), memberSchema); schemas.put(anotherSchema.getFullName(), anotherSchema); GenericAvroBinding binding = avroCatalog.getGenericMultiBinding(schemas); Iterator<KeyValueVersion> iter = kvStore.multiGetIterator(...); for (KeyValueVersion kvv : iter) { GenericRecord object = binding.toObject(kvv.getValue()); String schemaName = object.getSchema().getFullName(); if (schemaName.equals(memberSchema.getFullName())) { ... } else if (schemaName.equals(anotherSchema.getFullName())) { ... } else { ... } }
A special use case for a generic multiple schema binding is when the
application treats values dynamically based on their schema, rather than
using a fixed set of known schemas. The AvroCatalog.getCurrentSchemas()
method can be used to obtain a map of the
most current schemas, which can be passed to AvroCatalog.getGenericMultiBinding(java.util.Map<java.lang.String, org.apache.avro.Schema>)
.
For example, the following code fragment demonstrates reading values with
different schemas using a generic multiple schema binding. Note that in a
long running application, it is possible that a schema may be added and used
to store a key-value pair, after the binding has been created. The
application may handle this possibility by catching SchemaNotAllowedException
.
GenericAvroBinding binding = avroCatalog.getGenericMultiBinding(avroCatalog.getCurrentSchemas()); Iterator<KeyValueVersion> iter = kvStore.storeIterator(...); while (iter.hasNext()) { KeyValueVersion kvv = iter.next(); GenericRecord object; try { object = binding.toObject(kvv.getValue()); } catch (SchemaNotAllowedException e) { // In this example, ignore values with a schema that was not // known at the time the binding was created. continue; } String schemaName = object.getSchema().getFullName(); if (schemaName.equals(memberSchema.getFullName())) { ... } else if (schemaName.equals(anotherSchema.getFullName())) { ... } else { ... } }
Modifier and Type | Method and Description |
---|---|
GenericRecord |
toObject(Value value)
|
Value |
toValue(GenericRecord object)
Before doing a write operation, the user calls
toValue passing
an object she wishes to store. |
GenericRecord toObject(Value value) throws SchemaNotAllowedException, IllegalArgumentException
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 this
binding (and was specified when the binding was created).
In other words, this method transforms the serialized data in the value
parameter to conform to the schema of the GenericRecord
that is returned.
toObject
in interface AvroBinding<GenericRecord>
toObject
in interface ValueBinding<GenericRecord>
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 schemaGenericRecord
instance. The GenericContainer.getSchema()
method will return the reader schema, which is
the schema that was specified when this binding was created.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.Value toValue(GenericRecord object) throws SchemaNotAllowedException, UndefinedSchemaException, IllegalArgumentException
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 GenericRecord
object
parameter; it is returned by GenericContainer.getSchema()
and normally specified when creating a GenericData.Record
object. The writer schema
must be one of the schemas specified when this binding was created.
In other words, this method returns serialized data that conforms to the
schema of the given GenericRecord
.
toValue
in interface AvroBinding<GenericRecord>
toValue
in interface ValueBinding<GenericRecord>
object
- the GenericRecord
instance the user wishes to
store, or at least serialize.Value
is
serialized Avro data, packaged in an internal format that includes a
reference to the Avro schemaSchemaNotAllowedException
- if the schema associated with the
object
parameter is not allowed with this 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.Copyright (c) 2011, 2015 Oracle and/or its affiliates. All rights reserved.