Source code for confluent_kafka.schema_registry.avro

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# Copyright 2020 Confluent Inc.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# See the License for the specific language governing permissions and
# limitations under the License.
from io import BytesIO
from json import loads
from struct import pack, unpack

from fastavro import (parse_schema,

from . import (_MAGIC_BYTE,
from confluent_kafka.serialization import (Deserializer,

class _ContextStringIO(BytesIO):
    Wrapper to allow use of StringIO via 'with' constructs.


    def __enter__(self):
        return self

    def __exit__(self, *args):
        return False

def _schema_loads(schema_str):
    Instantiates a Schema instance from a declaration string

        schema_str (str): Avro Schema declaration.

    .. _Schema declaration:

        Schema: Schema instance

    schema_str = schema_str.strip()

    # canonical form primitive declarations are not supported
    if schema_str[0] != "{":
        schema_str = '{"type":"' + schema_str + '"}'

    return Schema(schema_str, schema_type='AVRO')

[docs]class AvroSerializer(Serializer): """ AvroSerializer serializes objects in the Confluent Schema Registry binary format for Avro. AvroSerializer configuration properties: +---------------------------+----------+--------------------------------------------------+ | Property Name | Type | Description | +===========================+==========+==================================================+ | | | Registers schemas automatically if not | | ``auto.register.schemas`` | bool | previously associated with a particular subject. | | | | Defaults to True. | +---------------------------+----------+--------------------------------------------------+ | | | Callable(SerializationContext, str) -> str | | | | | | ```` | callable | Instructs the AvroSerializer on how to construct | | | | Schema Registry subject names. | | | | Defaults to topic_subject_name_strategy. | +---------------------------+----------+--------------------------------------------------+ Schemas are registered to namespaces known as Subjects which define how a schema may evolve over time. By default the subject name is formed by concatenating the topic name with the message field separated by a hyphen. i.e. {topic name}-{message field} Alternative naming strategies may be configured with the property ````. Supported subject name strategies: +--------------------------------------+------------------------------+ | Subject Name Strategy | Output Format | +======================================+==============================+ | topic_subject_name_strategy(default) | {topic name}-{message field} | +--------------------------------------+------------------------------+ | topic_record_subject_name_strategy | {topic name}-{record name} | +--------------------------------------+------------------------------+ | record_subject_name_strategy | {record name} | +--------------------------------------+------------------------------+ See `Subject name strategy <>`_ for additional details. Note: Prior to serialization all ``Complex Types`` must first be converted to a dict instance. This may handled manually prior to calling :py:func:`SerializingProducer.produce()` or by registering a `to_dict` callable with the AvroSerializer. See ```` in the examples directory for example usage. Args: schema_str (str): Avro `Schema Declaration. <>`_ schema_registry_client (SchemaRegistryClient): Schema Registry client instance. to_dict (callable, optional): Callable(object, SerializationContext) -> dict. Converts object to a dict. conf (dict): AvroSerializer configuration. """ # noqa: E501 __slots__ = ['_hash', '_auto_register', '_known_subjects', '_parsed_schema', '_registry', '_schema', '_schema_id', '_schema_name', '_subject_name_func', '_to_dict'] # default configuration _default_conf = {'auto.register.schemas': True, '': topic_subject_name_strategy} def __init__(self, schema_str, schema_registry_client, to_dict=None, conf=None): self._registry = schema_registry_client self._schema_id = None # Avoid calling registry if schema is known to be registered self._known_subjects = set() if to_dict is not None and not callable(to_dict): raise ValueError("to_dict must be callable with the signature" " to_dict(SerializationContext, object)->dict") self._to_dict = to_dict # handle configuration conf_copy = self._default_conf.copy() if conf is not None: conf_copy.update(conf) self._auto_register = conf_copy.pop('auto.register.schemas') if not isinstance(self._auto_register, bool): raise ValueError("auto.register.schemas must be a boolean value") self._subject_name_func = conf_copy.pop('') if not callable(self._subject_name_func): raise ValueError(" must be callable") if len(conf_copy) > 0: raise ValueError("Unrecognized properties: {}" .format(", ".format(conf_copy.keys()))) # convert schema_str to Schema instance schema = _schema_loads(schema_str) schema_dict = loads(schema.schema_str) parsed_schema = parse_schema(schema_dict) # The Avro spec states primitives have a name equal to their type # i.e. {"type": "string"} has a name of string. # This function does not comply. # schema_name = parsed_schema.get('name', schema_dict['type']) self._schema = schema self._schema_name = schema_name self._parsed_schema = parsed_schema
[docs] def __call__(self, obj, ctx): """ Serializes an object to the Confluent Schema Registry's Avro binary format. Args: obj (object): object instance to serializes. ctx (SerializationContext): Metadata pertaining to the serialization operation. Note: None objects are represented as Kafka Null. Raises: SerializerError: if any error occurs serializing obj Returns: bytes: Confluent Schema Registry formatted Avro bytes """ if obj is None: return None subject = self._subject_name_func(ctx, self._schema_name) # Check to ensure this schema has been registered under subject_name. if self._auto_register and subject not in self._known_subjects: # The schema name will always be the same. We can't however register # a schema without a subject so we set the schema_id here to handle # the initial registration. self._schema_id = self._registry.register_schema(subject, self._schema) self._known_subjects.add(subject) elif not self._auto_register and subject not in self._known_subjects: registered_schema = self._registry.lookup_schema(subject, self._schema) self._schema_id = registered_schema.schema_id self._known_subjects.add(subject) if self._to_dict is not None: value = self._to_dict(obj, ctx) else: value = obj with _ContextStringIO() as fo: # Write the magic byte and schema ID in network byte order (big endian) fo.write(pack('>bI', _MAGIC_BYTE, self._schema_id)) # write the record to the rest of the buffer schemaless_writer(fo, self._parsed_schema, value) return fo.getvalue()
[docs]class AvroDeserializer(Deserializer): """ AvroDeserializer decodes bytes written in the Schema Registry Avro format to an object. Note: ``Complex Types`` are returned as dicts. If a more specific instance type is desired a callable, ``from_dict``, may be registered with the AvroDeserializer which converts a dict to the desired type. See ```` in the examples directory in the examples directory for example usage. Args: schema_str (str): Avro reader schema declaration. schema_registry_client (SchemaRegistryClient): Confluent Schema Registry client instance. from_dict (callable, optional): Callable(dict, SerializationContext) -> object. Converts dict to an instance of some object. See Also: `Apache Avro Schema Declaration <>`_ `Apache Avro Schema Resolution <>`_ """ __slots__ = ['_reader_schema', '_registry', '_from_dict', '_writer_schemas'] def __init__(self, schema_str, schema_registry_client, from_dict=None): self._registry = schema_registry_client self._writer_schemas = {} self._reader_schema = parse_schema(loads(schema_str)) if from_dict is not None and not callable(from_dict): raise ValueError("from_dict must be callable with the signature" " from_dict(SerializationContext, dict) -> object") self._from_dict = from_dict
[docs] def __call__(self, value, ctx): """ Decodes a Confluent Schema Registry formatted Avro bytes to an object. Arguments: value (bytes): bytes ctx (SerializationContext): Metadata pertaining to the serialization operation. Raises: SerializerError: if an error occurs ready data. Returns: object: object if ``from_dict`` is set, otherwise dict. If no value is supplied None is returned. """ # noqa: E501 if value is None: return None if len(value) <= 5: raise SerializationError("Message too small. This message was not" " produced with a Confluent" " Schema Registry serializer") with _ContextStringIO(value) as payload: magic, schema_id = unpack('>bI', if magic != _MAGIC_BYTE: raise SerializationError("Unknown magic byte. This message was" " not produced with a Confluent" " Schema Registry serializer") writer_schema = self._writer_schemas.get(schema_id, None) if writer_schema is None: schema = self._registry.get_schema(schema_id) prepared_schema = _schema_loads(schema.schema_str) writer_schema = parse_schema(loads( prepared_schema.schema_str)) self._writer_schemas[schema_id] = writer_schema obj_dict = schemaless_reader(payload, writer_schema, self._reader_schema) if self._from_dict is not None: return self._from_dict(obj_dict, ctx) return obj_dict