Avor -- python

Avro schemas are defined using JSON. Schemas are composed of primitive types (null, boolean, int, long, float, double, bytes, and string) andcomplex types (record, enum, array, map, union, and fixed). You can learn more about Avro schemas and types from the specification, but for now let‘s start with a simple schema example, user.avsc:

{"namespace": "example.avro",
 "type": "record",
 "name": "User",
 "fields": [
     {"name": "name", "type": "string"},
     {"name": "favorite_number",  "type": ["int", "null"]},
     {"name": "favorite_color", "type": ["string", "null"]}
 ]
}
      

This schema defines a record representing a hypothetical user. (Note that a schema file can only contain a single schema definition.) At minimum, a record definition must include its type ("type": "record"), a name ("name": "User"), and fields, in this case name, favorite_number, and favorite_color. We also define a namespace ("namespace": "example.avro"), which together with the name attribute defines the "full name" of the schema (example.avro.User in this case).

Fields are defined via an array of objects, each of which defines a name and type (other attributes are optional, see the record specification for more details). The type attribute of a field is another schema object, which can be either a primitive or complex type. For example, the name field of our User schema is the primitive type string, whereas the favorite_number and favorite_color fields are both unions, represented by JSON arrays. unions are a complex type that can be any of the types listed in the array; e.g., favorite_number can either be an int or null, essentially making it an optional field.

Data in Avro is always stored with its corresponding schema, meaning we can always read a serialized item, regardless of whether we know the schema ahead of time. This allows us to perform serialization and deserialization without code generation. Note that the Avro Python library does not support code generation.

Try running the following code snippet, which serializes two users to a data file on disk, and then reads back and deserializes the data file:

import avro.schema
from avro.datafile import DataFileReader, DataFileWriter
from avro.io import DatumReader, DatumWriter

schema = avro.schema.parse(open("user.avsc").read())

writer = DataFileWriter(open("users.avro", "w"), DatumWriter(), schema)
writer.append({"name": "Alyssa", "favorite_number": 256})
writer.append({"name": "Ben", "favorite_number": 7, "favorite_color": "red"})
writer.close()

reader = DataFileReader(open("users.avro", "r"), DatumReader())
for user in reader:
    print user
reader.close()
      

This outputs:

{u‘favorite_color‘: None, u‘favorite_number‘: 256, u‘name‘: u‘Alyssa‘}
{u‘favorite_color‘: u‘red‘, u‘favorite_number‘: 7, u‘name‘: u‘Ben‘}
      

Let‘s take a closer look at what‘s going on here.

schema = avro.schema.parse(open("user.avsc").read())
      

avro.schema.parse takes a string containing a JSON schema definition as input and outputs a avro.schema.Schema object (specifically a subclass of Schema, in this case RecordSchema). We‘re passing in the contents of our user.avsc schema file here.

writer = DataFileWriter(open("users.avro", "w"), DatumWriter(), schema)
      

We create a DataFileWriter, which we‘ll use to write serialized items to a data file on disk. The DataFileWriter constructor takes three arguments:

• The file we‘ll serialize to
• A DatumWriter, which is responsible for actually serializing the items to Avro‘s binary format (DatumWriters can be used separately fromDataFileWriters, e.g., to perform IPC with Avro TODO: is this true??).
• The schema we‘re using. The DataFileWriter needs the schema both to write the schema to the data file, and to verify that the items we write are valid items and write the appropriate fields.

writer.append({"name": "Alyssa", "favorite_number": 256})
writer.append({"name": "Ben", "favorite_number": 7, "favorite_color": "red"})
        

We use DataFileWriter.append to add items to our data file. Avro records are represented as Python dicts. Since the field favorite_color has type["int", "null"], we are not required to specify this field, as shown in the first append. Were we to omit the required name field, an exception would be raised. Any extra entries not corresponding to a field are present in the dict are ignored.

reader = DataFileReader(open("users.avro", "r"), DatumReader())
        

We open the file again, this time for reading back from disk. We use a DataFileReader and DatumReader analagous to the DataFileWriter andDatumWriter above.

for user in reader:
    print user
        

The DataFileReader is an iterator that returns dicts corresponding to the serialized items.

Avro RPC

Server:

from BaseHTTPServer import BaseHTTPRequestHandler, HTTPServer
import avro.ipc as ipc
import avro.protocol as protocol
import avro.schema as schema

PROTOCOL = protocol.parse(open("../avro/mail.avpr").read())

class MailResponder(ipc.Responder):
    def __init__(self):
        ipc.Responder.__init__(self, PROTOCOL)

    def invoke(self, msg, req):
        if msg.name == ‘send‘:
            message = req[‘message‘]
            return ("Sent message to " + message[‘to‘]
                    + " from " + message[‘from‘]
                    + " with body " + message[‘body‘])
        else:
            raise schema.AvroException("unexpected message:", msg.getname())

class MailHandler(BaseHTTPRequestHandler):
  def do_POST(self):
    self.responder = MailResponder()
    call_request_reader = ipc.FramedReader(self.rfile)
    call_request = call_request_reader.read_framed_message()
    resp_body = self.responder.respond(call_request)
    self.send_response(200)
    self.send_header(‘Content-Type‘, ‘avro/binary‘)
    self.end_headers()
    resp_writer = ipc.FramedWriter(self.wfile)
    resp_writer.write_framed_message(resp_body)

server_addr = (‘localhost‘, 9090)

if __name__ == ‘__main__‘:
    server = HTTPServer(server_addr, MailHandler)
    server.allow_reuse_address = True
    server.serve_forever()

Client:

    print("Result: " + requestor.request(‘send‘, params))

    # cleanup
    client.close()

First run the server as:

./start_server.py

Then run the client as:

./send_message.py avro_user pat Hello_World