On a related note, is there a tool that can check the backwards compatibility
of schemas? I found some old messages talking about it, but no actual tool. I
guess I could hack it together using some functions in the Avro library.
Lukas
From: Burak Emre
Sent: Tuesday, February 3, 2015 9:01 AM
To: user@avro.apache.org
Subject: Re: Adding new field with default value to an Avro schema
@Sean thanks for the explanation.
I have multiple writers but only one reader and the only schema migration
operation is adding a new field so I thought that I may use the same schema for
all dataset since the ordering will be same in all of them even though some may
contain extra fields which is also defined in schema definition.
Actually I wanted to avoid using an external database for sequential schema ids
since it would make the system more complex than it should be in my case but it
seems this is the only option for now.
--
Burak Emre
Koc University
On Tuesday 3 February 2015 at 18:22, Sean Busbey wrote:
Schema evolution in Avro requires access to both the schema used when writing
the data and the desired Schema for reading the data.
Normally, Avro data is stored in some container format (i.e. the one in the
spec[1]) and the parsing library takes care of pulling the schema used when
writing out of said container.
If you are using Avro data in some other location, you must have the writer
schema as well. One common use case is a shared messaging system focused on
small messages (but that doesn't use Avro RPC). In such cases, Doug Cutting has
some guidance he's previously given (quoted with permission, albeit very late):
A best practice for things like this is to prefix each Avro record
with a (small) numeric schema ID. This is used as the key for a
shared database of schemas. The schema corresponding to a key never
changes, so the database can be cached heavily. It never gets very
big either. It could be as simple as a .java file, with the
constraint that you'd need to upgrade things downstream before
upstream, or as complicated as an enterprise-wide REST schema service
(AVRO-1124). A variation is to use schema fingerprints as keys.
Potentially relevant stuff:
https://issues.apache.org/jira/browse/AVRO-1124
http://avro.apache.org/docs/current/spec.html#Schema+Fingerprints
If you take the integer schema ID approach, you can use Avro's built in
utilities for zig-zap encoding, which will ensure that most of the time your
identifier only takes a small amount of space.
[1]: http://avro.apache.org/docs/current/spec.html#Object+Container+Files
On Tue, Feb 3, 2015 at 5:57 AM, Burak Emre emrekaba...@gmail.com wrote:
I added a field with a default value to an Avro schema which is
previously used for writing data. Is it possible to read the previous data
using only new schema which has that new field at the end?
I tried this scenario but unfortunately it throws EOFException while
reading third field. Even though it has a default value and the previous fields
is read successfully, I'm not able to de-serialize the record back without
providing the writer schema I used previously.
Schema schema = Schema.createRecord(test, null, avro.test, false);
schema.setFields(Lists.newArrayList(
new Field(project, Schema.create(Type.STRING), null, null),
new Field(city,
Schema.createUnion(Lists.newArrayList(Schema.create(Type.NULL),
Schema.create(Type.STRING))), null, NullNode.getInstance())
));
GenericData.Record record = new GenericRecordBuilder(schema)
.set(project, ff).build();
GenericDatumWriter w = new GenericDatumWriter(schema);
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
BinaryEncoder encoder = EncoderFactory.get().binaryEncoder(outputStream, null);
w.write(record, encoder);
encoder.flush();
schema = Schema.createRecord(test, null, avro.test, false);
schema.setFields(Lists.newArrayList(
new Field(project, Schema.create(Type.STRING), null, null),
new Field(city,
Schema.createUnion(Lists.newArrayList(Schema.create(Type.NULL),
Schema.create(Type.STRING))), null, NullNode.getInstance()),
new Field(newField,
Schema.createUnion(Lists.newArrayList(Schema.create(Type.NULL),
Schema.create(Type.STRING))), null, NullNode.getInstance())
));
DatumReaderGenericRecord reader = new GenericDatumReader(schema);
Decoder decoder =
DecoderFactory.get().binaryDecoder(outputStream.toByteArray(), null);
GenericRecord result = reader.read(null, decoder);
--
Sean
