What you're bringing up is a common problem for Kafka, but I think we
can separate a couple of the issues here.
First, I think it is a good idea to include the fingerprint with the
encoded bytes, regardless of how schemas are coordinated. There's no way
to reliably read those bytes if you don't know the write-side schema.
Even if you plan on only ever having one schema and coordinate by never
changing it, I think it is better to have a check that the schema you're
reading with matches the write side. It is better to have a reasonable
failure (unknown write schema) than to read corrupt data or fail while
deserializing.
The second problem is schema coordination. There are a few approaches
here, including a schema repository service like you suggest or
SchemaStore like Martin mentioned. I think we should keep the two
concerns separate because there are different ways of tackling this
issue without necessarily running a service.
rb
On 12/23/2015 03:19 AM, Niels Basjes wrote:
Yes, as a generic crossplatform solution this makes a lot of sense. It s
easy to build and stops consuming the messages as soon as the fingerprint
changes.
In my corporate reality I see that a source of messages puts them into
Kafka, then several consumers read and deserialize them in near-realtime.
In the static situation of "the schema doesn't change" having a fingerprint
of the schema would suffice.
But now the schema changes. I would like to deploy the new version of the
source system, and consume these new messages using the schema migration
idea. I.e. read the records as long as they are backwards compatible.
As a general assumption: it is impossible to deploy a new version of all
consuming applications at the same time.
I want as little down time as possible and I do not want to lose any
messages.
Is this possible with a simple fingerprint?
The only direction I can think of right now is by having a central registry
that contains all schemas ever used, indexed by their fingerprint.
What alternative solutions can we think of?
Niels
On Wed, 23 Dec 2015, 01:42 Sean Busbey <bus...@cloudera.com> wrote:
Including a schema fingerprint at the start
1) reuses stuff we have
2) gives a language independent notion of compatibility
3) doesn't bind how folks get stuff in/out of the single record form.
--
Sean Busbey
On Dec 22, 2015 06:52, "Niels Basjes" <ni...@basjes.nl> wrote:
I was not clear enough in my previous email.
What I meant is to 'wrap' the application schema in a serialization
wrapper
schema that has a field indicating the "schema classname".
That (generic setup) combined with some generated code in the schema
classes should yield a solution that supports schema migration.
Niels
On Tue, Dec 22, 2015 at 11:55 AM, Niels Basjes <ni...@basjes.nl> wrote:
Thanks for pointing this out.
This is exactly what I was working on.
The way I solved the 'does the schema match' question at work is by
requiring that all schema's start with a single text field "schema
classname" being the full class name of the class that was used to
generate
it.
That way we can have newer versions of the schema and still be able to
unpack them. In this form the classname is essentially an indicator if
schema migration is possible; even though the schemas are different.
What do you think of this direction?
Niels
On Mon, Dec 21, 2015 at 11:30 PM, Ryan Blue <b...@cloudera.com> wrote:
Niels,
This sounds like a good idea to me to have methods like this. I've had
to
write those methods several times!
The idea is also related to AVRO-1704 [1], which is a suggestion to
standardize the encoding that is used for single records. Some
projects
have been embedding the schema fingerprint at the start of each
record,
for
example, which would be a helpful thing to do.
It may also be a good idea to create a helper object rather than
attaching new methods to the datum classes themselves. In your example
below, you have to create a new encoder or decoder for each method
call. We
could instead keep a backing buffer and encoder/decoder on a class
that
the
caller instantiates so that they can be reused. At the same time, that
would make it possible to reuse the class with any data model and
manage
the available schemas (if embedding the fingerprint).
I'm thinking something like this:
ReflectClass datum = new ReflectClass();
ReflectData model = ReflectData.get();
DatumCodec codec = new DatumCodec(model, schema);
# convert datum to bytes using data model
byte[] asBytes = codec.toBytes(datum);
# convert bytes to datum using data model
ReflectClass copy = codec.fromBytes(asBytes);
What do you think?
rb
[1]: https://issues.apache.org/jira/browse/AVRO-1704
On 12/18/2015 05:01 AM, Niels Basjes wrote:
Hi,
I'm working on a project where I'm putting Avro records into Kafka
and
at
the other end pull them out again.
For that purpose I wrote two methods 'toBytes' and 'fromBytes' in a
separate class (see below).
I see this as the type of problem many developers run into.
Would it be a good idea to generate methods like these into the
generated
Java code?
This would make it possible to serialize and deserialize singles
records
like this:
byte [] someBytes = measurement.toBytes();
Measurement m = Measurement.fromBytes(someBytes);
Niels Basjes
P.S. possibly not name it toBytes but getBytes (similar to what the
String
class has)
public final class MeasurementSerializer {
private MeasurementSerializer() {
}
public static Measurement fromBytes(byte[] bytes) throws
IOException {
try {
DatumReader<Measurement> reader = new
SpecificDatumReader<>(Measurement.getClassSchema());
Decoder decoder =
DecoderFactory.get().binaryDecoder(bytes,
null);
return reader.read(null, decoder);
} catch (RuntimeException rex) {
throw new IOException(rex.getMessage());
}
}
public static byte[] toBytes(Measurement measurement) throws
IOException {
try {
ByteArrayOutputStream out = new ByteArrayOutputStream();
Encoder encoder =
EncoderFactory.get().binaryEncoder(out,
null);
SpecificDatumWriter<Measurement> writer = new
SpecificDatumWriter<>(Measurement.class);
writer.write(measurement, encoder);
encoder.flush();
out.close();
return out.toByteArray();
} catch (RuntimeException rex) {
throw new IOException(rex.getMessage());
}
}
}
--
Ryan Blue
Software Engineer
Cloudera, Inc.
--
Best regards / Met vriendelijke groeten,
Niels Basjes
--
Best regards / Met vriendelijke groeten,
Niels Basjes
--
Ryan Blue
Software Engineer
Cloudera, Inc.
