Hi Yacine,
I believe Ryan was mentioning that if you start from a schema `[“null”, “Car”]`
then rather than add a new bus branch to the union, you could update the car’s
schema to include new bus fields. For example (using IDL notation):
> // Initial approach
> // Evolved to union { null, Car, Bus }
> record Car {
> string vin;
> }
> record Bus {
> string vin;
> int capacity;
> }
>
> // Alternative approach
> // Here evolution wouldn't change the union { null, Vehicle }
> // Note also that this is actually a compatible evolution from the first
> approach, even under current rules (using aliases for name changes).
> record Vehicle {
> string vin; // (Would be optional if not all branches had it.)
> union { null, int } capacity; // The new field is added here.
> }
Pushing this further, you could also directly start upfront with just a
“Vehicle” record schema with an optional `car` field and add new optional
fields as necessary (for example a `bus` field). This gives you much greater
flexibility for evolving readers and writers separately (but you lose the
ability to enforce that at most one field is ever present in the record).
I agree that Avro’s current evolution rules are more geared towards reader
evolution: evolving writers while keeping readers constant is difficult (namely
because they don't support adding branches to unions or symbols to enums).
However, adding a completely new and separate "compatibility mode” has a high
complexity cost; both for implementors (the separate classes you mention) and
users (who must invoke them specifically). It would be best to keep evolution
rules universal.
Maybe we could extend the logic behind field defaults? Enum schemas could
simply contain a default symbol attribute. For unions, it’s a bit trickier, but
we should be able to get around with a default branch attribute which would
allow evolution if and only if the newly added branches can be read as the
default branch. Assuming the attribute names don’t already exist, this would be
compatible with the current behavior.
I think this use-case is common enough that it’d be worth exploring (for
example, Rest.li [1] adds an `$UNKNOWN` symbol to somewhat address this issue;
though not completely unfortunately [2]).
-Matthieu
[1] https://github.com/linkedin/rest.li
[2]
https://github.com/linkedin/rest.li/wiki/Snapshots-and-Resource-Compatibility-Checking#why-is-adding-to-an-enum-considered-backwards-incompatible
> On Mar 29, 2016, at 2:40 AM, Yacine Benabderrahmane
> <[email protected]> wrote:
>
> Hi Ryan,
>
> Just a little up^^. Could you please (or anyone else) give me a little
> feedback ?
> Thanks in advance.
>
> Regards,
> Yacine.
>
> 2016-03-21 17:36 GMT+01:00 Yacine Benabderrahmane <[email protected]
>> :
>
>> Hi Ryan,
>>
>> Thank you for giving feedback. I will try in the following to provide you
>> some more details about the addressed problem.
>>
>> But before that, just a brief reminder of the context. Avro has been
>> chosen in this project (and by many other ones for sure) especially for a
>> very important feature: enabling forward and backward compatibility
>> management through schema life-cycle. Our development model involves
>> intensive usage of this feature, and many heavy developments are made in
>> parallel streams inside feature teams that share the same schema, provided
>> the evolution of the latter complies with the stated compatibility rules.
>> This implies that all the entities supported by the Avro schema must
>> support the two-way compatibility, including unions. However, in the
>> special case of the union, this two-way compatibility is not well supported
>> by the current rules. Let me explain you the basement of our point of view,
>> it remains quite simple.
>>
>> The use case is to have, for example,
>> - a first union version A:
>> { "name": "Vehicle",
>> "type": ["null", "Car"] }
>> - a new version of it B:
>> { "name": "Vehicle",
>> "type": ["null", "Car", "Bus"] }
>> For being forward compatible, an evolution of the union schema must
>> guarantee that an old reader reading with A can read the data written with
>> the new schema B. Getting an error just means that the forward
>> compatibility feature is broken. But this is not actually the case (and
>> this behavior is not suitable), because the old reader has a correct schema
>> and this schema has evolved naturally to version B to incorporate a new
>> Vehicle type. Not knowing this new type must not produce an error, but just
>> give the reader a default value, which means: "Either the data is not there
>> or you do not know how to handle it".
>>
>> This is thought while keeping in mind that in an object-oriented code
>> modeling, a union field is seen as a class member with the higher level
>> generic type ("Any" (scala) or "Object" (java5+)...). Therefore, it is
>> natural for a modeler / programmer to implement the ability of not getting
>> the awaited types and using some default value of known type. To give a
>> more complete specification, the new mode of compatibility has to impose
>> one rule: the union default value must not change through versions and the
>> corresponding type must be placed at the top of the types list. This is
>> much easier to handle by development streams, because it is addressed once
>> for all in the very beginning of the schema life-cycle, than the fact to
>> oblige a number of teams, among which some are just not in place anymore,
>> to update the whole code just because another dev team has deployed a new
>> version of the union in the schema.
>>
>> Now, for being backward compatible, the reader with B must always be able
>> to read data written with schema A. Even if the type included in the data
>> is not known, so it gets the default value and not an error.
>>
>> I understand that getting an error could make sense when the requested
>> field is not present. However, this behavior:
>>
>> - is very restrictive, meaning: this obliges the old reader to update
>> its code for integrating the new schema, while he is not managing to do it
>> for many reasons: development stream of next delivery is not finished, or
>> not engaged, or not even planned - in the case of old and stable code
>> - breaks the forward compatibility feature: the older reader is not
>> able to read the new version of the union without getting an error
>> - breaks the backward compatibility feature: the new reader is not
>> able to read an old version containing unknown types of the union without
>> getting an error
>>
>> By the way, what do you exactly mean by "pushing evolution lower" and
>> "update the record"? Could you please give me an example of the trick you
>> are talking about?
>>
>> Just to be a bit more precise, we are not targeting to use a "trick". This
>> life-cycle management should be included in a standard so to keep the
>> software development clean, production safe and compliant with a complex
>> product road-map.
>>
>> Finally, you seem to be concerned by the "significant change to the
>> current evolution rules". Well, we actually do not change these rules, they
>> keep just the same. All we are proposing is to introduce a *mode* where
>> the rules of union compatibility change. This mode is materialized by a
>> minimum and thin impact of the existing classes without any change in the
>> behavior, all the logic of the new compatibility mode is implemented by new
>> classes that must be invoked specifically. But you would better see it in
>> the code patch.
>>
>> Looking forward to reading your feedback and answers.
>>
>> Regards,
>> Yacine.
>>
>>
>> 2016-03-17 19:00 GMT+01:00 Ryan Blue <[email protected]>:
>>
>>> Hi Yacine,
>>>
>>> Thanks for the proposal. It sounds interesting, but I want to make sure
>>> there's a clear use case for this because it's a significant change to the
>>> current evolution rules. Right now we guarantee that a reader will get an
>>> error if the data has an unknown union branch rather than getting a
>>> default
>>> value. I think that makes sense: if the reader is requesting a field, it
>>> should get the actual datum for it rather than a default because it
>>> doesn't
>>> know how to handle it.
>>>
>>> Could you give us an example use case that requires this new logic?
>>>
>>> I just want to make sure we can't solve your problem another way. For
>>> example, pushing evolution lower in the schema usually does the trick:
>>> rather than having ["null", "RecordV1"] => ["null", "RecordV1",
>>> "RecordV2"], it is usually better to update the record so that older
>>> readers can ignore the new fields.
>>>
>>> Thanks,
>>>
>>> rb
>>>
>>> On Mon, Mar 14, 2016 at 7:30 AM, Yacine Benabderrahmane <
>>> [email protected]> wrote:
>>>
>>>> Hi all,
>>>>
>>>> In order to provide a solution to the union schema evolution problem,
>>> as it
>>>> was earlier notified in the thread "add a type to a union
>>>> <
>>>>
>>> http://search-hadoop.com/m/F2svI1IXrQS1bIFgU1/union+evolution&subj=add+a+type+to+a+union
>>>>> "
>>>> of the user mailing list, we decided, for the needs of the reactive
>>>> architecture we have implemented for one of our clients, to implement an
>>>> evolution of the compatibility principle of Avro when using Unions. For
>>>> reminder, the asked question was about the way to handle the case where
>>> a
>>>> reader, using an old version of a schema that includes a union, reads
>>> some
>>>> data written with a new version of the schema where a type has been
>>> added
>>>> to the union.
>>>>
>>>> As answered by Martin Kleppman in that thread, one way to handle this
>>> kind
>>>> of evolution (a new version of the schema adds a new type type in a
>>> union)
>>>> would be to ensure that all the development streams have integrated the
>>> new
>>>> schema B before deploying it in the IT schema referential.
>>>> However, in big structures involving strongly uncorrelated teams (in the
>>>> product life-cycle point of view), this approach appears to be quite
>>>> impracticable, causing production stream congestion, blocking behavior
>>>> between teams, and a bunch of other
>>>> unwanted-counter-agile-/-reactive-phenomena...
>>>>
>>>> Therefore, we had to implement a new *compatibility* *mode* for the
>>> unions,
>>>> while taking care to comply with the following rules:
>>>>
>>>> 1. Clear rules of compatibility are stated and integrated for this
>>>> compatibility mode
>>>> 2. The standard Avro behavior must be kept intact
>>>> 3. All the evolution implementation must be done without introducing
>>> any
>>>> regression (all existing tests of the Avro stack must succeed)
>>>> 4. The code impact on Avro stack must be minimized
>>>>
>>>> Just to give you a very brief overview (as I don't know if this is
>>> actually
>>>> the place for a full detailed description), the evolution addresses the
>>>> typical problem where two development streams use the same schema but in
>>>> different versions, in the case described shortly as follows:
>>>>
>>>> - The first development stream, called "DevA", uses the version A of
>>> a
>>>> schema which integrates a union referencing two types, say "null" and
>>>> "string". The default value is set to null.
>>>> - The second development team, called "DevB", uses the version B,
>>> which
>>>> is an evolution of the version A, as it adds a reference to a new
>>> type
>>>> in
>>>> the former union, say "long" (which makes it "null", string" and
>>> "long")
>>>> - When the schema B is deployed on the schema referential (in our
>>> case,
>>>> the IO Confluent Schema Registry) subsequently to the version A
>>>> - The stream "DevA" must be able to read with schema A, even if
>>> the
>>>> data has been written using the schema B with the type "long" in
>>>> the union.
>>>> In the latter case, the read value is the union default value
>>>> - The stream "DevB" must be able to read/write with schema B,
>>> even if
>>>> it writes the data using the type "long" in the union
>>>>
>>>> The evolution that we implemented for this mode includes some rules that
>>>> are based on the principles stated in the Avro documentation. It is even
>>>> more powerful than showed in the few lines above, as it enables the
>>> readers
>>>> to get the default value of the union if the schema used for reading
>>> does
>>>> not contain the type used by the writer in the union. This achieves a
>>> new
>>>> mode of forward / backward compatibility. This evolution is for now
>>> working
>>>> perfectly, and should be on production in the few coming weeks. We have
>>>> also made an evolution of the IO Confluent Schema Registry stack to
>>> support
>>>> it, again in a transparent manner (we also intend to contribute to this
>>>> stack in a second / parallel step).
>>>>
>>>> In the objective of contributing to the Avro stack with this new
>>>> compatibility mode for unions, I have some questions about the
>>> procedure:
>>>>
>>>> 1. How can I achieve the contribution proposal? Should I directly
>>>> provide a patch in JIRA and dive into the details right there?
>>>> 2. The base version of this evolution is 1.7.7, is it eligible to
>>>> contribution evaluation anyway?
>>>>
>>>> Thanks in advance, looking forward to hearing from you and giving you
>>> more
>>>> details.
>>>>
>>>> Kind Regards,
>>>> --
>>>> *Yacine Benabderrahmane*
>>>> Architect
>>>> *OCTO Technology*
>>>> <http://www.octo.com>
>>>> -----------------------------------------------
>>>> Tel : +33 6 10 88 25 98
>>>> 50 avenue des Champs Elysées
>>>> 75008 PARIS
>>>> www.octo.com
>>>>
>>>
>>>
>>>
>>> --
>>> Ryan Blue
>>> Software Engineer
>>> Netflix
>>>
>>
>>
