Hi,
At La Poste Pôle Colis we use Avro in our new reactive architecture (kafka,
spark streaming, Cassandra, elasticsearch, play framework).
In our modeling we used the type union to bring together in one schema all
trace events of a package (arrival, departure, transportation, ...) at the body
attribute.
Example :
{
"namespace" : "fr.laposte.colis.schema.pivot.message",
"name" : "Message",
"type" : "record",
"doc" : "Cette structure défini les caractéristiques de base d'un message.
Peut(doit) être spécialisée pour un usage particulier",
"fields" : [
{
"name" :
"header",
"type" :
"fr.laposte.colis.schema.pivot.common.message.MessageHeader",
"doc" : "Entête
du message"
},{
"name" : "body",
"type" :
["fr.laposte.colis.schema.pivot.announcement.AnnouncementEventMessageBody",
"fr.laposte.colis.schema.pivot.delivery.DeliveryEventMessageBody",
"fr.laposte.colis.schema.pivot.handling.HandlingEventMessageBody",
"fr.laposte.colis.schema.pivot.crm.CrmEventMessageBody",
"fr.laposte.colis.schema.pivot.customs.transport.CustomsTransportMessageBody",
"fr.laposte.colis.schema.pivot.customs.consignment.CustomsContainerEventMessageBody",
"fr.laposte.colis.schema.pivot.customs.consignment.CustomsParcelEventMessageBody",
"fr.laposte.colis.schema.pivot.rest.common.Rest",
"fr.laposte.colis.schema.pivot.reject.RejectMessageBody",
"fr.laposte.colis.schema.pivot.dpmo.defectrequest.DefectRequestEventMessageBody",
"fr.laposte.colis.schema.pivot.dpmo.defectresult.DefectResultEventMessageBody",
"fr.laposte.colis.schema.timeout.TimeoutMessageBody",
"fr.laposte.colis.schema.notification.Notification"
],
"doc" :
"Abstraction du corps de message. Peut-être substitué par tout type dérivé du
type MessageBody"
}
]
}
However, as well explained at
(https://martin.kleppmann.com/2012/12/05/schema-evolution-in-avro-protocol-buffers-thrift.html)
: "Union types are powerful, but you must take care when changing them. If you
want to add a type to a union, you first need to update all readers with the
new schema, so that they know what to expect. Only once all readers are
updated, the writers may start putting this new type in the records they
generate"
My question : is a default value for field "body" is sufficient so that if the
reader encounters a union branch it does not know about, it can substitute the
default value (see
http://grokbase.com/t/avro/user/11b3bn6r6z/does-extending-union-break-compatibility)
?
Thank you in advance for your help.
