On Tue, May 30, 2017 at 9:06 AM, Yacov Manevich <m.yaco...@gmail.com> wrote:
> but there are some corner cases where that would not be true
>>
>
> Can you elaborate please?
>
Sure, the main thing is that protocol buffers do not have a canonical
format, and there are many ways to create a valid protocol buffer in
serialized form that will not match what you would get from serializing a
message the ordinary way. For example, you can take two serialized protocol
buffers and concatenate them together and get back a valid proto
representing the merge of the original two. In that case, un-marshaling and
re-marshaling that concatenated message would often result in a smaller
size even if there were no unknown fields.
Another thing to keep in mind is that we are planning to reintroduce
support for preserving unknown fields in proto3, so the behavior around
unknown fields is going to change soon.
> Why do you want to detect which schema was used to serialize the message
> anyway?
>
> because I have a project where we have a distributed system that part of
> it might be upgraded, but another part might not (depends on the costumer /
> admin) and I want to prevent an earlier version of the software to process
> information that came from a later version, because I want the earlier
> version to process the information only when it is upgraded.
>
I see; in that case I would recommend adding a field to the message that
explicitly indicates whether the message is in the old form or new form.
> On Tuesday, 30 May 2017 18:59:04 UTC+3, Adam Cozzette wrote:
>>
>> I am not that familiar with the Go implementation of protobuf, but my
>> guess is that this would be somewhat difficult to do reliably. If you
>> un-marshal and re-marshal the message and get a smaller size, then usually
>> that would indicate there were some unknown fields, but there are some
>> corner cases where that would not be true. Why do you want to detect which
>> schema was used to serialize the message anyway?
>>
>> On Sun, May 28, 2017 at 10:27 AM, Yacov Manevich <m.ya...@gmail.com>
>> wrote:
>>
>>> Hi all.
>>>
>>> I am in need of knowing whether I have received a protobuf message
>>> (protobuf version 3)
>>> that has additional fields than the schema that I currently have
>>> contains.
>>> Any pointers on how to do so, specifically in golang?
>>>
>>>
>>> *The longer version:*
>>>
>>> Let's assume I have a protobuf schema at version v1:
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> *message Foo { Bar bar = 1; Zig z = 2; Zag zz =
>>> 3;}message Bar { uint64 n = 1; bytes b = 2; bytes d =
>>> 3;}*
>>>
>>> *Zig *and *Zag *are not relevant
>>>
>>> and at some point in the future I will be forced to upgrade it to v2 by
>>> adding another field to *Bar*:
>>>
>>>
>>>
>>>
>>>
>>>
>>> *message Bar { uint64 n = 1; bytes b = 2; bytes d =
>>> 3; uint64 nn = 4;}*
>>>
>>> Let's also assume that from some reason I can only make modifications to
>>> the code but not to the proto structure at version v1.
>>> What I want is to be able to support backward and forward compatibility.
>>>
>>> - Backward compatibility is easy - if the message was created at v1,
>>> and the application (written in golang) is at version v2, the nn field
>>> will have the zero value and I can have the application act accordingly.
>>> - Forward compatibility - I want to be able to know at the runtime
>>> of application v1 that given bytes of a serialized message, that whether
>>> it
>>> was created with a protobuf schema of v1 or v2.
>>> How can I do this?
>>> What I thought of this far is un-marshaling the bytes to a message,
>>> and then marshalling back to bytes and comparing the length of the two
>>> buffers I have - if the new buffer is smaller than the first one, it
>>> means
>>> the message
>>> I got contains additional fields that my protobuf schema (v1) wasn't
>>> able to recognize.
>>>
>>>
>>> Is there a more elegant and sure way to do so?
>>>
>>>
>>>
>>> Regards and thanks.
>>>
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>>
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