Protocol Buffers are a serialisation solution developed by Google providing a platform and language neutral mechanism to send and receive structured data across the wire. Protocol Buffers encode data into dense binary objects that reduce packet sizes, enabling faster data exchange but with the disadvantage that the binary objects are not easily human readable.
Protocol Buffers allow developers to define how they want their application
data to be structured and the Remote Procedural Calls (RPC) to be made
available for a particular service. An example of a Protocol Buffer
proto3) definition follows with a “Greeter” service that takes
a user’s name and returns a customised greeting.
Very often we rely on inconsistent code at the boundaries of our applications
where rarely we enforce strict typing and structure of the data we exchange.
Being able to encapsulate the business semantics of an application within a
simple definition strengthens those boundaries providing a means to enforce
business logic. This functions well when all data is required to be present in
each message, however in less trivial applications business logic demands for
the ability to define values that may be nullable; something not possible with
proto3 using primitive data types. In contrast previous versions of Protocol
proto2, allowed for optional fields providing nullable
support for any field type.
To understand why, it may help to think about for what reasons
proto3 both omit fields from the encoded data. In
proto3 fields always have
a value and as such are never left unset. Because of this
proto3 can achieve
a smaller payload by not transferring fields set to their default values. This
incurs a form of compression, saving a few bytes on message exchange.
proto2 specification, in addition to the above, keeps track of whether
the current value was explicitly set on a field. It is this feature of
that allows for the optional field types. If the current value was not
explicitly set then the value is not transferred. This of course carries a
small penalty of needing extra storage space to transfer flags indicating
fields that have been explicitly set.
In light of this
proto3 has been designed to distinguish between the absence
of a primitive typed field and its default value through the use of the
official value wrapper
that form part of the
proto3 “standard library”.
The main difference here is the addition of the
StringValue submessage that
is a simple wrapper around a
proto3, submessages can be set to
nil allowing developers to use
wrappers for fields where the value may be optional. This adds a small amount
of overhead since wrapper values end up consuming an extra byte per field
because of the additional message layer. In addition the presence of the field
must be checked before it is used.
Despite this, one still cannot guarantee that the value in the wrapper was
explicitly set or defaulted to its zero value. However this moves the problem
of supporting optional values further away from the
proto3 specification and
more toward developer conventions. Typically if a submessage is
nil then an
application consuming the message can assume the field was deliberately (or
perhaps accidentally) omitted. If however the submessage has a non-nil value
then it must be assumed that whatever value is held by the wrapper was intended
to be consumed.