[Development] QtCS 2018 - Serialisation session notes

[Thiago Macieira]

Link: https://wiki.qt.io/QtCS2018_Serialisation

=== Binary JSON ===

* Was created for qtjsondb
* fast reading
* mmap()able
* Deprecate soon (Qt 5.12?)
** Remove from QJsonDocument in Qt 6
** Provide compat API to read
*** BSJON → JSON → parse again

=== QJsonDocument ===

* Limited to 128 MB of RAM footprint (due to Binary JSON)
* Needs to be fixed before Qt 6
* Can use the same backend as QCborValue
* Complaint: updating in-place is not easy
** If you update an entry in a node, you have to update the chain leading to 
it
** QCborValue has the same problem, as it was designed to have the same API

    QJsonValue foo = ...;
    foo["s"] = "value";             // does not compile
    foo.toObject()["s"] = "value";  // updates the temporary!
    
    // must write instead:
    QJsonObject fooObject = foo.toObject();
    fooObject["s"] = "value";
    setFoo(fooObject);

=== Common DOM API? ===

* Should we have a common API for manipulating trees of JSON-like values?
* QCborValue is a superset of QJsonValue
** But it would be confusing to users to use CBOR classes to manipulate JSON
** Generic name?
* What other uses would this API have?
** Replace QJSValue (QtQml)?
** Replace QVariantMap?
** Entry point for a Protobuf API?
* How to make sure people can't insert combinations not allowed in the output?
** For example, associative containers using integers as keys in JSON
** Do they do that? Maybe they won't write such code
*** They know what their content is used for
** We could use templates, specialising for CBOR, JSON, etc.
** We could have a wrapper class that has inlines and provides only the 
possible combinations

* QCborValue integrated last Friday into QtCore 5.12
** Need to know in the next few weeks if we keep it for 5.12
** Can yank it out and move to a new module for Tech Preview

=== Serialising Qt state ===

* Needed by Qt Remote Objects
* Slightly different from CBOR and JSON purposes
** Not about a standardised representation of a data model
** More about transmitting state from two independent processes of the same 
application
* Currently using QDataStream
** Has a lot of problems, can't really detect errors and not extensible enough
* Need more exploration, no conclusion

=== Protobuf ===

* Need volunteers to write a Proof of Concept
** plugin to protoc?

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center