Improve error handling if a job is annotated by several users

[nmanovic]

Now if a job is modified by several clients in parallel it will lead to an exception on server side (#190). Users complain about such behaviour.

• It is better to keep a version of annotation inside DB and pass it to client during initialization. When a client wants to save work it will update the version and sent it together with annotations. Thus server will clearly understand that there is a conflict and provide meaningful error message.

• Also it is good to have "force" flag to override existing annotations in DB explicitly. Thus user will be able to save annotations (delete old and add new).

• As discussed maybe ID should be unique inside a frame (high part of 64-int should contain frame number, low part of the int should contain a local id). Thus it will be easy to annotate in parallel different frames. But such approach works bad for tracks (users can draw different tracks for one object on different frames).

[nmanovic]

Andrei Spiridonov poxyu 06:40
Is it possible for several people to annotate same video/image in parallel? I tested it, but got error with status 400. Thank you!

poxyu , it is possible but you need to have several jobs for that (segment_size parameter when you create an annotation task). You got 400 error because we track ids of objects during annotations (aka client_id) and they become inconsistent if several persons annotate the same job. We know about the issue and think how to gracefully solve it.