[aerogear-dev] sync

Summers Pittman supittma at redhat.com
Mon Jan 6 11:30:09 EST 2014 

On 01/05/2014 11:42 PM, Douglas Campos wrote:
> On Thu, Jan 02, 2014 at 10:13:50AM -0500, Summers Pittman wrote:
>> On 12/20/2013 06:34 AM, Erik Jan de Wit wrote:
>>> So we already agreed that CouchDB has some nice features, but why
>>> wouldn't we just use pouchdb for javascript and touchdb? I've just
>>> tested the javascript and it syncs with the server perfectly, even
>>> if there are some things missing we could add them because these
>>> projects are open source.
>> Because wrapping CouchDB and friends to AeroGear with no other reason
>> than to create a org.jboss.sync name space is dumb.
> can't +9001 enough
>
>> Also the how of sync is still a bit far off. I'm still interested in the
>> "what" of sync. What use cases are we supporting, what features are we
>> supporting, what are our goals, what does a working finished sync impl
>> look like, etc.
> drafted here: https://gist.github.com/qmx/8278287
>
> # aerogear-sync
>
> While I was reviewing Summers' code and ideas, I realized that I really wanted
> everything he did, but as a second step after we nail down the basics.
>
> ## basics?
>
> Since we've been catering the enterprise market, this essentially means we need
> to get the __boring__ stuff right first, then move over to the __shiny__ stuff,
> like realtime data sync, update policies & friends.
>
> ### data model
>
> For starters, I think that the most important thing that needs to be agreed
> upon is the data model and the atomic operations around it. As previous
> discussed, I really like CouchDB's datamodel -- and hate erlang ;)
>
> `{_id:<guid>, content:<arbitrary json>, rev:<last revision>}`
>
> #### JS
>
> Well, it's JSON, it _Just Works_&trade;
>
> #### Java
>
> I didn't want to pick on Java, but since its fame forces me to it. First stab
> (a courtesy of our friend Dan Bevenius):
>
>      public interface Document {
>          public String id;
>          public String content;
>          public String rev;
>      }
>      
> We naturally want to kick this a notch, and use objects instead of plain strings:
>
>      public interface Document<T, ID> {
>      	public ID id;
>      	public T content;
>      	public String rev;
>      }
>      
> In this case, we can use the convention requiring that `T` is any **object
> serializable to  JSON**. `ID` is a convenience shorthand since it's a
> **GUID/UUID**. I think this key isn't necessarily a natural key (a surrogate
> key instead).
>
> #### Objective-C
>
> <volunteers needed> ;)
>
> ### API levels
>
> As soon as we have a rough data-model defined, we can start dabbling around
> different API levels to be served:
>
> (parts **I think** are potentially deliverable for a 1.0)
>
> - level 0: explodes when there's a conflict
> - level 1: semi-automatic conflict resolution via something like google's diff-match-patch
> - level 2: business rules determine who wins a conflicting update (supervisor
> wins over normal user)
>
> (parts **I think** are potentially deliverable for a 2.0)
>
> - level 3: real-time updates via diff-match-patch
> - level 4: real-time updates via OT/EC
>
> All those proposed API operations should be serializable, meaning I can
> potentially keep doing changes offline then just replying them to the server
> when online.
>
> ### transport
>
> Since we know about the future-looking ideas on v2.0, it would be really nice
> for us to specify a very simple/dumb JSON-based protocol for those change
> messages. Something that could accomodate both the full document updates and
> the OT/EC incremental bits too. I have no ideas on this, tbh.
>
> ## boring usecases
>
> ### scenario 1
>
> Building inspector system - we have mobile apps that store relevant info and
> are bound to be accessed on places where we won't have any kind of connection,
> or very poor signal.
>
> You can have several inspectors screening the same building simultaneously.
>
> Let's say we have Agnes and Joe are doing the fire extinguisher inspection in a
> new hospital building. Technically each fire extinguisher has its own
> identifier and can be an independent document. In this case we would have no
> conflict happening.
>
> Now they start finding expired fire extinguishers and start to add them to the
> report. This report could potentially have two divergent lists of fire
> extinguishers to be replenished/revalidated, as the building's compliance
> status.
>
> ### scenario 2
>
> Census system - we have mobile apps focused on offline data collection. We have
> the previous year's info that needs to be updated on the server. The
> interviewee needs to take a call, then asks the interviewer to come back later.
> This results in two sets of changes for the same document, stacked together,
> which should work flawlessly.
>
> Any other ideas/comments?

I'll add now :)

## Transactions

These are the most basic parts of sync I can think of that our system 
should be able to do/manage.  Our internal representation of the client 
documents and collections should make implementing this automatically 
and without user intervention as simple as possible

* Detect Change
     When a user changes her local data, the system should note the 
change and generate a sync message to send to the server. This can be 
done automatically or manually but SHOULD be done automatically.

* Send update
     When a sync message is ready to be sent, and the system allows for 
it to be sent (network available, not in blackout window from 
exponential backoff, etc) then sync message should be sent.  This SHOULD 
be done automatically.

* Receive Update
     When a client updates it data and successfully syncs to the remote 
server, the remote server will notify all of the relevant clients.  The 
client must automatically and without user intervention receive this 
update and either act on it or store it for later processing.

* Apply Update
     Once a client application has an update message from the server, it 
can apply the message correctly to the data on it.  This should be done 
automatically as part of receiving the update, but it may be done 
manually or may be delayed and automatically executed later.

* Detect Conflict
     When applying an update fails, the system must detect this.  The 
system will provide state to the application and/or the user to handle 
the conflict.  The user MUST NOT have to check for conflicts on her own.

* Resolve Conflict
     There must be a mechanism for resolving a conflict.  The CAN be 
done automatically using default resolvers provided by AeroGear, by 
using a resolver provided by the developer/user, or by the app user 
selecting the correct merge.  This will probably generate a new sync 
message.



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