>What happens when a client on a mobile device loses its connection for a short period of time and then reconnects?
The edits/patches/operations on the client would be stacked up and later when an connection is available that stack of edits/patches/operations would be sent to the server.
 
>What if load balancing cause the client to reconnect to a different process in the cluster?
If we allow for different types of what we currently call a DataStore to be pluggable, then it would be possible for that client to reconnect to a different instance with the assumption here that the underlying datastore is distributed.
 
>BTW, please tell me if the whole purpose of AeroGear's Data Sync feature is to satisfy this and only this scenario. If so, then I apologize for being a distraction.)
No, it is not the only scenario. We want to satisfy as many scenarios as possible, but we also have to start somewhere and preferable start small and build out from there. In the roadmap, we outlined as the first steps what is called conflict resolution. This is what our initial focus will be on and it will require minimal server side changes to existing server application (no server side state for clients). Our hope is that doing this might gain us some early adopters. This does not rule out that this could not be evolve into something like you've described in you email above, more about this later.
 
> If the client goes offline and then back online, the Data Sync functionality in the SDK would figure out which of the local entities the client has changed, and request from the server the latest revisions for each of them. Data Sync then merges the local changes onto the latest revisions (perhaps asking the user to manually resolve anything that cannot be automatically resolved), computes the new effective changes, and then sends a single batch request to the server to apply them. Some, all or none changes are accepted, and likely the application then has to decide whether to continue the process again, revert to the server’s versions, etc.
This sounds simliar to what we have discussed as conflict resolution in the Data Sync roadmap. It differs somewhat, like we don't specify anything with regard to batches as the goal it to limit the changes need for existing server implementations, in that the server only detects a conflict and does not try to apply any effective changes/operations/patches but instead works with complete objects/documents.
 
>The SDK would also make use of subscriptions/notifications so that it can be told immediately when entities are changed.
This is considered as a later part of conflict resolution where we notifications are planned to be used.
 
>The kinds of requests needed to support data sync in this scenario are fairly basic, so the server doesn’t have to be that complicated. Best of all, the server is not required to maintain any client-specific state.
This does sound like what we are trying to do with the conflict resolution approach. I know that it is even more basic than what you have described here but we could extend the roadmap to include such features.
 
I think there will be use cases for both the conflict resolution and the data sync approaches and users that might be quite happy with conflict resolution. I think it would make sense to split the two roadmaps/specs into separate ones. At the moment it looks, and was the original idea, that this would involve into the "real-time" datasync solution. But have these separate would hopefully make this clear that these are independent and can evolve independently.
Does this sound alright with everyone?
 
Thanks,
 
/Dan


On 5 September 2014 20:32, Randall Hauch <rhauch@redhat.com> wrote:
This does sounds quite interesting, Lukáš. If AeroGear uses Differential Synchronization, this approach might make it more efficient for the server to implement the behavior.

But I’m still bothered by the thought that AeroGear’s Data Sync feature uses any algorithm that requires the server to maintain client-specific state. 

First of all, servers that maintain no client state across requests will *always* scale much better than servers that maintain client-specific state across requests. Maintaining client-specific state requires extra work, consumes more memory, and complicates clustering, failover, and fault tolerance.

Secondly, if a server is maintaining client-specific state, how long does that client state have to be maintained before assuming the client is no longer there? What happens when a client on a mobile device loses its connection for a short period of time and then reconnects? What if load balancing cause the client to reconnect to a different process in the cluster? Bottom line is that it adds quite a bit of complexity.

Finally, I completely understand the benefits of using Differential Synchronization where many clients are collaborating on a single document. That’s the “collaborative editor” scenario, and surely there are apps that need this functionality. I’m just not convinced that this is the most common use case. In fact, I think it’s relatively uncommon, and I don’t think LiveOak will support it in the near term. (BTW, please tell me if the whole purpose of AeroGear's Data Sync feature is to satisfy this and only this scenario. If so, then I apologize for being a distraction.)

My understanding of the Data Sync feature, though, is that it is should be applicable to other scenarios. IMO, the far more common scenario is:

  • The server manages (many) millions of small entities, each of which is a domain-specific aggregate JSON document. Examples of different kinds of entities include: customers, books, insurance claims, catalog items, restaurants, purchases, tasks, etc. A single customer entity would aggregate most/all the information about the customer, including the name, phone numbers, addresses, profile information, domain-specific privileges (e.g., authorized to log issues or call support), etc. Note that these aggregate entities do not correspond to the entities in a traditional RDBMS/JPA/Hibernate app, where you’d have much finer-grained records (separate tables for customer, customer addresses, customer phone numbers, customer privileges, etc.).
  • At any point in time there may be 1Ks or 10Ks of clients reading dozens of documents/entities and sending changes to some of them (likely in batch operations). IOW, any one document might be concurrently modified by a relatively small number of clients.

I think it’s possible to support Data Sync in this scenario while maintaining no client-specific state on the server (other than while processing a single request).

The Data Sync functionality of the SDK used in the clients would work with local persistence and enable the app to function online or offline. This includes making it possible to edit entities and create new entities while offline, though it does not matter to Data Sync which subset of entities is persisted locally. If the client goes offline and then back online, the Data Sync functionality in the SDK would figure out which of the local entities the client has changed, and request from the server the latest revisions for each of them. Data Sync then merges the local changes onto the latest revisions (perhaps asking the user to manually resolve anything that cannot be automatically resolved), computes the new effective changes, and then sends a single batch request to the server to apply them. Some, all or none changes are accepted, and likely the application then has to decide whether to continue the process again, revert to the server’s versions, etc.  (The SDK would also make use of subscriptions/notifications so that it can be told immediately when entities are changed.)

What the batch operations entail is dependent upon the needs of the client app and capabilities of the server. If a server supports batch operations that contain the entire entity, then the server will almost certainly need to put revision identifiers in each entity sent to the client, so that when the client sends back a modified representation entity, the server can tell whether that document has since been modified by another client. If so, the server might rely upon client preferences (see below) to know whether to merge the changes (using a simple algorithm or more useful application-specific logic), overwrite, or fail to update the entity.

On the other hand, the server might support batch operations with partial updates, where the client only sends for each entity only the set of fields that are to be changed or removed. Conflicts and merging with this approach are easier and less-likely to be application specific, though client preferences (see below) might still dictate whether or not to merge based upon revision numbers. It also might make it easier for Data Sync, since the SDK simply has to record for each modified entity only those modified fields (no matter how long the client was offline). To keep the SDK simple, a best practice might be to ensure clients always modify together those fields that must be consistent. For example, in a contacts application, if a user modifies the first name of a contact, the app might tell the SDK to modify the first name and last name fields, even though the user didn’t modify the last name. When synchronized, both the first name and last name fields would be sent to the server as a single update. This helps ensure that even when multiple clients are concurrently updating the same set of fields to different values, the result of applying all of those changes will exactly match the state as set by one of the clients.

It’s also likely that for batch operations to work well for many different kinds of applications, the server may support multiple policies that specify for a given batch operation the atomicity, consistency, and isolation guarantees. One policy might ensure that the entire batch either completely succeeds only when there are no revision conflicts, otherwise it completely fails. Another policy might be eventually-consistent in the sense that the changes in every batch operations will eventually be applied, though this may require the server to have application-specific conflict resolution logic. And there are policies that are somewhere in-between. For example, imagine a server that supports “public” collections whose entities can be read/updated by any user, and “private” collections that expose only the entities that are readable and editable by that user. The likelihood of a concurrent update conflict on a private entity is quite small, since it’s possible only when different changes made on different devices are submitted at exactly the same time. On the other hand, the likelihood of a concurrent update conflict on a public entity is much higher. The server may allow updates to both public and private entities within a single batch operation, and a save policy that might update all private entities atomically (they all succeed or they all fail) while updates to *each* public entity is atomic (some might succeed while others might fail).

Any given server will likely support only a few of these policies. But either way, the server has to report back to the client the outcome of the batch request: which entities were updated, which were not, and the reason why each of the rejected updates failed (because other entity changes were rejected, because of a conflict, etc.). The SDK’s Data Sync functionality would use those results to know how to update the local persisted representations, and to start trying to resolve any failures.

The kinds of requests needed to support data sync in this scenario are fairly basic, so the server doesn’t have to be that complicated. Best of all, the server is not required to maintain any client-specific state.

Thoughts? Am I way off-base?

Best regards,

Randall




On Sep 4, 2014, at 10:38 AM, Lukáš Fryč <lukas.fryc@gmail.com> wrote:

Hey guys,


TL;DR: there are ways how to make the memory footprint of Differential Synchronization (DS) very low; if we assume that JSON patches are reversible and we create cumulative patches from series of individual patches, we can use (smart, garbage-collactable) revision control for storage of documents that server need to maintain.

----

I had an idea in my head that for couple of days as I was becoming familiar with how Differential Synchronization (DS) works and studying Dan's and Luke's impl.

I share the concern that Randall expressed in earlier thread - the DS in its pure version doesn't scale for huge amount of users when connected to one server.

Sure, the algorithm can be scaled trivially by adding new nodes that uses very same algorithm as is used for client<->server. But that doesn't mean it is something that we should do regularly to get more memory available.


Instead, I was thinking about limiting a memory that server needs to maintain in any point in time.


In pure DS, server have to maintain all copies of documents that clients ever requested. Even worse, it has to maintain two copies - "shadow" and "backup",

So, in worst case, 2 x N copies of document will be maintained by the server for N clients.

-----

The DS algorithm doesn't tell us how the "shadow" and "backup" documents should be stored.

We can transparently plug in a storage that will be clever about how to use "backups" and "shadow".

-----

CONCEPTS:

1) REVISION CONTROL

In order to limit a number of documents we need to maintain in server's memory, we can come with a system where just last known state is remembered in full version. Together with last known version, we remember also history of the document for each revision as it was patched by clients.

But server doesn't have to maintain full history of the document, it maintains just revisions that are known to its clients (that revision is known by DS for each client out of the box).

2) GARBAGE COLLECTION

Server trivially knows what versions are mantained by client, so it can get rid of those revisions that are no longer needed.

First, it can cut all the history that is older than the oldest version of document throughout all its clients.

Secondly, it can accumulate the serious of patches between states so that it doesn't have to maintain full history, but just revisions that is known to any of those clients.

----

DIAGRAM:

I've attached a picture for illustration of the revision control / garbage collection, it does not illustrate algorithm itself (Dan did awesome job describing DS here [3]).

In the attached diagram, you can see four clients with different revisions:

Client 1: revision X
Client 2: revision X+3
Client 3: revision X+3
Client 4: revision X+4

If any of the clients reaches server, server can reconstruct the document (shadow and backup) for given client revision and the DS algorithm then can operate as normally (that's why it is transparent from algorithmic point of view).

If server comes to garbage collection, it can scan through available client revisions. As it identifies, that the oldest known revision is X, it can remove patch for X-1 and less as they are no longer needed.

Later, it can even more optimize, and accumulate patches so that the only information he knows is how to get to the revision for each participating client. In the picture, he can compute cumulative patch for getting from state X to state X+3, as X+1 and X+2 themselves are not needed.

(Patch accumulation is actually something that Google Wave does in its Operational Transformation implementation, just there it is called cumulative Operations).

----

Then it comes to caching strategies, you can maintain caches of things like last recently used revisions (they don't need to be computed each time). You can store some documents and their revision history to the disk (fully or partially), etc.

----

THE USE CASE:


1000 employees maintain one document - Contact List - on their smartphones

400 of those employees has mobile data plan, so they want to synchronize almost immediately as they are notified about changes. The rest of the devices is connected rather sparely.

If one employee changes data, 600 devices are notified about the change immediately (some on data plan, some through Wifi, etc.) Those clients all have version say X+3, because they are synchronized proactively.

These clients reach the server in say <10 seconds after that employee did the change.

First client reaches server and ask for version X+3 (it probably is still in the memory, but if it isn't, it is deconstructed and placed into MRU cache). All of the others reaches the server and re-synchronize themselves against the one copy of backup/shadow that is in the memory already.

The other clients will reach the server later, as they are connecting to Wifi, say in 5 minutes to 24 hours. They will have even older versions, such as X. Those clients will require more computation, but at the end the server can resolve them and through their revisions out of the window.

At the end, there are clients that didn't connected for days, maybe months - I suggest we don't actually use DS here and fallback to e.g. slow synchronization (show me what you have, I will show you mine), because maintaining full history for longer period of days may be too resource demanding (configurable?).

Obviously, 1000 is not much, but this can scale to pretty decent numbers if we use hybrid approach (fallback to other sync strategies if revision is already forgotten by a server).

----

WORST CASE:

we still have to maintain all the document revisions, but practically it won't happen :-)

----

ASSUMPTIONS:

1. patches are reversible (we are able to compute reverse patch for each patch sent by client)
2. patches are cumulative (we are able to compute aggregated patch from series of patches)

For 1, it seems many of JSON-patch libraries actually implement it (Java, JavaScript) [1, 2]

For 2, it again seems some libraries implement it and if not, we can implement it or even use brute-force - compute a patch between two documents


----

It's not the only way how to make it scale, but it does solve the problem pretty independently of the DS algorithm and still leaves the algorithm clean and simple.



Cheers,

~ Lukas


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