Discard an already saved entity - breeze

Question

I have a distributed system where users can make changes into one single database. To illustrate the problem, let's assume we have the following entities:
public class Product{
public int Id{get;set;}
public List<ProductOwner> ProductOwners{get;set;}
}
public class ProductOwner{
public int ProductId { get; set; }
[ForeignKey("ProductId")]
[Inversroperty("ProductOwners")]
public Product Product{ get; set; }
public int OwnerId { get; set; }
[ForeignKey("OwnerId")]
public Owner Owner{ get; set; }
}
public class Owner{
public int Id{get;set;}
}
Let's also assume we have two users, UserOne and UserTwo connected to the system.
UserOne adds Product1 and assigns Owner1 as an owner. As a result, a new ProductOwner1 is created with key=[Product1.Id, Owner1.Id]
UserTwo does the same operation, another instance ProductOwner2 with key=[Product1.Id, Owner1.Id] is created. This will result in an EF exception on the server side, which is expected, as a row with key=[Product1.Id, Owner1.Id] already exists in the database.
Question
The issue above can be partly resolved by having some sort of real time data refresh on both UserOne and UserTwo machines (I am already doing this) and running a validation task on the server to ignore and not save entities that are already in the DB.
The remaining issue is how to tell Breeze on 'userTwo' machine to mark ProductOwner2 as saved and change its state from Added to Unchanged?

Answer 1

I think this is an excellent question and has been raised enough that I wanted to chime in on how I would do it given the above scenario in hopes others can find a good way to accomplish this from a Breeze.js perspective as well. This answer doesn't really address server logic so it is incomplete at best.
Step 1 - Open a web socket
First and foremost we need some way to tell the other connected clients that there has been a change. SignalR is a great way to do this if you are using the ASP.NET MVC stack and there are a bunch of other tools.
The point is that we don't need to have a great way of passing data down and forcing it in to the client's cache, we just need a lightweight way to tell the client that some information has changed and if they are concerned with this to refresh something. My recommendation in this area would be to use a payload that tells the client either what entity type and Id changed or give a resource to the client to let them know what collection of entities to refresh. Two examples of a JSON payload that would work well here -
{
"entityChanges": [
{
"id": "123",
"type": "product",
"new": false
},
{
"id": "234",
"type": "product",
"new": true
}
],
collectionChanges: [
{
"type": "productOwners"
}
]
}
In this scenario we are simply telling the client that the products with Ids of 123 and 234 have changed, and that 234 happens to be a new entity. We aren't pushing any data about what properties have changed to the client as that is their responsibility to decide whether to refresh or requery for data. There is also the possibility of telling the client to refresh a whole collection like in the second array but I will focus on the first example.
Step 2 - Handle the changes
Ok we got a payload from our web socket that we need to pass to some analyzer to decide whether to requery. My recommendation here is to check if that entity exists in cache, and if so, refresh it. If a flag comes down in the JSON that says it is a new entity we probably also need to requery it. Here is some basic logic -
function checkForChanges (payload) {
var parsedJson = $.parse(payload);
$.each(parsedJson.entityChanges, function (index, item) {
// If it is a new entity,
if (item.new === true) {
// Go get it from the database
manager.fetchEntityByKey(item.type, item.id)
.then(fetchSucceeded).fail(fetchFailed);
} else {
// Check local cache first
var localentity = manager.getEntityByKey(item.type, item.id);
// And if we have a local copy already,
if (localentity) {
// Go refresh it from the database
manager.fetchEntityByKey(item.type, item.id)
.then(fetchSucceeded).fail(fetchFailed);
}
}
}
}
Now there is probably some additional logic in your application that need to be handled but in a nut shell we are -
Opening up a lightweight connection to the client to listen for changes only
Creating a handler for when those changes occur
Applying some logic on how to query for or refresh the data
Some considerations here are you may want to use different merge strategies depending on various conditions. For instance if the entity already has changes you may want to preserve changes, where as if it is a entity that is always in a state of flux you may want to overwrite changes.
http://www.breezejs.com/sites/all/apidocs/classes/MergeStrategy.html
Hope this provides some insight, and if it doesn't answer your question directly I apologize for crowding up the answers : )

Answer 2

Would it be possible to catch the entity framework / unique key constraint error on the breeze client and react by creating a new entity manager (using the createEmptyCopy method), loading the relevant ProductOwner records and using them to determine which ProductOwner records in the original entityManager need to be set "unchanged" using the entity's entityAspect's setUnchanged method. Once this "synchronization" is done the save changes can be retried.
In other words, the client is optimistic the save will succeed but can recover if necessary. The server remains oblivious to the potential race condition and has no custom code.
A brute force approach, apologies if I'm stating the obvious.