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Facing Lost Updates while trying to Horizontally Scale a Spring Boot APP connected to Postgres

The Architecture I am working on today consists of 2 instances of the same springboot app connected to a single datasource i.e PostgreSQL Database.
For all database queries I rely heavily on Spring Data JPA. I use the JpaRepository Interface to perform actions like findById , save etc.
The Spring Boot application mostly behaves like an Events ingestor, whose primary task is to take in requests and make updates in the database.
The Load balancer directs requests alternatively two each application server.
It is highly likely that 2 or more incoming concurrent requests need to access the same row/entity in the Database.
Today, even though we say repository.saveAndFlush(), we observe that the final save happened with a stale entity i.e some columns are not updated with the info from previous incoming requests.
Can someone point me in the right direction with the best design and spring data features to avoid such inconsistent states in the DB ?

spring mvc crud example without database

Is it possible to create a CRUD system without database in spring mvc framework?
If possible then which way?
I made an application where i can save and update a single value but i need to save and update a list of value.

HSQLDB offers in memory DB, you can use it to do crud operations for your unit tests. You can maintain spring configuration for the Unit tests and another spring configuration for deployed code. The db configuration can be different for both, so, you run the data updates on the in-memory databse when running tests and actual database when deployed to server. Spring takes care of this seamlessly.
http://hsqldb.org/ spring boot has this inbuild
Other ways to test out your code is to using mockito where you test your code on mock data/objects.
Update: there is something called DBUnit, just found out, it can also help you test against fake db

You Can recreate a CRUD operation using File Systems In Java using a File or Excel as a Database and and then Using File Handling or Apache POI in order to Create Update or Delete.

How to create Single Page Application (SPA) in java with hibernate and MySQL

I'm trying to choose best set of tools to create Single Page Applications. I would like to be able to use Java and Hibernate on server side along with MySQL. But what about ajax layer of SPA?
Maybe I have entirely wrong idea about that and Java and Hibernate make no sense in this case? But than how to implement complex server side operations?

It's rather question of UI part. The page asks a server for pages data. Java + hibernate could be used to implement service (e.g. REST service which returns data in JSON format). You can use SpringMVC to implement the service.
AJAX calls the service and process obtained data.

You can use angularJS for creating Single Page Application. this is the best framework i found for SPA development so far and using in most of projects developing which needs SPA.
for sample application refer to github
and detail about angularJS

Are there any design patterns that could work in this scenario?

We have a system (Java web application) that's been in active development / maintenance for a long time now (something like ten years).
What we're looking at doing is implementing a RESTful API to the web app. This web application, using Jersey, will be a separate project with the intent that it should be able to run alongside the main application or deployed in the cloud.
Because of the nature and age of our application, we've had to implement a (somewhat) comprehensive caching layer on top of the database (postgres) to help keep load down. Anyway, for the RESTful API, the idea is that GET requests will go to the cache first instead of the database to keep load of the database.
The cache will be populated in a way to help ensure that most things registered API users will need should be in there.
If there is a cache miss, the needed data should be retrieved from the database (also being entered into the cache in the process).
Obviously, this should remain transparent from the RESTful endpoint methods in my code. We've come up with the idea of creating a 'Broker' to handle communications with the DB and the cache. The REST layer will simply pass across ids (if looking to retrieve) or populated Java objects (if looking to insert / update) and the broker will take care of retrieving / updating / invalidating, etc.
There is also the issue of extensibility. To begin with, the API will be living alongside the rest of servers so access to the database won't be an issue however if we deploy to the cloud, we're going to need a different Broker implementation that will communicate with the system (namely the database) in a different manner (potentially through the use of an internal API).
I already have a rough idea on how I can implement this but it struck me that is probably a problem for which a suitable pattern could exist. If I could follow an established pattern as opposed to coming up with my own solution, that'll probably be a better choice. Any ideas?

Ehcache has an implementation of just such a cache that it calls a SelfPopulatingCache.
Requests are made to the cache, not to the database. Then if there is a cache miss Ehcache will call the database (or whatever external data source you have) on your behalf.
You just need to implement a CacheEntryFactory which has a single method:
Object createEntry(Object key) throws Exception;
So as the name suggests, Ehcache implements this concept with a pretty standard factory pattern...

There's no pattern. Just hide the initial DB services behind interfaces, build tests around their intended behavior, then switch in an implementation that uses the caching layer. I guess dependency injection would be the best thing to help you do that?

Sounds like decorator pattern will suit your need: http://en.wikipedia.org/wiki/Decorator_pattern.
You can create an DAO interface for data access, something like:
Value get(long id);
And firstly create a direct DB implementation, then create a Cache implementation which calls underlying DAO instance, in this case it should be the DB implementation.
The Cache implementation will try to get value from its own managed Cache, and from underlaying DAO if it fails.
So both of your old application or the REST will only see DAO interface, without knowing any implemntation details, and in future you can change the implementation freely.

The best design pattern for transparently caching HTTP requests is to use an HTTP cache.

Empty Hibernate cache on demand

I'm writing a soap web service: Jboss + Hibernate + Java. Database on PostrgreSQL. After publishing the webservice, it runs perfectly.
For testing purposes I change data on the database by opening pgAdmin, and changing the values on the rows by hand. Now, the problem is, Hibernate is not aware of those changes. Not until I re-publish the web service.
Is there any way to tell Hibernate to empty the cache or reload the data from the database so it will take the last values available?
Thanks!

I'm assuming you're talking about the second level cache...
The Cache API exposes several methods allowing to evict various regions. Check all the evictXxx() methods. Use SessionFactory#getCache() to obtain the Cache.