Codility Lessions 4 Sorting - NumberOfDiscIntersections Simple Solution

Essentially, we treat the input as a set of line segments with different start and end values. If we sort the lines by their start values, and iterate from the smallest start value line, then for a given line segment, what it overlaps can be calculated by counting the lines which start before the end of the give line segment. After we have to remove this line's start value from the start values array, otherwise we get duplicate counts from other line segments overlapping with the one we already counted. When we remove, any duplicate start values won't matter. I tried to implement this using a Line class, creating two arrayLists for start values and lines. Which is somewhat lots of code.

But for this question, there is another input property we can use. Notice that for the input array A, the end value of A[j] line cannot be smaller than the start value of A[i] line if i<j. So we can just sort the start values array and iterate through input array A. For any A[i] and A[j] where i<j, if end value of A[i]  is larger than start value of A[j], they must overlap.

Also, be careful with integer overflow.

A Simple Multi-Tenancy Implementation with Hibernate and Spring

For our implementation of the multi-tenant application, Each tenant's data is kept in a physically separate database instance. For each database instance a connection pool is set up when the application starts. For any http request that is not authenticated, a default connection pool is used, pointing to where the user login info is stored. After login the tenant identifier is stored in the Spring Authentication object, so that the corresponding connection pool for that user can be identified. For Spring Security login, we use hibernate, entity manager, and UserDetailService for user authentication.

The User class implements UserDetails interface, so that after user is authenticated we can retrieve this user instance from security context.



For Spring Security, we have to implements UserDetailService.


Then we have to config Persistence configurations.


Notice that we don't have to config a DataSource bean. Instead, we added additional configuration properties for hibernate. We have to create both MyTenantIdentifierResolver and MyMultiTenantConnectionProvider.

First let's look at the MyTenantIdentifierResolver, which essentially output a string for MyMultiTenantConnectionProvider class to choose the right connection pool.


Inside MyMultiTenantConnectionProvider, we first initialize a map containing all connection pools for all tenant. For the overridden method selectConnectionProvider(String s), the input string is provided by MyTenantIdentifierResolver, and the output is a ConnectionProvider object containing a datasource for the specific tenant.



Lastly all we need to define is a custom ConnectionProvider to hold a DataSource.

Use Reflection for Spring Data Jpa Specification to enable filter on all attributes of entities

Problem:

We have a large number of domain entities that we want to expose to front end. Front end would like the users to be able to filter on those entities using almost all attributes that those entities have.

Also, when the entities form one to one or one to many relations, the users should be able to filter on the attributes of the related entities as well.

If we use Spring Data JpaRepository interface, we can filter entities using Query Methods:


However if the User entity has 5 attributes, then there are 31 possible combinations of those 5 attributes that the user should be able to filter the results, which means 31 methods have to be defined. And we have to decide which method to be invoked in the controller class based on URL parameters. It will be the same situation using @query.

Solution:

So instead we have to use JpaSpecificationExecutor for our repositories and pass a Specification instance to the findAll() method of entity repositories. We create a BaseSpecification class to transform url filter parameters into predicats.

First we try to make every attribute of the entities filterable. To do that, we have to read the attribute name from the url query string and test whether the query key is one of the entity's attributes. We achieve this by using the Reflection API:
But we also want to be able to filter on attributes from the entities that are associated with the entity to be filterd. We design the api as such: http://localhost:8080/demo/api/user?address.street=5th will find all users whose address has the attribute Street and has the value 5th.

At this point the front end rises the requirement that the legacy front end code couldn't provide the entity name that is associated with the entity to be filtered. So the to find the users whose address has attribute street and value 5th, the query string looks like this: http://localhost:8080/demo/api/user?street=5th

In this case we have to search if the url query key is one of the attributes from the filtered entity or from entities associate with it. We can achieve this search using either Breadth First Search or Depth Firsts Search.


Here the search output a Pair. The Path object is later to be used to build the Criteria Predicate. And the Class object tells us the type of the attribute used for filtering. If the type is Date for example, we can generate Date specific Criteria Predicates.