Forem: Adya Shukla

Big Word : Multi Tenancy

Adya Shukla — Sun, 30 Aug 2020 14:21:39 +0000

What is Multi Tenancy ?
It sounds like a big big word but its explanation is quite simple.
SO what it is !!!
According to wikipedia,
"The term "software multitenancy" refers to a software architecture in which a single instance of software runs on a server and serves multiple tenants. Systems designed in such manner are often called shared (in contrast to dedicated or isolated)."
Simply said it is a software architecture pattern where a single application is shared by multiple customers and they own separate databases.

It is quite common for many businesses to use Single Tenant approach. So what is that?
In simple words, Single Tenancy is where a single application has a single customer.

Now comes the point what does multi tenancy provides us that single tenancy does not and vice versa :

Multi tenancy reduces the cost
Adding new customers becomes easy
Maintaining the same application becomes easier
Also it maximizes resource usage

On the other hand,

Single tenancy provides an option for customization where in single tenants can customize their app according to their needs.
Single tenants can decide what option or update to they want for their product.

Obviously there is no preference guideline to choose between the two as it truly depends on the requirement.
The goal of this small article was to make familiar with both the options we have.
Hope it helps !

Overview of Queries type in Spring Data JPA

Adya Shukla — Wed, 26 Aug 2020 02:13:43 +0000

This post deals with types of JPA query available. JPA queries are of 3 types mainly :

Query
NativeQuery
CriteriaQuery

1 Query: It is written in Java Persistence Query Language (JPQL) syntax and used to perform Crud operations.
Query query = getEntityManager().createQuery("SELECT m FROM MenuEntity m WHERE m.id=:id");

There are two subtypes of query as well :
TypedQuery
NamedQuery

a . TypedQuery : JPA can't infer what query result type will be and thus we have to cast the type. JPA provides a special Query sub-type known as a TypedQuery. This is always preferred to use if we know our Query result type beforehand.

TypedQuery<Menu> query
= getEntityManager().createQuery("SELECT m FROM Menu m WHERE m.id=:id", Menu.class); **
In above query we can see Menu type assigned to TypedQuery.

b. NamedQuery : We provide NamedQuery to entity class, which gives a quick way to read Entity related queries. All NamedQueries must have a unique name. If there is a single NamedQuery then we can use @NamedQuery on the entity class, but if there are multiple then we use @NamedQueries and add comma separated @NamedQuery in it.

For example :
@Entity
@NamedQuery(name="Menu.findAll", query="SELECT m FROM Menu m")
public class Menu {
...
}

2 .NativeQuery : NativeQuery is very simple to use as it provide provides flexibility to write queries that are not provided by JPQL syntax.

Select m from Menu m

3 .CriteriaQuery : Criteria queries are mainly useful to build dynamic queries whose exact structure is only known at runtime.

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery q = cb.createQuery(Menu.class);
Root c = q.from(Menu.class);
q.select(c);

What is the difference between using JPQL and Criteria Queries?
JPQL queries are defined as simple strings similar to SQL queries, but criteria queries are defined by creating Java objects that represents query elements.
Whenever we have to build dynamic queries then criteria queries are preferred else its good to use JPQL for simple queries.
Also when we use criteria queries then we can detect errors at compile time itself.

Docker Basics

Adya Shukla — Sun, 23 Aug 2020 12:16:30 +0000

As a newbie the first question that comes to mind is What is Docker ?
So, A docker is a command line program, a background daemon and set of remote services that simplifies installation, running, publishing or removing software.

Docker is NOT virtualization.

Running docker means running two programs : docker CLI and docker daemon.
Docker containers are isolated with 8 aspects:
1 . PID namespace : process identifiers and capabilities
2 . UTS namespace : host and domain space
3 . MNT namespace : File system access and structure
4 . IPC namespace : Process communication over shared memory
5 . NET namespace : Network access and structure
6 . USR namespace : User name and identifiers
7 . chroot() : controls location of file system root
8 . cgroups : Resource protection

What are Images ?
A bundled snapshot of all the files that should be available to a program running inside a container.

What is container ?
It is a running or stopped instance of some image.

We can create as many container from an image but containers that are created from same image don't share changes to their file systems.
Here are few basic commands to run :

1 . docker run hello-world
You will get output as :
Hello from Docker!
This message shows that your installation appears to be working correctly.

2 . docker run --detach --name web nginx : latest
or
docker run -d --name web nginx : latest
where -d or --detach will start the program in background

3 . To check currently running containers :
docker ps
This will only display running containers but if you want to see all including those in exited state then :
docker ps -a

4 . To check docker logs :
docker logs

5 . To continue watching logs :
docker logs --follow

These are few very basics of the docker. Hope it helps !!!