H2 Database features

Benefits of H2
** Can be incorporated into Java programs **
- Embedded and server modes; in-memory databases

The main features of H2 are:

Very fast, open source, JDBC API

Embedded and server modes; in-memory databases

Browser based Console application

Small footprint: around 2 MB jar file size

	H2	Derby	HSQLDB	MySQL	PostgreSQL
Pure Java	Yes	Yes	Yes	No	No
Memory Mode	Yes	Yes	Yes	No	No
Encrypted Database	Yes	Yes	Yes	No	No

From H2 Database Engine

** Pure java **: Made in Java
- ex: H2/Derby/HSQLDB
Since it can be embedded in your own application, it will start up automatically when the application is started **
Embedded / server modes / in-memory databases can be used

server modes

If you start the h2 server, you can access it from other servers as well.

in-memory databases modes

Run in-memory
Start with h2.start (); in a Java program on the Java virtual machine
In-memory data is not saved in HDD or externally but saved in memory when data is ejected by SQL statement with insert statement.
** Disappears on reboot **
** Cannot be seen from other servers because it is only in memory **
In-memory, the DB will be cleared the next time you start it, so it is often used in development!

Embedded modes

Like in-memory, it runs inside a Java program on a Java virtual machine
Data is saved externally on HDD / SSD
** Can be accessed from outside **
** Data remains even after a crash **
The same unit test cannot be performed if the DB is updated when the DB unit test is performed.

Relationship between H2 and JDBC

In the past, there were many cases where the DB was directly accessed from Application using a library specific to the DB such as Oracle ** (such as using OracleLib in C ++).
With this method ** DB cannot be changed freely and products cannot be rearranged ** (Oracle version or MySQL version cannot be used)
→ In Java ** JDBC is solved! ** **
DB access basics
JDBC: ** SQL abstraction mechanism **
Access the specified DB (Oracle, etc.) based on JDBC jdbc_url instead of hitting Oracle directly
A further ** abstraction ** by accessing the Oracle library by JDBC and accessing the RDB
If you read jdbc_url and write h2, JDBC will access H2DB through the h2 library.
The app and RDB were directly connected (** tightly coupled ), but now the app and RDB are ** indirect ** via JDBC ( abstraction module **)! (Becomes ** loosely coupled **)
The app doesn't have to be involved in what the DB is, it only needs to know where to access it by URL (JDBC accesses the DB by URL)
No matter what the RDB is **, the app doesn't need to change it and JDBC will do it nicely **!