What is a load-balaner?
You cannot combine these three features all together “Cheap, Fast, and Good.
The CAP theorem applies a similar type of logic to distributed systems—namely, that a distributed system can deliver only two of three desired characteristics: consistency, availability, and partition tolerance (the ‘C,’ ‘A’ and ‘P’ in CAP).
A distributed system is a network that stores data on more than one node (physical or virtual machines) at the same time. Because all cloud applications are distributed systems, it’s essential to understand the CAP theorem when designing a cloud app so that you can choose a data management system that delivers the characteristics your application needs most.
Let’s take a detailed look at the three distributed system characteristics to which the CAP theorem refers.
Consistency
Consistency means that all clients see the same data at the same time, no matter which node they connect to. For this to happen, whenever data is written to one node, it must be instantly forwarded or replicated to all the other nodes in the system before the write is deemed ‘successful.’
Availability
Availability means that any client making a request for data gets a response, even if one or more nodes are down. Another way to state this—all working nodes in the distributed system return a valid response for any request, without exception.
Partition tolerance
A partition is a communications break within a distributed system—a lost or temporarily delayed connection between two nodes. Partition tolerance means that the cluster must continue to work despite any number of communication breakdowns between nodes in the system.
The microservice architecture pattern. Requests often span multiple services. Each service handles a request by performing one or more operations, e.g. database queries, publishes messages, etc.
How to understand the behavior of an application and troubleshoot problems?
Implement each business transaction that spans multiple services as a saga. A saga is a sequence of local transactions. Each local transaction updates the database and publishes a message or event to trigger the next local transaction in the saga. If a local transaction fails because it violates a business rule then the saga executes a series of compensating transactions that undo the changes that were made by the preceding local transactions.
There are two ways of coordinating sagas:
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POST /orders request and creates an Order in a PENDING state
POST /orders request and creates an Order in a PENDING stateJava 9 introduces a new level of abstraction above packages, formally known as the Java Platform Module System (JPMS), or “Modules” for short.
Java 9 modules are one of the biggest changes in the structure of Java.
What is Module?
A Module is a group of closely related packages and resources along with a new module descriptor file (module-info.java).
In other words, it's a “package of Java Packages” abstraction that allows us to make our code even more reusable.
"Why we need Java SE 9 Module System” means the problems of the Current Java SE8 or earlier System.
Java SE 9 Module System is going to provide the following benefits
We know what JDK software contains. After installing JDK 8 software, we can see a couple of directories like bin, jre, lib etc in Java Home folder. However, Oracle Corp has changed this folder structure a bit differently as shown below.
As of now, Java 9 Module System has 95 modules in Early Access JDK. Oracle Corp has separated JDK jars and Java SE Specifications into two sets of Modules.
Java 9 Module System has a “java.base” Module. It’s known as Base Module. It’s an Independent module and does NOT dependent on any other modules. By default, all other Modules are dependent on this module. That’s why “java.base” Module is also known as The Mother of Java 9 Modules. It’s the default module for all JDK Modules and User-Defined Modules just like Object class is a base class of all the other classes.
Module Descriptor
When we create a module, we include a descriptor file that defines several aspects of our new module:
There are four types of modules in the new module system:
We can see what these modules are by typing into the command line:
To set up a module, we need to put a special file at the root of our packages named module-info.java.
This file is known as the module descriptor and contains all of the data needed to build and use our new module.
And all public types exported from a dependency are accessible by our module when we use this directive.
Requires Static
Sometimes we write code that references another module, but that users of our library will never want to use.
For instance, we might write a utility function that pretty-prints our internal state when another logging module is present. But, not every consumer of our library will want this functionality, and they don't want to include an extra logging library.
In these cases, we want to use an optional dependency. By using the requires static directive, we create a compile-time-only dependency:
We commonly work with libraries to make our lives easier.
But, we need to make sure that any module that brings in our code will also bring in these extra ‘transitive' dependencies or they won't work.
Luckily, we can use the requires transitive directive to force any downstream consumers also to read our required dependencies:
Exports
By default, a module doesn't expose any of its API to other modules. This strong encapsulation was one of the key motivators for creating the module system in the first place.
Our code is significantly more secure, but now we need to explicitly open our API up to the world if we want it to be usable.
We use the exports directive to expose all public members of the named package:
Exports... To
We can use exports…to to open up our public classes to the world.
But, what if we don't want the entire world to access our API?
We can restrict which modules have access to our APIs using the exports…to the directive.
Similar to the exports directive, we declare a package as exported. But, we also list which modules we are allowing to import this package as a requires. Let's see what this looks like:
Uses
A service is an implementation of a specific interface or abstract class that can be consumed by other classes.
We designate the services our module consumes with the uses directive.
Note that the class name we use is either the interface or abstract class of the service, not the implementation class:
Provides With
A module can also be a service provider that other modules can consume.
The first part of the directive is the provides keyword. Here is where we put the interface or abstract class name.
Next, we have the with directive where we provide the implementation class name that either implements the interface or extends the abstract class.
Here's what it looks like put together:
Open
We mentioned earlier that encapsulation was a driving motivator for the design of this module system.
Before Java 9, it was possible to use reflection to examine every type and member in a package, even the private ones. Nothing was truly encapsulated, which can open up all kinds of problems for developers of the libraries.
Because Java 9 enforces strong encapsulation, we now have to explicitly grant permission for other modules to reflect on our classes.
If we want to continue to allow full reflection as older versions of Java did, we can simply open the entire module up:
If we need to allow reflection of private types, but we don't want all of our code exposed, we can use the opens directive to expose specific packages.
But remember, this will open the package up to the entire world, so make sure that is what you want:
Okay, so reflection is great sometimes, but we still want as much security as we can get from encapsulation. We can selectively open our packages to a pre-approved list of modules, in this case, using the opens…to directive:
We know, Java SE 7 has introduced a new exception-handling construct: Try-With-Resources to manage resources automatically. The main goal of this new statement is “Automatic Better Resource Management”. Java SE 9 is going to provide some improvements to this statement to avoid some more verbosity and improve some Readability.
Java SE 7 example
How does try with resources work?
try-with-resources is available to any class that implements the AutoCloseable interface. In the above example, BufferedReader implements an AutoCloseable interface.
switch statement to support the following new features:yield in Java 13)Sealed classes and interfaces restrict which other classes or interfaces may extend or implement them.
For background information about sealed classes and interfaces, see JEP 409.
One of the primary purposes of inheritance is code reuse: When you want to create a new class and there is already a class that includes some of the code that you want, you can derive your new class from the existing class. In doing this, you can reuse the fields and methods of the existing class without having to write (and debug) them yourself.
However, what if you want to model the various possibilities that exist in a domain by defining its entities and determining how these entities should relate to each other? For example, you're working on a graphics library. You want to determine how your library should handle common geometric primitives like circles and squares. You've created a Shape class that these geometric primitives can extend. However, you're not interested in allowing any arbitrary class to extend Shape; you don't want clients of your library declaring any further primitives. By sealing a class, you can specify which classes are permitted to extend it and prevent any other arbitrary class from doing so.
Declaring Sealed Classes
To seal a class, add the sealed modifier to its declaration. Then, after any extends and implements clauses, add the permits clause. This clause specifies the classes that may extend the sealed class.
Constraints on Permitted Subclasses
Permitted subclasses have the following constraints:
They must be accessible by the sealed class at compile time.
For example, to compile Shape.java, the compiler must be able to access all of the permitted classes of Shape: Circle.java, Square.java, and Rectangle.java. In addition, because Rectangle is a sealed class, the compiler also needs access to FilledRectangle.java.
They must directly extend the sealed class.
They must have exactly one of the following modifiers to describe how it continues the sealing initiated by its superclass:
final: Cannot be extended further
sealed: Can only be extended by its permitted subclasses
non-sealed: Can be extended by unknown subclasses; a sealed class cannot prevent its permitted subclasses from doing this
For example, the permitted subclasses of Shape demonstrate each of these three modifiers: Circle is final while Rectangle is sealed and Square is non-sealed.
They must be in the same module as the sealed class (if the sealed class is in a named module) or in the same package (if the sealed class is in the unnamed module, as in the Shape.java example).
For example, in the following declaration of sample.Shape, its permitted subclasses are all in different packages. This example will compile only if Shape and all of its permitted subclasses are in the same named module.