AWS Application Integration Services: SQS, SNS and SWF

We will look at some basic points of AWS Application Integration services like SQS, SNS and SWF.

SQS (Simple Queue Service)

1. Fully managed message queues for microservices, distributed systems, and serverless applications.

2. Enables application components and microservices to communicate with each other.

3. Pull based system

4. Queue Types: Standard and FIFO

5. Can be used with Redshift, DynamoDB, RDS, EC2, ECS, Lambda, S3 and SNS.

6. Multiple copies of every message are stored redundantly across multiple availability zones so that they are available whenever needed.

For more details, you can read documentation

SNS (Simple Notification Service)

1. Fully managed pub/sub messaging service for microservices, distributed systems, and serverless applications.

2. Push-based, many-to-many messaging.

3. Publishers to Topic: EC2, S3, RDS, CloudWatch

4. Subscribers to Topic: Serverless functions (Lambda), Queues (SQS), HTTP/S endpoints and distributed systems. Additionally, SNS fans out notifications to end users via mobile push messages, SMS, and email.

5. SNS uses cross availability zone message storage to provide high message durability. 

6. SNS Topic owners can keep sensitive data secure by setting topic policies that restrict who can publish and subscribe to a topic.

SWF (Simple Workflow Service)

1. SWF lets you write your application components and coordination logic in any programming language and run them in the cloud or on-premises.

2. SWF creates a logical separation between tasks and components and acts as a task coordinator. 

AWS Storage Services: S3, Glacier, EBS, EFS, FSx and Storage Gateway

AWS provides following services under storage section:

1. S3
2. Glacier
3. EFS
4. FSx
5. Storage Gateway

Following are some basic and important points about AWS Storage services:

S3

1. Cloud storage service like Dropbox and Google Drive.

2. Object based storage not block level (like EBS and EFS). Data is treated as object. Single object in S3 can be uploaded up to 5TB in multi-part. You cannot install OS and software on it.

3. Buckets: Data is stored in buckets which are similar to Windows folders. Bucket name must be in lower case and alphanumeric. Bucket name must be unique globally. By default bucket is private.

4. Versioning: Versioning takes more space as each version is saved individually in same or different bucket. Versioning must be done for cross-region replication. Once versioning is enabled, it can’t be disabled, only suspended.

5. Storage Class: 

• Standard (Frequently accessed data, minimum storage duration: 30 days,  99.999999999% durability (11 times 9))
• Intelligent-Tiering (Long-lived data with changing or unknown access patterns)
• Standard-IA (Long-lived, infrequently accessed data, minimum storage duration: 30 days, 99.999999999% durability (11 times 9))
• One Zone-IA (Long-lived, infrequently accessed, non-critical data)
• Glacier (Data archiving with retrieval times ranging from minutes to hours, minimum storage duration: 90 days)
• Reduced Redundancy (Not recommended, Frequently accessed, non-critical data which even if get lost, it does not hamper you)

6. Encryption: 

• SSE-S3 (uses AES 256 encryption methods)
• SSE-KMS (Key Management Service)
•  SSE-C (Client Side Encryption)

7. Bucket URL syntax: https://s3.regionname.amazonaws.com/bucketname/objectname

8. Eventual Consistency: When we upload a new file to S3, it becomes available immediately, but when we perform overwrite and delete operation, there is some delay which is known as eventual consistency. When a file upload to S3 is successful, it returns HTTP200 status.

9. Security: Data is secured using ACL (Access Control List) and Bucket Policies at Bucket or Object level. You can write custom bucket policies using JSON.

10. Data Transfer Acceleration: Enables quick upload of data to S3 bucket over long distance using CloudFront.

11. Lifecycle Management: You can manage transition of file from one storage class to another using Lifecycle rules. For example, you can move a file from Standard Storage Class to Infrequently Access Storage Class after some days (min 30 days) if it is not frequently used now. Similarly, if you want to archive this file after some days (min 30 days), you can further move it to Glacier.

12. Static Website Hosting: You can host static website and customize the URL using Route53.

Glacier 

1. Data Backup and Archive

2. Types of data: Hot Data (which we need on daily basis), Cold Data (which we don’t need on daily basis, archive this data to Glacier).

3. Delay in retrieval time and may take 3-5 hours.

4. Minimum storage duration in Glacier is 90 days. Archives deleted before 90 days incur a pro-rated charge equal to the storage charge for the remaining days.

EBS

1. Elastic Block Storage (just like Hard Disk of your laptop and can only be used by mounting on an EC2 instance unlike S3).

2. Backup of EBS volumes is called Snapshot and is done in incremental fashion. You can also take point-in-time snapshots of your EBS volumes and save it on S3.

3. To take backup of Root EBS (where OS is running), you must stop it first for data integrity.

4. Root EBS can’t be encrypted and “Delete on Termination” is checked by default.

5. To share snapshots between AWS accounts, make sure snapshots MUST NOT be encrypted.

6. Multiple Availability Zone is NOT supported.

7. Cannot attach one EBS volume to multiple EC2. Use EFS for this.

8. RAID0, RAID1 and RAID10 (combination of both) are preferred. RAID5 is discouraged.

9. EBS Volume Types

• General Purpose (SSD) (gp2) volumes can burst to 3000 IOPS, and deliver a consistent baseline of 3 IOPS/GiB. 
• Provisioned IOPs (SSD) (io1) volumes can deliver up to 64000 IOPS, and are best for EBS-optimized instances. 
• Max Throughput Optimized HDD (ST1) – For frequent accessed data
• Max Cold HDD (SC1) – For IA (in-frequent accessed data)
• Magnetic volumes, previously called standard volumes, deliver 100 IOPS on average, and can burst to hundreds of IOPS. Lowest cost

EFS

1. Elastic File System somewhat like EBS. 

2. EFS can be mounted on several EC2 instances and on-premise servers at the same time unlike EBS.

3. EFS currently only works with Linux, not with Windows.

4. EBS has fixed amount of storage while EFS can be scaled whenever required.

5. Coming soon, the Amazon EFS Infrequent Access storage class.

6. EBS and EFS cannot be used as an origin for CDN unlike S3.

7. EBS and EFS are faster than S3 as these are directly mounted on EC2.

Storage Gateway

1. Integrates on-premise datacenter storage with cloud storage.

2. It connects to AWS storage services, such as Amazon S3, Amazon Glacier, and Amazon EBS, providing storage for files, volumes, and virtual tapes in AWS.

3. Storage Gateway is downloaded and installed at on-premise.

4. Caching and monitoring of data using Storage Gateway.

5. File Gateway: Simple file storage using NFS (Network File System) protocol.

6. Volume Gateway: Hard disk / block storage, cached mode (frequent access data is in cache of Volume Gateway and entire data is in cloud) and storage mode (entire data is in data center and asynchronously backed up to cloud).

7. Tape Gateway

AWS Database Services: RDS, DynamoDB, ElastiCache, Neptune and Redshift

AWS provides following services under database section:

1. RDS

2. DynamoDB
3. ElastiCache

4. Neptune

5. Redshift

Following are some basic and important points about AWS Database services:

RDS

1. Relational Database Service. Supports Aurora, MySQL, MariaDB, PostgreSQL, Oracle, MS SQL Server.

2. Backup and Restore methods: Automated (done by AWS automatically, backs up data with transaction logs) and Snapshots (manual process, usually done by system admins).

3. To improve DB performance, you can use ElastiCache, DAX and Read Replicas.

Aurora 

1. Combination of MySQL and PostgreSQL (RDBMS based).

2. Up to 5 times faster than standard MySQL databases and 3 times faster than standard PostgreSQL databases.

3. Automatically scales up to 64TB on SSD per database instance.

4. Replicates 6 copies of database across 3 Availability Zones.

5. Each DB cluster can have up to 15 READ replicas.

6. Failover takes less than 30 seconds.

7. Backs up database to S3.

8. You can monitor database performance using Amazon CloudWatch.

DynamoDB

1. DynamoDB (Dynamo Database or DDB) is Amazon NoSQL Database.

2. DynamoDB Security is provided by Fine-Grained Access Control (FGAC) mechanism. FGAC is based on the AWS IAM.

3. DynamoDB Triggers integrate with AWS Lambda.

4. DynamoDB Streams provides a 24-hour chronological sequence of updates to items in a table. AWS Lambda can read updates to a table from a stream.

5. Dynamo DB Accelerator (DAX) is in-memory database cache for Dynamo DB.

Neptune

1. Graph based database

Redshift 

1. Data Warehouse and Reporting System in the Amazon Cloud.

2. Use OLAP (Online Analytical Processing), SQL and BI tools to analyze the data.

3. Redshift Spectrum extends the power of Redshift to query unstructured data in S3 (without loading your data into Redshift).

ElastiCache

1. In-memory database cache in the Amazon Cloud for fast performance.

2. ElastiCache Engines: Redis, Memcached

DMS

1. Database Migration Service with zero/negligible downtime.

2. Supports homogenous (example: Oracle to Oracle) and heterogeneous (example: Oracle to Aurora or MySQL) database migration.

Use of Private Constructor in OOPS

If we set access specifier of a constructor to private, then that constructor can only be accessed inside the class. A private constructor is mainly used when you want to prevent the class instance from being created outside the class. 

This is mainly in the case of singleton class. Singleton classes are employed extensively in concepts like Networking and Database Connectivity. Using private constructor we can ensure that no more than one object can be created at a time. 

Example of Private Constructor in a Singleton Class

public class SingletonClass

{

    public static SingletonClass singletonClass;

    private SingletonClass() 

    {

    }

    public static SingletonClass getInstance() 

    {

        if(singletonClass == null) 

        {

            singletonClass = new SingletonClass();

        }

        return singletonClass;

    }

}

A class with private constructor cannot be inherited.

If we don’t want a class to be inherited, then we make the class constructor private. So, if we try to derive another class from this class then compiler will flash an error. Why compiler will flash an error? 

We know the order of execution of constructor in inheritance that when we create an object of a derived class then first constructor of the base call will be called and then constructor of derived class. Since base class constructor is private, hence, derived class will fail to access base class constructor.

We can also use sealed class to stop a class to be inherited. Sealed class provide more flexible and readable way to stop inheritance.

Diamond Problem in Multiple Inheritance in OOPS

The diamond problem occurs when two super classes of a class have a common base class. 

Suppose there are four classes A, B, C and D. Class B and C inherit class A. Now class B and C contains one copy of all the functions and data members of class A. Class D is derived from class B and C. Now class D contains two copies of all the functions and data members of class A. One copy comes from class B and another copy comes from class C.

Let’s say class A has a function with name display(). So class D have two display() functions as I have explained above. If we call display() function using class D object then ambiguity occurs because compiler gets confused that whether it should call display() that came from class B or from class C. If you will compile above program then it will show error.

This kind of problem is called diamond problem as a diamond structure is formed (see the image).

That is why major programming languages like C#, Java and Delphi don't have multiple inheritance because it can lead to diamond problem and rather than providing some complex way to solve it, there are better ways through which we can achieve the same result as multiple inheritance. We can use interfaces to resolve this problem.

C++ supports multiple inheritance.

Notice that the above problem with multiple class inheritance can also come with only three classes where all of them has at least one common method.

Because of this problem we can not extend two classes for implementing multiple inheritance and to resolve this problem of multiple inheritance in object oriented programming we use interfaces for implementing the functionality of multiple inheritance. 

As we know we do not define a function but only declare that function in an interface. So if we use interfaces we can extend one class and one or more interfaces or we can implement more than one interfaces at a time to use the functionality of multiple inheritance and we can escape from diamond problem.

How to declare Static/Class variables, properties, functions and procedures in Delphi?

In Delphi, we do have static variables, properties, functions and procedures, but instead of the word "static" we refer the word "class". In this tutorial, I have shown the syntax of declaring the static variables, properties, functions and procedures in Delphi. We don't have to use "static" keyword while declaring the static variables and properties, just use "class" keyword. While declaring static functions and procedures, we have to use both "class" and "static" keyword.

Declare a static/class variable in Delphi

type

  TStaticDemoClass = class(TObject)

  public

    class var StaticVar: integer;

  end;

Declare a static/class property in Delphi

type

  TStaticDemoClass = class(TObject)

  private

    class var FStaticVar: integer;

  public

    class property StaticVar: integer read FStaticVar write FStaticVar;

  end;

Now we can use above StaticVar variable/property anywhere in the unit like this:

procedure TForm1.Button1Click(Sender: TObject);

var a:integer;

begin

  TStaticDemoClass.StaticVar := 1;

end;

We can also use static/class functions and procedures in Delphi. Below example, I have copied from documentation:

type

  TMyClass = class

    strict private

      class var FX: Integer;

    strict protected  

      //Note: accessors for class properties must be declared class static.

      class function GetX: Integer; static;

      class procedure SetX(val: Integer); static;

    public

      class property X: Integer read GetX write SetX;

      class procedure StatProc(s: String); static;

  end;

TMyClass.X := 17;

TMyClass.StatProc('Hello');

You need to use "static" keyword after function / procedure declaration. Classes can have static class methods -- i.e. methods that can be called from a class type. Class static methods can be accessed without an object reference. Unlike ordinary class methods, class static methods have no Self parameter at all. They also cannot access any instance members. They still have access to class fields, class properties, and class methods. Also unlike class methods, class static methods cannot be declared virtual.

VCL Hierarchy in Delphi: Types of Controls in Delphi: TWinControls and TGraphicControls

In VCL (Visual Component Library) hierarchy, you will find TObject at the top, then TPersistent, then TComponent and then TControl.

TObject >> TPersistent >> TComponent >>TControl

VCL Hierarchy in Delphi

TObject is the base class from which all classes descend.

TPersistent class descends directly from TObject. The special characteristic of TPersistent is that it is an abstract class that defines the methods that allow it to be streamed.

TComponent is the base class from which all components descend. Non-visual components are descendants of TComponent. For example: TTimer

TControl is the base class from which all the visual components descend. For example: TEdit, TListBox, TComboBox etc.

Again, there are two types of controls (TControl): Window Controls (TWinControl) and Graphic Controls (TGraphicControl)

Following is the difference between TWinControl and TGraphicControl in Delphi:

TWinControls

TWinControls can receive input focus and they can be parents to other controls.

Example: TEdit, TListBox and TComboBox

TGraphicControl

TGraphicControls differ from TWinControls in that they cannot receive input focus. Also, they cannot be parents to other controls.

Example: TLabel

TGraphicControls are used when you want to display something on the form.

Two key advantages of TGraphicControls over TWinControls:

1. TGraphicControls don't use up system resources since they don't require a window handle.

2. They are a bit faster at drawing than their TWinControl counterparts since their painting is not subject to the windows message dispatching system. Instead, they piggyback on their parent's paint process.

For more details, please refer to documentation.