Posted in Software Engineering

AWS – System Manager

AWS System manager has powerful features to manages our EC2 – instances, following are the overview

AWS Systems Manager Patch Manager

AWS Systems Manager Patch Manager automates the process of patching managed instances with security-related updates. For Linux-based instances, we can also install patches for non-security updates. We can patch fleets of Amazon EC2 instances or our on-premises servers and virtual machines (VMs) by operating system type. This includes supported versions of Windows, Ubuntu Server, Red Hat Enterprise Linux (RHEL), SUSE Linux Enterprise Server (SLES), Amazon Linux, and Amazon Linux 2. We can scan instances to see only a report of missing patches, or we can scan and automatically install all missing patches.

AWS Systems Manager State Manager

AWS Systems Manager State Manager is primarily used as a secure and scalable configuration management service that automates the process of keeping our Amazon EC2 and hybrid infrastructure in a state that our define. This does not handle patch management, unlike AWS Systems Manager Patch Manager. With the State Manager, we can configure our instances to boot with a specific software at start-up; download and update agents on a defined schedule; configure network settings and many others, but not the patching of our EC2 instances.

AWS Systems Manager Session Manager

 AWS Systems Manager Session Manager is primarily used to comply with corporate policies that require controlled access to instances, strict security practices, and fully auditable logs with instance access details but not for applying OS patches.

AWS Systems Manager Maintenance Windows

AWS Systems Manager Maintenance Windows let us define a schedule for when to perform potentially disruptive actions on our instances such as patching an operating system, updating drivers, or installing software or patches. Each Maintenance Window has a schedule, a maximum duration, a set of registered targets (the instances that are acted upon), and a set of registered tasks. We can also specify dates that a Maintenance Window should not run before or after, and we can specify the international time zone on which to base the Maintenance Window schedule.

Posted in Software Engineering



For the web domain registration, use Amazon Route 53 and then register a 2048-bit RSASHA256 encryption key from a third-party certificate service. Enable Domain Name System Security Extensions (DNSSEC) by using a 3rd party DNS provider that uses customer managed keys. Register the SSL certificates in ACM and attach them to the Application Load Balancer. Configure the Server Name Identification extension in all user requests to the website.


Attackers sometimes hijack traffic to internet endpoints such as web servers by intercepting DNS queries and returning their own IP addresses to DNS resolvers in place of the actual IP addresses for those endpoints. Users are then routed to the IP addresses provided by the attackers in the spoofed response, for example, to fake websites. You can protect your domain from this type of attack, known as DNS spoofing or a man-in-the-middle attack, by configuring Domain Name System Security Extensions (DNSSEC), a protocol for securing DNS traffic.

AWS Route 53 – Registered Domain

Amazon Route 53 supports DNSSEC for domain registration. However, Route 53 does not support DNSSEC for DNS service, regardless of whether the domain is registered with Route 53. If you want to configure DNSSEC for a domain that is registered with Route 53, you must either use another DNS service provider or set up your own DNS server.

Posted in Software Engineering

What is Backpressure?

Backpressuring is simply the process of handling a fast item producer. If an Observable produces 1_000_000 items per second how a subscriber which can handle only 100 items per second does process the items? The Observable class has an unbounded buffer size, it means it will buffer everything and pushes it to the subscriber, that’s where you get the OutOfMemoryException.
By applying Backpressure onto a stream, it’ll be possible to handle items as needed, unnecessary items can be discarded or even let the producer know when to create and push the new items.

What is the problem?

bigTable.selectAll() // <=== or a hot observable like mouse movement

.map(/** mapping **/)


.subscribe(data -> { doSomethingHere(data) });

The problem of the code above is, it’s fetching all the rows from the database and pushes it to downstream, which results in high memory usage because it buffers all the data into memory. Do we want all of the data? Yes! but do we need all of it at once? No

I see this kind of usage in lots of projects, I’m pretty sure most of us have done something like this before even though knowing something is wrong, querying a database and assuming there won’t be lots of data, then it will and we end up with a poor app performance in production. If we are lucky enough we’ll get an OOM Exception but most of the times the app behaves slow and sluggish.

What is the solution?

Backpressure to rescue!! back in RxJava 1, the Observable class was responsible for the backpressure of streams, since RxJava 2 there is a separate class for handling the backpressure, Flowable.

How to create a Flowable?

There are multiple ways for creating a backpressure stream:

  1. Converting the Observable to Flowable with the x.toFloawable() method
Observable.range(1, 1_000_000).toFlowable(BackpressureStrategy.Drop)

With the Drop strategy the downstream doesn’t get all the one million items, it gets the items as it handles the previous items.

example output:
drops some of the items here
drops again
and so on

Note if you subscribe without changing the schedular you will get the whole one million items, since it’s synchronous the producers is blocked by the subscriber.


There are a few backpressure strategies:

  • Drop: Discards the unrequested items if it exceeds the buffer size
  • Buffer: Buffers all the items from the producer, watch for OOMs
  • Latest: Keeps only the most recent item
  • Error: throws a MissingBackpressureException in case of over emission
  • Missing: No strategy, it would throw a MissingBackpressureException sooner or later somewhere on the downstream

2. Use the Flowable.create() factory method:

We won’t get much more functionality than the x.toFlowable() here. let’s skip this one.

3. Use the Flowable.generate() factory method:

This is what we were looking for, the generate method has a few overloads, this is the one which can satisfy our needs.


() -> 0, //initial state

(state, emitter) -> { //current state


return current + 1; // next state



This code generates a stream of positive numbers: 0,1,2,3,4,5,6,…

The first parameter is a Callable to return an initial state. The second one is a BiFunction which gets called upon on every request to create a new item, its parameters are the current state and an emitter. So let’s apply it to our database code:

Flowable<List<Data>> select(int page, int pageSize) {

return Flowable.generate(

() -> page, //initial page

(currentPage, emitter) -> {

emitter.onNext("Select * From myTable LIMIT $pageSize OFFSET ${page * pageSize}"));

return currentPage + 1; // next page



Why there is no string templating in recent java releases, java 9, 10, 11?!! WTH Java

Now we can call it like this:, 10)

.map(/** mapping **/)

.flatMap(items -> {}, 1)// <== 1 indicates how many concurrent task should be executed

// observeOn uses a default 128 buffer size so we overwrite it

.observeOn(Schedulers.single(), false, 1)

.subscribe(new DefaultSubscriber<List<Data>>() {


protected void onStart() {

// super.onStart(); the the default implementation requests Long.MAX_VALUE




public void onNext(List<Data> data) {


request(1); // if you want more data



public void onError(Throwable t) {




public void onComplete() {




That’s all there’s to it. 😃

Posted in Software Engineering

Transaction Isolation

As we know that, in order to maintain consistency in a database, it follows ACID properties. Among these four properties (Atomicity, Consistency, Isolation and Durability) Isolation determines how transaction integrity is visible to other users and systems. It means that a transaction should take place in a system in such a way that it is the only transaction that is accessing the resources in a database system.
Isolation levels define the degree to which a transaction must be isolated from the data modifications made by any other transaction in the database system. A transaction isolation level is defined by the following phenomena –

  • Dirty Read – A Dirty read is the situation when a transaction reads a data that has not yet been committed. For example, Let’s say transaction 1 updates a row and leaves it uncommitted, meanwhile, Transaction 2 reads the updated row. If transaction 1 rolls back the change, transaction 2 will have read data that is considered never to have existed.
  • Non Repeatable read – Non Repeatable read occurs when a transaction reads same row twice, and get a different value each time. For example, suppose transaction T1 reads data. Due to concurrency, another transaction T2 updates the same data and commit, Now if transaction T1 rereads the same data, it will retrieve a different value.
  • Phantom Read – Phantom Read occurs when two same queries are executed, but the rows retrieved by the two, are different. For example, suppose transaction T1 retrieves a set of rows that satisfy some search criteria. Now, Transaction T2 generates some new rows that match the search criteria for transaction T1. If transaction T1 re-executes the statement that reads the rows, it gets a different set of rows this time.

Based on these phenomena, The SQL standard defines four isolation levels :

  1. Read Uncommitted – Read Uncommitted is the lowest isolation level. In this level, one transaction may read not yet committed changes made by other transaction, thereby allowing dirty reads. In this level, transactions are not isolated from each other.
  2. Read Committed – This isolation level guarantees that any data read is committed at the moment it is read. Thus it does not allows dirty read. The transaction holds a read or write lock on the current row, and thus prevent other transactions from reading, updating or deleting it.
  3. Repeatable Read – This is the most restrictive isolation level. The transaction holds read locks on all rows it references and writes locks on all rows it inserts, updates, or deletes. Since other transaction cannot read, update or delete these rows, consequently it avoids non-repeatable read.
  4. Serializable – This is the Highest isolation level. A serializable execution is guaranteed to be serializable. Serializable execution is defined to be an execution of operations in which concurrently executing transactions appears to be serially executing.

The Table is given below clearly depicts the relationship between isolation levels, read phenomena and locks :

Anomaly Serializable is not the same as Serializable. That is, it is necessary, but not sufficient that a Serializable schedule should be free of all three phenomena types.

Posted in Software Engineering

Threadpool executor


Original post:

Analysis When ThreadPool Executor is constructed There is a RejectedExecutionHandler parameter.

Rejected Execution Handler is an interface:

public interface RejectedExecutionHandler {
    void rejectedExecution(Runnable r, ThreadPoolExecutor executor);

There is only one way. When the number of threads to be created is greater than the maximum number of threads in the thread pool, the new task is rejected and the method in this interface is called.

This interface can be implemented by itself to handle these tasks that exceed the number.

ThreadPool Executor itself has provided four rejection strategies:

  1. Caller Runs Policy
  2. AbortPolicy
  3. Discard Policy
  4. Discard Oldest Policy.

These four rejection strategies are very simple when you look at the implementation method.


The default rejection strategy in ThreadPool Executor is AbortPolicy. Throw an exception directly.

private static final RejectedExecutionHandler defaultHandler =
    new AbortPolicy();

The following is his realization:

public static class AbortPolicy implements RejectedExecutionHandler {
    public AbortPolicy() { }
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        throw new RejectedExecutionException("Task " + r.toString() +
                                             " rejected from " +

Simple and rude, throw a RejectedExecutionException exception directly, and don’t perform this task.


First, customize a Runnable to give each thread a name. Next, use this Runnable.

static class MyThread implements Runnable {
        String name;
        public MyThread(String name) {
   = name;
        public void run() {
            try {
            } catch (InterruptedException e) {
            System.out.println("thread:"+Thread.currentThread().getName() +" implement:"+name +"  run");

Then we construct a thread pool with a core thread of 1 and a maximum number of threads of 2. The rejection strategy is AbortPolicy

ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 2, 0, 
        new LinkedBlockingDeque<Runnable>(2), 
        new ThreadPoolExecutor.AbortPolicy());
for (int i = 0; i < 6; i++) {
    System.out.println("Adding the _____________"+i+"Tasks");
    executor.execute(new MyThread("thread"+i));
    Iterator iterator = executor.getQueue().iterator();
    while (iterator.hasNext()){
        MyThread thread = (MyThread);

The output is:

Analyse the process.

  1. When the first task is added, it is executed directly and the task list is empty.
  2. When adding the second task, because the LinkedBlocking Deque is used and the core thread is executing the task, the second task will be placed in the queue with thread 2 in the queue.
  3. When the third task is added, it will also be placed in the queue, where there are threads 2 and 3.
  4. When adding the fourth task, because the core task is still running and the task queue is full, Hu directly creates a new thread to perform the fourth task. At this time, there are two threads running in the thread pool, reaching the maximum number of threads. There are threads 2 and 3 in the task queue.
  5. When the fifth task is added, there is no place to store and execute it anymore, and it will be rejected by the thread pool to execute the rejected Execution method of the rejected Execution method of the AbortPolicy, which throws an exception directly.
  6. Ultimately, only four threads can run. The latter were rejected.


CallerRunsPolicy calls the thread in the current thread pool to execute the rejected task after the task is refused to be added.

The following is his realization:

public static class CallerRunsPolicy implements RejectedExecutionHandler {
    public CallerRunsPolicy() { }
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {

It’s also simple, run directly.


ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 2, 30,
        new LinkedBlockingDeque<Runnable>(2),
        new ThreadPoolExecutor.AbortPolicy());

According to the above operation, output

Note that the fifth task, Task 5, is also rejected by the thread pool, so the rejected Execution method of CallerRunsPolicy is executed, which directly executes the run method of the task. So you can see that Task 5 is executed in the main thread.

It can also be seen from this that because the fifth task runs in the main thread, the main thread is blocked, so that when the fifth task is finished and the sixth task is added, the first two tasks are finished and there are idle threads, so thread 6 can be added to the thread pool to execute.

The disadvantage of this strategy is that it may block the main thread.


This strategy is much simpler to handle. Looking at the implementation, we can see that:

public static class DiscardPolicy implements RejectedExecutionHandler {
    public DiscardPolicy() { }
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {

This thing did nothing.

Therefore, using this rejection strategy, tasks rejected by thread pools will be discarded directly, no exception will be discarded and no execution will be performed.


ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 2, 30,
        new LinkedBlockingDeque<Runnable>(2),
        new ThreadPoolExecutor.DiscardPolicy());


As you can see, the tasks 5 and 6 added later will not be executed at all, and there will be no response, and they will be discarded directly.


The role of the Discard Oldest Policy strategy is to abandon the oldest task in the task queue when the task refuses to be added, that is, to join the queue first, and then to add the new task.

public static class DiscardOldestPolicy implements RejectedExecutionHandler {
    public DiscardOldestPolicy() { }
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {

In rejected Execution, the first joined task is popped up from the task queue, a position is vacated, and then the execute method is executed again to join the task in the queue.


ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 2, 30,
        new LinkedBlockingDeque<Runnable>(2),
        new ThreadPoolExecutor.DiscardOldestPolicy());

The output is:

As you can see,

  1. When the fifth task is added, it is rejected by the thread pool. At this point, there are tasks 2 and 3 in the task queue.
  2. At this point, the rejection strategy will make the first task in the task queue pop up, that is task 2.
  3. Then the rejected task 5 is added to the human task queue, and then the task queue becomes task 3 and task 5.
  4. When the sixth task is added, because of the same process, task 3 in the queue is discarded and task 6 is added. Task 5 and task 6 are added to the queue.
  5. Therefore, only 1, 4, 5, 6. Task 2 and task 3 times abandoned, will not be executed.

Custom Denial Policy

Looking at the four rejection strategies provided by the previous system, we can see that the implementation of the rejection strategy is very simple. The same is true of self-writing.

For example, if you want the rejected task to be executed in a new thread, you can write as follows:

static class MyRejectedExecutionHandler implements RejectedExecutionHandler {
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        new Thread(r,"New Threads"+new Random().nextInt(10)).start();

Then use it normally:

ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 2, 30,
        new LinkedBlockingDeque<Runnable>(2),
        new MyRejectedExecutionHandler());


Tasks 5 and 6 that were rejected were found to be executed in the new thread.