State, Time and Concurrency

State, time, and concurrency are critical to building scalable software. Yet I had very little context over what they were and how they relate with each other when I started.

So this is the post that I would have wanted when I started.

State and Time

First, let's understand what state is.

It's a piece of data that changes over time.

Some examples of state in programming:

• Current time
• Location of a character in a game.
• Bank Balance
• No of. employees at an org
• ....

I hope you get the drift?

Now you might be wondering how they are related? Well, if you read the definition it's right there "changes over time".

Lets take a code sample:

let balance = 4; /* State */

function incBalance(amt) {
  balance += amt;
}

incBalance(4); /* Output 8 */
incBalance(4); /* Output 12 */

Here the variable balance is a piece of state which is updated by the function incBalance.

I called the function incBalance twice and each time the output was different. Let's look at how the execution went down:

The first time it was run:

incBalance(4);

/* let's execute the function by 
replacing the argument amt with the value i.e. 4*/

balance += 4;

/* the current value of variable balance is 4, so the result is */

balance = 8;

The second time it was run:

incBalance(4);

/* let's execute the function by 
replacing the argument amt with the value i.e. 4*/

balance += 4;

/* the current value of variable balance is 8, so the result is */

balance = 12;

So the function incBalance was updating a piece of state called balance and each time it was run, the output was dependent on the past value.

Another way of looking at this is whenever there is a stateful function (a function dependent on a piece of state) the output is not just dependent on the inputs but also when the function is run. In other terms, it is dependent on time.

A function that doesn't depend on the state is called a "pure" function.

TLDR:

When we introduce state we introduce dependency on time.

Time and Concurrency

Definition of concurrency from a popular StackOverflow answer:

Concurrency is when two or more tasks can start, run, and complete in overlapping time periods

Again from the definition itself, we can see "overlapping time periods".

Let's look at some code again:

let balance = 10;

function incBalance(amt) {
  balance += amt;
}

setTimeout(() => incBalance(8), 1000 * 3);
setTimeout(() => incBalance(8), 1000 * 1);
setTimeout(() => incBalance(8), 1000 * 2);

console.log({ balance }); /* Output 10 */

When you execute the above the output should be the value 10, which should be confusing?

In the above code sample, we are trying to achieve concurrency by using the setTimeout function. Basically, incBalance will be called concurrently three times at different time intervals so when all of them are done the final balance value should be 34 instead of 10.

So what is going on here and how do we get the expected result?

We need to ensure that all the concurrent calls are executed before we print the result.

How do we keep track of the fact whether all that needs executing is executed?

By using - State!

Let's update the above code and include some state to make sure all the concurrent stuff is completed at a given time:

let balance = 10;

/* state */
let task_completed = {
  task1: false,
  task2: false,
  task3: false,
};

function incBalance(amt) {
  balance += amt;
}

function concurrentIncBalance(taskName) {
  incBalance(8);
  task_completed[taskName] = true;
  /* time */
  if (
    task_completed["task1"] &&
    task_completed["task2"] &&
    task_completed["task3"]
  ) {
    console.log("Finished running all three tasks and the result is:");
    console.log({ balance });
  }
}

/* concurrency */
setTimeout(() => concurrentIncBalance("task1"), 1000);
setTimeout(() => concurrentIncBalance("task2"), 1000);
setTimeout(() => concurrentIncBalance("task3"), 1000);

Phew! See just adding a few requirements can make the code so much more complex!

The above callback based approach is a good segue to learning about Promises.

I hope this helps, thanks for reading.