Just a collection of tips to make working with Postgres (and other SQL-like databases) easier

Measurement Unit

Take this case for an example, you have a product table and you want to store product information like its size and weight by adding these columns to describe such properties: width, length, height and weight.

So what’s the problem with the above table? We are assuming that the size props (width, height and length) are measured in cm and the weight is measured in kg. Usually, we could put this logic in application layer to make sure we convert everything to cm and kg before inserting into the database. However, it could lead to even more problems

• What will happen if a new dev join the team? How can you make sure that person will know when to convert and when not?
• What will happen if a dev using pound join the team?
• What will happen if a dev accidentally assume the value in weight is in mg?
• Sometimes, you could do a double conversion, making thing worse.
• You need to remember adding comment to every place in your code, just to remind people which measurement unit that function is using.
• Which data type to choose? Integer, of course, is not a good choice. However, working with real, double, decimal or numeric is always harder compare to int. They could cause some problems with parsing and datatype for languages/libraries like Nodejs.

Leetcode: Squares of Sorted Array

Given an integer array nums sorted in non-decreasing order, return an array of the squares of each number sorted in non-decreasing order.

Example 1:

Input: nums = [-4,-1,0,3,10]
Output: [0,1,9,16,100]
Explanation: After squaring, the array becomes [16,1,0,9,100].
After sorting, it becomes [0,1,9,16,100].

Example 2:

Input: nums = [-7,-3,2,3,11]
Output: [4,9,9,49,121]

Constraints:

1 <= nums.length <= 104
-104 <= nums[i] <= 104
nums is sorted in non-decreasing order.

An example about configuring PubSub BigQuery Subscription with Pulumi

• BigQuery Subscription
• Configure BigQuery Dataset and Table
• Configure BigQuery Subscription
• Application Logging
• Query data from BigQuery

BigQuery Subscription

It’s hard to view the content of the messages that were published to a topic because the application has already processed and acknowledged them before you can do anything. Usually, you have to create another test subscription for the messages to be replicated to and then pull messages from that test subscription. However, the Google PubSub UI doesn’t provide any way to pull specific message by id. The GCloud Console UI is a frustrating UI itself, slow to load and had to pull several times to find the necessary messages.

Google offers BigQuery Subscription, a solution to that issue and also to provide a long term storage for your messages so you can troubleshoot and do complex query later. In this post, I’m going to show a sample BigQuery Subscription workflow with Pulumi.

Configure BigQuery Dataset and Table

First, you need to create a BigQuery Dataset and a BigQuery Table following the schema defined here. You can do it manually on the UI or via Pulumi

BigQuery Dataset

const pubsubDatasetId = `pubsub`;

export const pubsubDataset = new gcp.bigquery.Dataset(
  `my-dataset`,
  { datasetId: pubsubDatasetId }
);

BigQuery Table (a bit messy since the schema has to be defined in JSON string)

export const messageTable = new gcp.bigquery.Table(
  `my-table`,
  {
    datasetId: pubsubDatasetId,
    tableId: `message-values`,
    // if you don't want other people to accidentally delete is, set to true
    deletionProtection: true,
    schema: `
  [
    {
      "name": "data",
      "type": "STRING",
      "mode": "NULLABLE",
      "description": "The message body"
    },
    {
      "name": "subscription_name",
      "type": "STRING",
      "mode": "NULLABLE",
      "description": ""
    },
    {
      "name": "message_id",
      "type": "STRING",
      "mode": "NULLABLE",
      "description": ""
    },
    {
      "name": "publish_time",
      "type": "TIMESTAMP",
      "mode": "NULLABLE",
      "description": ""
    },
    {
      "name": "attributes",
      "type": "STRING",
      "mode": "NULLABLE",
      "description": "Message attributes as JSON string"
    }
  ]
  `,
  },
  {
    dependsOn: [pubsubDataset],
  }
);

Ok, the story is that, I’m really bad at css. I have never worked on building any frontend component and I was given a task to build the Custom Checkbox component with Reactjs from scratch. Here is how…

• 1. The basic HTML and CSS
  • Prepare the structure
  • Styling the Checkbox
  • Styling different states
• 2. The Reactjs component
• 3. Handle other Checkbox states

1. The basic HTML and CSS

Prepare the structure

Here is how you usually create a checkbox with pure html and css. To avoid any complicated event handler, I will simply wrap the <label> tag around, which allows clicking on any element inside to transfer the event to the corresponding <input> element without any Javascript needed.

<label class="mylabel">
  <input class="myinput" type="checkbox" name="checkbox" />
  <div class="mylabel">Checkbox label</div>
</label>

.mylabel {
  display: flex;
  gap: 5px;
  align-items: center;
  margin: 2px;
}

Try the live example in the below iframe (or direct link)

just a blog post for summarising my algorithm course

• The Problem
• Interface
• Quick Find
• Quick Union
• Weighted Quick Union
• Quick Union with Path Compression

The Problem

Given a data structure organised as a set of N objects, is there a path connecting 2 objects?

// union: connect 2 objects
// connected: whether 2 objects are connected?
union(4, 3);
union(3, 8);
union(6, 5);
union(9, 4);
union(2, 1);

connected(0, 7) return false;
connected(8, 9) return true;

union(5, 0);
union(7, 2);
union(6, 1);
union(1, 0);

connected(0, 7) return true;

Can only answer the question with Yes or No. The Dynamic Connectivity implementation cannot answer the exact path between 2 objects. It can only answer whether there are any paths connecting 2 objects.

Leetcode: Binary Search

This is so trivial. I just put it here so I can look up faster.

Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists, then return its index. Otherwise, return -1. You must write an algorithm with O(log n) runtime complexity.

Example 1

Input: nums = [-1,0,3,5,9,12], target = 9
Output: 4
Explanation: 9 exists in nums and its index is 4

Example 2

Input: nums = [-1,0,3,5,9,12], target = 2
Output: -1
Explanation: 2 does not exist in nums so return -1

Constraints

1 <= nums.length <= 104
-104 < nums[i], target < 104
All the integers in nums are unique.
nums is sorted in ascending order.

Just an interview question 🙃

The interview question: Re-implement Javascript Promise from scratch. The implementation should support chaining (asynchronously, of course).

Promise API

It has been too long since I started using async/await. I almost forgot these Promise APIs.

Let’s revisit the basic Javascript Promise APIs

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 300);
});

// try catch each one separately
myPromise
  .then(handleResolvedA, handleRejectedA)
  .then(handleResolvedB, handleRejectedB)
  .then(handleResolvedC, handleRejectedC);

// or catch the first one if any
myPromise
  .then(handleResolvedA)
  .then(handleResolvedB)
  .then(handleResolvedC)
  .catch(handleRejectedAny);

Leetcode: Merge Two Sorted Lists

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

Part 2: Refactor a legacy Worker Base - Part 2 - Scope Management

Rewrite the WorkerBase class

After fixing the Scope management problem, it’s time to rewrite the WorkerBase class in a way that the components are loosely coupling, composable and detachable. The solution turned out to be a very simple approach. It’s the middleware design that is very common in popular Web server frameworks (ASP.Net Core, Express.js, Koa.js,…).

In case you don’t know what a middleware is, read ASP.Net Core Middleware.

After analyzing the legacy WorkerBase class, I found that they could be organized into these middlewares

• Exception handling middleware
• Logging middleware
• Message queue behaviors middleware
• …

Part 1: Refactor a legacy Worker Base - Part 1 - The long lasting pain

The refactoring solution I presented in this post is the solution that I wrote in C#. It doesn’t mean you cannot do this in Nodejs. It is just because the team is migrating away from Nodejs to C#. We are familiar with these tools and they are already available as standard pattern in C#.

First thing first: An IOC Container

We learnt this at university. Why the heck did we forget this? Is it because the program language allows us to make this mistake so easily or is it because of the community that encourages the bad behaviors everywhere?

As I mentioned earlier, scope management of the legacy codebase is awful. We actually used a function to surround the scope of a message and the derived class is actually just a collection of function, not a scope container. Every time we want to activate a new method, we have to pass all the parameters downstream.

class WorkerBase {
  async start() {
    let message;
    do {
      message = await pullMessages(1);
      const context = this.buildContext(message);

      // this processMessage function wrap the scope of a message
      await this.processMessage(message, context);
    } while (message != null);
  }
}

// Worker service 1
class Worker1 extends WorkerBase {
  myProp = 1;

  async processMessage(message, context) {
    logic1(message, context);
    logic2(message, context);

    myProp++; // this will mutate myProp and affect other message
  }

  logic1(message, context) {}

  logic2(message, context) {}
}

We wrote JS in an OOP way but didn’t apply the OOP best practices!