KafkaKRU 밋업 리뷰: Event Sourcing부터 리더 파티션 밸런싱까지​

2024년 11월 21일, 서울 중구 삼화타워에서 열린 KafkaKRU 밋업에 참석했습니다. 사실 대기자 명단에 있었어서 참석이 어려운 상태였던 것 같지만, 열정으로 봐주셔서 다행히 쫓겨나지는 않았습니다. 결과적으로는 예상을 훨씬 뛰어넘는 값진 시간이었어요.

Overview​

• 참여 날짜: 2024년 10월 24일 목 (day 3)
• 장소: Kakao AI Campus

Overview​

Do you use cloud storage across multiple devices? If so, you've probably noticed the gradual increase of conflict files.

Conflict files that keep piling up whenever you turn around

Conflict files tend to accumulate for various reasons, such as making edits before files are synced or experiencing network delays.

Personally, I like to keep things tidy, so I regularly delete these dummy files. However, today I find the repetitive task a bit tedious. So, I thought I'd write a shell script to automate the process and show off my developer skills.

Recently, I undertook the task of moving my blog to a new platform. As I encountered various issues, I jotted down potential solutions, thinking they might be useful to others. Here’s a detailed account of the migration process.

Overview​

• Do you use a variety of programming languages rather than just one?
• Have you ever felt fatigued from memorizing commands for multiple package managers like sdkman, rvm, nvm, etc.?
• Would you like to manage your development environment more quickly and conveniently?

With mise, you can use the exact version of any language or tool you need, switch between different versions, and specify versions for each project. By specifying versions in a file, you can reduce communication costs among team members about which version to use.

Until now, the most famous tool in this field was asdf[^fn-nth-1]. However, after starting to use mise recently, I found that mise offers a slightly better user experience. In this post, I will introduce some simple use cases.

Not sure if it's intentional, but even the web pages look similar.

mise-en-place, mise​

mise (pronounced 'meez') is a tool for setting up development environments. The name comes from a French culinary term that roughly translates to "setting" or "putting in place." It means having all your tools and ingredients ready before you start cooking.

Here are some of its simple features:

Overview​

Implementing a server application that can handle multiple client requests simultaneously is now very easy. Just using Spring MVC alone can get you there in no time. However, as an engineer, I am curious about the underlying principles. In this article, we will embark on a journey to reflect on the considerations that were made to implement a multi-connection server by questioning the things that may seem obvious.

Socket​

The first destination is 'Socket'. From a network programming perspective, a socket is a communication endpoint used like a file to exchange data over a network. The description 'used like a file' is important because it is accessed through a file descriptor (fd) and supports I/O operations similar to files.

To implement a server application using sockets, you need to go through the following steps:

In PostgreSQL, the FOR UPDATE lock is used to explicitly lock rows in a table while performing a SELECT query within a transaction. This lock mode is typically used to ensure that the selected rows do not change until the transaction is completed, preventing other transactions from modifying or locking these rows in a conflicting manner.

For example, it can be used to prevent other customers from changing data while a specific customer is going through the ticket booking process.

The cases we will examine in this article are somewhat special:

• How does select for update behave if there is a mix of locked reads and unlocked reads?
• If a lock is used initially, is it possible for other transactions to read?
• Can consistent reading of data be guaranteed even if reading methods are mixed?

In PostgreSQL, the select for update clause operates differently depending on the transaction isolation level. Therefore, it is necessary to examine how it behaves at each isolation level.

Let’s assume a scenario where data is being modified when the following data exists.

What are Network Packets?​

How do we transmit data over a network? Establishing a connection with the recipient and sending the data all at once might seem like the most straightforward approach. However, this method becomes inefficient when handling multiple requests because a single connection can only maintain one data transfer at a time. If a connection is prolonged due to a large data transfer, other data will have to wait.

To efficiently handle the data transmission process, networks divide data into multiple pieces and require the receiving end to reassemble them. These fragmented data structures are called packets. Packets include additional information to allow the receiving end to reassemble the data in the correct order.

While transmitting data in multiple packets enables efficient processing of many requests through packet switching, it can also lead to various errors such as data loss or incorrect delivery order. How should we debug such issues? 🤔

Situation​

• As the codebase grows, the number of configuration values required for running a Spring application is increasing.
• While most situations are validated with test code, there are times when testing with bootRun locally is necessary.

Complication​

• Want to separate configuration values into environment variables for better management.
• .env files are typically ignored in Git, making version tracking difficult and prone to fragmentation.
  • Need a way to synchronize files across multiple machines.

Question​

• Is there a convenient method that minimizes friction among developers and is easy to apply?
  • Preferably a familiar method for easier maintenance.
• Can the version of .env files be managed?
• Is the learning curve low?
  • Want to avoid a situation where the solution is more complex than the problem.
• Can it be applied directly to the production environment?

Answer​

AWS S3​

• Updating .env files is convenient with AWS CLI.
• Version control of .env files can be done through snapshots.
• AWS S3 is a service familiar to most developers and has a low learning curve.
• In the AWS ECS production environment, system variables can be directly applied using S3 ARNs.

.

..

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Is that all?​

If that's it, the article might seem a bit dull, right? Of course, there are still a few issues remaining.

Which Bucket is it in?​

When using S3, it's common to end up with many buckets due to file structure optimization or business-specific categorization.

aws s3 cp s3://something.service.com/enviroment/.env .env

If the .env file is missing, you'll need to download it using AWS CLI as shown above. Without someone sharing the bucket with you in advance, you might have to search through all buckets to find the environment variable file, which could be inconvenient. The intention was to avoid sharing, but having to receive something to share again might feel a bit cumbersome.

Too many buckets. Where's the env...?

Automating the process of exploring buckets in S3 to find and download the necessary .env file would make things much easier. This can be achieved by writing a script using tools like fzf or gum.

Spring Boot Requires System Environment Variables, Not .env...​

Some of you may have already noticed that Spring Boot reads system environment variables to fill in placeholders in YAML files. However, using just the .env file won't apply the system environment variables, thus not being picked up during Spring Boot's initialization process.

Let's briefly look at how it works.

# .env
HELLO=WORLD

# application.yml
something:
    hello: ${HELLO} # Retrieves the value from the HELLO environment variable on the OS.

@Slf4j
@Component
public class HelloWorld {

    @Value("${something.hello}")
    private String hello;

    @PostConstruct
    public void init() {
        log.info("Hello: {}", hello);
    }
}

SystemEnvironmentPropertySource.java

You can see that the placeholder in @Value is not resolved, causing the bean registration to fail and resulting in an error.

Just having a .env file doesn't register it as a system environment variable.

To apply the .env file, you can either run the export command or register the .env file in IntelliJ's run configurations. However, using the export command to register too many variables globally on your local machine can lead to unintended behavior like overwriting, so it's recommended to manage them individually through IntelliJ's GUI.

IntelliJ supports configuring .env files via GUI.

The placeholder is resolved and applied correctly.

Final Answer - The Real Final One​

Phew, the long process of problem identification and scoping has come to an end. Let's summarize the workflow once more and introduce a script.

1. Use an automation script to find and download the appropriate .env from S3.
2. Set the .env as system environment variables.

The shell script is written to be simple yet stylized using gum.

Full Code

#!/bin/bash

S3_BUCKET=$(aws s3 ls | awk '{print $3}' | gum filter --reverse --placeholder "Select...") # 1.

# Choose deployment environment
TARGET=$(gum choose --header "Select a environment" "Elastic Container Service" "EC2")
if [ "$TARGET" = "Elastic Container Service" ]; then
    TARGET="ecs"
else
    TARGET="ec2"
fi

S3_BUCKET_PATH=s3://$S3_BUCKET/$TARGET/

# Search for the env file
ENV_FILE=$(aws s3 ls "$S3_BUCKET_PATH" | grep env | awk '{print $4}' | gum filter --reverse --placeholder "Select...") # 2.

# Confirm
if (gum confirm "Are you sure you want to use $ENV_FILE?"); then
    echo "You selected $ENV_FILE"
else
    die "Aborted."
fi

ENV_FILE_NAME=$(gum input --prompt.foreground "#04B575" --prompt "Enter the name of the env file: " --value ".env" --placeholder ".env")
gum spin -s meter --title "Copying env file..." -- aws s3 cp "$S3_BUCKET_PATH$ENV_FILE" "$ENV_FILE_NAME" # 3.

echo "Done."

1. Use gum filter to select the desired S3 bucket.
2. Search for items containing the word env and assign it to a variable named ENV_FILE.
3. Finalize the object key of the .env file and proceed with the download.

I've created a demo video of the execution process.

Demo

After this, you just need to apply the .env file copied to the current directory to IntelliJ as mentioned earlier.

Conclusion​

• The script is easy to maintain due to its simplicity.
• Team response has been very positive.
• Developers appreciate aesthetics.
• For sensitive credentials, consider using AWS Secret Manager.

This article discusses the inefficient existing implementation and documents the methods attempted to improve it.

Existing Issues​

While it wasn't impossible to join tables scattered across multiple databases in a single query, it was challenging...

1. Is a specific coordinate within area "a"?
2. Writing join queries was difficult due to tables existing on physically different servers
   1. Why the need for a single query? Due to the large size of the data to be queried, I wanted to minimize the amount loaded into application memory as much as possible.
3. Since DB joins were not possible, application joins were necessary, resulting in around 24 billion loops (60000 * 40000)
   1. Although processing time was minimized through partitioning, CPU load remained high due to the loops.
4. Through the migration process of merging physically different databases into one, the opportunity for query optimization was achieved as joins became possible.

Approach​

Given that the primary reason for not being able to use database joins had been resolved, I actively considered utilizing index scans for geometry processing.

• Using PostGIS's GIST index allows for creating a spatial index similar to R-tree, enabling direct querying through index scans.
• To use spatial indexing, a column of type geometry is required.
• While latitude and longitude coordinates were available, there was no geometry type, so it was necessary to first create geometry POINT values using the coordinates.

To simulate this process, I prepared the exact same data as in the live DB and conducted experiments.

First, I created the index:

CREATE INDEX idx_port_geom ON port USING GIST (geom);

Then, I ran the PostGIS contains function:

SELECT *
FROM ais AS a
JOIN port AS p ON st_contains(p.geom, a.geom);

Awesome...

Results​

Before Applying Spatial Index​

1 minute 47 seconds to 2 minutes 30 seconds

After Applying Spatial Index​

0.23 milliseconds to 0.243 milliseconds

I didn't prepare a capture, but before applying the index, queries took over 1 minute and 30 seconds.

Let's start with the conclusion and then delve into why these results were achieved.

GiST (Generalized Search Tree)​

A highly useful index for querying complex geometric data, the internal structure is illustrated below.

The idea of an R-tree is to divide the plane into rectangles to encompass all indexed points. Index rows store rectangles and can be defined as follows:

"The point we are looking for is inside the given rectangle."

The root of the R-tree contains several of the largest rectangles (which may intersect). Child nodes contain smaller rectangles included in the parent node, collectively encompassing all base points.

In theory, leaf nodes should contain indexed points, but since all index rows must have the same data type, rectangles reduced to points are repeatedly stored.

To visualize this structure, let's look at images for three levels of an R-tree. The points represent airport coordinates.

Level one: two large intersecting rectangles are visible.

Two intersecting rectangles are displayed.

Level two: large rectangles are split into smaller areas.

Large rectangles are divided into smaller areas.

Level three: each rectangle contains as many points as to fit one index page.

Each rectangle contains points that fit one index page.

These areas are structured into a tree, which is scanned during queries. For more detailed information, it is recommended to refer to the following article.

Conclusion​

In this article, I briefly introduced the specific conditions, the problems encountered, the efforts made to solve them, and the basic concepts needed to address these issues. To summarize:

• Efficient joins using indexes could not be performed on physically separated databases.
• By enabling physical joins through migration, significant performance improvements were achieved.
• With the ability to utilize index scans, overall performance was greatly enhanced.
• There was no longer a need to unnecessarily load data into application memory.
• CPU load due to loops was alleviated.

Reference​

• Spatial Indexing
• https://dbknowledge.tistory.com/48
• https://medium.com/postgres-professional/indexes-in-postgresql-5-gist-86e19781b5db