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Alex Xu

@alexxubyte

Jan 21

Metrics collection is a popular system design interview question. There are two ways metrics data can be collected, pull or push. It is a routine debate. In this post, we will take a look at the pull model. 1/8

Figure 1 shows data collection with a pull model over HTTP. We have dedicated metric collectors which pull metrics values from the running applications periodically. 2/8

In this approach, the metrics collector needs to know the complete list of service endpoints to pull data from. One naive approach is to use a file to hold DNS/IP information for every service endpoint on the “metric collector” servers. 3/8

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Alex Xu

@alexxubyte

Jan 18

One picture is worth more than a thousand words. This is what happens when you buy a product using Paypal/bank card under the hood.1/8

To understand this, we need to digest two concepts: 𝐜𝐥𝐞𝐚𝐫𝐢𝐧𝐠 & 𝐬𝐞𝐭𝐭𝐥𝐞𝐦𝐞𝐧𝐭. Clearing is a process that calculates who should pay whom with how much money; while settlement is a process where real money moves between reserves in the settlement bank. 2/8

Let’s say Bob wants to buy an SDI book from Claire’s shop on Amazon.

- Pay-in flow (Bob pays Amazon money):
1.1 Bob buys a book on Amazon using Paypal.
1.2 Amazon issues a money transfer request to Paypal.3/8

Read 8 tweets

Alex Xu

@alexxubyte

Jan 5

Here is a simplified design diagram that explains how a stock exchange such as Nasdaq or Newyork exchange works under the hood: #developers #systemdesign

Step 1: A client places an order via the broker’s web or mobile app.

Step 2: The broker sends the order to the exchange.1/6

Step 3: The exchange client gateway performs operations such as validation, rate limiting, authentication, normalization, etc., and sends the order to the order manager.

Step 4-5: The order manager performs risk checks based on rules set by the risk manager.2/6

Step 6: Once risk checks are passed, the order manager checks if there is enough money in the wallet.

Step 7 - 9: The order is sent to the matching engine. The matching engine sends back the execution result if a match is found. 3/6

Read 6 tweets

Alex Xu

@alexxubyte

Jan 4

One of the most serious problems a payment system can have is to 𝐝𝐨𝐮𝐛𝐥𝐞 𝐜𝐡𝐚𝐫𝐠𝐞 𝐚 𝐜𝐮𝐬𝐭𝐨𝐦𝐞𝐫. When we design the payment system, it is important to guarantee that the payment system executes a payment order exactly-once.1/6

At the first glance, exactly-once delivery seems very hard to tackle, but if we divide the problem into two parts, it is much easier to solve. An operation is executed exactly-once if:

1. It is executed at least once.
2. At the same time, it is executed at most once.2/6

We now explain how to implement at least once using retry and at most once using idempotency check.

𝐑𝐞𝐭𝐫𝐲
Occasionally, we need to retry a payment transaction due to network errors or timeout. Retry provides the at-least-once guarantee. 3/6

Read 6 tweets

Alex Xu

@alexxubyte

Jan 3

Here is what happens when you click the Buy button on Amazon or any of your favorite shopping websites.

1. When a user clicks the “Buy” button, a payment event is generated and sent to the payment service.

2. The payment service stores the payment event in the database. 1/6

3. Sometimes a single payment event may contain several payment orders. For example, you may select products from multiple sellers in a single checkout process. The payment service will call the payment executor for each payment order. 2/6

4. The payment executor stores the payment order in the database.

5. The payment executor calls an external PSP to finish the credit card payment. 3/6

Read 6 tweets

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