What is Curbo?

Curbo is a mobility marketplace, that allows users to buy and sell cars in a digital way similar to Amazon. It is available on the web and mobile platforms such as Android and iOS.

Requirements

Our system should meet the following requirements:

Functional requirements

We will design our system for two types of users: Buyers and Sellers.

Buyers

• Buyers should be able to see all the cars in the platform with details and pricing information.

• Buyers should be able to book a test drive to buy a car.

• Buyers should be able to see the location of the spot.

Sellers

• Sellers should be able to accept or deny the Buyer's requested test drive.

• Once a seller accepts the test drive, they should see the preferred location of the buyer.

• Sellers should be able to mark the test drive as complete.

Non-Functional requirements

• High reliability.

• High availability with minimal latency.

• The system should be scalable and efficient.

Extended requirements

• Customers can rate the buy/sell process after it's completed.

• Payment processing.

• Metrics and analytics.

Estimation and Constraints

Let's start with the estimation and constraints.

Traffic

Let us assume we have 100 million daily active users (DAU) with 1 million buyers and on average our platform enables 10 million requests daily.

If on average each user performs 10 actions (such as requesting available cars, loans, book test drives, etc.) we will have to handle 1 billion requests daily.

100 𝑚𝑖𝑙𝑙𝑖𝑜𝑛×10 𝑎𝑐𝑡𝑖𝑜𝑛𝑠=1 𝑏𝑖𝑙𝑙𝑖𝑜𝑛/𝑑𝑎𝑦100 million×10 actions\=1 billion/day

What would be Requests Per Second (RPS) for our system?

1 billion requests per day translate into 12K requests per second.

1 𝑏𝑖𝑙𝑙𝑖𝑜𝑛(24 ℎ𝑟𝑠×3600 𝑠𝑒𝑐𝑜𝑛𝑑𝑠)=∼12𝐾 𝑟𝑒𝑞𝑢𝑒𝑠𝑡𝑠/𝑠𝑒𝑐𝑜𝑛𝑑(24 hrs×3600 seconds)1 billion​=∼12K requests/second

### Storage

If we assume each message on average is 400 bytes, we will require about 400 GB of database storage every day.

1 𝑏𝑖𝑙𝑙𝑖𝑜𝑛×400 𝑏𝑦𝑡𝑒𝑠=∼400 𝐺𝐵/𝑑𝑎𝑦1 billion×400 bytes\=∼400 GB/day

And for 10 years, we will require about 1.4 PB of storage.

400 𝐺𝐵×10 𝑦𝑒𝑎𝑟𝑠×365 𝑑𝑎𝑦𝑠=∼1.4 𝑃𝐵400 GB×10 years×365 days\=∼1.4 PB

### Bandwidth

As our system is handling 400 GB of ingress every day, we will a require minimum bandwidth of around 4 MB per second.

400 𝐺𝐵(24 ℎ𝑟𝑠×3600 𝑠𝑒𝑐𝑜𝑛𝑑𝑠)=∼5 𝑀𝐵/𝑠𝑒𝑐𝑜𝑛𝑑(24 hrs×3600 seconds)400 GB​=∼5 MB/second

### High-level estimate

Here is our high-level estimate:

What kind of database should we use?

While our data model seems quite relational, we don't necessarily need to store everything in a single database, as this can limit our scalability and quickly become a bottleneck.

We will split the data between different services, each owning a particular table. Then we can use a relational database such as PostgreSQL or a distributed NoSQL database such as MongoDB for our use case.

High-level design

Now let us do a high-level design of our system.

Architecture

We will be using microservices architecture since it will make it easier to horizontally scale and decouple our services. Each service will have ownership of its data model. Let's try to divide our system into some core services.

Customer Service

This service handles customer-related concerns such as authentication and customer information.

Buyers Service

This service handles buyer-related concerns such as authentication and buyer information.

Car Service

This service will be responsible for car matching and quadtree aggregation.

Test Drive Service

This service handles sale-related functionality in our system.

Payment Service

This service will be responsible for handling payments in our system.

Notification Service

This service will simply send push notifications to the users.

Analytics Service

This service will be used for metrics and analytics use cases.

What about inter-service communication and service discovery?

Since our architecture is microservices-based, services will be communicating with each other as well. Generally, REST or HTTP performs well but we can further improve the performance using gRPC which is more lightweight and efficient.

Service discovery is another thing we will have to take into account. We can also use a service mesh that enables managed, observable, and secure communication between individual services.

Quadtrees

A Quadtree is a tree data structure in which each internal node has exactly four children. They are often used to partition a two-dimensional space by recursively subdividing it into four quadrants or regions. Each child or leaf node stores spatial information. Quadtrees are the two-dimensional analog of Octrees which are used to partition three-dimensional space.

Quadtrees enable us to search points within a two-dimensional range efficiently, where those points are defined as latitude/longitude coordinates or as cartesian (x, y) coordinates.

We can save further computation by only subdividing a node after a certain threshold.

Quadtree seems perfect for our use case, we can update the Quadtree every time we receive a new location update from the driver. To reduce the load on the quadtree servers we can use an in-memory datastore such as Redis to cache the latest updates. And with the application of mapping algorithms such as the Hilbert curve, we can perform efficient range queries to find nearby drivers for the customer.

How to find the best cars nearby?

Once we have a list of nearby cars from the Quadtree servers, we can perform some sort of ranking based on parameters like average ratings, relevance, past customer feedback, etc. This will allow us to broadcast notifications to the best available seller first.

Dealing with high demand

In cases of high demand, we can use the concept of Surge Pricing. Surge pricing is a dynamic pricing method where prices are temporarily increased as a reaction to increased demand and mostly limited supply. This surge price can be added to the base price of the car.

Payments

Handling payments at scale is challenging, to simplify our system we can use a third-party payment processor like Stripe or PayPal. Once the payment is complete, the payment processor will redirect the user back to our application and we can set up a webhook to capture all the payment-related data.

Notifications

Push notifications will be an integral part of our platform. We can use a message queue or a message broker such as Apache Kafka with the notification service to dispatch requests to Firebase Cloud Messaging (FCM) or Apple Push Notification Service (APNS) which will handle the delivery of the push notifications to user devices.

Detailed design

It's time to discuss our design decisions in detail.

Data Partitioning

To scale out our databases we will need to partition our data. Horizontal partitioning (aka Sharding) can be a good first step. We can shard our database either based on existing partition schemes or regions. If we divide the locations into regions using let's say zip codes, we can effectively store all the data in a given region on a fixed node. But this can still cause uneven data and load distribution, we can solve this using Consistent hashing.

Metrics and Analytics

Recording analytics and metrics is one of our extended requirements. We can capture the data from different services and run analytics on the data using Apache Spark which is an open-source unified analytics engine for large-scale data processing. Additionally, we can store critical metadata in the views table to increase data points within our data.

Caching

In a location services-based platform, caching is important. We have to be able to cache the recent locations of the customers and drivers for fast retrieval. We can use solutions like Redis or Memcached but what kind of cache eviction policy would best fit our needs?

Which cache eviction policy to use?

Least Recently Used (LRU) can be a good policy for our system. In this policy, we discard the least recently used key first.

How to handle cache miss?

Whenever there is a cache miss, our servers can hit the database directly and update the cache with the new entries.

Identify and resolve bottlenecks

Let us identify and resolve bottlenecks such as single points of failure in our design:

• "What if one of our services crashes?"

• "How will we distribute our traffic between our components?"

• "How can we reduce the load on our database?"

• "How to improve the availability of our cache?"

• "How can we make our notification system more robust?"

To make our system more resilient we can do the following:

• Running multiple instances of each of our services.

• Introducing load balancers between clients, servers, databases, and cache servers.

• Using multiple read replicas for our databases.

• Multiple instances and replicas for our distributed cache.

• Exactly once delivery and message ordering is challenging in a distributed system, we can use a dedicated message broker such as Apache Kafka or NATS to make our notification system more robust.