339 lines
15 KiB
Markdown
339 lines
15 KiB
Markdown
# Design Amazon's sales rank by category feature
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*Note: This document links directly to relevant areas found in the [system design topics](https://github.com/donnemartin/system-design-primer-interview#index-of-system-design-topics-1) to avoid duplication. Refer to the linked content for general talking points, tradeoffs, and alternatives.*
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## Step 1: Outline use cases and constraints
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> Gather requirements and scope the problem.
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> Ask questions to clarify use cases and constraints.
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> Discuss assumptions.
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Without an interviewer to address clarifying questions, we'll define some use cases and constraints.
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### Use cases
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#### We'll scope the problem to handle only the following use case
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* **Service** calculates the past week's most popular products by category
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* **User** views the past week's most popular products by category
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* **Service** has high availability
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#### Out of scope
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* The general e-commerce site
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* Design components only for calculating sales rank
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### Constraints and assumptions
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#### State assumptions
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* Traffic is not evenly distributed
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* Items can be in multiple categories
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* Items cannot change categories
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* There are no subcategories ie `foo/bar/baz`
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* Results must be updated hourly
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* More popular products might need to be updated more frequently
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* 10 million products
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* 1000 categories
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* 1 billion transactions per month
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* 100 billion read requests per month
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* 100:1 read to write ratio
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#### Calculate usage
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**Clarify with your interviewer if you should run back-of-the-envelope usage calculations.**
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* Size per transaction:
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* `created_at` - 5 bytes
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* `product_id` - 8 bytes
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* `category_id` - 4 bytes
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* `seller_id` - 8 bytes
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* `buyer_id` - 8 bytes
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* `quantity` - 4 bytes
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* `total_price` - 5 bytes
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* Total: ~40 bytes
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* 40 GB of new transaction content per month
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* 40 bytes per transaction * 1 billion transactions per month
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* 1.44 TB of new transaction content in 3 years
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* Assume most are new transactions instead of updates to existing ones
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* 400 transactions per second on average
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* 40,000 read requests per second on average
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Handy conversion guide:
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* 2.5 million seconds per month
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* 1 request per second = 2.5 million requests per month
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* 40 requests per second = 100 million requests per month
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* 400 requests per second = 1 billion requests per month
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## Step 2: Create a high level design
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> Outline a high level design with all important components.
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## Step 3: Design core components
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> Dive into details for each core component.
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### Use case: Service calculates the past week's most popular products by category
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We could store the raw **Sales API** server log files on a managed **Object Store** such as Amazon S3, rather than managing our own distributed file system.
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**Clarify with your interviewer how much code you are expected to write**.
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We'll assume this is a sample log entry, tab delimited:
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```
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timestamp product_id category_id qty total_price seller_id buyer_id
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t1 product1 category1 2 20.00 1 1
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t2 product1 category2 2 20.00 2 2
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t2 product1 category2 1 10.00 2 3
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t3 product2 category1 3 7.00 3 4
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t4 product3 category2 7 2.00 4 5
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t5 product4 category1 1 5.00 5 6
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...
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```
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The **Sales Rank Service** could use **MapReduce**, using the **Sales API** server log files as input and writing the results to an aggregate table `sales_rank` in a **SQL Database**. We should discuss the [use cases and tradeoffs between choosing SQL or NoSQL](https://github.com/donnemartin/system-design-primer-interview#sql-or-nosql).
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We'll use a multi-step **MapReduce**:
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* **Step 1** - Transform the data to `(category, product_id), sum(quantity)`
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* **Step 2** - Perform a distributed sort
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```
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class SalesRanker(MRJob):
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def within_past_week(self, timestamp):
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"""Return True if timestamp is within past week, False otherwise."""
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...
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def mapper(self, _ line):
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"""Parse each log line, extract and transform relevant lines.
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Emit key value pairs of the form:
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(category1, product1), 2
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(category2, product1), 2
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(category2, product1), 1
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(category1, product2), 3
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(category2, product3), 7
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(category1, product4), 1
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"""
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timestamp, product_id, category_id, quantity, total_price, seller_id, \
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buyer_id = line.split('\t')
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if self.within_past_week(timestamp):
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yield (category_id, product_id), quantity
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def reducer(self, key, value):
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"""Sum values for each key.
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(category1, product1), 2
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(category2, product1), 3
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(category1, product2), 3
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(category2, product3), 7
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(category1, product4), 1
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"""
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yield key, sum(values)
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def mapper_sort(self, key, value):
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"""Construct key to ensure proper sorting.
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Transform key and value to the form:
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(category1, 2), product1
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(category2, 3), product1
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(category1, 3), product2
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(category2, 7), product3
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(category1, 1), product4
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The shuffle/sort step of MapReduce will then do a
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distributed sort on the keys, resulting in:
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(category1, 1), product4
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(category1, 2), product1
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(category1, 3), product2
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(category2, 3), product1
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(category2, 7), product3
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"""
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category_id, product_id = key
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quantity = value
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yield (category_id, quantity), product_id
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def reducer_identity(self, key, value):
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yield key, value
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def steps(self):
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"""Run the map and reduce steps."""
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return [
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self.mr(mapper=self.mapper,
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reducer=self.reducer),
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self.mr(mapper=self.mapper_sort,
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reducer=self.reducer_identity),
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]
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```
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The result would be the following sorted list, which we could insert into the `sales_rank` table:
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```
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(category1, 1), product4
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(category1, 2), product1
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(category1, 3), product2
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(category2, 3), product1
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(category2, 7), product3
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```
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The `sales_rank` table could have the following structure:
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```
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id int NOT NULL AUTO_INCREMENT
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category_id int NOT NULL
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total_sold int NOT NULL
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product_id int NOT NULL
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PRIMARY KEY(id)
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FOREIGN KEY(category_id) REFERENCES Categories(id)
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FOREIGN KEY(product_id) REFERENCES Products(id)
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```
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We'll create an [index](https://github.com/donnemartin/system-design-primer-interview#use-good-indices) on `id `, `category_id`, and `product_id` to speed up lookups (log-time instead of scanning the entire table) and to keep the data in memory. Reading 1 MB sequentially from memory takes about 250 microseconds, while reading from SSD takes 4x and from disk takes 80x longer.<sup><a href=https://github.com/donnemartin/system-design-primer-interview#latency-numbers-every-programmer-should-know>1</a></sup>
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### Use case: User views the past week's most popular products by category
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* The **Client** sends a request to the **Web Server**, running as a [reverse proxy](https://github.com/donnemartin/system-design-primer-interview#reverse-proxy-web-server)
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* The **Web Server** forwards the request to the **Read API** server
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* The **Read API** server reads from the **SQL Database** `sales_rank` table
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We'll use a public [**REST API**](https://github.com/donnemartin/system-design-primer-interview##representational-state-transfer-rest):
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```
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$ curl https://amazon.com/api/v1/popular?category_id=1234
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```
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Response:
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```
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{
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"id": "100",
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"category_id": "1234",
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"total_sold": "100000",
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"product_id": "50",
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},
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{
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"id": "53",
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"category_id": "1234",
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"total_sold": "90000",
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"product_id": "200",
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},
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{
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"id": "75",
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"category_id": "1234",
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"total_sold": "80000",
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"product_id": "3",
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},
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```
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For internal communications, we could use [Remote Procedure Calls](https://github.com/donnemartin/system-design-primer-interview#remote-procedure-call-rpc).
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## Step 4: Scale the design
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> Identify and address bottlenecks, given the constraints.
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**Important: Do not simply jump right into the final design from the initial design!**
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State you would 1) **Benchmark/Load Test**, 2) **Profile** for bottlenecks 3) address bottlenecks while evaluating alternatives and trade-offs, and 4) repeat. See [Design a system that scales to millions of users on AWS]() as a sample on how to iteratively scale the initial design.
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It's important to discuss what bottlenecks you might encounter with the initial design and how you might address each of them. For example, what issues are addressed by adding a **Load Balancer** with multiple **Web Servers**? **CDN**? **Master-Slave Replicas**? What are the alternatives and **Trade-Offs** for each?
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We'll introduce some components to complete the design and to address scalability issues. Internal load balancers are not shown to reduce clutter.
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*To avoid repeating discussions*, refer to the following [system design topics](https://github.com/donnemartin/system-design-primer-interview#) for main talking points, tradeoffs, and alternatives:
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* [DNS](https://github.com/donnemartin/system-design-primer-interview#domain-name-system)
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* [CDN](https://github.com/donnemartin/system-design-primer-interview#content-delivery-network)
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* [Load balancer](https://github.com/donnemartin/system-design-primer-interview#load-balancer)
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* [Horizontal scaling](https://github.com/donnemartin/system-design-primer-interview#horizontal-scaling)
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* [Web server (reverse proxy)](https://github.com/donnemartin/system-design-primer-interview#reverse-proxy-web-server)
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* [API server (application layer)](https://github.com/donnemartin/system-design-primer-interview#application-layer)
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* [Cache](https://github.com/donnemartin/system-design-primer-interview#cache)
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* [Relational database management system (RDBMS)](https://github.com/donnemartin/system-design-primer-interview#relational-database-management-system-rdbms)
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* [SQL write master-slave failover](https://github.com/donnemartin/system-design-primer-interview#fail-over)
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* [Master-slave replication](https://github.com/donnemartin/system-design-primer-interview#master-slave-replication)
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* [Consistency patterns](https://github.com/donnemartin/system-design-primer-interview#consistency-patterns)
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* [Availability patterns](https://github.com/donnemartin/system-design-primer-interview#availability-patterns)
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The **Analytics Database** could use a data warehousing solution such as Amazon Redshift or Google BigQuery.
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We might only want to store a limited time period of data in the database, while storing the rest in a data warehouse or in an **Object Store**. An **Object Store** such as Amazon S3 can comfortably handle the constraint of 40 GB of new content per month.
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To address the 40,000 *average* read requests per second (higher at peak), traffic for popular content (and their sales rank) should be handled by the **Memory Cache** instead of the database. The **Memory Cache** is also useful for handling the unevenly distributed traffic and traffic spikes. With the large volume of reads, the **SQL Read Replicas** might not be able to handle the cache misses. We'll probably need to employ additional SQL scaling patterns.
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400 *average* writes per second (higher at peak) might be tough for a single **SQL Write Master-Slave**, also pointing to a need for additional scaling techniques.
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SQL scaling patterns include:
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* [Federation](https://github.com/donnemartin/system-design-primer-interview#federation)
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* [Sharding](https://github.com/donnemartin/system-design-primer-interview#sharding)
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* [Denormalization](https://github.com/donnemartin/system-design-primer-interview#denormalization)
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* [SQL Tuning](https://github.com/donnemartin/system-design-primer-interview#sql-tuning)
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We should also consider moving some data to a **NoSQL Database**.
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## Additional talking points
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> Additional topics to dive into, depending on the problem scope and time remaining.
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#### NoSQL
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* [Key-value store](https://github.com/donnemartin/system-design-primer-interview#)
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* [Document store](https://github.com/donnemartin/system-design-primer-interview#)
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* [Wide column store](https://github.com/donnemartin/system-design-primer-interview#)
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* [Graph database](https://github.com/donnemartin/system-design-primer-interview#)
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* [SQL vs NoSQL](https://github.com/donnemartin/system-design-primer-interview#)
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### Caching
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* Where to cache
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* [Client caching](https://github.com/donnemartin/system-design-primer-interview#client-caching)
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* [CDN caching](https://github.com/donnemartin/system-design-primer-interview#cdn-caching)
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* [Web server caching](https://github.com/donnemartin/system-design-primer-interview#web-server-caching)
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* [Database caching](https://github.com/donnemartin/system-design-primer-interview#database-caching)
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* [Application caching](https://github.com/donnemartin/system-design-primer-interview#application-caching)
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* What to cache
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* [Caching at the database query level](https://github.com/donnemartin/system-design-primer-interview#caching-at-the-database-query-level)
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* [Caching at the object level](https://github.com/donnemartin/system-design-primer-interview#caching-at-the-object-level)
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* When to update the cache
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* [Cache-aside](https://github.com/donnemartin/system-design-primer-interview#cache-aside)
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* [Write-through](https://github.com/donnemartin/system-design-primer-interview#write-through)
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* [Write-behind (write-back)](https://github.com/donnemartin/system-design-primer-interview#write-behind-write-back)
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* [Refresh ahead](https://github.com/donnemartin/system-design-primer-interview#refresh-ahead)
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### Asynchronism and microservices
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* [Message queues](https://github.com/donnemartin/system-design-primer-interview#)
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* [Task queues](https://github.com/donnemartin/system-design-primer-interview#)
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* [Back pressure](https://github.com/donnemartin/system-design-primer-interview#)
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* [Microservices](https://github.com/donnemartin/system-design-primer-interview#)
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### Communications
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* Discuss tradeoffs:
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* External communication with clients - [HTTP APIs following REST](https://github.com/donnemartin/system-design-primer-interview#representational-state-transfer-rest)
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* Internal communications - [RPC](https://github.com/donnemartin/system-design-primer-interview#remote-procedure-call-rpc)
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* [Service discovery](https://github.com/donnemartin/system-design-primer-interview#service-discovery)
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### Security
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Refer to the [security section](https://github.com/donnemartin/system-design-primer-interview#security).
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### Latency numbers
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See [Latency numbers every programmer should know](https://github.com/donnemartin/system-design-primer-interview#latency-numbers-every-programmer-should-know).
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### Ongoing
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* Continue benchmarking and monitoring your system to address bottlenecks as they come up
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* Scaling is an iterative process
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