system-design-primer/solutions/system_design/pastebin/README.md

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# Design Pastebin.com (or Bit.ly)
*Note: This document links directly to relevant areas found in the [system design topics](https://github.com/donnemartin/system-design-primer#index-of-system-design-topics) to avoid duplication. Refer to the linked content for general talking points, tradeoffs, and alternatives.*
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**Design Bit.ly** - is a similar question, except pastebin requires storing the paste contents instead of the original unshortened url.
## Step 1: Outline use cases and constraints
> Gather requirements and scope the problem.
> Ask questions to clarify use cases and constraints.
> Discuss assumptions.
Without an interviewer to address clarifying questions, we'll define some use cases and constraints.
### Use cases
#### We'll scope the problem to handle only the following use cases
* **User** enters a block of text and gets a randomly generated link
* Expiration
* Default setting does not expire
* Can optionally set a timed expiration
* **User** enters a paste's url and views the contents
* **User** is anonymous
* **Service** tracks analytics of pages
* Monthly visit stats
* **Service** deletes expired pastes
* **Service** has high availability
#### Out of scope
* **User** registers for an account
* **User** verifies email
* **User** logs into a registered account
* **User** edits the document
* **User** can set visibility
* **User** can set the shortlink
### Constraints and assumptions
#### State assumptions
* Traffic is not evenly distributed
* Following a short link should be fast
* Pastes are text only
* Page view analytics do not need to be realtime
* 10 million users
* 10 million paste writes per month
* 100 million paste reads per month
* 10:1 read to write ratio
#### Calculate usage
**Clarify with your interviewer if you should run back-of-the-envelope usage calculations.**
* Size per paste
* 1 KB content per paste
* `shortlink` - 7 bytes
* `expiration_length_in_minutes` - 4 bytes
* `created_at` - 5 bytes
* `paste_path` - 255 bytes
* total = ~1.27 KB
* 12.7 GB of new paste content per month
* 1.27 KB per paste * 10 million pastes per month
* ~450 GB of new paste content in 3 years
* 360 million shortlinks in 3 years
* Assume most are new pastes instead of updates to existing ones
* 4 paste writes per second on average
* 40 read requests per second on average
Handy conversion guide:
* 2.5 million seconds per month
* 1 request per second = 2.5 million requests per month
* 40 requests per second = 100 million requests per month
* 400 requests per second = 1 billion requests per month
## Step 2: Create a high level design
> Outline a high level design with all important components.
![Imgur](http://i.imgur.com/BKsBnmG.png)
## Step 3: Design core components
> Dive into details for each core component.
### Use case: User enters a block of text and gets a randomly generated link
We could use a [relational database](https://github.com/donnemartin/system-design-primer#relational-database-management-system-rdbms) as a large hash table, mapping the generated url to a file server and path containing the paste file.
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Instead of managing a file server, we could use a managed **Object Store** such as Amazon S3 or a [NoSQL document store](https://github.com/donnemartin/system-design-primer#document-store).
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An alternative to a relational database acting as a large hash table, we could use a [NoSQL key-value store](https://github.com/donnemartin/system-design-primer#key-value-store). We should discuss the [tradeoffs between choosing SQL or NoSQL](https://github.com/donnemartin/system-design-primer#sql-or-nosql). The following discussion uses the relational database approach.
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* The **Client** sends a create paste request to the **Web Server**, running as a [reverse proxy](https://github.com/donnemartin/system-design-primer#reverse-proxy-web-server)
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* The **Web Server** forwards the request to the **Write API** server
* The **Write API** server does does the following:
* Generates a unique url
* Checks if the url is unique by looking at the **SQL Database** for a duplicate
* If the url is not unique, it generates another url
* If we supported a custom url, we could use the user-supplied (also check for a duplicate)
* Saves to the **SQL Database** `pastes` table
* Saves the paste data to the **Object Store**
* Returns the url
**Clarify with your interviewer how much code you are expected to write**.
The `pastes` table could have the following structure:
```
shortlink char(7) NOT NULL
expiration_length_in_minutes int NOT NULL
created_at datetime NOT NULL
paste_path varchar(255) NOT NULL
PRIMARY KEY(shortlink)
```
We'll create an [index](https://github.com/donnemartin/system-design-primer#use-good-indices) on `shortlink ` and `created_at` 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#latency-numbers-every-programmer-should-know>1</a></sup>
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To generate the unique url, we could:
* Take the [**MD5**](https://en.wikipedia.org/wiki/MD5) hash of the user's ip_address + timestamp
* MD5 is a widely used hashing function that produces a 128-bit hash value
* MD5 is uniformly distributed
* Alternatively, we could also take the MD5 hash of randomly-generated data
* [**Base 62**](https://www.kerstner.at/2012/07/shortening-strings-using-base-62-encoding/) encode the MD5 hash
* Base 62 encodes to `[a-zA-Z0-9]` which works well for urls, eliminating the need for escaping special characters
* There is only one hash result for the original input and and Base 62 is deterministic (no randomness involved)
* Base 64 is another popular encoding but provides issues for urls because of the additional `+` and `/` characters
* The following [Base 62 pseudocode](http://stackoverflow.com/questions/742013/how-to-code-a-url-shortener) runs in O(k) time where k is the number of digits = 7:
```
def base_encode(num, base=62):
digits = []
while num > 0
remainder = modulo(num, base)
digits.push(remainder)
num = divide(num, base)
digits = digits.reverse
```
* Take the first 7 characters of the output, which results in 62^7 possible values and should be sufficient to handle our constraint of 360 million shortlinks in 3 years:
```
url = base_encode(md5(ip_address+timestamp))[:URL_LENGTH]
```
We'll use a public [**REST API**](https://github.com/donnemartin/system-design-primer#representational-state-transfer-rest):
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```
$ curl -X POST --data '{ "expiration_length_in_minutes": "60", \
"paste_contents": "Hello World!" }' https://pastebin.com/api/v1/paste
```
Response:
```
{
"shortlink": "foobar"
}
```
For internal communications, we could use [Remote Procedure Calls](https://github.com/donnemartin/system-design-primer#remote-procedure-call-rpc).
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### Use case: User enters a paste's url and views the contents
* The **Client** sends a get paste request to the **Web Server**
* The **Web Server** forwards the request to the **Read API** server
* The **Read API** server does the following:
* Checks the **SQL Database** for the generated url
* If the url is in the **SQL Database**, fetch the paste contents from the **Object Store**
* Else, return an error message for the user
REST API:
```
$ curl https://pastebin.com/api/v1/paste?shortlink=foobar
```
Response:
```
{
"paste_contents": "Hello World"
"created_at": "YYYY-MM-DD HH:MM:SS"
"expiration_length_in_minutes": "60"
}
```
### Use case: Service tracks analytics of pages
Since realtime analytics are not a requirement, we could simply **MapReduce** the **Web Server** logs to generate hit counts.
**Clarify with your interviewer how much code you are expected to write**.
```
class HitCounts(MRJob):
def extract_url(self, line):
"""Extract the generated url from the log line."""
...
def extract_year_month(self, line):
"""Return the year and month portions of the timestamp."""
...
def mapper(self, _, line):
"""Parse each log line, extract and transform relevant lines.
Emit key value pairs of the form:
(2016-01, url0), 1
(2016-01, url0), 1
(2016-01, url1), 1
"""
url = self.extract_url(line)
period = self.extract_year_month(line)
yield (period, url), 1
def reducer(self, key, value):
"""Sum values for each key.
(2016-01, url0), 2
(2016-01, url1), 1
"""
yield key, sum(values)
```
### Use case: Service deletes expired pastes
To delete expired pastes, we could just scan the **SQL Database** for all entries whose expiration timestamp are older than the current timestamp. All expired entries would then be deleted (or marked as expired) from the table.
## Step 4: Scale the design
> Identify and address bottlenecks, given the constraints.
![Imgur](http://i.imgur.com/4edXG0T.png)
**Important: Do not simply jump right into the final design from the initial design!**
State you would do this iteratively: 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](https://github.com/donnemartin/system-design-primer/blob/master/solutions/system_design/scaling_aws/README.md) 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?
We'll introduce some components to complete the design and to address scalability issues. Internal load balancers are not shown to reduce clutter.
*To avoid repeating discussions*, refer to the following [system design topics](https://github.com/donnemartin/system-design-primer#index-of-system-design-topics) for main talking points, tradeoffs, and alternatives:
* [DNS](https://github.com/donnemartin/system-design-primer#domain-name-system)
* [CDN](https://github.com/donnemartin/system-design-primer#content-delivery-network)
* [Load balancer](https://github.com/donnemartin/system-design-primer#load-balancer)
* [Horizontal scaling](https://github.com/donnemartin/system-design-primer#horizontal-scaling)
* [Web server (reverse proxy)](https://github.com/donnemartin/system-design-primer#reverse-proxy-web-server)
* [API server (application layer)](https://github.com/donnemartin/system-design-primer#application-layer)
* [Cache](https://github.com/donnemartin/system-design-primer#cache)
* [Relational database management system (RDBMS)](https://github.com/donnemartin/system-design-primer#relational-database-management-system-rdbms)
* [SQL write master-slave failover](https://github.com/donnemartin/system-design-primer#fail-over)
* [Master-slave replication](https://github.com/donnemartin/system-design-primer#master-slave-replication)
* [Consistency patterns](https://github.com/donnemartin/system-design-primer#consistency-patterns)
* [Availability patterns](https://github.com/donnemartin/system-design-primer#availability-patterns)
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The **Analytics Database** could use a data warehousing solution such as Amazon Redshift or Google BigQuery.
An **Object Store** such as Amazon S3 can comfortably handle the constraint of 12.7 GB of new content per month.
To address the 40 *average* read requests per second (higher at peak), traffic for popular content 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. The **SQL Read Replicas** should be able to handle the cache misses, as long as the replicas are not bogged down with replicating writes.
4 *average* paste writes per second (with higher at peak) should be do-able for a single **SQL Write Master-Slave**. Otherwise, we'll need to employ additional SQL scaling patterns:
* [Federation](https://github.com/donnemartin/system-design-primer#federation)
* [Sharding](https://github.com/donnemartin/system-design-primer#sharding)
* [Denormalization](https://github.com/donnemartin/system-design-primer#denormalization)
* [SQL Tuning](https://github.com/donnemartin/system-design-primer#sql-tuning)
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We should also consider moving some data to a **NoSQL Database**.
## Additional talking points
> Additional topics to dive into, depending on the problem scope and time remaining.
#### NoSQL
* [Key-value store](https://github.com/donnemartin/system-design-primer#key-value-store)
* [Document store](https://github.com/donnemartin/system-design-primer#document-store)
* [Wide column store](https://github.com/donnemartin/system-design-primer#wide-column-store)
* [Graph database](https://github.com/donnemartin/system-design-primer#graph-database)
* [SQL vs NoSQL](https://github.com/donnemartin/system-design-primer#sql-or-nosql)
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### Caching
* Where to cache
* [Client caching](https://github.com/donnemartin/system-design-primer#client-caching)
* [CDN caching](https://github.com/donnemartin/system-design-primer#cdn-caching)
* [Web server caching](https://github.com/donnemartin/system-design-primer#web-server-caching)
* [Database caching](https://github.com/donnemartin/system-design-primer#database-caching)
* [Application caching](https://github.com/donnemartin/system-design-primer#application-caching)
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* What to cache
* [Caching at the database query level](https://github.com/donnemartin/system-design-primer#caching-at-the-database-query-level)
* [Caching at the object level](https://github.com/donnemartin/system-design-primer#caching-at-the-object-level)
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* When to update the cache
* [Cache-aside](https://github.com/donnemartin/system-design-primer#cache-aside)
* [Write-through](https://github.com/donnemartin/system-design-primer#write-through)
* [Write-behind (write-back)](https://github.com/donnemartin/system-design-primer#write-behind-write-back)
* [Refresh ahead](https://github.com/donnemartin/system-design-primer#refresh-ahead)
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### Asynchronism and microservices
* [Message queues](https://github.com/donnemartin/system-design-primer#message-queues)
* [Task queues](https://github.com/donnemartin/system-design-primer#task-queues)
* [Back pressure](https://github.com/donnemartin/system-design-primer#back-pressure)
* [Microservices](https://github.com/donnemartin/system-design-primer#microservices)
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### Communications
* Discuss tradeoffs:
* External communication with clients - [HTTP APIs following REST](https://github.com/donnemartin/system-design-primer#representational-state-transfer-rest)
* Internal communications - [RPC](https://github.com/donnemartin/system-design-primer#remote-procedure-call-rpc)
* [Service discovery](https://github.com/donnemartin/system-design-primer#service-discovery)
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### Security
Refer to the [security section](https://github.com/donnemartin/system-design-primer#security).
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### Latency numbers
See [Latency numbers every programmer should know](https://github.com/donnemartin/system-design-primer#latency-numbers-every-programmer-should-know).
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### Ongoing
* Continue benchmarking and monitoring your system to address bottlenecks as they come up
* Scaling is an iterative process