Part 1:

前言：

最近看了一些关于短址（short URL）方面的一些博客，有些博客说到一些好的东西，但是，也不是很全，所以，这篇博客算是对其它博客的一个总结吧。

 介绍：

短址，顾名思义，就是把长的 URL 转成短的 URL, 现在提供这种服务的有很多公司，我们以google家的 URL shortener 服务： http://goo.gl/ 为例。

首先我们到 http://goo.gl/，然后把本文博客的地址http://blog.csdn.net/beiyeqingteng 输入进去，最后它会返回一个更短的URL，http://goo.gl/Jfs6q 。如下图所示：

URL 解析：

当我们在浏览器里输入 http://goo.gl/Jfs6q 时，DNS首先解析获得http://goo.gl/的IP地址。当DNS获得IP地址以后（比如：74.125.225.72），会向这个地址发送HTTP GET请求，查询 Jfs6q， 这个时候，http://goo.gl/服务器会把请求通过HTTP 301转到对应的长URL http://blog.csdn.net/beiyeqingteng 。后面的解析过程就和平常网址解析是一样的了。

短址本质：

短址本质上是实现了一个映射函数 f: X -> Y 。而这个映射函数必须同时具有两个特点：

1. 如果 x1 != x2, 则 f (x1) != f(x2);

2. 对于每一个 y, 能够找到唯一的一个 x 使得 f(x) = y;

对于任何的线性函数，比如 f(x) = 2x，都满足这样的条件。

好了，如果了解了短址的本质，我们再来看它是如何实现的。

注明：在google URL shortener 服务中，它允许一个长 url 对应多个短的url。这可能是出于安全上的考虑。在本文中，我们不考虑这种情况。

 实现：

短址的长度一般设为 6 位，而每一位是由 [a - z, A - Z, 0 - 9] 总共 62 个字母组成的，所以6位的话，总共会有 62^6 ~= 568亿种组合，基本上够用了。在google URL shortener 服务中，短址长度为 5，大概有9亿多种组合.

 假设我们用数据库来保存长地址和短地址的映射，那么，在表 LongtoShortURL 中，我们会有三列：

1. ID，int,  自动增长；

2. LURL，varchar,  // 长URL；

3. SURL, varchar,  // 短URL。

 现在我们考虑通过如何长URL得到唯一的短URL。

 在讲具体算法以前，先提一个问题：10进制数和16进制数是否满足刚刚提到的映射函数 f: X -> Y中的两个条件？

答案： 是。

 本文的思路也是利用进制之间的转换。因为我们总共有 62 个字母，我们可以自创一种进制，叫做 62 进制。其规则如下：

1 0  → a
2 1  → b
3 ...
4 25 → z
5 ...
6 52 → 0
7 61 → 9

所以，对于每一个长地址，我们可以根据它的ID，得到一个6位的 62 进制数，这个6位的 62 进制数就是我们的短址。具体实现如下：

 1 public List<Integer> base62(int id) {
 3     List<Integer> value = http://www.mamicode.com/new LinkedList<Integer>();
 4     while (id > 0) {
 5         int remainder = id % 62;
 6         value.add(0, remainder);
 7         id = id / 62;
 8     }    
 9     return value;
10 }

举例： 

对于 ID = 138，通过 base62(138), 我们得到 value = http://www.mamicode.com/[2, 14]。根据上面的对应规则表，我们可以得到其对应的短址为：aaaabn 。(由 value 得到具体的短址，可以通过switch 语句得到，因为代码太长，在此略过。)

 当我们想通过短址找到所对应的长地址，方法也很简单，就是把62进制数转成10进制数即可，这样我们就可以得到长地址的ID了。

比如，对于短址aaae9a，其62进制为[0, 0, 0, 4,61,0] ，则其长地址的ID 为[0, 0, 0, 4,61,0] = 0×62^5+ 0×62^4 + 0×62^3 + 4×62^2 + 61×62^1 + 0×62^0 = 1915810。有了ID，我们自然就可以得到长地址了。

Part 2:

Question

How to create a TinyURL system?

If you are not familiar with TinyRUL, I’ll briefly explain here. Basically, TinyURL is a URL shortening service, a web service that provides short aliases for redirection of long URLs. There are many other similar services like Google URL Shortener, Bitly etc..

For example, URL http://blog.gainlo.co/index.php/2015/10/22/8-things-you-need-to-know-before-system-design-interviews/ is long and hard to remember, TinyURL can create a alias for it – http://tinyurl.com/j7ve58y. If you click the alias, it’ll redirect you to the original URL.

So if you would design this system that allows people input URLs with short alias URLs generated, how would you do it?

High-level idea

Let’s get started with basic and high-level solutions and we can keep optimizing it later on.

At first glance, each long URL and the corresponding alias form a key-value pair. I would expect you to think about something related to hash immediately.

Therefore, the question can be simplified like this – given a URL, how can we find hash function F that maps URL to a short alias:
F(URL) = alias
and satisfies following condition:s

1. Each URL can only be mapped to a unique alias
2. Each alias can be mapped back to a unique URL easily

The second condition is the core as in the run time, the system should look up by alias and redirect to the corresponding URL quickly.

 Basic solution

To make things easier, we can assume the alias is something like http://tinyurl.com/<alias_hash> and alias_hash is a fixed length string.

If the length is 7 containing [A-Z, a-z, 0-9], we can serve 62 ^ 7 ~= 3500 billion URLs. It’s said that there are ~644 million URLs at the time of this writing.

To begin with, let’s store all the mappings in a single database. A straightforward approach is using alias_hash as the ID of each mapping, which can be generated as a random string of length 7.

Therefore, we can first just store <ID, URL>. When a user inputs a long URL “http://www.gainlo.co”, the system creates a random 7-character string like “abcd123” as ID and inserts entry <“abcd123”, “http://www.gainlo.co”> into the database.

In the run time, when someone visits http://tinyurl.com/abcd123, we look up by ID “abcd123” and redirect to the corresponding URL “http://www.gainlo.co”.

Performance VS flexibility

There are quite a few follow-up questions for this problem. One thing I’d like to further discuss here is that by using GUID (Globally Unique Identifier) as the entry ID, what would be pros/cons versus incremental ID in this problem?

If you dig into the insert/query process, you will notice that using random string as IDs may sacrifice performance a little bit. More specifically, when you already have millions of records, insertion can be costly. Since IDs are not sequential, so every time a new record is inserted, the database needs to go look at the correct page for this ID. However, when using incremental IDs, insertion can be much easier – just go to the last page.

So one way to optimize this is to use incremental IDs. Every time a new URL is inserted, we increment the ID by 1 for the new entry. We also need a hash function that maps each integer ID to a 7-character string. If we think each string as a 62-base numeric, the mapping should be easy (Of course, there are other ways).

On the flip side, using incremental IDs will make the mapping less flexible. For instance, if the system allows users to set custom short URL, apparently GUID solution is easier because for whatever custom short URL, we can just calculate the corresponding hash as the entry ID.

Note: In this case, we may not use random generated key but a better hash function that maps any short URL into an ID, e.g. some traditional hash functions like CRC32, SHA-1 etc..

Cost

I can hardly not ask about how to evaluate the cost of the system. For insert/query, we’ve already discussed above. So I’ll focus more on storage cost.

Each entry is stored as <ID, URL> where ID is a 7-character string. Assuming max URL length is 2083 characters, then each entry takes 7 * 4 bytes + 2083 * 4 bytes = 8.4 KB. If we store a million URL mappings, we need around 8.4G storage.

If we consider the size of database index and we may also store other information like user ID, date each entry is inserted etc., it definitely requires much more storage. 

Multiple machines

Apparently, when the system has developed to certain scale, a single machine is not capable to store all the mappings. How do we scale with multiple instances?

The more general problem is how to store hash mapping across multiple machines. If you know distributed key-value store, you should know that this can be a very complicated problem. I will discuss only high-level ideas here and if you are interested in all those details, I’d recommend you read papers like Dynamo: Amazon’s Highly Available Key-value Store.

In a nutshell, if you want to store a huge amount of key-value pairs across multiple instances, you need to design a lookup algorithm that allows you to find the corresponding machine for a given lookup key.

For example, if the incoming short alias is http://tinyurl.com/abcd123, based on key “abcd123” the system should know which machine stores the database that contains entry for this key. This is exactly the same idea of database sharding.

A common approach is to have machines that act as a proxy, which is responsible for dispatching requests to corresponding backend stores based on the lookup key. Backend stores are actually databases that store the mapping. They can be split by various ways like use hash(key) % 1024 to divide mappings to 1024 stores.

There are tons of details that can make the system complicated, I’ll just name a few here:

• Replication. Data stores can crash for various random reasons, therefore a common solution is having multiple replicas for each database. There can be many problems here: how to replicate instances? How to recover fast? How to keep read/write consistent?
• Resharding. When the system scales to another level, the original sharding algorithm might not work well. We may need to use a new hash algorithm to reshard the system. How to reshard the database while keeping the system running can be an extremely difficult problem.
• Concurrency. There can be multiple users inserting the same URL or editing the same alias at the same time. With a single machine, you can control this with a lock. However, things become much more complicated when you scale to multiple instances.

Reference:

http://stackoverflow.com/questions/742013/how-to-code-a-url-shortener (本文算法来源)

http://blog.sina.com.cn/s/blog_65db99840100lg4n.html

http://blog.csdn.net/beiyeqingteng

http://blog.gainlo.co/index.php/2016/03/08/system-design-interview-question-create-tinyurl-system/

Design Tiny URL