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Why do sites like YouTube, Imgur and most others use random characters as their content ids rather than just sequential numbers, like those created by auto-increment in MySQL?

To explain what I mean:

In the URL: https://www.youtube.com/watch?v=QMlXuT7gd1I

The QMlXuT7gd1I at the end indicates the specific video on that page, but I'm assuming that video also has a unique numeric id in the database. Why do they create and use this alphanumeric string rather than just use the video's database id?

I'm creating a site which identifies content in the URL like above, but I'm currently using just the DB id. I'm considering switching to random strings because all major sites do it, but I'd like to know why this is done before I implement it.

Thanks!

user3471040
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    You won't know for sure in the case of YouTube. It is likely that the alpha numeric identifier is a hashed value of a combination of various meta data elements of the video. `hash(ID, TIME, LENGTH)` – Ryan Dec 18 '14 at 06:53
  • It enables their use as a secret key, allowing for [unlisted videos](https://support.google.com/youtube/answer/157177?hl=en). – Jeremy Dec 18 '14 at 06:53

4 Answers4

9

Some sites do that because of sharding.

When you have only one process (one server) writing, it is possible to make an auto-increment id without having duplicate ids, but when you have multiple servers (with multiple processes) writing content, like youtube, it's not possible to use autoincrement id anymore. The costs of synchronization to avoid duplication would be huge.

For example, if you read mongodb's ocjectid documentation you can see this structure for the id: a 4-byte value representing the seconds since the Unix epoch, a 3-byte machine identifier, a 2-byte process id, and a 3-byte counter, starting with a random value.

At the end, it's only 12 byte. The thing is when you represent in hexadecimal, it seems like 24 bytes, but that is only when you show it.

Another advantage of this system is that the timestamp is included in the id, so you can decouple the id to get the timestamp.

Christian Gollhardt
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Noogic
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First this is not a random string, it is a base calculation which is depended on the id. They go this way, because Alphanumeric has a bigger base

Something like 99999999 could be 1NJCHR

Take a look here, and play with the bases, and learn more about it.

You will see it is way more shorter. That is the only reason i can imagine, someone would go this way, and it makes sense, if you have ids like 54389634589347534985348957863457438959734

As self and Cameron commented/answered there are chances (especialy for youtube) that there are additional security parameters like time and lenght are calculated into it in some way, so you are not able to guess an identifier.

Community
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Christian Gollhardt
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In addition to Christian's answer above, using a base calculation, hashed value or other non-numeric identifier has the advantage of obscuring your db's size from competitors.

Even if you stayed with numeric and set your auto_increment to start at 50,000, increase by 50, etc., educated guesses can still be made at the db's size and growth. Non-numeric options don't eliminate this possibility, but they inhibit it to a certain extent.

Cameron Hurd
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there are major chances for malicious inputs by end users, and by not using ids users cant guess ids and thus can't guess how large db is. However other's answers on base calculation explains well.

shyammakwana.me
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