Question

i wanted to implement simple picture url, like facebook has done it.

if you check on http://developers.facebook.com/docs/reference/api/using-pictures/

you will see, they used photo, even and application with id only, but event, application and photo id are all three different table id’s.

FB picture: http://graph.facebook.com/331218348435/picture

how they know which id is for which category (event, application or photo)?

Answer 1

Each object in Facebook could have it’s own unique identifier. Meaning, for example, if a FB group has an ID of 42 then an application, group, or any other object can NOT have that same id. This can allow the web application to map an ID directly to a URL for a picture.

Facebook is using a 64-bit identifier which is likely encoded with information. Therefore, I don’t think you will ever see an id as small as 42. The information could be in encoded in the following manner:

64 bits total
1st 16 bits = Shard ID (where is the object is stored)
2nd 16 bits = Type ID (app, group, person)
Remaining 32 bits = Local unique ID (incremental id for example)

Using some bit-wise logic you can extract the type from the identifier. Although it probably isn’t needed if you have a direct 1 to 1 mapping of ID to picture URL.

I don’t actually know how FB’s back end is structured. The above is just an example implementation. If you have tables with 32 bit id’s, you can still convert to 64 bit with an encoded type and making all identifiers unique regardless of object type.

It might be overkill, especially if you have an existing structure in production. I am merely attempting to explain how it could be done and how possibly Facebook does it.