问题
I've been wondering how facebook manages the database design for all the different things that you can "like". If there is only one thing to like, this is simple, just a foreign key to what you like and a foreign key to who you are.
But there must be hundreds of different tables that you can "like" on facebook. How do they store the likes?
回答1:
If you want to represent this sort of structure in a relational database, then you need to use a hierarchy normally referred to as table inheritance. In table inheritance, you have a single table that defines a parent type, then child tables whose primary keys are also foreign keys back to the parent.
Using the Facebook example, you might have something like this:
User
------------
UserId (PK)
Item
-------------
ItemId (PK)
ItemType (discriminator column)
OwnerId (FK to User)
Status
------------
ItemId (PK, FK to Item)
StatusText
RelationshipUpdate
------------------
ItemId (PK, FK to Item)
RelationshipStatus
RelationTo (FK to User)
Like
------------
OwnerId (FK to User)
ItemId (FK to Item)
Compound PK of OwnerId, ItemId
In the interest completeness, it's worth noting that Facebook doesn't use an RDBMS for this sort of thing. They have opted for a NoSQL solution for this sort of storage. However, this is one way of storing such loosely-coupled information within an RDBMS.
回答2:
Facebook does not have traditional foreign keys and such, as they don't use relational databases for most of their data storage. Simply, they don't cut it for that.
However they use several NoSQL type data stores. The "Like" is most likely attributed based on a service, probably setup in an SOA style manner throughout their infrastructure. This way the "Like" can basically be attributed to anything they want it to be associated with. All this, with vast scalability and no tightly coupled relational issues to deal with. Something that Facebook, can't really afford to deal with at the volume they operate.
They could also be using an AOP (Aspect Oriented Programming) style processing mechanism to "attach" a "Like" to anything that may need one at page rendering time, but I get the notion that it is asynchronous processing via JavaScript against an SOA style web service or other delivery mechanism.
Either way, I'd love to hear how they have this setup from an architecture perspective myself. Considering their volume, even the simple "Like" button becomes a significant implementation of technology.
回答3:
You can have a table with Id, ForeignId and Type. Type can be anything like Photo, Status, Event, etc… ForeignId would be the id of the record in the table Type. This makes possible for both comments and likes. You only need one table for all likes, one for all comments and the one I described.
Example:
Items
Id | Foreign Id | Type
----+-------------+--------
1 | 322 | Photo
4 | 346 | Status
Likes
Id | User Id | Item Id
----+-------------+--------
1 | 111 | 1
Here, user with Id 111 likes the photo with Id 322.
Note: I assume you are using an RDBMS, but see Adron's answer. Facebook does not use an RDBMS for most of their data.
回答4:
I'm pretty sure Facebook does not store "like" information as how some other suggested it using RDBMS. With millions of users and possibly thousands of like, we're looking at thousands of rows to join here which would impact performance.
The best approach here is to append all "likes" in a single row. For example, a table with user_like_id column of text datatype. Then all id's who liked the post is appended. In this case, you only query one row and you got everything. This will be a lot faster than joining tables and getting counts.
EDIT: I haven't been here on this site lately and I just discovered this answer has been downvoted. Well, here's an example post with like count and their avatars. This is my design where I just implemented what I'm talking about.
The two components here are 1.) XREF table and 2.) JSON object.
The likes are still stored on a XREF table. But at the same time, data is appended on JSON object and stored on a text column on the post table.
Why did I store the likes info on a text column as JSON? So that there's no need to do db lookup/joins for the likes. If someone unlike the post, the JSON object is just updated.
Now I don't know why this answer is downvoted by some users here. This answer provides quick data retrieval. This is close to NoSQL approach which is how FB access data. In this case, there's no need for extra joins/lookup to get likes info.
And here's the table that holds the likes. It's just a simple XREF mapping between user and item table.
来源：https://stackoverflow.com/questions/7487476/facebook-like-data-structure