# Design Photo Sharing App

<http://blog.gainlo.co/index.php/2016/03/01/system-design-interview-question-create-a-photo-sharing-app/>

Photo-sharing service like Instagram, Flickr, where users can upload photos to share them with other users

## Data Sharding

**a. Partitioning based on UserID**

find the shard number by UserID % 10

Issues with this partition scheme:

* How would we handle hot users?
* Some users upload a lot more photos, creating non-uniform distribution of storage
* What if we cannot store all pictures of a user on one shard?
* Unavailability of all of the user's data if that shard is down or higher latency if it's serving high load

**b. Partitioning based on PhotoID**

Generate unique PhotoID, and find a shard number through “PhotoID % 10”

**How can we generate PhotoIDs?**

One solution could be that we **dedicate** a **separate** **database** instance to **generate** **auto-incrementing IDs**.

**Wouldn’t this key generating DB be a single point of failure?**

Yes, it would be. A workaround for that could be defining two such databases with one generating even numbered IDs and the other odd numbered. For the MySQL, the following script can define such sequences:

```
KeyGeneratingServer1:
auto-increment-increment = 2
auto-increment-offset = 1

KeyGeneratingServer2:
auto-increment-increment = 2
auto-increment-offset = 2
```

## Ranking and News Feed Generation

sorting/merging/ranking

**Pre-generating the News Feed: (UserNewsFeed table)**

We can have dedicated servers that are continuously generating users’ News Feeds and storing them in a ‘**UserNewsFeed**’ **table**. So whenever any user needs the latest photos for their News Feed, we will simply query this table and return the results to the user.


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