What approaches and methods should be used when working with sharded databases?

When working with sharded databases, the following approaches are important:

• Choosing a sharding strategy: horizontal (dividing by rows) or vertical (dividing by columns). Horizontal sharding is more commonly used for scaling.
• Defining the sharding key: the key should evenly distribute data across shards to avoid "hot" shards.
• Query routing: the application should be able to direct queries to the correct shard using the sharding key.
• Ensuring consistency: when distributing data, it is important to consider potential issues with transactions and consistency between shards.
• Monitoring and balancing: regularly monitor the load and redistribute data if necessary.

Example in Go for selecting a connection to the appropriate shard by key:

func getShardConnection(userID int) *sql.DB {
    shardID := userID % numberOfShards
    return shardConnections[shardID]
}

Here, numberOfShards is the number of shards, and shardConnections is an array or map with connections to the databases.