Implementing Vertical Sharding
Vertical sharding is fine, but how can we actually implement it? 🤔
Vertical Sharding
Vertical sharding is splitting a database by the tables. Shards will hold a subset of tables. For example, all payments-related tables go to one shard, while all auth-related tables go to another.
So, how to implement it?
Need for a configuration store
For our API servers to talk to the correct database we would need a configuration store that holds the information for all the tables mapped to the database server that holds it.
For example, the Users table is present on DB1 while Transactions on DB2
Whenever the request comes, the API servers first check the config to find which DB holds the table and then fire the SQL query to that specific database for the table.
Reactive update
All API servers will cache the configuration to avoid an expensive network call to get the database ensuring we get a solid boost to the performance.
When a table is moved from one database server to another, the configuration will be updated and hence the changes would need to be reactively propagated to all the API servers. Hence our config store needs to support reactive communication.
This is where we choose Zookeeper which is resilient and battle-tested to achieve this.
Moving tables
Say, we are moving table T2
from database server DB1 to DB2. Moving the table from one server to another is done in 4 simple steps.
Dump the table T2
We first dump the table T2
from DB1 transactionally using the utility mysqldump
that not only dumps the table data but also records the position in the binlog
. This is like taking a point-in-time snapshot of the table.
Restore the dump
We now restore the dump to database DB2. This way we will have a database server with the table T2
containing data till a certain point in time.
Sync table T2
on DB1 and DB2
We now setup the replication from DB1 to DB2 specifically for sync changes happening on table T2
. It is done through a custom job that will use the recorded binlog
position and start syncing from it.
Cutover
Once the table T2
is synced with almost 0 replication lag on DB1 and DB2 we cutover. We first rename the table to T2_bak
and update the config in Zookeeper.
As we rename the table any queries going to DB1 for table T2
will start throwing "Table not found" errors, but as Zookeeper will propagate the changes to all API servers they would use DB2 to fire any query on table T2
, thus completing the table movement.
This is how you can implement vertical sharding.
Here's the video of my explaining this in-depth 👇 do check it out
Sharding is super-important when you want to handle the traffic that cannot be handled through one server. Sharding comes in two flavors - Horizontal and Vertical. In horizontal sharding, we split the table by rows and keep them on separate servers. In vertical sharding, we distribute the tables across multiple database servers.
For example, keeping all the payments-related tables in one database server, and all the auth-related tables in another. Vertical sharding comes in super handy when we are moving from monolith to microservices. All this sounds simple yet awesome theoretically, but would we actually implement it?
In this video, we take an in-depth look, not at the theoretical side of vertical sharding, but at the implementation side of it. We will see how Vertical Sharding is implemented with minimal downtime and what are the exact steps to do it.
Outline:
00:00 Agenda
03:17 Introduction to Vertical Sharding
05:23 Implementing Vertical Sharding
05:55 Picking a configuration store
10:34 Moving a table from one server to another
18:58 Summarizing the overall flow
You can also
Subscribe to the YT Channel Asli Engineering
Listen to this on the go on Spotify
Thank you so much for reading 🖖 If you found this helpful, do spread the word about it on social media; it would mean the world to me.
You can also follow me on your favourite social media LinkedIn, and Twitter.
Yours truly,
Arpit
arpitbhayani.me
Until next time, stay awesome :)