The second part of this story explains how sharding was performed and the technique of rebalancing used. Read the full story here.
Every startup’s fondest dream is to somehow grow exponentially but still stay nimble and super efficient. However that’s easier said than done. The 32GB RAM that is more than capable of handling the load today is going to look like a joke a week later. And with the financial freedom of a startup, you can only take one step at a time.
At Freshdesk, our customer base grew by 400 percent in the last year. And the number of requests per week boomed from 2 million to 65 million.
These are really cool numbers for a 3-year-old startup but from an engineering perspective, it’s closer to nightmare than dream come true. We scaled left right and center (but mostly upwards) in a really short amount of time, using a whole bunch of vertical techniques. Sure, we eventually had to shard our databases just to keep up, but some of these techniques helped us stay afloat, for quite a while.
We tried to scale in the most straightforward way there is, by increasing the RAM, CPU and I/O. We travelled from Medium Instance Amazon EC2 First Generation to High Memory Quadruple Extra Large. It effectively increased our RAM from 3.75 GB to 64 GB. Then we figured that the amount of RAM we add and the CPU cycles do not correlate with the workload we get out of the instance. So we stayed put at 64GB.
The Read/write split
Since Freshdesk is a heavy read application (4:1; end user portals, APIs and loads of third party integrations tend to do that to you), we used MYSQL replication and distributed the reads between master and slave to accommodate them. Initially, we had different slaves getting selected for different queries using a round robin algorithm, but that quickly proved ineffective as we had no control over which query hit which DB. We worked around this by marking dedicated roles for each slave. For example, we used a slave for background processing jobs and another for report generation and so on (Seamless Database Pool, a Rails plugin, should do the job but if you’re an Engineyard user, I’d suggest you check out this cookbook).
As expected, the R/W split increased the number of I/Os we performed on our DBs but it didn’t do much good for the number of writes per second.
MySQL 5 has a built-in partitioning capability so all you have to do is, just choose the partition key and the number of partitions and the table will be partitioned, for you, automatically. However, if you’re thinking about going for MySQL partitioning, here are a couple of things you should keep in mind:
1. You need to choose the partition key carefully or alter the current schema to follow the MySQL partition rules.
2. The number of partitions you start with will affect the I/O operations on the disk directly.
3. If you use a hash-based algorithm with hash-based keys, you cannot control who goes where. This means you’ll be in trouble if two or more noisy customers fall within the same partition.
4. You need to make sure that every query contains the MySQL partition key. A query without the partition key ends up scanning all the partitions. Performance takes a dive as expected.
Post-partitioning, our read performance increased dramatically but, as expected, our number of writes didn’t increase much.
Some objects like support agent details change only 3-4 times in their lifetime. So, we started caching ActiveRecord objects and as well as html partials (bits and pieces of HTML) using Memcached. We chose Memcached because it scales well with multiple clusters. The Memcached client you use actually makes a lot of difference in the response time so we ended up going with dalli.
Another way we try to keep the response time low is by using different storage engines for different purposes. For example, we use Amazon RedShift for analytics and data mining and Redis, to store state information and background jobs for Resque. But because Redis can’t scale or fallback, we don’t use it for atomic operations.
Scaling vertically can only get you so far. Even as we tried various techniques, we knew it was only a matter of time before we scaled horizontally. And the rate at which we were growing didn’t give us much time to ponder over whether it was a good decision or not. So before our app response times could sky rocket and the status quo changed, we sharded our databases. But that story’s for another day.