[quantumf]

I think you're understating how tough it can be.

There are applications that

  * are mature and complex

  * with 100s of tables

  * serving millions of users

  * have to be broken into multiple micro-services

  * have developer resource constraints

So you're easily looking at a 1-2 year project, not 1-8 weeks.

You've also ignored some of the complexities, such as resharding (moving data between shards), which may significantly add to the cost of the project.

[kakwa_]

Also, when architecturing for shards, you must take into account availability.

Having several shards can lower the availability of your application if it cannot handle the absence of a shard.

For example if you have 99.9% availability on your individual DBs, and if you split it up into 10 shards, availability will drop to 99% (8 hours VS 3 days of downtime a year).

To handle that, you need to add replication and automatic fail-overs, adding even more complexity.

[jhwjhw]

At Prosperworks we offer a CRM which integrates closely with G Suite applications like Gmail and Calendar.

We consider our app to be maturing if not mature. It is certainly complex - we integrate with dozens of partners and external APIs. We have 80 tables and 300k LoC of Rails code which is runs on several TB of PostgresSQL data. We have not broken our app into multiple micro-services. Like everybody, we always feel that our developer resources are constrained.

Our data model is very similar to the CRM example in Ozgun's article: _mostly_ we have a master customer table and a wide halo of tables which are directly or transitively associated with customers. We called this the "company sharding domain". Since we allow one user to be associated with multiple accounts, we shard our user table independently: there is a smaller halo of tables in the "user sharding domain". And we have a handful of global tables for content and feature configuration in the "unsharded domain".

We kicked off our migration project from unsharded Postgres to sharded CitusCloud in early Q4 2016. We had one dev work on it solid for one quarter updating our code to be shard-ready. Then another 1.5 devs joined for a month in the final build up to the physical migration. We migrated in late Feb 2017, then consumed perhaps another 3 dev-months on follow-up activities like stamping out some distributed queries which we had unwisely neglected and updating our internal process for our brave new world.

Two years ago at another company I was tech lead on a migration of two much larger Mongo collections to sharded Mongo. That was a larger PHP application which was organized partly into microservices. That effort had a similar cost: as I recall I spent one quarter and two other devs spent about one month, and there were some post-migration follow-up costs as well.

I am confident that real world applications of significant complexity can be migrated from unsharded to sharded storage with a level of effort less than 1 year. I admit that 8 weeks feels fast but I'm sure I could have done it if we had been willing to tie up more devs.

Why were these efforts easier than 2 years? Because we didn't have to build the sharding solution itself - those came off the shelf from some great partners (shout outs to CitusData and mLabs). We just had to update our applications to be shard-smart and coordinate a sometimes complicated physical migration, derisking, and cutover process.

That said, I can imagine the work growing slowly but linearly in the number of tables, and quickly but linearly in the number of micro-services.

[ozgune]

I used to think similarly several years ago. I now think differently for the following reasons:

* Citus and other technologies can now provide features that do a lot of the heavy lifting. Some examples are resharding, shard rebalancing, and the high availability features mentioned below.

* My estimates are for B2B (multi-tenant) apps. For those apps, we found that the steps you need to take in re-modeling your data and changing your app are fairly similar. At Citus, we used to shy away when we saw 200-300 tables. These days, complex apps and schemas have become commonplace.

* We saw dozens of complex B2B databases migrate in similar time frames. Yes, some took longer - I'm in the tech business and always an optimist. :)

I also don't want to generalize without knowing more about your setup. If you drop me a line at ozgun @ citusdata.com, happy to chat more!