Sunday, November 3, 2013

Scaling pains changing ldap to mysql was like changing engine of a nascar during race

When you have a big customer base then you get lots of interesting problems like :
  1. Customers trying to store 400K files in one folder
  2. Customers trying to dump 100+ TB of files in one account
  3. Customers with more than half of their data in Trash
  4. Customers wanting to create> 64K users in an account
  5. Customers with 25K+ users trying to do search/load/sort on users listing
Our startup used to use LDAP to store users and customer metadata and Mysql to store files/folder metadata. 

Lately we had been hit by #4 and #5 item and that is causing scaling issues because no matter what we do ldap write performance sucks when we go beyond some million users.  Earlier ldap used to scale because we had 30-40 ldaps but to reduce ops management issues we consolidated then to 4 per DC and it worked initially but lately with #4 and #5 its not scaling fine.  Ldap is an alien to most programmers, there are only 2-3 guys in the team that knows a little bit about it so most of the times it remains orphan and being involved in every prod issue regarding ldap is not fun.

So we started on a project to replace ldap with mysql but problem is that ldap was in the company from day one so replacing it isnt that easy.  We use SOA and there are many moving parts that uses ldap, converting ldap to mysql required upgrading all services in all data centers at same time which sounds both risky and a hard sell to ops/management team.  So how do you change an engine of a running race car?

Well thats where interface based programming and feature flags comes into picture.

We have all our code related to ldap in one layer implementing DirectoryService Interface



To move to mysql we introduced two fields on each customer ldap_url and mysql_url and prepopulated ldap_url for all customers. We then created RoutingDirectoryService that for each request will determine whether customer is on ldap or mysql and routes the call accordingly to LdapDirectoryService or SqlDirectoryService.

We had to follow the same pattern in all other services and some were written in python so we had to duplicate the effort.

But luckily we were able to achieve the objective with less disruption to release process and we released the code in sleeper mode to prod one service at a time. The code was in sleeper mode because no customers were moved to mysql until we upgraded all services to use router logic.

Last night we migrated 100 customers to mysql and so far we saw only one issue.  We would continue to migrate more customer over the month  and eventually when ldap is gone we would remove RoutingDirectoryService.

4 years ago if we had done this then it would have been easy as no of customers were less but now we have so many customers that  we need to go feature flag way. Also customers rely on us 24X7 so we cant take risk of changing the engines of all car in a running race at the same time so we are now doing one car at a time :) and putting them back on race track.

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