Azure Front Door custom domain quote limit and solutions

When you reach the quotes of Azure Services you need to roll up your sleeves and go back to the design board. 

Business context

A company has around 20-50 products that are available in the EU, APAC and all US states. Each product can have around 5-15 different presentation web site (including custom domain) in each country. 

Technical constraint

The customer is using Azure and one of the technical objectives is to use only Azure Services, without any other 3rd party providers. Azure Front Door is in front of their API, used to map all the custom domains redirection to the main domain and to manage the security rules. The security rules are changed often (every day) - at least 3-4 rules per day, making the WAF component of Azure Front Door a goldmine for the operational team.

US: 20 products X 10 custom domains X 50 US states =10000 web site and 1000 custom domains

          EU: 20 products X 10 custom domains X 10 EU countries =2000 web site and 1000 custom domains 

The reality is not that we have 12000 custom domains in the EU and the US. For now, there are around 900 and the forecast for the next 3 years is up to 2000 custom domains in total. 

The current quote of the custom domain for Azure Front Door and the rest of Azure services is 500 per instance. This is a hard limit and cannot be changed. To be able to handle 900 or 2000 custom domains we need to find another method to map the domains.

Possible solutions

The top 3 options available, that are easy to implement are:

1. Spin-up Azure VMs or a containerization solution and install a reserve proxy (Nginx). Use Azure Front Door for the main domain and for all security rules (WAF)
• Pros: easy to configure
• Cons: scalability, availability and the risk the reach the 500 custom domain limitation
2. Use a 3rd party provider in the front for custom domain redirect and keep Azure Front Door for the main domain and for all security rules (WAF). 
• Pros: low operational cost
• Cons: 3rd party, that is not part of Azure Services
3. Use multiple Azure Front Doors, one as the main, for the main domain and all security rules (WAF). Configure multiple Azure Front Doors in the front of the main one only for custom domain redirects. 
• Pros: easy to manage and all services are part of Azure portfolio
• Cons: Multiple Azure Front Door instances might add an extra complexity layer

Option no. 3 with multiple instances of Azure Front Door looks to be the winner. Easy to configure, scalable and cost prediction done easily. For example, for 900 custom domains, we need:

• 1 X Premium instance for the main domain and WAF / €139
• 2 X Standard instances for the custom domain mapping / €29.4 (2 X €14.7)

For 2000 custom domains, we need to increase the no. of Standard instances:

• 1 X Premium instance for the main domain and WAF / €139
• 2 X Standard instances for the custom domain mapping / €58.8 (4 X €14.7)

Another advantage of this solution is the segregation of roles. The role that manages the WAF and security rules is different from the one that is managing the custom domain. You can even extend the concept and have Azure Front Door instances per region for the custom domains, fully managed by the local teams.