I can speak to the back-end stack. We're mostly a rails app and we've picked off services as required. Here's a breakdown.

rails

Our rails server is the largest part of the stack. Although we are using React for new front-end dev, the majority of our dev is done with server-rendered templates. Here are some stats:

+----------------------+--------+-------+---------+---------+-----+-------+
| Name                 |  Lines |   LOC | Classes | Methods | M/C | LOC/M |
+----------------------+--------+-------+---------+---------+-----+-------+
| Controllers          |  11934 |  9282 |     200 |    1376 |   6 |     4 |
| Helpers              |   1406 |  1138 |       0 |     197 |   0 |     3 |
| Models               |   5855 |  4515 |     119 |     696 |   5 |     4 |
| Mailers              |    701 |   581 |      16 |      61 |   3 |     7 |
| Javascripts          | 226460 |134085 |       0 |   13209 |   0 |     8 |
| Libraries            |     52 |    38 |       2 |       4 |   2 |     7 |
| Concern specs        |    246 |   176 |       2 |       1 |   0 |   174 |
| Controller specs     |   3806 |  3116 |       0 |      36 |   0 |    84 |
| Helper specs         |    160 |   108 |       1 |       0 |   0 |     0 |
| Lib specs            |   1657 |  1331 |       0 |      10 |   0 |   131 |
| Mailer specs         |    186 |   157 |       0 |       0 |   0 |     0 |
| Model specs          |   2694 |  2064 |       0 |      11 |   0 |   185 |
| Policy specs         |    244 |   198 |       0 |       1 |   0 |   196 |
| Serializer specs     |     99 |    94 |       0 |       1 |   0 |    92 |
| Service specs        |  12231 |  9460 |       8 |     172 |  21 |    53 |
+----------------------+-------+--------+---------+---------+-----+-------+
| Total                | 267731 |166343 |     348 |   15775 |  45 |     8 |
+----------------------+-------+--------+---------+---------+-----+-------+
  Code LOC: 149639     Test LOC: 16704     Code to Test Ratio: 1:0.1

micro-services

We have a number of services that do various things. Here's a quick rundown:

• spawner (rails): starts/stops docker containers for Projects.
• spawner_router (nginx): knows which project container is running on which node.
• async (sidekiq): queueing services for screenshots, emails, spam, indexing, lots of other stuff.
• preview (nginx): nginx-only service for rending projects from editor and various views like debug, deployed.
• preview_cname (nginx): nginx-only service for rending CNAME'd projects.
• pen_preprocessor_ruby (padrino): coordinates Pen preprocessing. Delegates lambda calls for different preprocesor types.
• pen_slim (sinatra): does preprocessing for Slim (is on its own because is insecure, so on lockdown)
• pen_haml (sinatra): does preprocessing for HAML (is on its own because is insecure, so on lockdown)
• pen_preprocessor_node (express): preprocessor rendering for node services too slow to run in Lambda.
• screenshot (express): takes screenshots of Pens.
• fluentd (fluentd): log capture from Projects, forwards to rails > front-end via long-polling

technology

• docker-swarm: all services (except for Project gulp containers) are deployed via Swarm.
• swarm-classic: projects are launched into a classic swarm to avoid Swarm Mode (new-style swarm) limitations.
• rds: RDS MYSQL and we're moving to Aruoura
• redis: Elasticache via AWS
• phusion passenger: webserver, cause this has lots of tuning options

other stuff

I'm sure I'm forgetting stuff here. AMA! :)