Textin uses one-time-password (OTP) generators we call Clockers. Workers send the OTPs as a text (SMS) messages, and, based on that, we store this event as a checkin in a Percona SQL database. The OTP algorithm is HOTP (time-based). Our telephony services are provided by Nexmo and backend is written in PHP on Zend framework 2. Frontend is Jquery based and relies heavily on Ajax requests to provide data in real-time. There is also a RESTFUL JSON API. We're set up on AWS EC2 server running Ubuntu 14.04. For additional data security we perform continuous on-the-fly DB replication to a S3 bucket.