The Caching Server is a paid software. As HelloCaptcha provides a web service, we developed a caching server, which caches the CAPTCHA pool on your server. In this case you provide the infrastructure for serving CAPTCHAs, which leads to a higher SLA.
For each profile (user) we maintain a pool of CAPTCHAs. The pooling technique is introduced, because rendering an animated CAPTCHA takes time, and the on-demand render would not provide sufficient reaction time. Utilizing a caching server this pool is copied to your server and the CAPTCHAs will be served by your infrastructure. The animated CAPTCHAs are still rendered on our server, and copied to your server in the background. The benefit of this solution is that for instance an internet line breakdown between the two servers would not break your service.