Please help us help you and fill where relevant:
Your LimeSurvey version: [see right hand bottom of your LimeSurvey admin screen]
Own server or LimeSurvey hosting:
Theme:
==================
I have a use case for doing large-scale surveys. I need to be able to survey about 500,000 participants over a 6-8 week period (so about 50,000/week or 10,000/day). Peak load would be about 5000 concurrent participants. With appropriate server hardware, can Lime Survey support that level of load/concurrency?

thanks
Fred

LimeSurvey itself isn't holding you back. Your Webstack setup and hosting environment will need to cope.
5000 concurrent participants are not the same as 5000 concurrent connections. One participant can have more than one connections via the browser and 5000 participants are not clicking the next or submitbutton at the same time.

If you have a survey with images/video you have to think about the bandwidth of the connection as well. Is the server allowed to 100Mbit/s or more? Is that shared with others? If you have file based session ,make sure to have enough space for PHP session files. They are big in LimeSurvey and you get one created for every new browser hitting the survey.

Thanks for the reply. I am familiar with the "infrastructure" aspects of scaling (I use a platform to deliver this survey use case now, just looking for an alternative).

My question is more whether there is anything intrinsic to the Lime Survey architecture which would be a constraint (the sessions are a good example).

Does the platform scale horizontally? Is it designed to run in a single server environment or does it allow deployment on multiple web front end servers for redundancy/load balancing? Has anyone used the platform for this kind of load?

Thanks

I'm not aware of any setup with newer versions (LS3/LS5). Most setups are vertical scaling or perhaps have webserver and database on separate servers.

The most sophisticated setup discussed here was a few years ago.

BTW, latest high-load experience: 22k+ in first 15 mins after start, 75k+ total responses in 2 days.
Configuration used >> failover switch (nginx server) + 2 application clusters :
main cluster -- 2 load balancers (nginx) + 3 http servers (apache) + 1 cahing server (memcached) + 1 db server (maria)
reserve cluster (clone mirror instance) -- 1 load balancer + 2http + 1 cahing +1 db
This time All servers with automatic scale up/down from 2*2.4Ghz/4Gb to 2*24Ghz /32Gb

The thread is still valuable cause the main architecture of LimeSurvey hasn't changed that much.
forums.limesurvey.org/forum/design-issue...survey-takers#144504

You ignored the template for threadstarter. What LimeSurvey version do you use?
 

We are not currently using Lime Survey, that is why I didn't answer those questions. We have a 15-year-old home-built system that I am looking to replace and am investigating open-source and off-the-shelf options.

Thanks