How to have fast redirects with async db writes?

I am currently working on a node.js api deployed on aws with elastic beanstalk.

The api accepts a url with query parameters, saves the parameters on a db (in my case aws rds), and redirects to a new url without waiting for the db response.

The main priority by far for the api is the redirection speed and the ability to handle a lot of requests. The aim of this question is to get your suggestions on how to do that.

I ran the api through a service called blitz.io to see what load it could handle and this is the report I got from them: http://ift.tt/1eiY4Ov

The instance and the database are running on t2.micro and db.t2.micro respectively.

The api can handle the load if no write is performed on the db, but crashes under a certain load when it writes on the db (I shared the report for the latter case) even without waiting for the db responses.

I checked the logs and found the following error in /var/log/nginx/error.log:

*1254 socket() failed (24: Too many open files) while connecting to upstream

I am not familiar with how nginx works but I imagine that every db connection is seen as an open file. Hence, the error implies that we reach the limit for open files before being able to close the connections. Is that a correct interpretation? Why am I getting the error?

I increased the limit in the way suggested here: http://ift.tt/1HkQwC2 but it did not solve the problem.

At this point I am not sure what to do. Can I close the connections before getting a response from the db? Is it a hardware limitation? The writes to the db are tiny.

Thank you in advance for your help! :)