Serve dynamically generated image or saved image with Express

Question

I built an image delivery app which can serve images and resize/change image extension on-demand.

I use Express for the routing and Sharp to work on the images.

When an image is requested with a new size/extension, I want to save the generated image on the filesystem and retrieve the saved image the next time the same image is requested with the same parameters (sort of a cache).

Everything I built is working fine, but in production CPU usage is high. I suspect it is because I return an image with res.sendFile() after saving it on the filesystem.

At the moment here is my workflow :

• I check on the filesystem if the image is already saved with the wanted parameters (fs.access())
• If it exists, I res.sendFile the local image path
• If it doesn't exist, I generate it using Sharp, I save it on the filesystem and I res.sendFile the generated image path

I read res.sendFile doesn't use the system sendfile call, and is CPU-heavy.

How can I replace it ?

Here is what I found when googling :

• When I generate a new image, I can res.send() the buffer generated by Sharp instead of res.sendFile the saved image
• When the image is already saved, maybe I should use the static express middleware (http://expressjs.com/en/starter/static-files.html) but I don't know how I can dynamically call it and tell it the image path (based on the parameters provided in the request URL).