ESPAsyncWebServer : how to send a file ?

[christianw]

I have an ESP32 with an ESPAsyncWebServer which records data in a table (3600 rows maximum)

How do I get the web server to send the data as a CSV file to the browser?
Thanks in advance

[lbernstone]

If you are just sending a file from the flash, you can use the serveStatic method.
https://github.com/me-no-dev/ESPAsyncWe ... le-by-name
The mime type would be "text/csv".
I would recommend you add a max-age header at the frequency that you are collecting the data.

[christianw]

If you are just sending a file from the flash

Thanks but... datas are in RAM : I have a array to store them :

Code: Select all

struct SDATA {
  float temp;
  long  tmillis;
};

SDATA TabMesures[3600];

[lbernstone]

I assume this is data that you actually want to log anyhow. Write it to a file, and then staticServe that file.
If you want a lightweight data logger that's a bit more sophisticated, take a look at rrdTool

[christianw]

I prefer not to save the data permanently but to transmit it either for downloading or for display (using chart.js for example).

[boarchuz]

CSV is very basic, you could construct it on-the-fly from memory (assuming ESPAsyncWebServer exposes the right stuff to make it possible).

It could be as simple as this (pseudocode obviously):

Code: Select all

http_server.on(HTTP_GET, "/data.csv") {
    response.header.set("Content-Type", "text/csv");
    // TODO: set Content-Length or use chunked encoding
    response.begin();
    for(int row = 0; row < ARRAY_SIZE(TabMesures); ++row) {
        char buffer[20];
        sprintf(buffer, "%.2f", TabMesures[row].temp);
        response.send(buffer);
        response.send(',');
        sprintf(buffer, "%ld", TabMesures[row].tmillis);
        response.send(buffer);
        response.send('\n');
    }
    response.end();
}

[christianw]

I'll try this later, thanks.
But

Code: Select all

sprintf(buffer, "%.2f",...) 

won't work with an ESP32. I'll change to

Code: Select all

dtostrf()

but that's a detail.
On the other hand, I'm more worried about the RAM taken up: about 50 kB for the whole CSV

[lbernstone]

You can do a printf directly into the response method in a for loop. This will stream it, and likely is breaking it into mtu-sized chunks to deliver on the line.