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 :

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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):

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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

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

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

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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.

[zingzog]

I am working on a project that requires sending large files, .png, css, javascript, etc. from an ESP32 server to a client browser.

I extract the file content as bytes, store it as constant arrays in flash, and simply use the constant array as the data source in the server's send command, no RAM requirement. I wondered if a similar mechanism would work for you.

I created a char array as a variable, not constant.

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#define NUMVALUES 3600
#define CHARSPERVALUE 36
static char tabvals_csv[NUMVALUES * CHARSPERVALUE] = "";

and filled it with values created using sprintf.

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void fillTabMesures(){
  int n;
  char tabvalue[CHARSPERVALUE] = "";
  for (int i=0;i<(NUMVALUES);i++){
    float value = float(i)/100.0;
    unsigned long  tnow = millis();
    n = sprintf(tabvalue,"%7.2f,%22lu,\n", value, tnow);
    strcat(tabvals_csv,tabvalue);
  }
}

I set up a callback with:

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    server.on("/tabvals.csv",handletabvals);

and used the send command with the array as a parameter to send the file

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void handletabvals(){
    server.send(200, "text/csv", tabvals_csv);
}

Now, if I request http://buttons.local/tabvals.csv from my server using firefox on my PC I get a file save dialog asking where to save the file. The file loads to my spreadsheet program and gives output of the form:

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   0.00,                   793,
   0.01,                   793,
   0.02,                   793,
   0.03,                   793,
   0.04,                   793,

The disadvantage is that the array is larger than the raw values.
The advantages are:
- that the data don't have to be processed on the fly, they are sent directly by the server.
- no additional buffering/storage is needed to process the raw data to csv strings.

I am using ESP32-S3-DevKitC-1, and I don't have a problem with sprintf.

Andy