While developing the interfaces for SCADA Final Aggregator you face the issue of the private data protection: the UI is loaded with the javascript application that runs in the browser and requires authentication to access the SFA API functions. However, the application may contain components which an unauthorized user should not see: plans of the building, security cam footages, even the list of the managed items may be confidential.

One way to solve this problem is to use the frontend server for such content. However, frontend is not always necessary and, in our case, the content structure often requires the access rights to certain parts to be set on the front-end. Therefore, it may involve duplicating user base and difficult integration of the additional authentication methods.

In most cases, it would be sufficient to delineate access to such content with the help of SFA PVT server. The access rights to the certain files and catalogs are regulated with pvt parameter in SFA API keys.

The PVT server interface is available at http(s)://<IP_address_SFA:Port>/pvt, and the private content should be placed in pvt folder of EVA root directory.

pvt parameter of API keys supports MQTT-style wildcards, i.e.:

pvt = map.jpg, c1/#, +/content.js

will give the key access to map.jpg, all files and subfolders of c1 folder as well as content.js file in any first-level folder.

If the client is authenticated in advance, the future requests do not require k=APIKEY param.

Loading files from PVT Server

The file can be loaded with the following request:



  • k valid API key
  • f a full relative file path, i.e. map.jpg or c2/content.js

Receiving the file list

Use c=list request param to receive the file list by the specified mask:


The mask should be included in the pvt key access right parameter, for example

pvt = c1/*.json ; or c1/# for all files and masks

The complete request example:


which return JSON array:

     "name": "1.png",
     "size": 2443,
     "time": {
         "c": 1507735364.2441583,
         "m": 1507734605.1451921
     "name": "2.png",
     "size": 2231,
     "time": {
         "c": 1507735366.5561802,
         "m": 1507735342.923956


  • size file size (in bytes)
  • time/c inode creation time (ctime, UNIX timestamp)
  • time/m file modification time (mtime)

Receiving the newest and the oldest file

Use c=newest (c=oldest) param to do the typical job of the management interfaces - receiving the newest file from the specified folder.


Example: there is a monitoring camera that uploads a file to the folder on the server every 10 seconds. The uploaded files are named, i.e. TIMESTAMP.jpg or ID.jpg.

Connect the file with these images to pvt:

cd pvt
ln -sf /path/to/camerafolder cam1

and easily receive the newest file with the following request:


Image Processing

Use ic=resize to ask the server to preprocess the image file. To let the server process images, Python PIL (pillow) library should be installed. EVA installer automatically installs the library using pip3.

Please, make sure that system has at least libjpeg-dev and libjpeg8-dev before EVA setup, otherwise, PIL won’t work with JPEG images.

In case you miss this and server returns an error (“decoder not available”), reinstall pillow:

pip3 install --no-cache-dir -I pillow

If everything is installed correctly, you can receive the processed image using the following request:



  • X and Y - image maximum width/height
  • Q image quality
  • encode image encoder

I.e. let’s get an image pvt/cam/1.jpg, resize it to 800x600 as max, and convert to JPEG with 90% quality:


We may combine ic with c param, allowing us to receive the newest file by the mask. The request


will return the newest jpeg file from cam1 folder having scaled the image size to max 800x600 (proportionally) and reduced its quality to 50%. If the newest file cannot be processed (for example, the image isn’t completely loaded by cam yet and the file is locked), the server will attempt to process the previous one.

If the content is processed immediately before its loading by the interface, the server won’t need to generate the unnecessary images, especially if every client demands a specific format.

The maximum size of source file for the image processing is 10 megabytes.

Disabling cache

To ensure the request cashing is disabled, add nocache parameter with any value:


if you use this parameter for requests, web browser will not cache a file (if random value is used). Besides, the server will set Cache-Control, Expires and Pragma headers to the values which prohibit any caching.

Using frontend

If you work via frontend, you can use the pvt folder as a usual one (in case the authentication succeeded) by accessing files by their path without f parameter. Example for NGINX:

location / {
    rewrite ^/pvt/(.+)$ /pvt?f=$1 last;

The additional commands will keep working:

GET "http://eva.sfa.domain/pvt/data/cam3/*.jp*?c=newest&ic=resize:320x200x50:jpeg&nocache=1508070344872"

Remote content

SFA PVT can act as a proxy, fetching allowed resources in local network and displaying them to user.

This can be done with request


Param nocache is optional. If user is logged in, param k can be omitted.

Example: you have a chart on storage server in local network displaying storage usage. The chart is located at

Append the following permission string to API key:

rpvt =

This will grant access to all files on the specified host in /charts/ folder.

Then include remote chart in your interface:

<img src="/rpvt?k=APIKEY&f=" />

As you see, the remote client doesn’t need to have a direct access to web server, /rpvt API call acts for him as a content proxy.

To use remote content feature, you must follow the rules:

  • protocol (http/https) doesn’t need to be specified in rpvt API key param.
  • f param of /rpvt request may contain uri protocol (e.g. If the protocol is not specified, SFA uses plain HTTP without SSL.
  • You can not specify http(s) port in f param of /rpvt unless it’s also specified in rpvt API key param.
  • Avoid using rpvt = #, this will allow /rpvt to work as http proxy for any local and Internet resource and may open a security hole.