MATLAB: How to look at the payload of a webwrite response when the response is a 400 or other error

MATLAB

How do I look at the payload of a "webwrite" response when the response is a 400 or other error? It throws a MATLAB error, but doesn't display any of the error messages the server sent back as payload.

My code looks something like this:

json_request = struct('a', nan);
json_response = webwrite('http://localhost:8000/reports/', json_request, ...
    weboptions('MediaType', 'application/json', 'RequestMethod', 'post'))

Best Answer

So basically, you can't get the payload of an error response using "webwrite". However you can get this using the HTTP interface. It might look something like:

import matlab.net.http.*
r = RequestMessage;
resp = r.send(url); %replace url with your target address
show(resp)

Related Solutions

MATLAB: How to preemptively include a Basic Authentication header when working with “webread”/”webwrite”/”websave” in MATLAB R2019b

In MATLAB releases prior to MATLAB R2019b, when you specified a 'Username' and 'Password' MATLAB would preemptively pass along these in a Basic Authentication header in the first request which "webread" makes. In MATLAB R2019b this changed, MATLAB now follows the challenge and response workflows from RFC-7235 more closely; which basically means it first makes a request without credentials expecting the server to return an "401 - Unauthorized" response in combination with a 'WWW-Authenticate' header specifying which authentication methods it supports. MATLAB then makes a second request with that particular authentication method. This is a more secure approach but this breaks workflows working with servers like the one described above.

If you still want "webread"/"webwrite"/"websave" to preemptive include a Basic Authentication header in the first request, do not specify 'Username' and 'Password' property/value pairs in "weboptions", instead use 'HeaderFields' to specify your own Authentication header. A Basic Authentication header is very simple to form: it is simply the text 'Basic ', followed by [username] colon [password] in base64 encoded format. So in MATLAB you could for example write:

% Example username and password
username = 'myUser';
password = 'myPassword';
% Manually set Authorization header field in weboptions
options = weboptions('HeaderFields',{'Authorization',...
    ['Basic ' matlab.net.base64encode([username ':' password])]});
% Make a request using these options
webread('https://httpbin.org/hidden-basic-auth/myUser/myPassword', options)

Note httpbin.org is a service for testing HTTP requests, the 'hidden-basic-auth' endpoint behaves in a way similar to described above (the request succeeds if you preemptively include a Basic Authorization header but immediately returns "404 - Not Found" if no (valid) authentication header was present). The myUser and myPassword specified in the request URL here is part of this particular testing endpoint, when calling your actual server you would normally not include your username and password in the request URL in this way, also do not call httpbin.org with you real username and password.

This approach also works correctly in MATLAB releases prior to R2019b.

Further note that the workflow for the HTTP interface has not changed. In this interface, if you explicitly specify 'Username', 'Password' and exactly one authentication method using a Credentials object, the Authorization header was always- and still is- preemptively included in the first request. So you can still write:

import matlab.net.http.*
% Create a credentials object. If you specify exactly one Scheme, we
% immediately automatically add the Authorization header for this Scheme,
% if you do not specify a Scheme we first make a request without any
% Authorization header expecting a server challenge.
creds = Credentials('Username','myUser','Password','myPassword',...
    'Scheme', AuthenticationScheme.Basic);
% Add these credentials to the options
options = HTTPOptions('Credentials', creds);
% Create a request
req = RequestMessage('GET');
% Execute this request on some URL with these options
response = req.send('https://httpbin.org/hidden-basic-auth/myUser/myPassword',options);

MATLAB: Frequency response!!

Please see tfestimate (Signal Processing Toolbox), spa, spafdr, etfe (System Identification Toolbox) for functions that calculate the frequency response from time domain signals in a non-parametric (empirical) fashion.

Perhaps a more reliable way is to compute a dynamic linear model from the data and then obtain the frequency response by calling BODE or FREQRESP on it. For example:

z = iddata(I, V, Ts); % Ts is sample time

model = n4sid(z, 1:10); % state-space model or order between 1 and 10

H = freqresp(model, linspace(0, pi/Ts, 100)); % the frequency response

This requires System Identification Toolbox.

Best Answer

Related Solutions

MATLAB: How to preemptively include a Basic Authentication header when working with “webread”/​”webwrite”​/”websave” in MATLAB R2019b

MATLAB: Frequency response!!

Related Question

MATLAB: How to preemptively include a Basic Authentication header when working with “webread”/”webwrite”/”websave” in MATLAB R2019b