Custom Front Ends

Interacting with docassemble over HTTP

If you are a software developer, you can develop your own front end for docassemble. You can use docassemble’s API to communicate with docassemble sessions. It is slightly more complicated, but just as effective, to communicate with docassemble the same way that the web browser does.

You can extract a machine-readable version of any docassemble screen. If you add &json=1 to the end of the URL, or include json=1 as parameter of a POST request, a JSON representation of the screen will be returned. (In fact, json set to anything will cause a JSON representation to be returned.)

To communicate back with the server, you will need to mimic the way that the browser communicates with the server. The easiest way to figure this out is use your browser’s developer tools and inspect the POST requests that the browser sends to the server.

docassemble uses cookies and 302 redirects, so if you are using a library to send HTTP requests to docassemble, you need to make sure that cookies are stored for the life of the session, and make sure that your library will follow redirects. If your library does not store cookies, you will encounter an infinite redirect, because the first thing that docassemble does when a user connects to it is send a self-referential redirect with a cookie.

In general, you can set any variable in the interview by sending a POST request with parameters where the keys are base64-encoded variable names and the values are the values you want to assign to the variables. In Javascript, you can use the atob() and btoa() functions to convert between base64 and text. However, when using btoa(), you need to alter the results to remove newline and = characters when working with variable names. Thus, if your variable name is varName, call btoa(varName).replace(/[\\n=]/g, ''). In Python, you can use the encode_name() and decode_name() functions to convert to and from base64.

For example, if you want to set the variable favorite_fruit to 'apple', you would convert favorite_fruit to base64 using btoa('favorite_fruit').replace(/[\\n=]/g, '') or encode_name('favorite_fruit'), to get 'ZmF2b3JpdGVfZnJ1aXQ'. Then you would put the following key and value in your POST request:

  • ZmF2b3JpdGVfZnJ1aXQ: apple

The POST request needs to go to the interview URL, which will look like

In addition to including keys and values of variables, your requests should include the parameter json=1, so that the server knows to respond with JSON. In addition, your requests should feed back the following values from the previous JSON response you received:

  • csrf_token. This token is a security measure that protects against cross-site interference. See CSRF protection.
  • _question_name. This contains the name of the question to which you are providing data. In most cases, this is not used, but there are some question types for which it is important.
  • _datatypes. This is a way of telling the server the data types of the variables being set, so that the server knows which values are integers or dates rather than text values. The value is a base64-encoded JSON representation of a dictionary where the keys are base64-encoded variable names and the values are the names of variables’ datatypes.
  • _varnames. For certain types of questions, variable aliases are used. This base64-encoded JSON representation of a dictionary tells the server what this mapping is.

The _datatypes field is important if you are setting non-text values. For example, to set the variable likes_fruit to True, a boolean value, you would run btoa('likes_fruit').replace(/[\\n=]/g, '') to get the key name bGlrZXNfZnJ1aXQ, and then you would run btoa('{"bGlrZXNfZnJ1aXQ": "boolean"}') to get eyJiR2xyWlhOZlpuSjFhWFEiOiAiYm9vbGVhbiJ9. Then you would set the following keys and values in your POST request:

  • bGlrZXNfZnJ1aXQ: True
  • _datatypes: eyJiR2xyWlhOZlpuSjFhWFEiOiAiYm9vbGVhbiJ9

If you are uploading a file, use the multipart/form-data style of encoding POST parameters, and include one additional parameter:

  • _files. This is a base64-encoded JSON representation of a list where each element is a base64-encoded variable name for a file being uploaded.

The “name” of an uploaded file should simply be the base64-encoded variable name.

For example, if you wanted to upload a file into a variable user_picture, you would run btoa('user_picture').replace(/[\\n=]/g, '') to get 'dXNlcl9waWN0dXJl', and then you would run btoa('["dXNlcl9waWN0dXJl"]') to get 'WyJkWE5sY2w5d2FXTjBkWEpsIl0=', and you would set the following in your POST:

  • dXNlcl9waWN0dXJl: the file you are uploading, using the standard method for attaching files to a multipart/form-data POST request.
  • _files: WyJkWE5sY2w5d2FXTjBkWEpsIl0=

There is also a second way to upload files, which uses data URLs. To use this method, send a normal POST request, without multipart/form-data and without a traditional uploaded file, in which there is a key called _files_inline, which is set to base64-encoded JSON data structure containing the file or files you want to upload, and some information about them.

For example, suppose you want to upload a file to the variable user_picture. You would run btoa('user_picture').replace(/[\\n=]/g, '') to get 'dXNlcl9waWN0dXJl'. Then you would create a Javascript object (a Python dictionary) with two key-value pairs. In the first key-value pair, the key will be keys and the value will be a list containing the base64-encoded variable names of the variables to which you want to upload files. In the second key-value pair, the key will be values and the value will be an object (a Python dictionary) with the following keys:

  • name: the name of the file being uploaded, without a directory specified.
  • size: the number of bytes in the file.
  • type: the MIME type of the file being uploaded.
  • content: a data URL containing the contents of the file, using base64 encoding.

Here is an example of the data structure you would need to create:


Assuming that this data structure was stored in a Javascript variable data, you would set the POST parameter _files_inline to the result of btoa(JSON.stringify(data)).

In addition, when sending a POST request, include the parameter json and set it to 1, so that the response you get back is in JSON format.

The format of the JSON representation should be self-explanatory. Toggle the json=1 URL parameter to compare the HTML version of the screen to the JSON representation.

Example of “logging in” with JavaScript

If you want to programmatically “log in” to docassemble, you can use code like the following:

var myHeaders = new Headers();

var myInit = { method: 'GET',
               headers: myHeaders,
               mode: 'cors',
               cache: 'default',
               redirect: 'follow',
               credentials: 'include' };

var myRequest = new Request('', myInit);

fetch(myRequest).then(function(response) {
  var contentType = response.headers.get("content-type");
  if(contentType && contentType.includes("application/json")) {
    return response.json();
  throw new TypeError("Error: JSON not returned from sign-in site");
}).then(function(json) {
  var form = new FormData();
  form.append('next', '');
  form.append('csrf_token', json.csrf_token);
  form.append('email', '[email protected]');
  form.append('password', 'xxsecretxx');
  var loginInit = { method: 'POST',
                    headers: myHeaders,
                    mode: 'cors',
                    cache: 'default',
                    credentials: 'include',
                    redirect: 'follow',
                    body: form };
  var loginRequest = new Request('', loginInit);
  fetch(loginRequest).then(function(response) {
    var contentType = response.headers.get("content-type");
    if(contentType && contentType.includes("application/json")) {
      return response.json();
    throw new TypeError("Error: JSON not returned after signing in");
  }).then(function(json) {


Background information

If you are building a front end to docassemble, you do not need to know exactly how docassemble works internally, but it may help for you to know a little about its internal design.

The docassemble web application saves state in a two different ways.

First, there is the state that represents the relationship between the web browser and the docassemble server. This is controlled by Flask, using a standard Flask session ID. Redis is used as the backend for this session tracking system. (The flask_kvsession and simplekv.memory.redisstore packages are used.) This session tracking is necessary for the user login system, which is provided by the flask_user package. It is used on all pages of a docassemble site. It is necessary for CSRF protection. This session tracking system works through a cookie called session. This session ID is generated by Flask and only lasts for the life of the web browser session (unless the user chooses the “remember me” option). Old session IDs are automatically purged after a time.

Second, there is the state that represents the current step of a user’s docassemble interview. This is not related to Flask, and is unique to docassemble. Each step of the interview process (which is accessible to the user with the “Back” button, which reverts the interview state to the previous step) is stored as a row in a SQL table called userdict. A state of the interview is represented as a Python dictionary serialized with cPickle and stored in the dictionary column of the userdict table. This Python dictionary is used as the namespace in which interview code is evalutated. A stored interview session is uniquely identified by the interview file name (e.g., docassemble.eviction:data/questions/complaint.yml) and a session ID, which is a unique sequence of 32 random upper and lowercase characters. These are the values referred to in the i and session parameters produced by interview_url(). (In the userdict table, the interview file name is in the filename column and the session ID is in the key column.)

So, there are two session IDs, based on systems that are independent from one another. As a front-end developer, you will probably never need to worry about the Flask session ID, which is stored in the cookie called session; your HTTP library and the docassemble server can work with this session ID without any manual intervention from you. The session ID that matters is the docassemble session ID that, along with the filename of the YAML interview file, uniquely identifies an interview session.

By default, server-side encryption is used, which means that interview answers are stored encrypted on the server, and only a user who started the interview can access the interview answers. The encryption key is stored in a cookie called secret, and the value of the secret is not stored on the server anywhere. For a logged in user, the secret is based on the user’s password, and the cookie is set when the user logs in. Users who are not logged in have a random secret. When the user logs in or registers, existing stored interview states are re-encrypted with the new encryption key. (The same happens when a logged-in user changes his or her password.) If a user forgets his or her password, there is no way to retrieve the interview answers. If a user completes an interview without logging in, and the cookie containing the secret expires (as it will when the user closes the browser), there is no way for the interview answers to be retrieved again. Server-side encryption can be turned off in the interview by setting multi_user to True.

The docassemble web application is designed to use Ajax calls as much as possible. When a user first visits an interview, the browser sends a GET request, and the server responds with HTML. However, when the user interacts with the screen and moves from step to step in the interview, the browser sends Ajax POST request, and the server responds with JSON. Within this JSON is a string representing the HTML of the <body> of the new screen. The server knows it is dealing with an Ajax request because the browser includes ajax=1 among the parameters of every request.

How sessions work

All interviews take place using the root location / (unless the root of the whole site is changed in the configuration to something else).

A new user starts an interview by visiting / with an i parameter set to the interview name (e.g., docassemble.eviction:data/questions/complaint.yml).

If a new user visits / without an i parameter, the user will be directed to /list. However, if there is no dispatch directive in the configuration, the user will be redirected to the default interview.

A user can enter an on-going interview by visiting / with an i parameter and a session parameter.

Once the user enters an interview, the interview filename i and the session ID are cached in the Flask session data, so the web browser does not have to send them to the server with every request; the session cookie is the key that the server uses to know which session it is dealing with.

The current interview’s session ID is primarily used internally, out of sight of the front end, but it can be obtained by:

  • Calling interview_url(), which returns a URL that can be used to resume the interview, containing both the i and session parameters.
  • Calling docassemble.base.functions.get_uid(), which returns the session ID.
  • Calling interview_list() to get information about all of the sessions of a logged-in user.
  • Calling get_interview_variables() from JavaScript.

The web browser interface tries to make sure that the location bar always displays the i parameter. Users may bookmark the URL or copy and paste the URL to share it with other people. As long as the i parameter is present in the location bar, the link will generally behave in a way that meets the user’s expectations.

However, this is only done out of convenience to the user. Since the interview file name is stored in the user’s Flask session, it is not necessary to use the i parameter; in fact, if you try navigating to / with a web browser while using an interview, this will simply have the effect of refreshing the screen.

If the browser sends a request with a different i parameter than the one in the Flask session data, this has the effect of starting a new interview session. The old interview session is not deleted; it is still there, stored in SQL, and is accessible to a logged-in user on the /interviews page.

If the URL parameter reset=1 is included along with an i parameter in a GET request, the current interview session is erased from the server (if there is a current session), and the interview is started fresh.

If the URL parameter cache=0 is included in a GET request, the interview file will be re-read from the disk. This is how the Playground works; when you click the “Save and Run” button or the “Run” button, the browser is directed to a link that contains cache=0. (Parsing an interview YAML file is a computationally intensive task, so docassemble keeps interviews in memory whenever it can.)

While the web browser sends POST requests with ajax=1, a custom front end would probably not be interested in the information returned from such requests. Instead, a custom front end would send POST requests with json=1 and get back a JSON representation of the next screen of the interview.

Note that as a technical matter, a user does not need to go through the flask_user log-in process in order to resume an existing interview. Even if server-side encryption is used, if the appropriate secret cookie is present, the interview answers will be decrypted. So if you have a mechanism for remembering i, session, and secret, you can run interviews without logging in. And if server-side encryption is turned off in an interview, all you need is i and session. So, if you have such a mechanism for storing these values, the only benefit to going through the login process would be that the user of the interview would be that the interview could use functions like user_info() to do different things based on who the user is.

Cross-Origin Resource Sharing (CORS)

There are many security issues with sending sensitive data over HTTP. By default, HTTP libraries implement fairly strict security restrictions, which you can override by:

  1. Changing how you call the library; and
  2. Changing the headers returned by the web server.

In addition, docassemble uses Flask’s CSRF protection system. This is why you need to include a csrf_token with every POST request. By default, Flask’s CSRF protection mechanism scrutinizes the referer header of each request, and generates an error if the referer header is not set to an appropriate value. This security check can be turned off by setting require referer to False in the Configuration. However, CSRF protection itself cannot be turned off.

Depending on how your custom front end calls docassemble, you may need to change the headers that the docassemble web server returns. By setting the cross site domains directive in the Configuration to include, you will activate the following headers:

Access-Control-Allow-Credentials "true"

In some circumstances you can set cross site domains to include * in order to allow connections from anywhere. But when cookies are used, this is not allowed. See the “Credentialed requests and wildcards” section of Mozilla’s Cross-Origin Resource Sharing documentation.

Using the API avoids issues with CORS.