Writing a new application feature

The changes needed to add a new feature will vary, of course, but this document provides a general outline of what you may need to do, as well as an example of the specific steps needed to add a new feature: adding a new option to the application that is dynamically synced through the data system in real-time to all browsers the user may have open.

As you read this, you may find you need to learn about Zulip's real-time push system; the real-time push and events documentation has a detailed explanation of how everything works.

General Process

Files impacted

This tutorial will walk through adding a new feature to a Realm (an organization in Zulip). The following files are involved in the process:


  • zerver/model.py: Defines the database model.
  • zerver/views/realm.py: The view function that implements the API endpoint for editing realm objects.
  • zerver/lib/actions.py: Contains code for updating and interacting with the database.
  • zerver/lib/events.py: Ensures that the state Zulip sends to clients is always consistent and correct.


  • static/templates/settings/organization-permissions-admin.handlebars: defines the structure of the admin permissions page (checkboxes for each organization permission setting).
  • static/js/settings_org.js: handles organization setting form submission.
  • static/js/server_events_dispatch.js: handles events coming from the server (ex: pushing an organization change to other open browsers and updating the application's state).

Backend Testing

  • zerver/tests/test_realm.py: end-to-end API tests for updating realm settings.
  • zerver/tests/test_events.py: tests for possible race bugs in the zerver/lib/events.py implementation.

Frontend Testing

  • frontend_tests/casper_tests/10-admin.js: end-to-end tests for the organization admin settings pages.
  • frontend_tests/node_tests/dispatch.js

Adding a field to the database

Update the model: The server accesses the underlying database in zerver/models.py. Add a new field in the appropriate class.

Create and run the migration: To create and apply a migration, run:

./manage.py makemigrations
./manage.py migrate

Test your changes: Once you've run the migration, flush memcached on your development server (./scripts/setup/flush-memcached) and then restart the development server to avoid interacting with cached objects.

Backend changes

We have a framework that automatically handles many of the steps for the most common types of UserProfile and Realm settings. We refer to this as the property_types framework. However, it is valuable to understand the flow of events even if the property_types framework means you don't have to write much code for a new setting.

Database interaction: Add any necessary code for updating and interacting with the database in zerver/lib/actions.py. It should update the database and send an event announcing the change.

Application state: Modify the fetch_initial_state_data and apply_event functions in zerver/lib/events.py to update the state based on the event you just created.

Backend implementation: Make any other modifications to the backend required for your feature to do what it's supposed to do (this will be unique to the feature you're implementing).

New views: Add any new application views to zproject/urls.py, or update the appropriate existing view in zerver/views/. This includes both views that serve HTML (new pages on Zulip) as well as new API endpoints that serve JSON-formatted data.

Testing: At the very least, add a test of your event data flowing through the system in test_events.py and an API test (e.g. for a Realm setting, in test_realm.py).

Frontend changes

JavaScript: Zulip's JavaScript is located in the directory static/js/. The exact files you may need to change depend on your feature. If you've added a new event that is sent to clients, be sure to add a handler for it in static/js/server_events_dispatch.js.

CSS: The primary CSS file is static/styles/zulip.css. If your new feature requires UI changes, you may need to add additional CSS to this file.

Templates: The initial page structure is rendered via Jinja2 templates located in templates/zerver. For JavaScript, Zulip uses Handlebars templates located in static/templates. Templates are precompiled as part of the build/deploy process.

Zulip is fully internationalized, so when writing both HTML templates or JavaScript code that generates user-facing strings, be sure to tag those strings for translation.

Testing: There are two types of frontend tests: node-based unit tests and blackbox end-to-end tests. The blackbox tests are run in a headless browser using Casper.js and are located in frontend_tests/casper_tests/. The unit tests use Node's assert module are located in frontend_tests/node_tests/. For more information on writing and running tests, see the testing documentation.

Documentation changes

After implementing the new feature, you should document it and update any existing documentation that might be relevant to the new feature. For more information on the kinds of documentation Zulip has, see Documentation.

Example Feature

This example describes the process of adding a new setting to Zulip: a flag that allows an admin to require topics on stream messages (the default behavior is that topics can have no subject). This flag is an actual Zulip feature. You can review the original commit in the Zulip repo. (This commit displays the work of setting up a checkbox for the feature on the admin settings page, communicating and saving updates to the setting to the database, and updating the state of the application after the setting is updated. For the code that accomplishes the underlying task of requiring messages to have a topic, you can view this commit.)

Update the model

First, update the database and model to store the new setting. Add a new boolean field, mandatory_topics, to the Realm model in zerver/models.py.

# zerver/models.py

class Realm(ModelReprMixin, models.Model):
    # ...
    restricted_to_domain = models.BooleanField(default=True) # type: bool
    invite_required = models.BooleanField(default=False) # type: bool
+   mandatory_topics = models.BooleanField(default=False) # type: bool

The Realm model also contains an attribute, property_types, which other backend functions use to handle most realm settings without any custom code for the setting (more on this process below). The attribute is a dictionary, where the key is the name of the realm field and the value is the field's type. Add the new field to the property_types dictionary.

# zerver/models.py

class Realm(ModelReprMixin, models.Model)
  # ...
  # Define the types of the various automatically managed properties
    property_types = dict(
        # ...
+       mandatory_topics=bool,
        # ...

The majority of realm settings can be included in property_types. However, there are some properties that need custom logic and thus cannot use this framework. For example:

  • The realm authentication_methods attribute is a bitfield and needs additional code for validation and updating.
  • The allow_message_editing and message_content_edit_limit_seconds fields depend on one another, so they are also handled separately and not included in property_types.

When creating a realm property that is not a boolean, Text or integer field, or when adding a field that is dependent on other fields, do not add the field to the property_types dictionary. The steps below will point out where to write additional code for these cases.

Create the migration

Create the migration file: ./manage.py makemigrations. Make sure to commit the generated file to git: git add zerver/migrations/NNNN_realm_mandatory_topics.py (NNNN is a number that is equal to the number of migrations.)

If you run into problems, the Django migration documentation is helpful.

Test your migration changes

Apply the migration: ./manage.py migrate


shell $ ./manage.py migrate
Operations to perform:
  Synchronize unmigrated apps: staticfiles, analytics, pipeline
  Apply all migrations: zilencer, confirmation, sessions, guardian, zerver, sites, auth, contenttypes
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying zerver.NNNN_realm_mandatory_topics... OK

Once you've run the migration, restart memcached on your development server (/etc/init.d/memcached restart) and then restart the development server to avoid interacting with cached objects.

Handle database interactions

Next, we will implement the backend part of this feature. Like typical apps, we will need our backend to update the database and send some response to the client that made the request.

Beyond that, we need to orchestrate notifications about the setting change to other clients (or other users, if you will). Clients find out about settings through two closely related code paths. When a client first contacts the server, the server sends the client its initial state. Subsequently, clients subscribe to "events," which can (among other things) indicate that settings have changed.

For the backend piece, we will need our action to make a call to send_event to send the event to clients that are active. We will also need to modify fetch_initial_state_data so that the new field is passed to clients. See our event system docs for all the gory details.

Anyway, getting back to implementation details...

If you are working on a feature that is in the realm property_types dictionary, you will not need to add code to zerver/lib/actions.py, but we will describe what the process in that file does:

In zerver/lib/actions.py, the function do_set_realm_property takes in the name of a realm property to update and the value it should have. This function updates the database and triggers an event to notify clients about the change. It uses the field's type, specified in the Realm.property_types dictionary, to validate the type of the value before updating the property; this is primarily an assertion to help catch coding mistakes, not to check for bad user input.

After updating the given realm field, do_set_realm_property creates an 'update' event with the name of the property and the new value. It then calls send_event, passing the event and the list of users whose browser sessions should be notified as the second argument. The latter argument can be a single user (if the setting is a personal one, like time display format), members in a particular stream only or all active users in a realm.

# zerver/lib/actions.py

def do_set_realm_property(realm, name, value):
  # type: (Realm, str, Union[Text, bool, int]) -> None
  """Takes in a realm object, the name of an attribute to update, and the
  value to update.
  property_type = Realm.property_types[name]
  assert isinstance(value, property_type), (
      'Cannot update %s: %s is not an instance of %s' % (
          name, value, property_type,))

  setattr(realm, name, value)
  event = dict(
  send_event(event, active_user_ids(realm))

If the new realm property being added does not fit into the property_types framework (such as the authentication_methods field), you'll need to create a new function to explicitly update this field and send an event. For example:

# zerver/lib/actions.py

def do_set_realm_authentication_methods(realm, authentication_methods):
    # type: (Realm, Dict[str, bool]) -> None
    for key, value in list(authentication_methods.items()):
        index = getattr(realm.authentication_methods, key).number
        realm.authentication_methods.set_bit(index, int(value))
    event = dict(
    send_event(event, active_user_ids(realm))

Update application state

zerver/lib/events.py contains code to ensure that your new setting is included in the data sent down to clients: both when a new client is loaded and when changes happen. This file also automatically handles realm settings in the property_types dictionary, so you would not need to change this file if your setting fits that framework.

The fetch_initial_state_data function is responsible for sending data when a client is loaded (data added to the state here will be available both in page_params in the browser, as well as to API clients like the mobile apps). The apply_event function in zerver/lib/events.py is important for making sure the state is always correct, even in the event of rare race conditions.

# zerver/lib/events.py

def fetch_initial_state_data(user_profile, event_types, queue_id, include_subscribers=True):
  # ...
  if want('realm'):
    for property_name in Realm.property_types:
        state['realm_' + property_name] = getattr(user_profile.realm, property_name)
    state['realm_authentication_methods'] = user_profile.realm.authentication_methods_dict()
    state['realm_allow_message_editing'] = user_profile.realm.allow_message_editing
    # ...

def apply_event(state, events, user_profile, include_subscribers):
  for event in events:
    # ...
    elif event['type'] == 'realm':
       field = 'realm_' + event['property']
       state[field] = event['value']
       # ...

If your new realm property fits the property_types framework, you don't need to change fetch_initial_state_data or apply_event. However, if you are adding a property that is handled separately, you will need to explicitly add the property to the state dictionary in the fetch_initial_state_data function. E.g., for authentication_methods:

# zerver/lib/events.py

def fetch_initial_state_data(user_profile, event_types, queue_id, include_subscribers=True):
  # ...
  if want('realm'):
      # ...
      state['realm_authentication_methods'] = user_profile.realm.authentication_methods_dict()
      # ...

For this setting, one won't need to change apply_event since its default code for realm event types handles this case correctly, but for a totally new type of feature, a few lines in that function may be needed.

Add a new view

You will need to add a view for clients to access that will call the actions.py code to update the database. This example feature adds a new parameter that will be sent to clients when the application loads and should be accessible via JavaScript. There is already a view that does this for related flags: update_realm in zerver/views/realm.py. So in this case, we can add our code to the existing view instead of creating a new one.

You'll need to add a parameter for the new field to the update_realm function in zerver/views/realm.py (and add the appropriate mypy type annotation).

# zerver/views/realm.py

def update_realm(request, user_profile, name=REQ(validator=check_string, default=None),
             # ...,
+            mandatory_topics=REQ(validator=check_bool, default=None),
             # ...):
+            # type: (HttpRequest, UserProfile, ..., Optional[bool], ...
  # ...

If this feature fits the property_types framework and does not require additional validation, this is the only change to make to zerver/views/realm.py.

Text fields or other realm properties that need additional validation can be handled at the beginning of update_realm.

# zerver/views/realm.py

# Additional validation/error checking beyond types go here, so
# the entire request can succeed or fail atomically.
if default_language is not None and default_language not in get_available_language_codes():
    raise JsonableError(_("Invalid language '%s'" % (default_language,)))
if description is not None and len(description) > 100:
    return json_error(_("Realm description cannot exceed 100 characters."))
# ...

The code in update_realm loops through the property_types dictionary and calls do_set_realm_property on any property to be updated from the request.

If the new feature is not in property_types, you will need to write code to call the function you wrote in actions.py that updates the database with the new value. E.g., for authentication_methods, we created do_set_realm_authentication_methods, which we will call here:

# zerver/views/realm.py

# import do_set_realm_authentication_methods from actions.py
from zerver.lib.actions import (
    # ...
# ...
# ...
if authentication_methods is not None and realm.authentication_methods_dict() != authentication_methods:
        do_set_realm_authentication_methods(realm, authentication_methods)
        data['authentication_methods'] = authentication_methods
# ...

This completes the backend implementation. A great next step is to write the backend tests.

Backend Tests

To test the new setting syncs correctly with the property_types framework, one usually just needs to add a line in each of test_events.py and test_realm.py with a list of values to switch between in the test. In the case of a boolean field, no action is required, because those tests will correctly assume that the only values to test are True and False.

In test_events.py, the function that runs tests for the property_types framework is do_set_realm_property_test, and in test_realm.py, it is do_test_realm_update_api.

One still needs to add a test for whether the setting actually controls the feature it is supposed to control, however.

Update the front end

After completing the process of adding a new feature on the back end, you should make the required front end changes: in this case, a checkbox needs to be added to the admin page (and its value added to the data sent back to server when a realm is updated) and the change event needs to be handled on the client.

To add the checkbox to the admin page, modify the relevant template, static/templates/settings/organization-permissions-admin.handlebars (omitted here since it is relatively straightforward).

Then add the new form control in static/js/admin.js.

// static/js/admin.js

function _setup_page() {
    var options = {
        realm_name: page_params.realm_name,
        realm_description: page_params.realm_description,
        realm_restricted_to_domain: page_params.realm_restricted_to_domain,
        realm_invite_required: page_params.realm_invite_required,
        // ...
+       realm_mandatory_topics: page_params.mandatory_topics,
        // ...

The JavaScript code for organization settings and permissions can be found in static/js/settings_org.js.

There is a front-end version of property_types, which reduces the code needed on the front end for a new feature.

Add the new feature to the property_types object in settings_org.js. The key should be the setting name and the value should be an object with the following keys:

  • type
  • checked_msg (what message the user sees when they enable the setting)
  • unchecked_msg (what message the user sees when they disable the setting)
// static/js/settings_org.js

var property_types = {
    settings: {
        // ...
    permissions: { // ...
+       mandatory_topics: {
+           type: 'bool',
+           checked_msg: i18n.t("Topics are required in messages to streams"),
+           unchecked_msg: i18n.t("Topics are not required in messages to streams"),

Additionally, any code needed to update the UI when the setting is changed should be written in a function inside settings_org.js. For example, when a realm description is updated, that value change should occur in other windows where the description field is visible:

# static/js/settings_org.js

exports.update_realm_description = function () {
    if (!meta.loaded) {


This ensures the appropriate code will run even if the changes are made in another browser window.

In the example of updating a mandatory_topics setting, most of the changes are on the backend, so no UI updates are required.

Finally, update server_events_dispatch.js to handle related events coming from the server. There is an object, realm_settings, in the function dispatch_normal_event. The keys in this object are setting names and the values are the UI updating functions to run when an event has occurred.

If there is no relevant UI change to make, the value should be noop (this is the case for mandatory_topics). However, if you had written a function in settings_org.js to update UI, that function should be the value in the realm_settings object.

// static/js/server_events_dispatch.js

function dispatch_normal_event(event) {
    switch (event.type) {
    // ...
    case 'realm':
      var realm_settings = {
          add_emoji_by_admins_only: settings_emoji.update_custom_emoji_ui,
          allow_edit_history: noop,
          // ...
+         mandatory_topics: noop,
          // ...

The rest of the dispatch_normal_events function updates the state of the application if an update event has occurred on a realm property and runs the associated function to update the application's UI, if necessary.

Front End Tests

A great next step is to write front end tests. There are two types of frontend tests: node-based unit tests and Casper end-to-end tests.

At the minimum, if you created a new function to update UI in settings_org.js, you will need to mock that function in frontend_tests/node_tests/dispatch.js. Add the name of the UI function you created to the following object with noop as the value:

# frontend_tests/node_tests/dispatch.js

set_global('settings_org', {
    reset_realm_default_language: noop,
    toggle_email_change_display: noop,
    toggle_name_change_display: noop,
    update_message_retention_days: noop,
    update_realm_description: noop,

Beyond that, you should add any applicable tests that verify the behavior of the setting you just created.

Update documentation

After you add a new view, you should document your feature. This feature adds new functionality that requires messages to have topics if the setting is enabled. A recommended way to document this feature would be to update and/or augment Zulip's user documentation to reflect your changes and additions.

At the very least, this will involve adding (or modifying) a Markdown file documenting the feature to templates/zerver/help/ in the main Zulip server repository, where the source for Zulip's user documentation is stored. For information on writing user documentation, see Zulip's general user guide documentation.

For a more concrete example of writing documentation for a new feature, see an example commit in the Zulip repo that documented a new realm feature, the current source, and the final rendered documentation.