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.

General Process in brief

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, restart memcached on your development server (/etc/init.d/memcached restart) and then restart run-dev.py to avoid interacting with cached objects.

Backend changes

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_events functions in zerver/lib/actions.py to update the state based on the event you just created.

Backend implementation: Make any other modifications to the backend required for your change.

New views: Add any new application views to zerver/urls.py. 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.

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 to static/js/server_events.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.

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.

Example Feature

This example describes the process of adding a new setting to Zulip: a flag that restricts inviting new users to admins only (the default behavior is that any user can invite other users). It is based on an actual Zulip feature, and you can review the original commit in the Zulip git repo. (Note that Zulip has since been upgraded from Django 1.6 to 1.8, so the migration format has changed.)

Update the model

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

--- a/zerver/models.py
+++ b/zerver/models.py
@@ -139,6 +139,7 @@ class Realm(ModelReprMixin, models.Model):
     restricted_to_domain = models.BooleanField(default=True) # type: bool
     invite_required = models.BooleanField(default=False) # type: bool
+    invite_by_admins_only = models.BooleanField(default=False) # type: bool
     create_stream_by_admins_only = models.BooleanField(default=False) # type: bool
     mandatory_topics = models.BooleanField(default=False) # type: bool

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_invite_by_admins_only.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.0026_realm_invite_by_admins_only... OK

Handle database interactions

Next, we will move on to implementing 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 to other clients (or other users, if you will) that our setting has changed. 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 future clients see the new changes.

Anyway, getting back to implementation details...

In zerver/lib/actions.py, create a new function named do_set_realm_invite_by_admins_only. This function will update the database and trigger an event to notify clients when this setting changes. In this case there was an existing realm|update event type which was used for setting similar flags on the Realm model, so it was possible to add a new property to that event rather than creating a new one. The property name matches the database field to make it easy to understand what it indicates.

The second argument to send_event is the list of users whose browser sessions should be notified. Depending on the setting, this can be a single user (if the setting is a personal one, like time display format), only members in a particular stream or all active users in a realm. :

# zerver/lib/actions.py

def do_set_realm_invite_by_admins_only(realm, invite_by_admins_only):
  realm.invite_by_admins_only = invite_by_admins_only
  event = dict(
  send_event(event, active_user_ids(realm))
  return {}

Update application state

You then need to add code that will handle the event and update the application state. In zerver/lib/actions.py update the fetch_initial_state and apply_events functions. :

def fetch_initial_state_data(user_profile, event_types, queue_id):
  # ...
  state['realm_invite_by_admins_only'] = user_profile.realm.invite_by_admins_only`

In this case you don’t need to change apply_events because there is already code that will correctly handle the realm update event type: :

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

Add a new view

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

# zerver/views/__init__.py

def home(request):
  # ...
  page_params = dict(
    # ...
    realm_invite_by_admins_only = register_ret['realm_invite_by_admins_only'],
    # ...

Since this feature also adds a checkbox to the admin page, and adds a new property the Realm model that can be modified from there, you also need to make changes to the update_realm function in the same file: :

# zerver/views/realm.py

def update_realm(request, user_profile, name=REQ(validator=check_string, default=None),
                 restricted_to_domain=REQ(validator=check_bool, default=None),
                 invite_required=REQ(validator=check_bool, default=None),
                 ...more arguments):

  # ...

  if invite_by_admins_only is not None and
    realm.invite_by_admins_only != invite_by_admins_only:
      do_set_realm_invite_by_admins_only(realm, invite_by_admins_only)
      data['invite_by_admins_only'] = invite_by_admins_only

Then 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/admin_tab.handlebars (omitted here since it is relatively straightforward). Then add code to handle changes to the new form control in static/js/admin.js. :

var url = "/json/realm";
var new_invite_by_admins_only =
data[invite_by_admins_only] = JSON.stringify(new_invite_by_admins_only);

  url: url,
  data: data,
  success: function (data) {
    # ...
    if (data.invite_by_admins_only) {
      ui.report_success("New users must be invited by an admin!", invite_by_admins_only_status);
    } else {
      ui.report_success("Any user may now invite new users!", invite_by_admins_only_status);
    # ...

Finally, update server_events.js to handle related events coming from the server. :

# static/js/server_events.js

function dispatch_normal_event(event) {
    switch (event.type) {
    # ...
    case 'realm':
        if (event.op === 'update' && event.property === 'invite_by_admins_only') {
            page_params.realm_invite_by_admins_only = event.value;

Any code needed to update the UI should be placed in dispatch_normal_event callback (rather than the channel.patch) function. This ensures the appropriate code will run even if the changes are made in another browser window. In this example most of the changes are on the backend, so no UI updates are required.