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. You may also find it beneficial to read Zulip's architectural overview. Zulip is a web application built using the Django framework, and some of the processes listed in this tutorial, such as database migrations and tests, use Django's tooling.

Zulip's directory structure will also be helpful to review when creating a new feature. Many aspects of the structure will be familiar to Django developers. Visit Django's documentation for more information about how Django projects are typically organized. And finally, the message sending documentation on the additional complexity involved in sending messages.

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:

Backend

zerver/models/realms.py: Defines the database model.
zerver/views/realm.py: Contains the view function that implements the API endpoint for editing Realm objects.
zerver/actions/realm_settings.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.

Frontend

web/src/state_data.ts: Defines the expected state of the data that is received from the Zulip server.
web/templates/settings/organization_settings_admin.hbs: Defines the structure of the organization settings admin modal.
web/src/admin.ts: Handles building the organization settings admin modal.
web/src/settings_org.ts: Handles form submission for changes to an organization's settings.
web/src/server_events_dispatch.js: Handles events coming from the server.

Backend testing

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

Frontend testing

web/e2e-tests/admin.test.ts: Contains end-to-end tests for the organization admin settings pages.
web/tests/dispatch.test.cjs: Unit tests for handling of events.

Documentation

zerver/openapi/zulip.yaml: Contains OpenAPI definitions for the Zulip REST API.
api_docs/changelog.md: Documentation listing all changes to the Zulip Server API.
starlight_help/...: The user-facing documentation (help center) for the application.

Adding a field to the database

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

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

bash

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

It's highly recommended to read our database migration documentation to learn more about creating and applying database migrations.

Test your changes: Once you've run the migration, restart the development server.

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/actions/realm_settings.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 field 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/TypeScript: Zulip's JavaScript and TypeScript sources are located in the directory web/src/. 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 web/src/server_events_dispatch.js.

CSS: The primary CSS file is web/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/app. For JavaScript, Zulip uses Handlebars templates located in web/templates. Templates are precompiled as part of the build/deploy process.

Zulip is fully internationalized, so when writing both HTML templates or JavaScript/TypeScript/Python 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 Chromium browser using Puppeteer and are located in web/e2e-tests/. The unit tests use Node's assert module are located in web/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 detailed information on the kinds of documentation Zulip has, see Documentation.

Help center documentation: You will likely need to at least update, extend and link to articles in the starlight_help/ directory that are related to your new feature. Writing help center articles provides more detailed information about writing and editing feature starlight_help/ directory articles.

API documentation: A new feature will probably impact the REST API documentation as well, which will mean updating zerver/openapi/zulip.yaml and modifying api_docs/changelog.md for a new feature level. Documenting REST API endpoints explains Zulip's API documentation system and provides a step by step guide to adding or updating documentation for an API endpoint.

Example feature

This example describes the process of adding a new realm setting to Zulip. For the purposes of this tutorial, this new setting will be called my_fantastic_feature.

:::{tip}

A useful tool that we'll highlight throughout this tutorial is git grep. If you're unfamiliar with using git grep, then check out this blog post for an introduction to it, as well as the official git grep documentation

:::

Update the model

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

diff

 # zerver/models/realms.py

 class Realm(models.Model):
     # ...
     require_unique_names = models.BooleanField(default=False)
     name_changes_disabled = models.BooleanField(default=False)
     email_changes_disabled = models.BooleanField(default=False)
     avatar_changes_disabled = models.BooleanField(default=False)

+    # My fantastic feature for the new feature tutorial.
+    my_fantastic_feature = models.BooleanField(default=False)

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.

diff

 # zerver/models/realms.py

 class Realm(models.Model)
     # ...
     # Define the types of the various automatically managed properties
     property_types: dict[str, type | UnionType] = dict(
         allow_message_editing=bool,
         avatar_changes_disabled=bool,
         # ...
+        my_fantastic_feature=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. Any properties that define a relationship to another model (e.g., a ForeignKey field) do not use the property_types framework, for example moderation_request_channel property and the can_add_custom_emoji_group property.

When creating a realm property that is not a boolean, character 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 using the Django makemigrations command: ./manage.py makemigrations. Make sure to commit the generated file to git: git add zerver/migrations/NNNN_my_fantastic_feature.py (NNNN is a number that is equal to the current number of migrations.)

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

Test your migration changes

Apply the migration using Django's migrate command, ./manage.py migrate:

console

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_my_fantastic_feature... OK

Once you've run the migration, restart the development server.

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_on_commit to send the event to clients that are active (The event is only sent after the current database transaction commits, hence the name). 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.

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/actions/realm_settings.py, but we will describe what the process in that file does:

In zerver/actions/realm_settings.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_on_commit, 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 channel only or all active users in an realm.

:::{tip}

Use git grep to find and read the do_set_realm_property function. Are there property_types that are handled differently in that function? Which ones? Does the function indicate why?

:::

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

python

# zerver/actions/realm_settings.py

def do_set_realm_stream(
    realm: Realm,
    field: Literal[
        "moderation_request_channel",
        "new_stream_announcements_stream",
        "signup_announcements_stream",
        "zulip_update_announcements_stream",
    ],
    stream: Stream | None,
    stream_id: int,
    *,
    acting_user: UserProfile | None,
) -> None:
    # We could calculate more of these variables from `field`, but
    # it's probably more readable to not do so.
    if field == "moderation_request_channel":
        old_value = realm.moderation_request_channel_id
        realm.moderation_request_channel = stream
        property = "moderation_request_channel_id"

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 app). 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.

python

# zerver/lib/events.py

def fetch_initial_state_data(
    user_profile: UserProfile | None,
    # ...
    if want("realm"):
        # The realm bundle includes both realm properties and server
        # properties, since it's rare that one would want one and not
        # the other. We expect most clients to want it.
        # ...
        for property_name in Realm.property_types:
            state["realm_" + property_name] = getattr(realm, property_name)

        for setting_name in Realm.REALM_PERMISSION_GROUP_SETTINGS:
            setting_group_id = getattr(realm, setting_name + "_id")
            state["realm_" + setting_name] = get_group_setting_value_for_register_api(
                setting_group_id, anonymous_group_membership_data_dict
            )
        # ...

def apply_event(
    user_profile: UserProfile,
    # ...
) -> None:
    # ...
    elif event["type"] == "realm":
        if event["op"] == "update":
            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.

For our example, 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).

diff

 # zerver/views/realm.py

@require_realm_admin
@typed_endpoint
def update_realm(
    request: HttpRequest,
    user_profile: UserProfile,
     *,
     allow_message_editing: Json[bool] | None = None,
     authentication_methods: Json[dict[str, Any]] | None = None,
     # ...
+    my_fantastic_feature: Json[bool] | None = None,
     # ...
 ):
     # ...

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.

python

# 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 '{language}'").format(language=default_language))
# ...

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 moderation_request_channel, we created do_set_realm_moderation_request_channel, which we will call here:

python

# zerver/views/realm.py

# import do_set_realm_authentication_methods from actions.py
from zerver.actions.realm_settings import (
    # ...
    do_set_realm_moderation_request_channel,
    # ...
)
# ...
# ...
    # Channel-valued settings are not yet fully supported by the
    # property_types framework, and thus have explicit blocks here.
    if moderation_request_channel_id is not None and (
        realm.moderation_request_channel is None
        or realm.moderation_request_channel.id != moderation_request_channel_id
    ):
        new_moderation_request_channel_id = None
        if moderation_request_channel_id >= 0:
            (new_moderation_request_channel_id, sub) = access_stream_by_id(
                user_profile, moderation_request_channel_id, require_content_access=False
            )
        do_set_realm_moderation_request_channel(
            realm,
            new_moderation_request_channel_id,
            moderation_request_channel_id,
            acting_user=user_profile,
        )
        data["moderation_request_channel_id"] = moderation_request_channel_id
# ...

This completes the backend implementation. A great next step is to write automated backend tests for your new feature.

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 zerver/tests/test_events.py and zerver/tests/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 would still need to add tests for whether the setting actually controls the feature it is supposed to control. At this time, our example, my_fantastic_feature, does not change any behavior on the backend, so we don't need to add any additional tests. If you're using this tutorial as a guide for creating an actual new feature, then at this point you should add any necessary backend tests.

Visit Zulip's Django testing documentation to learn more about the backend testing framework.

:::{tip}

Pick one of the other Realm.property_types settings and use git grep to find examples of backend tests that were written for that setting: git grep REALM_SETTING zerver/tests/.

:::

Also note that you may already need to update the API documentation for your new feature to pass new or existing backend tests at this point. The tutorial for writing REST API endpoints can be a helpful resource, especially the section on debugging schema validation errors.

What is my fantastic feature?

Since you've now set up the backend for your new fantastic feature, let's take a moment to discuss what that feature actually will be. Here's where you get to be creative!

Think of some fun, visual change that you could make to the Zulip web app in your development environment. Maybe toggling my_fantastic_feature will change the app font to Dingbats. Maybe it will change the color of the font in the message feed. Maybe it will make all user status emoji render upside down 🙃. That's up to you!

The next part of this tutorial will go through the process of setting up the toggle for turning your feature on and off via the organization settings admin modal in the web-app. After that, your final challenge will be to make the changes to the codebase to make your fantastic idea into a reality in your development environment.

Update the frontend

After completing the process of adding a new realm setting on the backend, you'll want make the required frontend changes.

Since we've updated the data sent for a realm to include a new property, we need to update the realm_schema on the frontend to match our changes in zerver/lib/events.py. This can be done in web/src/state_data.ts:

diff

 // web/src/state_data.ts

 // Sync this with zerver.lib.events.do_events_register.
 export const realm_schema = z.object({
     // ...
     realm_move_messages_within_stream_limit_seconds: z.nullable(z.number()),
+    realm_my_fantastic_feature: z.boolean(),
     realm_name_changes_disabled: z.boolean(),
     // ...

In order for an admin user to enable and disable our new realm setting, a checkbox needs to be added to the organization settings 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 organization settings admin modal, you'll need to modify one of the templates for the organization settings modal: web/templates/settings/organization.... If you're adding a non-checkbox field, you'll need to specify the type of the field via the data-setting-widget-type attribute in the Handlebars template.

For simplicity, let's add the checkbox for realm_my_fantastic_feature to the "Message feed" section in web/templates/settings/organization_settings_admin.hbs. Since there are a number of realm settings controlled via checkbox, there is a settings_checkbox partial template already set up for you to reuse.

:::{tip}

Use git grep to see where the other realm settings in the "Message feed" section of that template are in the web/ directory.

Also, if you're not familiar with the organization settings modal, log in as Iago in your development environment and follow the Zulip help center instructions for customizing organization settings.

:::

Then add the new form control in web/src/admin.ts.

diff

 // web/src/admin.ts

 export function build_page() {
     const options = {
        custom_profile_field_types: realm.custom_profile_field_types,
        full_name: current_user.full_name,
         // ...
+        realm_my_fantastic_feature: realm.realm_my_fantastic_feature,
         // ...

You'll also want to add a label to the admin_settings_label object in that same file. Note that these labels are marked for translation.

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 other than in settings page itself, the value should be noop. However, if you are writing a function to update the UI after a given setting has changed, your function should be referenced in the realm_settings of server_events_dispatch.js. See for example settings_emoji.update_custom_emoji_ui.

diff

 // web/src/server_events_dispatch.js

 function dispatch_normal_event(event) {
     switch (event.type) {
     // ...
     case 'realm':
         var realm_settings = {
             add_custom_emoji_policy: settings_emoji.update_custom_emoji_ui,
             allow_edit_history: noop,
             // ...
+            my_fantastic_feature: ???,
             // ...
         };

Checkboxes and other common input elements handle the UI updates automatically through the logic in settings_org.sync_realm_settings.

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.

Now that you've got the admin checkbox set up to toggle your new feature on and off, you should work on implementing your fantastic idea! Remember to use git grep to help as you become familiar with the codebase.

Manually test

Here are few important cases you should consider when manually testing your changes:

For organization settings where we have a "save/discard" model, make sure both the "Save" and "Discard changes" buttons are working properly when toggling the realm setting.
If your setting is dependent on another setting, carefully check that both are properly synchronized. For example, the input element for realm_waiting_period_threshold_custom_input is shown only when we have selected the custom time limit option in the realm_waiting_period_threshold dropdown.
Do some manual testing for the real-time synchronization of input elements across the browsers and just like "Discard changes" button, check whether dependent settings are synchronized properly (this is easy to do by opening two browser windows to the settings page, and making changes in one while watching the other).
Each subsection has independent "Save" and "Discard changes" buttons, so changes and saving in one subsection shouldn't affect the others.

Frontend tests

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

At the minimum, you'll want to update web/tests/lib/example_realm.cjs for the new realm setting that you've added.

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

:::{tip}

If you've identified a similar feature on the frontend to the one that you're creating, use your git grep skills to find and read through the frontend tests for that feature.

:::

Update documentation

If you're using this tutorial as a guide to add an actual new feature to Zulip, it's really important to make sure that your new feature is well documented.

A recommended way to document a new feature is to update and/or augment Zulip's existing help center documentation to reflect your changes and additions.

At the very least, this will involve modifying (or adding) a Markdown file documenting the feature in the starlight_help/ directory of the main Zulip server repository, where the source for Zulip's end user documentation is stored. Details about writing, editing and testing these files can be found in: Writing help center articles.

Also, new features will often impact Zulip's REST API documentation, which is found in zerver/openapi/zulip.yaml. You may have noticed this during the testing process as the Zulip test suite should fail if there is a change to the API without a corresponding update to the documentation.

The best way to understand writing and updating Zulip's API documentation is to read more about Zulip's REST API documentation process and OpenAPI configuration.

In particular, if there is an API change, you should use the create-api-changelog tool to create a file for the API changelog entry for your your new feature. The API feature level allows the developers of mobile clients and other tools using the Zulip API to programmatically determine whether the Zulip server they are interacting with supports a given feature; see the Zulip release lifecycle.

If you're using this tutorial to learn about the codebase and have created a fun, fantastic feature, share a screenshot and/or a GIF from your development environment of your unique changes to the web-app UI in the Zulip development community's new members channel!

Writing a new application feature

Writing a new application feature

General process

Files impacted

Adding a field to the database

Backend changes

Frontend changes

Documentation changes

Example feature

Update the model

Create the migration

Test your migration changes

Handle database interactions

Update application state

Add a new view

Backend tests

What is my fantastic feature?

Update the frontend

Manually test

Frontend tests

Update documentation

Share your fantastic feature