zulip/docs/hashchange-system.md

URL hashes and deep linking

Hashchange

The Zulip web application has a nice system of hash (#) URLs that can be used to deep-link into the application and allow the browser's "back" functionality to let the user navigate between parts of the UI. Some examples are:

• /#settings/your-bots: Bots section of the settings overlay.
• /#streams: Streams overlay, where the user manages streams (subscription etc.)
• /#streams/11/announce: Streams overlay with stream ID 11 (called "announce") selected.
• /#narrow/stream/android/subject/fun: Message feed showing stream "android" and topic "fun".

The main module in the frontend that manages this all is static/js/hashchange.js (plus hash_util.js for all the parsing code), which is unfortunately one of our thorniest modules. Part of the reason that it's thorny is that it needs to support a lot of different flows:

• The user clicking on an in-app link, which in turn opens an overlay. For example the streams overlay opens when the user clicks the small cog symbol on the left sidebar, which is in fact a link to /#streams. This makes it easy to have simple links around the app without custom click handlers for each one.
• The user uses the "back" button in their browser (basically equivalent to the previous one, as a link out of the browser history will be visited).
• The user clicking some in-app click handler (e.g. "Stream settings" for an invidual stream), that potentially does several UI-manipulating things including e.g. loading the streams overlay, and needs to update the hash without re-triggering the open animation (etc.).
• Within an overlay like the streams overlay, the user clicks to another part of the overlay, which should update the hash but not re-trigger loading the overlay (which would result in a confusing animation experience).
• The user is in a part of the webapp, and reloads their browser window. Ideally the reloaded browser window should return them to their original state.
• A server-initiated browser reload (done after a new version is deployed, or when a user comes back after being idle for a while, see notes below), where we try to preserve extra state (e.g. content of compose box, scroll position within a narrow) using the /#reload hash prefix.

When making changes to the hashchange system, it is essential to test all of these flows, since we don't have great automated tests for all of this (would be a good project to add them to the Casper suite) and there's enough complexity that it's easy to accidentally break something.

Here's some notes on how we handle these cases:

• hashchange.hashchanged is the function used to handle the hash, when it's changed by the browser (e.g. by clicking on a link to a hash or using the back button).
• hashchange.should_ignore is the function hashchange.hashchanged calls to make it possible for clicking on links within a given overlay to just be managed by code within that overlay, without reloading the overlay. It primarily checks whether the "main hash" (i.e. the first piece like settings for #settings/your-account) is an overlay.
• hashchange.do_hashchange is what is called when the user reloads the browser. If the hash is nonempty, it ensures the relevant overlay is opened or the user is narrowed as part of the page load process. It is also is called by hashchange.hashchanged when the hash changes outside the should_ignore boundaries, since the logic for that case is identical.
• reload.preserve_state is called when a server-initiated browser reload happens, and encodes a bunch of data like the current scroll position into the hash.
• reload.initialize handles restoring the preserved state after a reload where the hash starts with /#reload.

Server-initiated reloads

There are a few circumstances when the Zulip browser window needs to reload itself:

• If the browser has been offline for more than 10 minutes, the browser's event queue will have been garbage-collected by the server, meaning the browser can no longer get real-time updates altogether. In this case, the browser auto-reloads immediately in order to reconnect. We have coded an unsuspend trigger (based on some clever time logic) that ensures we check immediately when a client unsuspends; grep for unsuspend to see the code.

• If a new version of the server has been deployed, we want to reload the browser so that it will start running the latest code. However, we don't want server deploys to be disruptive. So, the backend preserves user-side event queues (etc.) and just pushes a special restart event to all clients. That event causes the browser to start looking for a good time to reload, based on when the user is idle (ideally, we'd reload when they're not looking and restore state so that the user never knew it happened!). The logic for doing this is in static/js/reload.js; but regardless we'll reload within 30 minutes unconditionally.

  An important detail in server-initiated reloads is that we desynchronize when browsers start attempting them randomly, in order to avoid a thundering herd situation bringing down the server.

All reloads

In addition to saving state as described above when reloading the browser, Zulip also does a few bookkeeping things on page reload (like cleaning up its event queue, and saving any text in an open compose box as a draft).