Server architecture

This chapter is a “deep dive” into Bokeh server’s internals. It assumes you’re already familiar with the information on Bokeh server in Running a Bokeh server.

You might want to read this if you are:

  • trying to work on the Bokeh codebase

  • writing your own custom server process to use rather than bokeh serve

A custom server process can add additional routes (web pages or REST endpoints) using Tornado’s web framework.

If an application developer uses bokeh serve, they typically should not need to import from bokeh.server at all. An application developer would only use the Server class if it is doing something specialized, such as a custom or embedded server process.

Applications, sessions, and connections

Each server contains one or more applications; you can think of an application as a session template, or a factory for sessions. Sessions have a 1-1 relationship with instances of bokeh.document.Document: each session has a document instance. When a browser connects to the server, it gets a new session; the application fills in the session’s document with whatever plots, widgets, or other content it desires. The application can also set up callbacks, to run periodically or to run when the document changes.

Applications are represented by the Application class. This class contains a list of Handler instances and optional metadata. Handlers can be created in lots of ways: from JSON files, from Python functions, from Python files, and perhaps many more ways in the future. The optional metadata is available as a JSON blob via the /metadata endpoint. For example, creating a Application instance with:

Application(metadata=dict(hi="hi", there="there"))

will have http://server/myapp/metadata return (application/json):

    "data": {
        "hi": "hi",
        "there": "there"
    "url": "/myapp"

Around each application, the server creates an ApplicationContext. Its primary role is to hold the set of sessions for the application.

Sessions are represented by the class ServerSession.

Each application has a route (called an app_path in the client API), and each session has an ID. The combination of the two specifies a Document instance (the server looks up the application by path, and then looks up the session by ID).

Each session has 0-N connections, represented by the ServerConnection class. Connections are websocket connections. In general, sessions last as long as they have connections, though they only expire after a timeout (to allow for page reloads and the like).

Applications and application handlers cannot access the Server ServerSession, or ApplicationContext directly; they have a much more limited interface defined in two pieces, ServerContext and SessionContext. ServerContext presents a limited interface to some aspects of ApplicationContext and Server, while SessionContext presents a limited interface to some aspects of ServerSession. Concrete implementations of these interfaces are BokehServerContext and BokehSessionContext.

Summarizing the object graph:

  • Server implemented by BokehTornado

  • has N ApplicationContext

  • has 1 Application capable of creating new sessions

  • has 1 path used to identify it in URLs

  • has 1 ServerContext representing the aspects of the server visible to application code

  • has N ServerSession

  • has 1 session ID which is a string naming the session

  • has 1 Document representing the session state

  • has N ServerConnection representing websockets attached to the session

  • has 1 SessionContext representing the aspects of the session visible to application code

Tornado IOLoop and Async Code

To work on the server, you’ll need an understanding of Tornado’s IOLoop and the tornado.gen module.

The Tornado documentation will be the best resource, but here are some quick things to know:

  • The Bokeh server is single-threaded, so it’s important not to write “blocking” code, meaning code that uses up the single thread while it waits for IO or performs a long computation. If you do this, you’ll rapidly increase the latency seen by users of your application. For example, if you block for 100ms every time someone moves a slider, and ten users are doing this at once, users could easily see 10*100ms=1s of lag with only ten users.

  • In Tornado, non-blocking code is implemented with Python native coroutrines, i.e. async def.

  • When no code is running, Tornado waits in its IOLoop (sometimes called a “main loop” or “event loop”), which means it’s waiting for something to happen. When something happens, IOLoop executes any callbacks that were interested in that event.

Applications and the IOLoop

We don’t want applications to touch the Tornado IOLoop directly to add callbacks, because when a session expires or an application is reloaded, we need the ability to remove all callbacks belonging to a session or application.

To enable this, applications should only add callbacks using the APIs on Document and ServerContext. Methods on those classes allow applications to add_periodic_callback, add_timeout_callback, and add_next_tick_callback. We intercept these callback additions and are able to remove them when we unload an application or destroy a session.


If you look at the Application class, there are two ways the server can call into it.

  1. the modify_document() method which does just what it says: it passes in the session’s Document and allows the application to modify it (perhaps adding some plots and widgets).

  2. a set of “hooks” on_server_loaded(), on_server_unloaded(), on_session_created(), on_session_destroyed().

The “hooks” are called “lifecycle hooks” since they happen at defined points in the lifetime of an application and a session.

Here are the steps in the lifecycle:

  1. When the server process starts up, it calls on_server_loaded() on each application.

  2. When a client connects with a previously-unused session ID, the server creates a ServerSession and calls on_session_created() with an empty Document, then modify_document() to initialize the Document. The on_session_created() can also initialize part of the Document if it likes. on_session_created() happens before modify_document().

  3. When there are no connections to a session, it will eventually time out, and on_session_destroyed() will be called.

  4. If the server process shuts down cleanly, it will call on_server_unloaded() on each application. This is probably rare in production: it’s typical for server processes to be killed by a signal. on_server_unloaded() may be more useful during development so that apps can be reloaded without leaking resources.

These hooks can add periodic or one-shot callbacks to the ServerContext. These callbacks may be asynchronous (using Tornado’s async IO facilities) and are able to update all live session documents.

Critical consideration when using ``on_server_loaded()``: Process-global is NOT the same as cluster-global. If you scale a Bokeh application, you’ll want a separate process for each CPU core, roughly. Processes in a cluster may not even be on the same machine. A server process can never assume that it knows about “all sessions that exist,” only “all sessions hosted in this process.”

Details of ServerSession

The session object handles most interaction between the client and the server.


The trickiest aspect of ServerSession may be locking. In general, we want one callback or one websocket request to be processed at a time; we don’t want to interleave them, because it would be difficult to implement callbacks and request handlers if they had to worry about interleaving.

So ServerSession does one thing at a time, controlled by ServerSession._lock, which is a Tornado lock.

If you’re familiar with locking and threads, the situation here is conceptually identical; but race conditions can only happen at “yield points” (when we return to the IOLoop) rather than at any point, and the lock is a Tornado lock rather than a thread lock.

The rule is: to touch ServerSession.document code must hold ServerSession._lock.

For callbacks added through the Document API, we automatically acquire the lock on the callback’s behalf before we execute the callback, and release it afterward.

Callbacks added through the ServerContext API, can only obtain a reference to the session document using SessionContext.with_locked_document(). It executes a provided function with the document lock held, passing the document to that function.


It is very easy to modify the server code in such a way that you’re touching the document without holding the lock. If you do this, things will break in subtle and painful-to-debug ways. When you touch the session document, triple-check that the lock is held.

Session security

We rely on session IDs being cryptographically random and difficult to guess. If an attacker knows someone’s session ID, they can eavesdrop on or modify the session. If you’re writing a larger web app with a Bokeh app embedded inside, this may affect how you design your larger app.

Session timeout

To avoid resource exhaustion, unused sessions will time out according to code in

Websocket protocol

The server has a websocket connection open to each client (each browser tab, in typical usage). The primary role of the websocket is to keep the session’s Document in sync between the client and the server.

There are two client implementations in the Bokeh codebase: one is a Python ClientSession, the other is a JavaScript ClientSession. Client and server sessions are mostly symmetrical. On both sides, we are receiving change notifications from the other side’s Document, and sending notification of changes made on our side. In this way, the two Document are kept in sync.

The Python implementation of the websocket protocol can be found in bokeh.server.protocol, though both the client side and the server side use it.

Websockets already implement “frames” for us, and they guarantee frames will arrive in the same order they were sent. Frames are strings or byte arrays (or special internal frame types, such as pings). A websocket looks like two sequences of frames, one sequence in each direction (“full duplex”).

On top of websocket frames, we implement our own Message concept. A Bokeh Message spans multiple websocket frames. It always contains a header frame, metadata frame, and content frame. These three frames each contain a JSON string. The code permits these three frames to be followed by optional binary data frames. In principle, this could allow, for example, for sending NumPy arrays directly from their memory buffers to the websocket with no additional copies. However, the binary data frames are not yet used in Bokeh.

The header frame indicates the message type and gives messages an ID. Message IDs are used to match replies with requests (the reply contains a field saying “I am the reply to the request with ID xyz”).

The metadata frame has nothing in it for now but could be used for debugging data or for another purpose in the future.

The content frame has the “body” of the message.

There aren’t many messages right now. A quick overview:

  • ACK is used for an initial handshake when setting up the connection

  • OK is a generic reply when a request doesn’t require any more specific reply

  • ERROR is a generic error reply when something goes wrong

  • SERVER-INFO-REQ and SERVER-INFO-REPLY are a request-reply pair where the reply contains information about the server, such as its Bokeh version

  • PULL-DOC-REQ asks to get the entire contents of the session’s Document as JSON, and PULL-DOC-REPLY is the reply containing said JSON.

  • PUSH-DOC sends the entire contents of the session’s Document as JSON, and the other side should replace its document with these new contents.

  • PATCH-DOC sends changes to the session’s document to the other side

Typically, when opening a connection, one side will pull or push the entire document; after the initial pull or push, the two sides stay in sync using PATCH-DOC messages.

Some current protocol caveats

  1. In the current protocol, conflicts where both sides change the same thing at the same time are not handled (the two sides can end up out-of-sync if this happens, because the two PATCH-DOC are in flight at the same time). It’s easy to devise a scheme to detect this situation, but it’s less clear what to do when it’s detected, so right now, we don’t detect it and do nothing. In most cases, applications should avoid this situation because even if we could make sense of it and handle it somehow, it would probably be inefficient for the two sides of the app to “fight” over the same value. (If real-world applications trip on this issue, we will have to figure out what they’re trying to do and devise a solution.)

  2. At the moment, we are not smart about patching collections; if there’s a Model property that’s a giant dictionary, we’ll send the whole giant dictionary whenever any entry in it changes.

HTTP endpoints

The server only supports a few HTTP routes; you can find them in bokeh.server.urls.

In brief:

  • /static/ serves Bokeh’s JS and CSS resources

  • /app_path/ serves a page that displays a new session

  • /app_path/ws is the websocket connection URL

  • /app_path/autoload.js serves a chunk of JavaScript that backs the bokeh.embed.server_document() and bokeh.embed.server_session() functionality

Bokeh server isn’t intended to be a general-purpose web framework. You can, however, pass new endpoints to Server using the extra_patterns parameter and the Tornado APIs.

Additional details


In general, whenever a model property is modified, the new value is first validated, and the Document is notified of the change. Just as models may have on_change callbacks, so can a Document. When a Document is notified of a change to one of its models, it will generate the appropriate event (usually a ModelChangedEvent) and trigger the on_change callbacks, passing them this new event. Sessions are one such callback, which will turn the event into a patch that can be sent across the web socket connection. When a message is received by the client or server session, it will extract the patch and apply it directly to the Document.

In order to avoid events bouncing back and forth between client and server (as each patch would generate new events, which would in turn be sent back), the session informs the Document that it was responsible for generating the patch and any subsequent events that are generated. In this way, when a Session is notified of a change to the document, it can check whether the event.setter is identical with itself and therefore skip processing the event.


In general, all the concepts above are agnostic as to how precisely the models and change events are encoded and decoded. Each model and its properties are responsible for converting their values to a JSON-like format, which can be sent across the websocket connection. One difficulty here is that one model can reference other models, often in highly interconnected and even circular ways. Therefore, during the conversion to a JSON-like format, all references by one model to other models are replaced with ID references. Additionally, models and properties can define special serialization behavior. One such example is the ColumnData property on a ColumnDataSource, which will convert NumPy arrays to a base64 encoded representation, which is significantly more efficient than sending numeric arrays in a string-based format. The ColumnData property serializable_value method applies this encoding, and the from_json method will convert the data back. Equivalently, the JS-based ColumnDataSource knows how to interpret the base64 encoded data and converts it to JavaScript typed arrays, and its attributes_as_json methods also knows how to encode the data. In this way, models can implement optimized serialization formats.


To test client-server functionality, use the utilities in bokeh.server.tests.utils.

Using ManagedServerLoop, you can start up a server instance in-process. Share server.io_loop with a client, and you can test any aspect of the server. Check out the existing tests for lots of examples. Anytime you add a new websocket message or HTTP endpoint, be sure to add tests!