Understanding Bokeh server¶
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 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 byBokehTornado
has N
ApplicationContext
has 1
Application
capable of creating new sessionshas 1 path used to identify it in URLs
has 1
ServerContext
representing the aspects of the server visible to application codehas N
ServerSession
has 1 session ID which is a string naming the session
has 1
Document
representing the session statehas N
ServerConnection
representing websockets attached to the sessionhas 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.
Lifecycle¶
If you look at the Application
class, there are two ways the
server can call into it.
the
modify_document()
method which does just what it says: it passes in the session’sDocument
and allows the application to modify it (perhaps adding some plots and widgets).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:
When the server process starts up, it calls
on_server_loaded()
on each application.When a client connects with a previously-unused session ID, the server creates a
ServerSession
and callson_session_created()
with an emptyDocument
, thenmodify_document()
to initialize theDocument
. Theon_session_created()
can also initialize part of theDocument
if it likes.on_session_created()
happens beforemodify_document()
.When there are no connections to a session, it will eventually time out, and
on_session_destroyed()
will be called.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.
Locking¶
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.
Warning
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 application_context.py
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 connectionOK
is a generic reply when a request doesn’t require any more specific replyERROR
is a generic error reply when something goes wrongSERVER-INFO-REQ
andSERVER-INFO-REPLY
are a request-reply pair where the reply contains information about the server, such as its Bokeh versionPULL-DOC-REQ
asks to get the entire contents of the session’sDocument
as JSON, andPULL-DOC-REPLY
is the reply containing said JSON.PUSH-DOC
sends the entire contents of the session’sDocument
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¶
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.)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 thebokeh.embed.server_document()
andbokeh.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¶
Events¶
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.
Serialization¶
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.
Testing¶
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!