In this chapter, we will discuss Bokeh Server. Bokeh architecture has a decoupled design in which objects such as plots and glyphs are created using Python and converted in JSON to be consumed by the BokehJS client library.
However, it is possible to keep the objects in python and in the browser in sync with one another with the help of the Bokeh Server. It enables response to User Interface (UI) events generated in a browser with the full power of python. It also helps automatically push server-side updates to the widgets or plots in a browser.
A Bokeh server uses Application code written in Python to create Bokeh Documents. Every new connection from a client browser results in the Bokeh server creating a new document, just for that session.
The following code renders a sine wave line glyph. The callback function update_data() updates ColumnDataSource data taking the instantaneous value of the slider as current frequency.
import numpy as np
from bokeh.io import curdoc
from bokeh.layouts import row, column
from bokeh.models import ColumnDataSource
from bokeh.models.widgets import Slider, TextInput
from bokeh.plotting import figure
N = 200
x = np.linspace(0, 4*np.pi, N)
y = np.sin(x)
source = ColumnDataSource(data = dict(x = x, y = y))
plot = figure(plot_height = 400, plot_width = 400, title = "sine wave")
plot.line('x', 'y', source = source, line_width = 3, line_alpha = 0.6)
freq = Slider(title = "frequency", value = 1.0, start = 0.1, end = 5.1, step = 0.1)
def update_data(attrname, old, new):
a = 1
b = 0
w = 0
k = freq.value
x = np.linspace(0, 4*np.pi, N)
y = a*np.sin(k*x + w) + b
source.data = dict(x = x, y = y)
freq.on_change('value', update_data)
curdoc().add_root(row(freq, plot, width = 500))
curdoc().title = "Sliders"
Next, start the server by following the command line −
Bokeh serve –show sliders.py
This server starts running and serving the application at localhost:5006/sliders. The console log shows the following display −
C:\Users\User>bokeh serve --show scripts\sliders.py 2019-09-29 00:21:35,855 Starting Bokeh server version 1.3.4 (running on Tornado 6.0.3) 2019-09-29 00:21:35,875 Bokeh app running at: http://localhost:5006/sliders 2019-09-29 00:21:35,875 Starting Bokeh server with process id: 3776 2019-09-29 00:21:37,330 200 GET /sliders (::1) 699.99ms 2019-09-29 00:21:38,033 101 GET /sliders/ws?bokeh-protocol-version=1.0&bokeh-session-id=VDxLKOzI5Ppl9kDvEMRzZgDVyqnXzvDWsAO21bRCKRZZ (::1) 4.00ms 2019-09-29 00:21:38,045 WebSocket connection opened 2019-09-29 00:21:38,049 ServerConnection created
Open your favorite browser and enter the above address as follows −
You can try and change the frequency to 2 by rolling the slider.
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