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Data flow

In Bliss, Data and Meta-data flow from detector to Redis data base, they are never kept locally in memory. During a Scan, every Data produced by detector included in a Scan will follow this rule. Data are kept for a maximum of 1 day (by default and for a maximum amount of 1GB. In any case the rule of this data base is not to store Data but to publish live Data to external process like on-line data analysis, final storage


Data Client(s)

It’s now possible to link On-line data analysis easily with a Bliss session.

Any client can subscribe to receive Data notification of a session. As soon as it can reach a beam-line Beacon server.

Already several client use this mechanism: Flint, External Writer and PyMca

Simple example

Bliss provide a Python module to archive Data subscription. The following example listen to a session called demo_session and follow the scan activity:

from import node

session_node = node.get_session_node('demo_session')
for event,*values in session_node.walk_on_new_events(include_filter='scan'):
    if event == event.NEW_NODE:
        scan = values[0]
        print(f'Scan {scan.db_name} started')
    elif event == event.END_SCAN:
        scan = values[0]
        print(f'Scan {scan.db_name} ends')


In near future, it would be even possible to change scanning strategy with the computation result of an one-line data analysis program. i.e: change the position range of scanning motor during the scan…

Simple data-analysis interaction

Just a simple example to demonstrate the possible communication between on-line data and a alignment scan.

In the following example, one-line data analysis program stop the alignment when it estimate that enough points is taken.

from bliss.config import channels
from import node

max_value = 0.
# communcation channel for bliss session
stop_alignment = channels.Channel('stop_alignment',default_value=False)
session_node = node.get_session_node('demo_session')
for event,*values in session_node.walk_on_new_events():
    if event == event.NEW_NODE:
        node = values[0]
        if node.type == 'scan':
        # reset max_value to 0
        # when a new scan starts.
            max_value = 0.
    elif event == event.NEW_DATA_IN_CHANNEL:
        channel = values[0]
        if not'gauss') > -1:
        data_value = channel.get(-1) # last value
        if data_value > max_value:
            max_value = data_value
        elif data_value < (max_value/2.):
            print(f"Stop Alignment scan max_value:{max_value} current_value:{data_value}")
            #stop the scan
            stop_alignment.value = True

A simple hook to stop the scan:

from bliss.config import channels
from bliss.scanning import scan

class StopAlignmentHook(scan.WatchdogCallback):
    def __init__(self):
        self._stop_flag = channels.Channel('stop_alignment',
    def on_scan_data(self,*args):
        if self._stop_flag.value:
            raise StopIteration

Run it

Start the simple on-line data analysis program.

Then run the scan has follow:

DEMO_SESSION [1]: s = ascan(robz,-5,5,500,.001,sim_ct_gauss_noise,save=False,run=False)
DEMO_SESSION [2]: s.set_watchdog_callback(StopAlignmentHook())