katsdptelstate 0.8

Karoo Array Telescope - Telescope State Client

MeerKAT Science Data Processor Telescope State

This is a client package that allows connection to the Redis database that stores telescope state information for the Science Data Processor of the MeerKAT radio telescope. This database is colloquially known as telstate.

The Redis database acts as a key-value store. Each key is either a sensor or an attribute. A sensor has multiple timestamped values organised into an ordered set. An attribute (or immutable key) has a single value without a timestamp that is not allowed to change.

The keys are strings and the values are Python objects serialised via MessagePack, which has been extended to support tuples, complex numbers and NumPy arrays. Older versions of the database stored the values as pickles, and the package warns the user if that’s the case. Keys can be retrieved from the telstate object using attribute syntax or dict syntax.

Databases can be accessed via one of two backends: a Redis client backend that allows shared access to an actual Redis server over the network (or a simulated server via fakeredis) and a simplified in-memory backend for stand-alone access. Both backends support loading and saving a Redis snapshot in the form of an RDB dump file.

It is possible to have multiple views on the same database (one per telstate instance). A view is defined as a list of prefixes acting as namespaces that group keys. When reading from the database, each prefix is prepended to the key in turn until a match is found. When writing to the database, the first prefix is prepended to the key. The first prefix therefore serves as the primary namespace while the rest are supplementary read-only namespaces.

Warning

WARNING: The standard warning about Python pickles applies. Never retrieve data from an untrusted telstate database with values encoded as pickles, or connect to such a database over an untrusted network. Pickle support can be disabled by setting KATSDPTELSTATE_ALLOW_PICKLE=0 in the environment, which should make it safe to connect to untrusted telstates. On the other hand, the package warning can be disabled for trusted databases by setting the environment variable KATSDPTELSTATE_ALLOW_PICKLE=1.

Getting Started

The simplest way to test out katsdptelstate is to use the in-memory backend. If you want to run a real Redis server you will need to install Redis (version 2.8.9 or newer) on a suitable machine on the network. For example, do this:

- macOS: ``brew install redis``
- Ubuntu: ``apt-get install redis-server``

Then pip install katsdptelstate and run a local redis-server. If you also want to load RDB files, do pip install katsdptelstate[rdb].

A Simple Example

import time
import katsdptelstate

# Connect to an actual Redis server via an endpoint or an URL
telstate = katsdptelstate.TelescopeState('localhost:6379')
telstate = katsdptelstate.TelescopeState('redis://localhost')
# Or use the in-memory backend (useful for testing)
telstate = katsdptelstate.TelescopeState()
# Load RDB file into Redis if katsdptelstate is installed with [rdb] option
telstate.load_from_file('dump.rdb')

# Attribute / dict style access returns the latest value
telstate.add('n_chans', 32768)
print(telstate.n_chans)  # -> 32768
print(telstate['n_chans'])  # -> 32768

# List all keys (attributes and sensors)
print(telstate.keys())  # -> ['n_chans']

# Sensors are timestamped underneath
st = time.time()
telstate.add('n_chans', 4096)
et = time.time()
telstate.add('n_chans', 16384)
# Time ranges can be used and are really fast
telstate.get_range('n_chans', st=st, et=et)  # -> [(4096, 1556112474.453495)]
# Add an item 10 seconds back
telstate.add('n_chans', 1024, ts=time.time() - 10)

# Attributes cannot be changed (only deleted)
telstate.add('no_change', 1234, immutable=True)
# Adding it again is OK as long as the value doesn't change
telstate.add('no_change', 1234, immutable=True)
# Simpler notation for setting attributes
telstate['no_change'] = 1234
# Will raise katsdptelstate.ImmutableKeyError
telstate['no_change'] = 456

# Create a new view with namespace 'ns' and standard underscore separator
view = telstate.view('ns')
# Insert a new attribute in this namespace and retrieve it
view['x'] = 1
print(view['x'])  # -> 1
print(view.prefixes)  # -> ('ns_', '')
print(view.keys())  # -> ['n_chans', 'no_change', 'ns_x']

History

0.8 (2019-05-06)

• The default encoding is now msgpack; warn on loading pickles (#75, #79)

• The default backend is now in-memory (#76)

• Add the ability to dump in-memory backend to an RDB file (#77)

• Construct from RDB file-like objects and Redis URLs (#80, #82)

• Report keys and prefixes to the user as strings (#73)

• Add IPython tab completion (#83)

• RDB reader and writer cleanup (#85, #86)

0.7 (2019-02-12)

• Introduce encodings and add msgpack encoding as alternative to pickle (#64, #65)

• Introduce backends and add in-memory backend as alternative to redis (#71, #72)

• Simplify setting attributes via __setitem__ (#68)

• Let keys be bytes internally, but allow specification as unicode strings (#63)

• The GitHub repository is now public as well

0.6 (2018-05-10)

• Initial release of katsdptelstate