salobj CSCs

Writing a CSC

A Commandable SAL Component (CSC) typically consists of one or more of the following components:


The CSC responds to commands and outputs events and telemetry. Make it a subclass of ConfigurableCsc if it can be configured via the start command, or BaseCsc if not. Most CSCs are configurable.

ts_ATDome is one of the simplest CSCs, and is a good example of how to write a configurable CSC.


If your CSC communicates with a low-level controller then it will need some kind of connection to that controller. Examples include:

  • ts_ATDome, one of the simplest CSCs, communicates to its low-level controller using a single client TCP/IP socket.

  • ts_hexapod and ts_rotator communicate to its low-level controller via a pair of TCP/IP client sockets, one for commands and the other for replies.

  • ts_MTMount communicates to its low-level controller via a pair of TCP/IP sockets, one a client, one a socket.

  • ts_FiberSpectrograph communicates to the spectrograph via USB.

If your CSC communicates with other SAL components then it will need one or more Remotes (one per SAL component). Examples of CSCs with Remotes include:

  • ts_ATDomeTrajectory: has one remote to command ATDome and another to listen to ATMCS’s target event. This is one of the simplest CSCs with remotes.

  • ts_Watcher: uses remotes to listen to all SAL components that it monitors.

  • ts_scriptqueue: uses a single remote to communicate with all queued scripts. This relies on the feature that specifying index=0 allows one to communicate with all instances of an indexed SAL component.

Large File Annex Writer

If your CSC writes data to the Large File Annex (LFA) or another S3 server, create an AsyncS3Bucket using AsyncS3Bucket.make_bucket_name to construct the bucket name. To upload data:

  • Call AsyncS3Bucket.make_key to construct a key.

  • Call AsyncS3Bucket.upload to upload the data. See the doc string for details on getting your data into the required format.

  • Fall back to writing your data to local disk if S3 upload fails.

  • Call evt_largeFileObjectAvailable.set_write to report the upload.

See ts_FiberSpectrograph for a simple example.

Note that users of your CSC will have to configure access to the S3 server in the standard way. Most of such configuration is standard and well documented on the internet. However, if the S3 server is not part of Amazon Web Services (AWS), you will also have to define the salobj-specific environment variable S3_ENDPOINT_URL; see Environment Variables for details.

A Model

If your CSC has complicated internal state or performs complicated computations then we strongly recommend that you encapsulate that behavior in a class that acts as a data model. On the other hand, if a low level controller does most of the work then you probably do not need a model. Examples of CSCs with models include:

  • ts_ATDomeTrajectory: the model is the algorithm used to control the dome. This is one of the simplest CSCs with a model.

  • ts_watcher: the model contains and manages all the rules and alarms.

  • ts_scriptqueue: the model contains the queue and information about each script on the queue.

A Simulator

If your CSC controls hardware then we strongly recommend that you support running in simulation mode. This allows unit testing of your CSC and integration testing with other CSCs.

If your CSC talks to a low level controller then consider simulating the low-level controller. That allows your CSC to use its standard communication protocol to communicate with the simulator, which in turn means that your unit test exercise more of your code. All the CSCs listed under “Connections” above have a simulator that works this way.

Basic Development

Read Developing with the lsstts/develop-env Docker Container especially “CSC Development”. For detailed advice for configuring your environment see SAL Development.

Clone the templates package and follow the instructions in its README file to create a new package.

CSC Details

Make your CSC a subclass of ConfigurableCsc if it can be configured via the start command, or BaseCsc if not. Most CSCs can be configured.

  • Specify the following class variables, if appropriate:

    • default_initial_state (State): Default initial state.

      The default value is State.STANDBY, which is appropriate for most CSCs. Set to State.OFFLINE for externally commandable CSCs.

    • enable_cmdline_state (bool):

      Set True to allow your CSC to be started from the command line in a state other than default_initial_state. If True:

      • BaseCsc.amain adds command-line arguments --state and, if your CSC is configurable, --override.

      • If your CSC is configurable, then you must add constructor argument override to your CSC and pass it by name to super().__init__.

      • Note: we recommend the override constructor argument for all configurable CSCs, because it is useful for unit tests, even if enable_cmdline_state is False.

      The default is False because CSCs should start in default_initial_state unless you have a good reason to do otherwise.

    • valid_simulation_modes (list of int): a list or tuple of valid simulation modes:

      • If your CSC does not support simulation then set valid_simulation_modes = [0]. The value 0 is always used for normal operation.

      • To implement nonzero simulation modes see simulation mode.

    • simulation_help (str): help for the --simulate command-line argument. Please provide this if your CSC has more than 2 valid values for simulation_mode (e.g. more than 0 for normal operation and 1 for simulation). If there are two valid values, the default help will probably suffice. If there is only one valid value then there will be no --simulate command-line argument and simulation_help will be ignored.

    • version (str): the version of your package. Failure to provide this will produce a deprecation warning for now, and will someday be an error. Typically set to version = __version__, where __version__ has been imported as follows: from . import __version__; this only works if sets __version__ before importing the module defining the CSC.

    • Here is an example:

      from lsst.ts import salobj
      from . import __version__
      class ATDomeCsc(salobj.ConfigurableCsc):
          """...(doc string)...
          valid_simulation_modes = [0, 1]
          version = __version__
  • Handling commands:

    • Your subclass must provide a do_<name> method for every command that is not part of the standard CSC command set, as well as the following optional standard commands, if you want to support them (these are rare):

      • abort. Use of this command is discouraged. It is usually better to provide CSC-specific commands to stop specific actions.

      • enterControl. This command is only relevant for externally commandable CSCs, and we have few salobj-based CSCs that are externally commandable.

      • setValue. This is strongly discouraged, for reasons given below.

    • Each do_<name> method should be asynchronous (async def do_<name>...). Synchronous (def do_<name>...) methods are allowed, but deprecated.

    • If the command will take a long time before completion then you should issue a CMD_INPROGRESS acknowledgement, e.g. by calling topics.ControllerCommand.ack_in_progress on the cmd_<name> instance.

    • Most commands should only be allowed to run when the summary state is State.ENABLED. To enforce this, put the following as the first line of your do_<name> method: self.assert_enabled().

    • Your CSC reports the command as unsuccessful if the do_<name> method raises an exception. The ack value depends on the exception; see topics.ControllerCommand for details.

    • Your CSC reports the command as successful when do_<name> finishes and returns None. If do_<name> returns an acknowledgement (instance of SalInfo.AckCmdType) instead of None then your CSC sends that as the final command acknowledgement.

    • If you want to allow more than one instance of the command running at a time, set self.cmd_<name>.allow_multiple_callbacks = True in your CSC’s constructor. See topics.ReadTopic.allow_multiple_callbacks for details and limitations of this attribute.

    • If a do_<name> method must perform slow synchronous operations, such as CPU-heavy tasks or blocking I/O, make the method asynchronous and call the synchronous operation in a thread using the run_in_executor method of the event loop.

    • do_ is a reserved prefix: all do_<name> attributes must match a command name and must be callable.

    • It is strongly discouraged to allow modifying configuration in any way other than the start command, because that makes it difficult to reproduce the current configuration and determine how it got that way.

  • Set the following event data in your constructor, if necessary:

    • If your CSC has individually versioned subsystems, then call self.evt_softwareVersions.set(subsystemVersions=...).

    • If your CSC outputs detailed configuration information (and all configurable CSCs should do that), then call: self.evt_configurationApplied.set(otherInfo=...) with a comma-separated list of the names of those events, each without the logevent_ prefix.

    • Note: for both of these events call set not set_put, because the parent class adds more information before outputting the event.

  • Override BaseCsc.handle_summary_state to handle tasks such as:

    • Constructing a model, if your CSC has one.

    • Constructing the simulator, if in simulation mode.

    • Starting or stopping a telemetry loop and other background tasks.

    • Connecting to or disconnecting from a low-level controller (or simulator).

    Here is a typical outline:

    async def handle_summary_state(self):
        if self.disabled_or_enabled:
            if self.model is None:
                self.model = ...
            if self.telemetry_task.done():
                self.telemetry_task = asyncio.create_task(self.telemetry_loop())
            if self.simulation_mode and self.simulator is None:
                self.simulator = ...
            if self.connection is None:
                self.connection = ...
            if self.connection is not None:
                await self.connection.close()
                self.connection = None
            if self.simulator is not None:
                await self.simulator.close()
                self.simulator = None
            if self.model is not None:
                await self.model.close()
                self.model = None
  • Override BaseCsc.close_tasks if you have background tasks to clean up when quitting. This is not strictly needed if you cancel your tasks in BaseCsc.handle_summary_state, but it allows you to close CSCs in the ENABLED or DISABLED state in unit tests without generating annoying warnings about pending tasks.

  • If you override BaseCsc.start (which runs once as the CSC starts up) be sure to call await super().start() at or very near the end of your override. This is because BaseCsc.start may call state transition commands, which will trigger calls to BaseCsc.handle_summary_state; thus your CSC should be as “started” as practical before calling await super().start().

  • Configurable CSCs (subclasses of ConfigurableCsc) must provide additional Configurable CSC Details.

  • Talking to other CSCs:

    • Your subclass should construct a Remote for any remote SAL component it wishes to listen to or command. Be sure to wait for it to be started before trying to use it. For example:

      # in your constructor:
      self.electrometer1 = salobj.Remote(name="Electrometer", index=1)
      # in your start method:
      await self.electrometer1.start_task
  • Summary state and error code:

    • BaseCsc provides a default implementation for all summary state transition commands that might suffice.

    • Most commands should only be allowed to run when the summary state is State.ENABLED. To check this, put the following as the first line of your do_<name> method: self.assert_enabled()

    • Call BaseCsc.fault to send your CSC into the State.FAULT summary state.

  • Detailed state (optional):

    • The detailedState event is unique to each CSC.

    • detailedState is optional, but strongly recommended for CSCs that are complex enough to have interesting internal state.

    • Report all information that seem relevant to detailed state and is not covered by summary state.

    • Detailed state should be orthogonal to summary state. You may provide an enum field in your detailedState event, but it is not required and, if present, should not include summary states.

Indexed CSCs

If your CSC has named SAL indexes specified in the <IndexEnumeration> field of SALSubsystems.xml in ts_xml:

  • Add an enum.IntEnum enum class named SalIndex to the ts_idl package, with entries that match those in SALSubsystems.xml. This gives users named access to the allowed indices without having to import your CSC package.

  • Specify that enum class as the index when calling BaseCsc.amain in your bin script. amain will then insist that the user provide an index and will reject invalid values. Here is an example bin script that runs the MT Hexapod CSCs:

    #!/usr/bin/env python
    import asyncio
    from lsst.ts.idl.enums.MTHexapod import SalIndex
    from lsst.ts.mthexapod import HexapodCsc
  • Similarly, if you use CscCommander then call CscCommander.amain with index=SalIndex.

Configurable CSC Details

Configurable CSCs (subclasses of ConfigurableCsc) must provide the following support, in addition to the standard CSC Details:

  • A schema in jsonschema format that defines the configuration and, if practical, provides a default value for each parameter. If all values have sensible defaults then your CSC can be configured without specifying a configuration file as part of the start command.

  • A configure method that accepts configuration as a struct-like object (a types.SimpleNamespace).

  • A get_config_pkg classmethod that returns ts_config_..., the package that contains configuration files for your CSC.

  • In that config package:

    • Add a directory whose name is the SAL component, and a subdirectory inside that whose name is your schema version, for example ATDome/v1/.

      In that subdirectory add the following:

    • Configuration files, if any. These are only required if your CSC’s default configuration (as defined by the default values specfied in the schema) is not adequate for normal operation modes.

    • A file named _labels.yaml which contains a mapping of label: configuration file name for each recommended configuration file. Label names must be valid Python identifiers and must not start with underscore; labels that break this rule are ignored (with a logged warning). If you have no configuration files then provide an empty _labels.yaml (empty except, preferably, for a comment saying there are no configuration files), in order to avoid a warning log message when your CSC is constructed.

    • Add a new test method to the test case in tests/ If your CSC package requires packages that are not part of the lsstts/develop-env Docker container then use an environment variable to find your package; see ts_config_ocs/tests/ for a few examples.

    • Run the new unit test, to make sure it works.

  • Add the config package to your eups table as a required dependency in your ups/<csc_pkg>.table file.

Standard State Transition Commands

Standard CSC commands and their associated summary state changes:

Unit Testing your CSC

  • Make a unit test case that inherits from BaseCscTestCase and asynctest.TestCase

  • Override the BaseCscTestCase.basic_make_csc method to construct and return your CSC. You may also construct other objects needed for your tests, with these caveats:

    • BaseCscTestCase.basic_make_csc can only return the CSC, so any other objects must be set as instance variables (e.g. = MyFoo(...).

    • If any of these objects need to be cleaned up at the end of the test, add a tearDown method that performs the cleanup.

    • In tearDown Do not assume that BaseCscTestCase.basic_make_csc was called, because some test methods may not need to construct a CSC. If you add attributes in BaseCscTestCase.basic_make_csc then you must check that they exist in tearDown. A simple way to handle this is to add a setUp method and initialize any such attributes to None, then in tearDown only perform cleanup if the attributes are not None.

  • In each test that needs a CSC call async with self.make_csc(...): to construct:

    • the CSC

    • self.remote: a remote that talks to the CSC.

    • Any other objects you construct in basic_make_csc.

See tests/ in this package (ts_salobj) for an example.

Externally Commandable CSCs

Externally commandable CSCs are CSC that can be controlled by some means other than SAL when in the State.OFFLINE state. The camera is one example of an externally commandable CSC.

BaseCsc and ConfigurableCsc are not externally commandable. They do not support the enterControl command and they quit in response to the exitControl command.

To write write an externally commandable CSC using lsst.ts.salobj do the following in your subclass of BaseCsc or ConfigurableCsc:

  • Override do_exitControl to not quit.

  • Add method do_enterControl and make it transition from State.OFFLINE to State.STANDBY

  • Add code for external control; this should only work in State.OFFLINE state.

Running a CSC

To run your CSC call on the amain class method. For example:

import asyncio

from lsst.ts.salobj import TestCsc

If you wish to provide additional command line arguments for your CSC, override the BaseCsc.add_arguments and BaseCsc.add_kwargs_from_args class methods.

Simulation Mode

CSCs should support a simulation mode if practical; this is especially important if the CSC talks to hardware.

To implement a simulation mode, first pick one or more non-zero values for the simulation_mode constructor argument (0 is reserved for normal operation) and document what they mean. It is quite common to support only one simulation mode, in which case the two allowed values are 0 and 1. However, you may support additional modes; you can even use a bit mask to supporting independently simulating different subsystems.

Set class variable valid_simulation_modes to a list of all supported simulation modes, including 0 for normal operation. If your CSC has just one simulation mode (the most common case):

valid_simulation_modes = (0, 1)

Then decide where to turn on your simulator; here are some common choices:

  • If your CSC communicates with a low-level controller and your simulator emulates that controller (which is strongly recommended), start the simulator where you connect to the low-level controller. This is often the configure method for configurable CSCs, or a custom connect method that you write and that you call from configure.

  • If your simulator should only run in certain states, then you may start and stop it in handle_summary_state.

  • If your simulator needs no configuration and can always be running, it is simplest to start it in start and stop it in close_tasks.

A deprecated way to handle simulation that you may see in older code was to not set class variable valid_simulation_modes. This required overriding three methods: BaseCsc.implement_simulation_mode, BaseCsc.add_arguments, and BaseCsc.add_kwargs_from_args. This is no longer recommended, and failing to set class variable valid_simulation_modes will result in a deprecation warning.

External Connections

If your CSC communicates with some other controller or system (by means other than SAL), I suggest you make or break the connection in BaseCsc.handle_summary_state (or a method called from there) as follows:

  • If the current state is DISABLED or ENABLED state and not already connected, then make the connection. If you support simulation mode then read that to determine if this is a real or a simulated connection.

  • If the current state is something else then disconnect.

Examples include the following (both of which have a simulation mode):

  • ts_ATDome talks to a TCP/IP controller

  • ts_FiberSpectrograph controls fiber spectrographs over USB.

Telemetry Loop Example

Here is an example of how to write a telemetry loop. This assumes ts_utils has been imported using from lsst.ts import utils.

  1. In the constructor (__init__): initialize:

self.telemetry_loop_task = utils.make_done_future()
self.telemetry_interval = 1  # seconds between telemetry output

Initializing telemetry_loop_task to an asyncio.Future that is already done makes it easier to test and cancel than initializing it to None.

  1. Define a telemetry_loop method, such as:

async def telemetry_loop(self):
    while True: and write telemetry...
        await asyncio.sleep(self.telemetry_interval)
  1. Start and stop the telemetry loop in BaseCsc.handle_summary_state, as described above.