SKA DS Manager Session

Dish Structure Controller Session Management

The Dish Structure Controller (DSC) has an arbitration mechanism to allow only one client to have authority and execute commands successfully. The DscCmdAuthority variable on the server indicates which client has authority to command the DSC. LMC has the lowest priority. The priority above LMC is the EGUI client, and above that is the Hand Held Panel (HHP) client. This means that either EGUI or HHP can take control of the DSC when LMC has authority. LMC can only take authority if there is no authority being held. If for some reason LMC loses connection without releasing authority, LMC will be able to retake authority and get a valid session id. This is done implicitly when executing a command. LMC uses a unique immutable user id specific to each dish to identify itself to the DSC. The immutable user id has the format LMC-<DISH_ID>-<UNIQUE HASH>; the UNIQUE HASH is an obfuscated string generated from the tango device trl.

On Initialisation

On initialisation, DSM (Dish Structure Manager) subscribes to the DscCmdAuthority variable, LMC will only attempt to take authority on command execution or when the TakeAuthority command is executed. This sequence is shown below:

On Command Execution

When executing commands a pre-check is done by inspecting DscCmdAuthority variable. If LMC has authority, the command will be sent. If the server reports NO_AUTHORITY, LMC will attempt to issue a ReTakeAuthority command, to reclaim authority before re-attempting to send the command. If no client has authority, DSM will attempt to take authority and proceed with the command execution. Commands will be rejected if a higher priority client has authority. This sequence is shown below:

$@startuml actor DM actor DSM title Command Execution DM -> DSM: Call TrackStart(...) DSM -> DSM: Check DscCmdAuthority variable alt DscCmdAuthority is LMC DSM -> DSC: Call TrackStart(session_id, ...) alt session_id is valid DSC -> DSM: Return success DSM -> DM: Return success else session_id is invalid DSC -> DSM: Return NoAuth DSM -> DSC: Call TakeAuth(...) DSC -> DSM: Return new session ID DSM -> DSM: Save new session ID DSM -> DSC: Call TrackStart(session_id, ...) DSC -> DSM: Return success DSM -> DM: Return success end else DscCmdAuthority is None DSM -> DSC: Call TakeAuth(...) DSC -> DSM: Return new session ID DSM -> DSM: Save new session ID DSM -> DSC: Call TrackStart(session_id, ...) DSC -> DSM: Return success DSM -> DM: Return success else DscCmdAuthority is something else DSM -> DM: Fail command with reason\n no Authority on DSC end note right of DSC - DSC: Dish structure controller Server - DM: Dish manager - DSM: Dish structure manager end note @enduml$

Manually

In addition to the automatic session handling implemented in the Dish Structure Manager, the TakeAuthority, ReleaseAuth and ReTakeAuthority commands have also been exposed directly as Tango commands. The same sequence sequence of steps as described above in On Command Execution will take place when attempting to use TakeAuthority. When releasing authority, if the session id being held by the DSM is valid then authority will be released and DscCmdAuthority will report NO_AUTHORITY. In order to retake authority, the ReTakeAuthority command can be used if LMC has taken authority previously but lost the session id.