# Exercises in Co-simulation timing

A few simple exercises about co-simulation timing

## Key Parameters

- **Period**: The minimum time resolution a federate will allow.
- **Offset**: a shift in the period. Allowed times for federate grants after time 0 are `offset+N*period`, where `N` is a non-negative integer.
- **Time_delta**: the minimum time between grants, i.e. if a federate is granted time T, the next possible time is `T+Time_delta`.

## Modifier Flags

- **Uninteruptible**: a federate can only be granted requested times
- **wait_for_current_time_update**: specify that a federate should wait until all federates executing at the current time have finished.

![Connectivity Diagram](https://github.com/GMLC-TDC/helics_doc_resources/raw/main/user_guide/timing_fed_setup.png "Exercise Federate Setup")

1.  Federation Setup \[A: period=1; B: period=2; C: period=3\]

    a. Following time 0, which federate could execute next? **_A at time 1_**

    b. If all federates execute at all allowed times, what is the next time Federate B could have access to data from Federate C. **_4_**

    c. What is the next time all federate will be able to execute simultaneously? **_6_**

    **Key Principle**: _Federates are interrupted if there is updated data available and allowed time prior to the requested time_

2.  Federation Setup \[A: period=1,`wait_for_current_time_update`; B: period=2; C: period=3\]

    a. At what time will the data from Federate B published at time 2, be available to Federate A? **_2_**

    b. Federate A requests time 4: Federate B publishes at time 2. What time is Federate A granted? **_2_**

    c. Federate A requests time 2: Federate B publishes at time 2. What time does Federate A receive the data? **_2_**

    e. If A did not have the `wait_for_current_time_update` flag active, what time would Federate A receive the data? **_3_**

    **Key Principle**: _Federates are granted the next allowed time after the time specified in a request if they are not interrupted._

3.  Federation Setup \[A: period=1; B: period=2,offset=1,time_delta=2; C: period=3\]

    a. After time=0 what is the next allowable time for Federate B? **_3_**, _time=1 is not allowed due to time_delta_

    b. Federate C requested a time of 4, what time is Federate C granted? **_6_**

4.  Federation Setup \[A: period=1; B: period=2,uninterruptible; C: period=3\]

    a. Federate C Publishes at time 3, Federate B requests time 6, what time will it be granted?**_6_**, _B cannot be granted anything other than 6 due to uninterruptible flag_

    b. If Federate B were not uninterruptible what time would it be granted? **_4_**

    ![Connectivity Diagram](https://github.com/GMLC-TDC/helics_doc_resources/raw/main/user_guide/timing_fed_setup.png "Exercise Federate Setup")

5.  Federation Setup \[A: period=1; B: period=2; C: period=3\], Federates will send an update when they have received an update from the all other connected federates. Federate A sends an update at time 0, what is the Update sequence

<center>

| Time | Federate(s) |
| ---- | :---------: |
| 3    |      C      |
| 4    |      B      |
| 4    |      A      |
| 6    |      C      |
| 6    |      B      |
| 6    |      A      |
| 9    |      C      |

</center>