Review L2VPN state machine

[italovalcy]

Reviewing the L2VPN state machine due to the problem observed on Issue atlanticwave-sdx/sdx-controller#461, we need to augment the state machine (states and transition) to accommodate situations already documented on the Provision Data Model Spec and Network Use cases file.

Here are some updates needed:

1. When on the DOWN state, we should allow a transition to MODIFYING state through the action MODIFY
2. When on the DOWN state, we should allow a transition to DELETED state through the action DELETE
3. When on the ERROR state, we should allow a transition to MODIFYING state through the action MODIFY
4. When on the DISABLED state, we should allow a transition to DELETED state through the action DELETE
5. When on the MODIFYING state, we should allow a transition to DELETED state through the action DELETE
6. Create a new state MAINTENANCE
7. When on the UP state, we should allow a transition to MAINTENANCE state through the action MW_START
8. When on the MAINTENANCE state, we should allow a transition to UP state through the action MW_END

Ccing @jab1982 and @YufengXin for your considerations and comments

[YufengXin]

Thanks, @italovalcy, just want to clarify a couple of things:

1. In the document, "Enable"/"Disable" are defined as state, while "Maintenance" is defined as status. I originally thought they are the same thing, so a "Disabled" state is defined in the SM. Could you pls clarify the difference? Would it be good to just change the name "Disabled" to "Maintenance" if "Enable"/"Disable" are not state and should not be in the SM?
2. Deletion vs. Archive: The document seems to indicate that a connection, ever provisioned successfully, should not be deleted from the DB, ie, they can always be queried, though resources (bw and vlan) are released from PCE/controller. And if the provision (modification) failed (eg, PCE or OXP failed to provision), the connection goes to "DOWN" state and is not saved in the DB. In this case, PCE would not hold resources (BW and vlan). If we change the name "DELETED" to "Archived" and "DOWN" to "DELETED", would the SM make more sense?

@congwang09 Please consider more scenarios and redraw the SM for further discussion. Thx

[italovalcy]

Thanks, @italovalcy, just want to clarify a couple of things:

1. In the document, "Enable"/"Disable" are defined as state, while "Maintenance" is defined as status. I originally thought they are the same thing, so a "Disabled" state is defined in the SM. Could you pls clarify the difference? Would it be good to just change the name "Disabled" to "Maintenance" if "Enable"/"Disable" are not state and should not be in the SM?

Hi @YufengXin , yes, Maintenance is a status, meaning that it cannot be changed by the user in a request. Instead, it is a result from a serie of changes/events on the system that leads to the status being "maintenance". On the other hand, enabled and disabled are state, meaning that the user can send a PATCH request to change a L2VPN to be disabled, or enabled.

Although the name "State Machine", the word state here does not directly translate to the "state" field of the L2VPN, otherwise we wouldn't have the "under provisioning", deleted, etc.

Clarify the usage on Maintenance, it has the following meaning: when the network operator configure a Interface in Maintenance Mode and that interface is also an endpoint for a certain L2VPN, that L2VPN will be marked as maintenance. This can also happen with Links, switches and -- more generically -- other resources.

3. Deletion vs. Archive: The document seems to indicate that a connection, ever provisioned successfully, should not be deleted from the DB, ie, they can always be queried, though resources (bw and vlan) are released from PCE/controller. And if the provision (modification) failed (eg, PCE or OXP failed to provision), the connection goes to "DOWN" state and is not saved in the DB. In this case, PCE would not hold resources (BW and vlan). If we change the name "DELETED" to "Archived" and "DOWN" to "DELETED", would the SM make more sense?>

It wouldn't make sense to change the names. One thing I notice on the sentence above was this: "connection goes to "DOWN" state and is not saved in the DB", connections that are down remain on the DB. However, if a connection requirements cannot be fulfilled during the creation (not modification!), it won't be saved to the DB. During the modification, the DB will store the previous information (not modified) in case of a modification that cannot be fulfilled.