doc: registrar package doc

Author: RawanMostafa08

docs/internals/registrar/readme.md

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+# Registrar package
+
+The `registrar` pkg is used to handle node registration on ThreeFold Grid. It is used by the `noded` module, which triggers the registration process by first collecting node capacity information, creating a Redis client, and then adding the `registrar` to the intercommunication process using `zbus` for remote procedure calls (RPC).
+re-registration occurs automatically every 24 hours or immediately after an IP address change, ensuring the node's information stays up-to-date.
+registration process can fail due to various reasons, such as RPC calls failure.
+
+The registration process includes:
+
+1. Collecting node information
+2. Creating/Ensuring a twinID exists for the node
+3. Registering the node on the blockchain
+
+## Error Handling
+
+`ErrInProgress`: Error raised if the node registration is still in progress.
+
+`ErrFailed`: Error raised if the node registration fails.
+
+## Constants
+
+### Node registration state constants
+
+`Failed`: Node registration failed
+
+`InProgress`: Node registration is in progress
+
+`Done`: Node registration is completed
+
+## Structs
+
+### State Struct
+
+used to store the state of the node registration.
+
+#### Fields
+
+`NodeID`: The ID of the node.
+
+`TwinID`: The twin ID of the node.
+
+`State`: The state of the node registration.
+
+`Msg`: The message associated with the node registration state.
+
+### RegistrationInfo Struct
+
+used to store the capacity, location, and other information of the node.
+
+#### Fields
+
+`Capacity`: The capacity of the node.
+
+`Location`: The location of the node.
+
+`SecureBoot`: State whether the node is booted via efi or not.
+
+`Virtualized`: State whether the node has hypervisor on it or not.
+
+`SerialNumber`: The serial number of the node.
+
+`GPUs`: The GPUs of the node.
+
+#### Methods
+
+`WithCapacity`: Set the capacity of the node, taking a `gridtypes.Capacity` as input.
+
+`WithLocation`: Set the location of the node, taking a `geoip.Location` as input.
+
+`WithSecureBoot`: Set the secure boot status of the node, taking a boolean as input.
+
+`WithVirtualized`: Set the virtualized status of the node, taking a boolean as input.
+
+`WithSerialNumber`: Set the serial number of the node, taking a string as input.
+
+`WithGPUs`: Set the GPUs of the node, taking a string as input.
+
+### Registrar Struct
+
+The registrar is used to register nodes on the ThreeFold Grid.
+
+#### Fields
+
+`state`: The state of the registrar.
+
+`mutex`: A mutex for synchronizing access to the registrar.
+
+#### Methods
+
+`NodeID`: Returns the node ID if the registrar is in the done state, otherwise returns an error.
+
+`TwinID`: Returns the twin ID if the registrar is in the done state, otherwise returns an error.
+
+## Functions
+
+### `NewRegistrar(ctx context.Context, cl Zbus.Client, env environment.Environment, info RegistrationInfo) *Registrar`
+
+Creates a new registrar with the given context, client, environment, and registration information, starts the registration process and returns the registrar.
+
+### `FailedState(err error) State`
+
+Returns a failed state with the given error.
+
+### `InProgressState() State`
+
+Returns an in progress state.
+
+### `DoneState(nodeID , twinID uint32) State`
+
+Returns a done state with the given node ID and twin ID.
+