modifications in several files to :

- have consistent error return types
- store the zone redundancy in a Lww
- print the error and message in the CLI (TODO: for the server Api, should msg be returned in the body response?)

src/api/admin/cluster.rs

@@ -162,7 +162,12 @@ pub async fn handle_apply_cluster_layout(
 	let param = parse_json_body::<ApplyRevertLayoutRequest>(req).await?;
 
 	let layout = garage.system.get_cluster_layout();
-	let layout = layout.apply_staged_changes(Some(param.version))?;
+	let (layout, msg) = layout.apply_staged_changes(Some(param.version))?;
+    //TODO : how to display msg ? Should it be in the Body Response ?
+    for s in msg.iter() {
+        println!("{}", s);
+    }
+
 	garage.system.update_cluster_layout(&layout).await?;
 
 	Ok(Response::builder()

src/garage/cli/layout.rs

@@ -188,18 +188,22 @@ pub async fn cmd_show_layout(
 
 		// this will print the stats of what partitions
 		// will move around when we apply
-		if layout.calculate_partition_assignation() {
+        match layout.calculate_partition_assignation() {
+            Ok(msg) => { 
+                for line in msg.iter() {
+                    println!("{}", line);
+                }
                 println!("To enact the staged role changes, type:");
                 println!();
                 println!("    garage layout apply --version {}", layout.version + 1);
                 println!();
                 println!(
                     "You can also revert all proposed changes with: garage layout revert --version {}",
-				layout.version + 1
+                    layout.version + 1)},
-			);
+            Err(Error::Message(s)) => {
-		} else {
+                println!("Error while trying to compute the assignation: {}", s);
-			println!("Not enough nodes have an assigned role to maintain enough copies of data.");
+			    println!("This new layout cannot yet be applied.");},
-			println!("This new layout cannot yet be applied.");
+            _ => { println!("Unknown Error"); },
         }
 	}
 
@@ -213,7 +217,10 @@ pub async fn cmd_apply_layout(
 ) -> Result<(), Error> {
 	let layout = fetch_layout(rpc_cli, rpc_host).await?;
 
-	let layout = layout.apply_staged_changes(apply_opt.version)?;
+	let (layout, msg) = layout.apply_staged_changes(apply_opt.version)?;
+    for line in msg.iter() {
+        println!("{}", line);
+    }
 
 	send_layout(rpc_cli, rpc_host, layout).await?;
 

src/rpc/layout.rs

@@ -7,7 +7,7 @@ use itertools::Itertools;
 
 use serde::{Deserialize, Serialize};
 
-use garage_util::crdt::{AutoCrdt, Crdt, LwwMap};
+use garage_util::crdt::{AutoCrdt, Crdt, LwwMap, Lww};
 use garage_util::data::*;
 use garage_util::error::*;
 
@@ -27,12 +27,10 @@ pub struct ClusterLayout {
 	pub version: u64,
 
 	pub replication_factor: usize,
-    #[serde(default="default_one")]
-    pub zone_redundancy: usize,
   
     //This attribute is only used to retain the previously computed partition size, 
     //to know to what extent does it change with the layout update.
-    #[serde(default="default_zero")]
+    #[serde(default="default_partition_size")]
     pub partition_size: u32,
 
 	pub roles: LwwMap<Uuid, NodeRoleV>,
@@ -51,17 +49,31 @@ pub struct ClusterLayout {
 	pub ring_assignation_data: Vec<CompactNodeType>,
 
 	/// Role changes which are staged for the next version of the layout
+    #[serde(default="default_layout_parameters")]
+    pub parameters: Lww<LayoutParameters>,
 	pub staging: LwwMap<Uuid, NodeRoleV>,
 	pub staging_hash: Hash,
 }
 
-fn default_one() -> usize{
+fn default_partition_size() -> u32{
-    return 1;
-}
-fn default_zero() -> u32{
     return 0;
 }
 
+fn default_layout_parameters() -> Lww<LayoutParameters>{
+    Lww::<LayoutParameters>::new(LayoutParameters{ zone_redundancy: 1})
+}
+
+///This struct is used to set the parameters to be used in the assignation computation
+///algorithm. It is stored as a Crdt.
+#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Debug, Serialize, Deserialize)]
+pub struct LayoutParameters {
+    pub zone_redundancy:usize,
+}
+
+impl AutoCrdt for LayoutParameters {
+	const WARN_IF_DIFFERENT: bool = true;
+}
+
 const NB_PARTITIONS : usize = 1usize << PARTITION_BITS;
 
 #[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Debug, Serialize, Deserialize)]
@@ -108,18 +120,24 @@ impl NodeRole {
 }
 
 impl ClusterLayout {
-	pub fn new(replication_factor: usize, zone_redundancy: usize) -> Self {
+	pub fn new(replication_factor: usize) -> Self {
+        
+        //We set the default zone redundancy to be equal to the replication factor,
+        //i.e. as strict as possible.
+        let default_parameters = Lww::<LayoutParameters>::new(
+            LayoutParameters{ zone_redundancy: replication_factor});
+
 		let empty_lwwmap = LwwMap::new();
 		let empty_lwwmap_hash = blake2sum(&rmp_to_vec_all_named(&empty_lwwmap).unwrap()[..]);
 
 		ClusterLayout {
 			version: 0,
 			replication_factor,
-            zone_redundancy,
             partition_size: 0,
 			roles: LwwMap::new(),
 			node_id_vec: Vec::new(),
 			ring_assignation_data: Vec::new(),
+            parameters: default_parameters,
 			staging: empty_lwwmap,
 			staging_hash: empty_lwwmap_hash,
 		}
@@ -132,6 +150,7 @@ impl ClusterLayout {
 				true
 			}
 			Ordering::Equal => {
+                self.parameters.merge(&other.parameters);
 				self.staging.merge(&other.staging);
 
 				let new_staging_hash = blake2sum(&rmp_to_vec_all_named(&self.staging).unwrap()[..]);
@@ -145,7 +164,7 @@ impl ClusterLayout {
 		}
 	}
 
-	pub fn apply_staged_changes(mut self, version: Option<u64>) -> Result<Self, Error> {
+	pub fn apply_staged_changes(mut self, version: Option<u64>) -> Result<(Self,Message), Error> {
 		match version {
 			None => {
 				let error = r#"
@@ -164,16 +183,14 @@ To know the correct value of the new layout version, invoke `garage layout show`
 		self.roles.merge(&self.staging);
 		self.roles.retain(|(_, _, v)| v.0.is_some());
 
-		if !self.calculate_partition_assignation() {
+        let msg = self.calculate_partition_assignation()?;
-			return Err(Error::Message("Could not calculate new assignation of partitions to nodes. This can happen if there are less nodes than the desired number of copies of your data (see the replication_mode configuration parameter).".into()));
-		}
 
 		self.staging.clear();
 		self.staging_hash = blake2sum(&rmp_to_vec_all_named(&self.staging).unwrap()[..]);
 
 		self.version += 1;
 
-		Ok(self)
+		Ok((self,msg))
 	}
 
 	pub fn revert_staged_changes(mut self, version: Option<u64>) -> Result<Self, Error> {
@@ -231,24 +248,24 @@ To know the correct value of the new layout version, invoke `garage layout show`
     }
 
     ///Given a node uuids, this function returns the label of its zone
-    pub fn get_node_zone(&self, uuid : &Uuid) -> Result<String,String> {
+    pub fn get_node_zone(&self, uuid : &Uuid) -> Result<String,Error> {
         match self.node_role(uuid) {
             Some(role) => return Ok(role.zone.clone()),
-            _ => return Err("The Uuid does not correspond to a node present in the cluster.".to_string())
+            _ => return Err(Error::Message("The Uuid does not correspond to a node present in the cluster.".into()))
         }
     }
     
     ///Given a node uuids, this function returns its capacity or fails if it does not have any
-    pub fn get_node_capacity(&self, uuid : &Uuid) -> Result<u32,String> {
+    pub fn get_node_capacity(&self, uuid : &Uuid) -> Result<u32,Error> {
         match self.node_role(uuid) {
             Some(NodeRole{capacity : Some(cap), zone: _, tags: _}) => return Ok(*cap),
-            _ => return Err("The Uuid does not correspond to a node present in the \
+            _ => return Err(Error::Message("The Uuid does not correspond to a node present in the \
-                    cluster or this node does not have a positive capacity.".to_string())
+                    cluster or this node does not have a positive capacity.".into()))
         }
     }
 
     ///Returns the sum of capacities of non gateway nodes in the cluster
-    pub fn get_total_capacity(&self) -> Result<u32,String> {
+    pub fn get_total_capacity(&self) -> Result<u32,Error> {
         let mut total_capacity = 0;
         for uuid in self.useful_nodes().iter() {
             total_capacity += self.get_node_capacity(uuid)?;
@@ -311,7 +328,8 @@ To know the correct value of the new layout version, invoke `garage layout show`
             let zones_of_p = nodes_of_p.iter()
                     .map(|n| self.get_node_zone(&self.node_id_vec[*n as usize])
                          .expect("Zone not found."));
-            if zones_of_p.unique().count() < self.zone_redundancy {
+            let redundancy = self.parameters.get().zone_redundancy;
+            if zones_of_p.unique().count() < redundancy {
                 return false;
             }
         }
@@ -354,7 +372,7 @@ impl ClusterLayout {
 	/// Among such optimal assignation, it minimizes the distance to
 	/// the former assignation (if any) to minimize the amount of
 	/// data to be moved. 
-	pub fn calculate_partition_assignation(&mut self, replication:usize, redundancy:usize) -> Result<Message,String> {
+	pub fn calculate_partition_assignation(&mut self) -> Result<Message,Error> {
 		//The nodes might have been updated, some might have been deleted.
 		//So we need to first update the list of nodes and retrieve the
 		//assignation.
@@ -362,12 +380,12 @@ impl ClusterLayout {
         //We update the node ids, since the node list might have changed with the staged
         //changes in the layout. We retrieve the old_assignation reframed with the new ids
         let old_assignation_opt = self.update_node_id_vec()?;
-        self.replication_factor = replication;
+        
-        self.zone_redundancy = redundancy;
+        let redundancy = self.parameters.get().zone_redundancy;
 
         let mut msg = Message::new();
         msg.push(format!("Computation of a new cluster layout where partitions are \
-        replicated {} times on at least {} distinct zones.", replication, redundancy));
+        replicated {} times on at least {} distinct zones.", self.replication_factor, redundancy));
 
         //We generate for once numerical ids for the zone, to use them as indices in the 
         //flow graphs.
@@ -381,6 +399,7 @@ impl ClusterLayout {
         //In this case, integer rounding plays a marginal role in the percentages of 
         //optimality.
         let partition_size = self.compute_optimal_partition_size(&zone_to_id)?;
+
         if old_assignation_opt != None  {
             msg.push(format!("Given the replication and redundancy constraint, the \
                 optimal size of a partition is {}. In the previous layout, it used to \
@@ -392,6 +411,12 @@ impl ClusterLayout {
         }
         self.partition_size = partition_size;
 
+        if partition_size < 100 {
+            msg.push("WARNING: The partition size is low (< 100), you might consider to \
+            give the nodes capacities in a smaller unit (e.g. Mb instead of Gb) to \
+            achieve a more tailored use of your storage ressources.".into());
+        }
+
         //We compute a first flow/assignment that is heuristically close to the previous
         //assignment
         let mut gflow = self.compute_candidate_assignment( &zone_to_id, &old_assignation_opt)?;
@@ -413,7 +438,7 @@ impl ClusterLayout {
 	/// None if the node is not present anymore.
 	/// We work with the assumption that only this function and calculate_new_assignation
 	/// do modify assignation_ring and node_id_vec.
-    fn update_node_id_vec(&mut self) -> Result< Option< Vec<Vec<usize> > > ,String> {
+    fn update_node_id_vec(&mut self) -> Result< Option< Vec<Vec<usize> > > ,Error> {
         // (1) We compute the new node list
         //Non gateway nodes should be coded on 8bits, hence they must be first in the list
 	    //We build the new node ids	 
@@ -423,8 +448,8 @@ impl ClusterLayout {
             .map(|(k, _, _)| *k).collect();
         
         if new_non_gateway_nodes.len() > MAX_NODE_NUMBER {
-            return Err(format!("There are more than {} non-gateway nodes in the new \
+            return Err(Error::Message(format!("There are more than {} non-gateway nodes in the new \
-                            layout. This is not allowed.", MAX_NODE_NUMBER).to_string());
+                            layout. This is not allowed.", MAX_NODE_NUMBER).into() ));
         }
 
 		let mut new_gateway_nodes: Vec<Uuid> = self.roles.items().iter()
@@ -449,8 +474,8 @@ impl ClusterLayout {
             return Ok(None);
         }
         if self.ring_assignation_data.len() != nb_partitions * self.replication_factor {
-            return Err("The old assignation does not have a size corresponding to \
+            return Err(Error::Message("The old assignation does not have a size corresponding to \
-                the old replication factor or the number of partitions.".to_string());
+                the old replication factor or the number of partitions.".into()));
         }
 
         //We build a translation table between the uuid and new ids
@@ -482,14 +507,14 @@ impl ClusterLayout {
 
     ///This function generates ids for the zone of the nodes appearing in 
     ///self.node_id_vec.
-    fn generate_zone_ids(&self) -> Result<(Vec<String>, HashMap<String, usize>),String>{
+    fn generate_zone_ids(&self) -> Result<(Vec<String>, HashMap<String, usize>),Error>{
         let mut id_to_zone = Vec::<String>::new();
         let mut zone_to_id = HashMap::<String,usize>::new();
 
         for uuid in self.node_id_vec.iter() {
             if self.roles.get(uuid) == None {
-                return Err("The uuid was not found in the node roles (this should \
+                return Err(Error::Message("The uuid was not found in the node roles (this should \
-                    not happen, it might be a critical error).".to_string());
+                    not happen, it might be a critical error).".into()));
             }
             match self.node_role(&uuid) {
                 Some(r) => if !zone_to_id.contains_key(&r.zone) && r.capacity != None {
@@ -504,14 +529,14 @@ impl ClusterLayout {
 
     ///This function computes by dichotomy the largest realizable partition size, given
     ///the layout.
-    fn compute_optimal_partition_size(&self, zone_to_id: &HashMap<String, usize>) -> Result<u32,String>{
+    fn compute_optimal_partition_size(&self, zone_to_id: &HashMap<String, usize>) -> Result<u32,Error>{
         let nb_partitions = 1usize << PARTITION_BITS;
         let empty_set = HashSet::<(usize,usize)>::new();
         let mut g = self.generate_flow_graph(1, zone_to_id, &empty_set)?;
         g.compute_maximal_flow()?;
         if g.get_flow_value()? < (nb_partitions*self.replication_factor).try_into().unwrap() {
-            return Err("The storage capacity of he cluster is to small. It is \
+            return Err(Error::Message("The storage capacity of he cluster is to small. It is \
-                       impossible to store partitions of size 1.".to_string());
+                       impossible to store partitions of size 1.".into()));
         }
 
         let mut s_down = 1;
@@ -545,14 +570,15 @@ impl ClusterLayout {
         return vertices;
     }
 
-    fn generate_flow_graph(&self, size: u32, zone_to_id: &HashMap<String, usize>, exclude_assoc : &HashSet<(usize,usize)>) -> Result<Graph<FlowEdge>, String> {
+    fn generate_flow_graph(&self, size: u32, zone_to_id: &HashMap<String, usize>, exclude_assoc : &HashSet<(usize,usize)>) -> Result<Graph<FlowEdge>, Error> {
         let vertices = ClusterLayout::generate_graph_vertices(zone_to_id.len(), 
                                                         self.useful_nodes().len());
         let mut g= Graph::<FlowEdge>::new(&vertices);
         let nb_zones = zone_to_id.len();
+        let redundancy = self.parameters.get().zone_redundancy;
         for p in 0..NB_PARTITIONS {
-            g.add_edge(Vertex::Source, Vertex::Pup(p), self.zone_redundancy as u32)?;
+            g.add_edge(Vertex::Source, Vertex::Pup(p), redundancy as u32)?;
-            g.add_edge(Vertex::Source, Vertex::Pdown(p), (self.replication_factor - self.zone_redundancy) as u32)?;
+            g.add_edge(Vertex::Source, Vertex::Pdown(p), (self.replication_factor - redundancy) as u32)?;
             for z in 0..nb_zones {
                 g.add_edge(Vertex::Pup(p) , Vertex::PZ(p,z) , 1)?;
                 g.add_edge(Vertex::Pdown(p) , Vertex::PZ(p,z) , 
@@ -574,7 +600,7 @@ impl ClusterLayout {
 
 
     fn compute_candidate_assignment(&self, zone_to_id: &HashMap<String, usize>, 
-        old_assoc_opt : &Option<Vec< Vec<usize> >>) -> Result<Graph<FlowEdge>, String > {
+        old_assoc_opt : &Option<Vec< Vec<usize> >>) -> Result<Graph<FlowEdge>, Error > {
         
         //We list the edges that are not used in the old association
         let mut exclude_edge = HashSet::<(usize,usize)>::new();
@@ -601,7 +627,7 @@ impl ClusterLayout {
         return Ok(g);
     }
 
-    fn minimize_rebalance_load(&self, gflow: &mut Graph<FlowEdge>, zone_to_id: &HashMap<String, usize>, old_assoc : &Vec< Vec<usize> >) -> Result<(), String > {
+    fn minimize_rebalance_load(&self, gflow: &mut Graph<FlowEdge>, zone_to_id: &HashMap<String, usize>, old_assoc : &Vec< Vec<usize> >) -> Result<(), Error > {
         let mut cost = CostFunction::new();
         for p in 0..NB_PARTITIONS {
             for n in old_assoc[p].iter() {
@@ -616,7 +642,7 @@ impl ClusterLayout {
         return Ok(());
     }
 
-    fn update_ring_from_flow(&mut self, nb_zones : usize, gflow: &Graph<FlowEdge> ) -> Result<(), String>{
+    fn update_ring_from_flow(&mut self, nb_zones : usize, gflow: &Graph<FlowEdge> ) -> Result<(), Error>{
         self.ring_assignation_data = Vec::<CompactNodeType>::new();
         for p in 0..NB_PARTITIONS {
             for z in 0..nb_zones {
@@ -631,8 +657,8 @@ impl ClusterLayout {
         }
 
         if self.ring_assignation_data.len() != NB_PARTITIONS*self.replication_factor {
-            return Err("Critical Error : the association ring we produced does not \
+            return Err(Error::Message("Critical Error : the association ring we produced does not \
-                       have the right size.".to_string());
+                       have the right size.".into()));
         }
         return Ok(());
     }
@@ -643,7 +669,7 @@ impl ClusterLayout {
     fn output_stat(&self , gflow : &Graph<FlowEdge>, 
                     old_assoc_opt : &Option< Vec<Vec<usize>> >,
                     zone_to_id: &HashMap<String, usize>, 
-                    id_to_zone : &Vec<String>) -> Result<Message, String>{
+                    id_to_zone : &Vec<String>) -> Result<Message, Error>{
         let mut msg = Message::new();
         
 		let nb_partitions = 1usize << PARTITION_BITS;

src/rpc/system.rs

@@ -196,7 +196,6 @@ impl System {
 		network_key: NetworkKey,
 		background: Arc<BackgroundRunner>,
 		replication_factor: usize,
-        zone_redundancy: usize,
 		config: &Config,
 	) -> Result<Arc<Self>, Error> {
 		let node_key =
@@ -226,7 +225,7 @@ impl System {
 					"No valid previous cluster layout stored ({}), starting fresh.",
 					e
 				);
-				ClusterLayout::new(replication_factor, zone_redundancy)
+				ClusterLayout::new(replication_factor)
 			}
 		};