feat: add doc and fix test

CONTRIBUTING.md

@@ -236,11 +236,11 @@ sequenceDiagram
     note right of Ledger: Calling Numscript interpreter
     Ledger-->>Database: Store transaction and update balances
     note left of Database: This diagram simplify the process of writing the transaction. More entities will be saved, <br/>and some computation can happen, depending on the enabled features. More on this later. 
-    Database->>Ledger: Transaction written
+    Database-->>Ledger: Transaction written
     Ledger->>Database: Commit SQL transaction
     note left of Database: Now, updated account balances are written to the database and any <br/>concurrent transaction waiting for those accounts will be unblocked. 
-    Database->>Ledger: Transaction committed
-    Ledger->>Client: Created transaction
+    Database-->>Ledger: Transaction committed
+    Ledger-->>Client: Created transaction
 ```
 
 ***Get and lock balances of bounded source accounts***
@@ -282,7 +282,43 @@ FOR UPDATE
 
 ## Import
 
-TODO
+```mermaid
+sequenceDiagram
+    actor Client
+    actor Ledger
+    actor Database
+    Client->>Ledger: Create transaction
+    Ledger->>Database: Start SQL transaction (Serialization Isolation level)
+    Database-->>Ledger: SQL transaction started
+    Ledger->>Database: Count logs
+    Database-->>Ledger: Count of logs
+    alt id count > 0
+        note left of Ledger: Import is only allowed on empty ledgers
+        Ledger-->>Client: Invalid state error
+    else 
+        Ledger->>Ledger: Read log stream
+        loop over log stream
+            Ledger->>Database: Apply appropriate actions (create transactions, update metadata...)
+            Database-->>Ledger: OK
+        end
+    end
+    Ledger->>Database: Commit SQL transaction
+    alt another action has been made on the ledger in the meantime 
+        Database-->>Ledger: Serialization error
+    else
+        Database-->>Ledger: SQL Transaction committed
+    end
+    Ledger->>Client: OK
+```
+
+To import a ledger, we use the [Serializable Isolation Level](https://www.postgresql.org/docs/7.2/xact-serializable.html).
+This isolation level is the strictest Postgres can offer.
+
+That's way, if another concurrent request write something on the ledger (a new transaction for example), the import request will fail with a serialization error.
+It is the case, because, whatever the ledger is configured, finally, when writing, we will ***always*** write a log describing the action, causing conflict with Serializable Isolation Level.
+
+As said, this isolation level is the strictest Postgres can offer, we could ask why we don't use it all the time.
+That's because, if we would do that, we would have frequent serialization errors, and we would need to retry very often, and probably creating a big bottleneck.
 
 # Testing strategy
 
@@ -318,7 +354,7 @@ This command will :
 $ earthly -P +tests
 ```
 
-Additionnally, the flag ```--coverage=true``` can be passed to generate coverage :
+Additionally, the flag ```--coverage=true``` can be passed to generate coverage :
 ```shell
 $ earthly -P +tests --coverage=true # Generated under cover.out
 ```

internal/transaction.go

@@ -159,6 +159,24 @@ func (tx Transaction) VolumeUpdates() []AccountsVolumes {
 		}
 	}
 
+	slices.SortStableFunc(ret, func(a, b AccountsVolumes) int {
+		switch {
+		case a.Account < b.Account:
+			return -1
+		case a.Account > b.Account:
+			return 1
+		default:
+			switch {
+			case a.Asset < b.Asset:
+				return -1
+			case a.Asset > b.Asset:
+				return 1
+			default:
+				return 0
+			}
+		}
+	})
+
 	return ret
 }
 

internal/transaction_test.go

@@ -39,7 +39,7 @@ func TestTransactionsVolumesUpdate(t *testing.T) {
 
 	require.Equal(t, []AccountsVolumes{
 		{
-			Account: "world",
+			Account: "payments:001",
 			Asset:   "COIN",
 			Input:   big.NewInt(100),
 			Output:  big.NewInt(100),
@@ -51,7 +51,7 @@ func TestTransactionsVolumesUpdate(t *testing.T) {
 			Output:  big.NewInt(100),
 		},
 		{
-			Account: "payments:001",
+			Account: "world",
 			Asset:   "COIN",
 			Input:   big.NewInt(100),
 			Output:  big.NewInt(100),

test/e2e/api_ledgers_import_test.go

@@ -42,6 +42,9 @@ var _ = Context("Ledger engine tests", func() {
 		BeforeEach(func() {
 			createLedgerRequest = operations.V2CreateLedgerRequest{
 				Ledger: "foo",
+				V2CreateLedgerRequest: &components.V2CreateLedgerRequest{
+					Features: ledger.MinimalFeatureSet,
+				},
 			}
 		})
 		JustBeforeEach(func() {
@@ -236,4 +239,4 @@ var _ = Context("Ledger engine tests", func() {
 			})
 		})
 	})
-})
+})