2 files changed, 13 insertions, 0 deletions
diff --git a/lib/handler.ml b/lib/handler.ml
index 121a0c5..4babe6e 100644
--- a/lib/handler.ml
+++ b/lib/handler.ml
@@ -8,6 +8,10 @@ let valid_debit value limit balance =
 let create_transaction client_id (db_pool : Query.pool) (request : Request.t) =
   Caqti_eio.Pool.use
     (fun conn ->
+      let module C = (val conn : Rapper_helper.CONNECTION) in
+      C.with_transaction
+      @@ fun () ->
+      let _ = Query.lock client_id conn in
       let client_opt =
         Option.join @@ Result.to_option @@ Query.find_client client_id conn
       in
diff --git a/lib/query.ml b/lib/query.ml
index 1fdc2c3..2dab927 100644
--- a/lib/query.ml
+++ b/lib/query.ml
@@ -58,6 +58,14 @@ module Q = struct
         |sql}
         record_out]
   ;;
+
+  let lock =
+    [%rapper
+      execute
+        {sql|
+          SELECT pg_advisory_xact_lock(%int{client_id})
+        |sql}]
+  ;;
 end
 
 let ( let* ) = Result.bind
@@ -80,3 +88,4 @@ let execute_transaction ~client_id ~(op : Operation.transaction_op) conn =
 let find_client id conn = Q.client ~id conn
 let balance client_id conn = Q.balance ~client_id conn
 let transactions client_id conn = Q.transactions ~client_id conn
+let lock client_id conn = Q.lock ~client_id conn