diff --git a/storage/database.go b/storage/database.go
index da92700..3951c52 100644
--- a/storage/database.go
+++ b/storage/database.go
@@ -7,6 +7,7 @@ import (
 	"github.com/simon987/task_tracker/config"
 	"io/ioutil"
 	"os"
+	"sync"
 )
 
 type Database struct {
@@ -14,12 +15,14 @@ type Database struct {
 	saveTaskStmt *sql.Stmt
 
 	workerCache map[int64]*Worker
+	assignMutex *sync.Mutex
 }
 
 func New() *Database {
 
 	d := Database{}
 	d.workerCache = make(map[int64]*Worker)
+	d.assignMutex = &sync.Mutex{}
 
 	d.init()
 
diff --git a/storage/task.go b/storage/task.go
index 51f8474..393cd12 100644
--- a/storage/task.go
+++ b/storage/task.go
@@ -106,42 +106,46 @@ func (database *Database) GetTaskFromProject(worker *Worker, projectId int64) *T
 
 	db := database.getDB()
 
-	row := db.QueryRow(`
-	UPDATE task
-	SET assignee=$1, assign_time=extract(epoch from now() at time zone 'utc')
-	WHERE id IN
-	(
-		SELECT task.id
-	FROM task
-	INNER JOIN project on task.project = project.id
-	LEFT JOIN worker_verifies_task wvt on task.id = wvt.task AND wvt.worker=$1
-	WHERE NOT project.paused AND assignee IS NULL AND project.id=$2 AND status=1
-		AND (project.public OR (
-		  SELECT a.role_assign and not a.request FROM worker_access a WHERE a.worker=$1 AND a.project=$2
-		))
-		AND wvt.task IS NULL
-	ORDER BY task.priority DESC
-	LIMIT 1
-	)
-	RETURNING id`, worker.Id, projectId)
-	var id int64
+	database.assignMutex.Lock()
 
+	row := db.QueryRow(`
+		UPDATE task
+		SET assignee=$1, assign_time=extract(epoch from now() at time zone 'utc')
+		WHERE task.id = (
+			SELECT task.id
+			FROM task
+			INNER JOIN project on task.project = project.id AND project.id=$2 AND not paused
+			LEFT JOIN worker_verifies_task wvt on task.id = wvt.task AND wvt.worker=$1
+			LEFT JOIN worker_access wa on project.id = wa.project AND wa.worker=$1
+			WHERE 
+				assignee IS NULL 
+				AND status=1
+				AND (project.public OR (wa.role_assign AND NOT request))
+				AND wvt.task IS NULL
+			ORDER BY task.priority DESC
+			LIMIT 1
+		)
+		RETURNING task.id`, worker.Id, projectId)
+
+	var id int64
 	err := row.Scan(&id)
+
 	if err != nil {
-		logrus.WithFields(logrus.Fields{
-			"worker": worker,
-		}).Trace("No task available")
+		database.assignMutex.Unlock()
 		return nil
 	}
 
 	row = db.QueryRow(`
-	SELECT task.id, task.priority, task.project, assignee, retries, max_retries,
-	        status, recipe, max_assign_time, assign_time, verification_count, project.priority, project.name,
-	       project.clone_url, project.git_repo, project.version, project.motd, project.public, COALESCE(project.chain,0),
-	       project.assign_rate, project.submit_rate
-	FROM task 
-	  INNER JOIN project project ON task.project = project.id
-	WHERE task.id=$1`, id)
+		SELECT task.id, task.priority, task.project, assignee, retries, max_retries,
+				status, recipe, max_assign_time, assign_time, verification_count, project.priority, project.name,
+			   project.clone_url, project.git_repo, project.version, project.motd, project.public, COALESCE(project.chain,0),
+			   project.assign_rate, project.submit_rate
+		FROM task 
+		  INNER JOIN project project ON task.project = project.id
+		WHERE task.id=$1`, id)
+
+	database.assignMutex.Unlock()
+
 	project := &Project{}
 	task := &Task{}
 	task.Project = project