Connection(localhost:27017[-4]) failed to write: context canceled - mongodb

Question

Here I read it's necessary to cancel the context. This is how my db.go looks like:
package db
import (
"context"
"fmt"
"log"
"os"
"time"
"go.mongodb.org/mongo-driver/mongo"
"go.mongodb.org/mongo-driver/mongo/options"
)
const (
connectionStringTemplate = "mongodb://%s:%s#%s"
)
var DB *mongo.Client
var Ctx context.Context
// Connect with create the connection to MongoDB
func Connect() {
username := os.Getenv("MONGODB_USERNAME")
password := os.Getenv("MONGODB_PASSWORD")
clusterEndpoint := os.Getenv("MONGODB_ENDPOINT")
connectionURI := fmt.Sprintf(connectionStringTemplate, username, password, clusterEndpoint)
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
client, err := mongo.NewClient(options.Client().ApplyURI(connectionURI))
if err != nil {
log.Printf("Failed to create client: %v", err)
}
err = client.Connect(ctx)
if err != nil {
log.Printf("Failed to connect to cluster: %v", err)
}
// Force a connection to verify our connection string
err = client.Ping(ctx, nil)
if err != nil {
log.Printf("Failed to ping cluster: %v", err)
}
DB = client
Ctx = ctx
log.Printf("Connected to MongoDB!")
}
I execute this in the main.go:
func main() {
// Configure
db.Connect()
defer db.DB.Disconnect(context.Background())
r := gin.Default()
// Routes
//r.GET("/movies", handlers.GetAllMoviesHandler)
r.POST("/movies", handlers.AddMovieHandler)
// listen and serve on 0.0.0.0:8080
r.Run()
}
and it works fine if my local mongodb is running.
Now when I use the client like this (I know the naming is still bad):
func AddMovie(movie *Movie) (primitive.ObjectID, error) {
movie.ID = primitive.NewObjectID()
result, err := db.DB.Database("movies").Collection("movies").InsertOne(db.Ctx, movie)
if err != nil {
log.Printf("Could not create movie: %v", err)
return primitive.NilObjectID, err
}
oid := result.InsertedID.(primitive.ObjectID)
return oid, nil
}
I get an error like this
{"msg":{"Code":0,"Message":"connection(localhost:27017[-4]) failed to write: context canceled","Labels":["NetworkError","RetryableWriteError"],"Name":"","Wrapped":{"ConnectionID":"localhost:27017[-4]","Wrapped":{}}}}
It works fine when I put the defer cancel() in comment but I suppose this is not correct?. What am I doing wrong here.
Update:
It works when I use:
result, err :=db.DB.Database("movies").Collection("movies").InsertOne(context.Background(), movie) instead of ctx. I don't fully understand the usage of the context in these use case. I'm not sure if I should do the Ctx = ctx in the Connect function either.