How to mock a ping command - database

Question

I'm using https://github.com/DATA-DOG/go-sqlmock
and trying to mock a connection to a db.
Now, I need to mock a ping command (for load balancing purposes). However, I can't find any information about how to do that.
For example, I would like to write a test like this
db, mock, _ := sqlmock.New()
// ExpectPing does not exist, but it is there anything similar?
mock.ExpectPing().WillReturnError("mock error")
err := db.Ping()
if err==nil{
t.Fatal("there should have been an error")
}
in addition, I need to inject this mocked object into a function. let's say New(*sql.DB) that outputs a new structure. so it must be compatible with *sql.DB struct. for this reason, sqlmock seems a good choice. However, I've not found the way to mock the ping command.
it is there any way to do this using this library?
if not, is there any other database/sql mock library that could do the trick?

Answer 1

Update: As of Jan 6, 2020, this functionality has been added to go-sqlmock, and is included in the v1.4.0 release.
Original answer:
No, there is "nothing similar." The Ping and PingContext methods depend on the driver implementing the Pinger interface, so you can't mimic it by, for example, expecting a 'SELECT 1' or something.
There is already an issue requesting to add this. It seems to not have gotten much attention. I suspect creating a PR (which is probably about 3 lines of code) would greatly increase the chance of having that feature added.
If your goal is to make a Ping fail, you should be able to mimic that by connecting to an invalid DB endpoint (either with or without sqlmock).

Answer 2

You certainly can mock db itself:
type mockedDB struct {
*sql.DB
}
func (db *mockedDB) Ping() error {
return errors.New("not implemented")
}
func Example_mockedDB_Ping() {
db, _, _ := sqlmock.New()
defer db.Close()
mdb := mockedDB{db}
fmt.Println("mdb.Ping(): ", mdb.Ping())
// Output: mdb.Ping(): not implemented
}
but I don't understand what is the purpose of such experiment.
In the same way you can put mock into new type and define ExpectPing on it.

Answer 3

I needed to mock the Ping() functionality as well. This is how I solved it:
type mockDB struct {
ReturnError error
}
func (db *mockDB) Ping() error {
return db.ReturnError
}
func (db *mockDB) PingContext(ctx context.Context) error {
return db.ReturnError
}
// Pinger to be able to mock & ask just for a ping
type Pinger interface {
PingContext(ctx context.Context) error
Ping() error
}
// DatabasePingCheck returns a Check that validates connectivity to a database/sql.DB using Ping().
func DatabasePingCheck(pinger Pinger, timeout time.Duration) Check {
return func() error {
ctx, cancel := context.WithTimeout(context.Background(), timeout)
defer cancel()
if pinger == nil {
return fmt.Errorf("pinger is nil")
}
return pinger.PingContext(ctx)
}
}