checkers

checkers

The health library comes with a number of built-in checkers for well known types of dependencies.

If a pre-built checker is not available, you can create your own checkers by implementing the ICheckable interface (which consists of a single method - Status() (interface{}, error)).

If you do create a custom-checker - consider opening a PR and adding it to the list of built-in checkers.

Built-in checkers

• HTTP
• Redis
• SQL DB
• Mongo
• Reachable

HTTP

The HTTP checker is a generic HTTP call executor. To make use of it, instantiate and fill out a HTTPConfig struct and pass it into checkers.NewHTTP(...).

The only required attribute is HTTPConfig.URL (*url.URL). Refer to the source code for all available attributes on the struct.

Redis

The Redis checker allows you to test that your server is either available (by ping), is able to set a value, is able to get a value or all of the above.

To make use of it, instantiate and fill out a RedisConfig struct and pass it to checkers.NewRedis(...).

The RedisConfig must contain a valid RedisAuthConfig and at least one check method (ping, set or get).

Refer to the godocs for additional info.

SQL DB

The SQL DB checker has implementations for the following interfaces:

• SQLPinger, which encloses PingContext in sql.DB and sql.Conn
• SQLQueryer, which encloses QueryContext in sql.DB, sql.Conn, sql.Stmt, and sql.Tx
• SQLExecer, which encloses ExecContext in sql.DB, sql.Conn, sql.Stmt, and sql.Tx

SQLConfig

The SQLConfig struct is required when using the SQL DB health check. It must contain an inplementation of one of either SQLPinger, SQLQueryer, or SQLExecer.

If SQLQueryer or SQLExecer are implemented, then Query must be valid (len > 0).

Additionally, if SQLQueryer or SQLExecer are implemented, you have the option to also set either the QueryerResultHandler or ExecerResultHandler functions. These functions allow you to evaluate the result of a query or exec operation. If you choose not to implement these yourself, the default handlers are used.

The default ExecerResultHandler is successful if the passed exec operation affected one and only one row.

The default QueryerResultHandler is successful if the passed query operation returned one and only one row.

SQLPinger

Use the SQLPinger interface if your health check is only concerned with your application's database connectivity. All you need to do is set the Pinger value in your SQLConfig.

	db, err := sql.Open("mysql", dsn)
	if err != nil {
		return err
	}

	sqlCheck, err := checkers.NewSQL(&checkers.SQLConfig{
		Pinger: db
	})
	if err != nil {
		return err
	}

	hc := health.New()
	healthCheck.AddCheck(&health.Config{
		Name:     "sql-check",
		Checker:  sqlCheck,
		Interval: time.Duration(3) * time.Second,
		Fatal:    true,
	})

SQLQueryer

Use the SQLQueryer interface if your health check requires you to read rows from your database. You can optionally supply a query result handler function. If you don't supply one, the default function will be used. The function signature for the handler is:

type SQLQueryerResultHandler func(rows *sql.Rows) (bool, error)

The default query handler returns true if there was exactly one row in the resultset:

	func DefaultQueryHandler(rows *sql.Rows) (bool, error) {
		defer rows.Close()

		numRows := 0
		for rows.Next() {
			numRows++
		}

		return numRows == 1, nil
	}

IMPORTANT: Note that your query handler is responsible for closing the passed *sql.Rows value.

Sample SQLQueryer implementation:

	// this is our custom query row handler
	func myQueryHandler(rows *sql.Rows) (bool, error) {
		defer rows.Close()

		var healthValue string
		for rows.Next() {
			// this query will ever return at most one row
			if err := rows.Scan(&healthValue); err != nil {
				return false, err
			}
		}

		return healthValue == "ok", nil
	}

	db, err := sql.Open("mysql", dsn)
	if err != nil {
		return err
	}

	// we pass the id we are looking for inside the params value
	sqlCheck, err := checkers.NewSQL(&checkers.SQLConfig{
		Queryerer:            db,
		Query:                "SELECT healthValue FROM some_table WHERE id = ?",
		Params:               []interface{}{1},
		QueryerResultHandler: myQueryHandler
	})
	if err != nil {
		return err
	}

	hc := health.New()
	healthCheck.AddCheck(&health.Config{
		Name:     "sql-check",
		Checker:  sqlCheck,
		Interval: time.Duration(3) * time.Second,
		Fatal:    true,
	})

SQLExecer

Use the SQLExecer interface if your health check requires you to update or insert to your database. You can optionally supply an exec result handler function. If you don't supply one, the default function will be used. The function signature for the handler is:

type SQLExecerResultHandler func(result sql.Result) (bool, error)

The default exec handler returns true if there was exactly one affected row:

	func DefaultExecHandler(result sql.Result) (bool, error) {
		affectedRows, err := result.RowsAffected()
		if err != nil {
			return false, err
		}

		return affectedRows == int64(1), nil
	}

Sample SQLExecer implementation:

	// this is our custom exec result handler
	func myExecHandler(result sql.Result) (bool, error) {
		insertId, err := result.LastInsertId()
		if err != nil {
			return false, err
		}

		// for this example, a check isn't valid
		// until after the 100th iteration
		return insertId > int64(100), nil
	}

	db, err := sql.Open("mysql", dsn)
	if err != nil {
		return err
	}

	sqlCheck, err := checkers.NewSQL(&checkers.SQLConfig{
		Execer:              db,
		Query:               "INSERT INTO checks (checkTS) VALUES (NOW())",
		ExecerResultHandler: myExecHandler
	})
	if err != nil {
		return err
	}

	hc := health.New()
	healthCheck.AddCheck(&health.Config{
		Name:     "sql-check",
		Checker:  sqlCheck,
		Interval: time.Duration(3) * time.Second,
		Fatal:    true,
	})

Mongo

Mongo checker allows you to test if an instance of MongoDB is available by using the underlying driver's ping method or check whether a collection exists or not.

To make use of it, initialize a MongoConfig struct and pass it to checkers.NewMongo(...).

The MongoConfig struct must specify either one or both of the Ping or Collection fields.

Reachable

The reachable checker is a generic TCP/UDP checker. Use it to verify that a configured address can be contacted via a request over TCP or UDP. This is useful if you do not care about a response from the target and simply want to know if the URL is reachable.

The only required attribute is ReachableConfig.URL (*url.URL). Refer to the source code for all available attributes on the struct.