I have an application which needs configuration and I’ve created a configuration struct and I’m entering the configuration as a parameter to the function. The problem is that the configuration struct becomes bigger (like monolith) and bigger and I move the config to different functions in my app and which doesn’t need all the fields, just few of them. My question is if there is better approach to implement it in Go.
After struggling to find good way I’ve found this article (which a bit old but hopefully still relevant) and I wonder how and if I can use it to solve my problem.
Functional options instead of config struct https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
I need to inject some configuration properties to my application in
For example for function run (which is entry point ) I need to inject the log level and some other env variable like port host
For function build I need to “inject” the build flavor and build type etc.
Any example for my content will be very helpful
- How to structure it in the code ?
- How to implement it?
update
I need some E2E example how can I use the functional approach for different configs in the same package and other packages
2 Answers
Answers 1
It sounds like you're looking for an alternative to passing around the same configuration monolith structure to every package and every function. There are many solutions to this problem (more than I'm going to list here), and which one is right for you requires more knowledge of your code and your goals than we have, so it's probably best if you decide. And it sounds like you're wondering whether Dave Cheney's post on functional options provides a solution and how to apply it.
If your application's configuration is static in that it's not likely to change (mutate) through different threads of execution, and you don't need to create multiple instances with different configurations in the same main, then one option is package level variables and package initialization. If you object to exported package variables, you can use unexported package variables and control access via exported functions. Say run and build are two different packages:
// package main import( "github.com/profilename/appname/build" "github.com/profilename/appname/run" ) func main() { // do something to get configuration values build.Initialize(buildFlavor, buildType) // any post-build-initialize-pre-run-initialize stuff run.Initialize(logLevel, port, host) // other processing build.PreBuild("title") // other build functions may rely on configuration build.Build() // other stuff run.ReadFiles(f1, f2) run.Validate(preferredBackupPort) // port availability, chance to log.Fatal out run.Run() // cleanup } // package run var Host string var LogLevel, Port int init() { Host = `localhost` Port = 8888 Loglevel = 1 } func Initialize(logLevel, port int, host string) { // validation, panic on failure LogLevel = logLevel Host = host Port = port } func Run() { // do something with LogLevel, Host, Port } But that doesn't solve the problem addressed in Dave Cheney's post. What if the user is running this without host, port, or buildType (or other configuration variables), because he doesn't need those features? What if the user wants to run multiple instances with different configurations?
Dave's approach is primarily intended for situations where you will not use package-level variables for configuration. Indeed, it is meant to enable several instances of a thing where each instance can have a different configuration. Your optional configuration parameters become a single variadic parameter where the type is a function that modifies a pointer to the thing being configured. For you, that could be
// package run type Runner struct { Port int // rest of runner configuration } func NewRunner(options ...func(*Runner)) (runner *Runner, err error) { // any setup for _, option := range options { err = option(runner) if err != nil { // do something } } return runner, err } // package main func main() { // do something to get configuration values port := func(runner *Runner) { runner.Port = configuredPort } // other configuration if applicable runner := run.NewRunner(port) // ... In a way, Dave's approach appears targeted at packages that will be used as very flexible libraries, and will provide application interfaces that users might wish to create several instances of. It allows for main definitions that launch multiple instances with different configurations. In that post he doesn't go into detail on how to process configuration input in the main or on a configuration package.
Note that the way the port is set in the resulting code above is not very different from this:
// package run type Runner struct { Port int // rest of runner configuration } // package main, func main() runner := new(run.Runner) runner.Port = configuredPort which is more traditional, probably easier for most developers to read and understand, and a perfectly fine approach if it suits your needs. (And you could make runner.port unexported and add a func (r *Runner) SetPort(p int) { r.port = p } method if you wanted.) It is also a design that has the potential, depending on implementation, to deal with mutating configuration, multiple threads of execution (you'll need channels or the sync package to deal with mutation there), and multiple instances.
Where the function options design Dave proposed becomes much more powerful than that approach is when you have many more statements related to the setting of the option that you want to place in main rather than in run -- those will make up the function body.
UPDATE Here's a runnable example using Dave's functional options approach, in two files. Be sure to update the import path to match wherever you put the run package.
Package run:
package run import( "fmt" "log" ) const( DefaultPort = 8888 DefaultHost = `localhost` DefaultLogLevel = 1 ) type Runner struct { Port int Host string LogLevel int } func NewRunner(options ...func(*Runner) error) (runner *Runner) { // any setup // set defaults runner = &Runner{DefaultPort, DefaultHost, DefaultLogLevel} for _, option := range options { err := option(runner) if err != nil { log.Fatalf("Failed to set NewRunner option: %s\n", err) } } return runner } func (r *Runner) Run() { fmt.Println(r) } func (r *Runner) String() string { return fmt.Sprintf("Runner Configuration:\n%16s %22d\n%16s %22s\n%16s %22d", `Port`, r.Port, `Host`, r.Host, `LogLevel`, r.LogLevel) } Package main:
package main import( "errors" "flag" "github.com/jrefior/run" // update this path for your filesystem ) func main() { // do something to get configuration values portFlag := flag.Int("p", 0, "Override default listen port") logLevelFlag := flag.Int("l", 0, "Override default log level") flag.Parse() // put your runner options here runnerOpts := make([]func(*run.Runner) error, 0) // with flags, we're not sure if port was set by flag, so test if *portFlag > 0 { runnerOpts = append(runnerOpts, func(runner *run.Runner) error { if *portFlag < 1024 { return errors.New("Ports below 1024 are privileged") } runner.Port = *portFlag return nil }) } if *logLevelFlag > 0 { runnerOpts = append(runnerOpts, func(runner *run.Runner) error { if *logLevelFlag > 8 { return errors.New("The maximum log level is 8") } runner.LogLevel = *logLevelFlag return nil }) } // other configuration if applicable runner := run.NewRunner(runnerOpts...) runner.Run() } Example usage:
$ ./program -p 8987 Runner Configuration: Port 8987 Host localhost LogLevel 1 Answers 2
I use this to define per package Config Structs which are easier to manage and are loaded at the app start.
Define your config struct like this
type Config struct { Conf1 package1.Configuration `group:"conf1" namespace:"conf1"` Conf2 package2.Configuration `group:"conf2" namespace:"conf2"` Conf3 Config3 `group:"conf3" namespace:"conf3"` GeneralSetting string `long:"Setting" description:"setting" env:"SETTING" required:"true"` } type Config3 struct { setting string } And use "github.com/jessevdk/go-flags" to pass either --config3.setting=stringValue cmd arguments, or ENV variables export CONFIG3_SETTING=stringValue:
type Configuration interface {} const DefaultFlags flags.Options = flags.HelpFlag | flags.PassDoubleDash func Parse(cfg Configuration) []string { args, _ := flags.NewParser(cfg, DefaultFlags).Parse() return args } And your main should look something like this:
func main() { // Parse the configuration. var cfg Config Parse(&cfg) service := NewService(cfg.Conf3.Setting) } 
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