nject

README

nject, npoint, nserve, & nvelope - dependency injection

Install:

go get github.com/muir/nject

---

This is a quartet of packages that together make up a most of a golang API server framework:

nject: type safe dependency injection w/o requiring type assertions.

npoint: dependency injection wrappers for binding http endpoint handlers

nvelope: injection chains for building endpoints

nserve: injection chains for for starting and stopping servers

Basic idea

Dependencies are injected via a call chain: list functions to be called that take and return various parameters. The functions will be called in order using the return values from earlier functions as parameters for later functions.

Parameters are identified by their types. To have two different int parameters, define custom types.

Type safety is checked before any functions are called.

Functions whose outputs are not used are not called. Functions may "wrap" the rest of the list so that they can choose to invoke the remaing list zero or more times.

Chains may be pre-compiled into closures so that they have very little runtime penealty.

nject example

func example() {
	// Sequences can be reused.
	providerChain := Sequence("example sequence",
		// Constants can be injected.
		"a literal string value",
		// This function will be run if something downstream needs an int
		func(s string) int {
			return len(s)
		})
	Run("example",
		providerChain,
		// The last function in the list is always run.  This one needs
		// and int and a string.  The string can come from the constant
		// and the int from the function in the provider chain.
		func(i int, s string) {
			fmt.Println(i, len(s))
		})
}

npoint example

CreateEndpoint is the simplest way to start using the npoint framework. It generates an http.HandlerFunc from a list of handlers. The handlers will be called in order. In the example below, first WriteErrorResponse() will be called. It has an inner() func that it uses to invoke the rest of the chain. When WriteErrorResponse() calls its inner() function, the db injector returned by InjectDB is called. If that does not return error, then the inline function below to handle the endpint is called.

mux := http.NewServeMux()
mux.HandleFunc("/my/endpoint", npoint.CreateEndpoint(
	WriteErrorResponse,
	InjectDB("postgres", "postgres://..."),
	func(r *http.Request, db *sql.DB, w http.ResponseWriter) error {
		// Write response to w or return error...
		return nil
	}))

WriteErrorResponse invokes the remainder of the handler chain by calling inner().

func WriteErrorResponse(inner func() nject.TerminalError, w http.ResponseWriter) {
	err := inner()
	if err != nil {
		w.Write([]byte(err.Error()))
		w.WriteHeader(500)
	}
}

InjectDB returns a handler function that opens a database connection. If the open fails, executation of the handler chain is terminated.

func InjectDB(driver, uri string) func() (nject.TerminalError, *sql.DB) {
	return func() (nject.TerminalError, *sql.DB) {
		db, err := sql.Open(driver, uri)
		if err != nil {
			return err, nil
		}
		return nil, db
	}
}

nvelope example

Nvelope provides pre-defined handlers for basic endpoint tasks. When used in combination with npoint, all that's left is the business logic.

type ExampleRequestBundle struct {
	Request     PostBodyModel `nvelope:"model"`
	With        string        `nvelope:"path,name=with"`
	Parameters  int64         `nvelope:"path,name=parameters"`
	Friends     []int         `nvelope:"query,name=friends"`
	ContentType string        `nvelope:"header,name=Content-Type"`
}

func Service(router *mux.Router) {
	service := npoint.RegisterServiceWithMux("example", router)
	service.RegisterEndpoint("/some/path",
		nvelope.LoggerFromStd(log.Default()),
		nvelope.InjectWriter,
		nvelope.EncodeJSON,
		nvelope.CatchPanic,
		nvelope.Nil204,
		nvelope.ReadBody,
		nvelope.DecodeJSON,
		func (req ExampleRequestBundle) (nvelope.Response, error) {
			....
		},
	).Methods("POST")
}

nserve example

On thing you might want to do with nserve is to use a Hook to trigger per-library database migrations using libschema.

First create the hook:

package myhooks

import "github.com/nject/nserve"

var MigrateMyDB = nserve.NewHook("migrate, nserve.Ascending)

In each library, have a create function:

package users

import(
	"github.com/muir/libschema/lspostgres"
	"github.com/muir/nject/nserve"
)

func NewUsersStore(app *nserve.App) *Store {
	...
	app.On(myhooks.MigrateMyDB, func(database *libschema.Database) {
		database.Migrations("MyLibrary",
			lspostgres.Script("create users", `
				CREATE TABLE users (
					id	bigint PRIMARY KEY,
					name	text
				)
			`),
		)
	})
	...
	return &Store{}
}

Then as part of server startup, invoke the migration hook:

package main

import(
	"github.com/muir/libschema"
	"github.com/muir/libschema/lspostgres"
	"github.com/muir/nject/nject"
)

func main() {
	app, err := nserve.CreateApp("myApp", users.NewUserStore, ...)
	schema := libschema.NewSchema(ctx, libschema.Options{})
	sqlDB, err := sql.Open("postgres", "....")
	database, err := lspostgres.New(logger, "main-db", schema, sqlDB)
	myhooks.MigrateMyDB.Using(database)
	err = app.Do(myhooks.MigrateMyDB)

Development status

This repo represents continued development of Blue Owl's nject base. Blue Owl's code has been in production use for years and has been unchanged for years. The core of nject is mostly unchanged. Nvelope and nserve are new.