Grisha Trubetskoy

Notes to self.

Building a Go Web App - Part 2

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This is a continuation of part 1. (There is also part 3 and part 4).

So our app is going to have two major parts to it: client and server. (What year is this?). The server side is going to be in Go, and the client side in JS. Let’s talk about the server side first.

The Go (Server) Side

The server side of our application is going to be responsible for initially serving up all the necessary JavaScript and supporting files if any, aka static assets and data in the form of JSON. That’s it, just two functions: (1) static assets and (2) JSON.

It’s worth noting that serving assets is optional: assets could be served from a CDN, for example. But what is different is that it is not a problem for our Go app, unlike a Python/Ruby app it can perform on par with Ngnix and Apache serving static assets. Delegating assets to another piece of software to lighten its load is no longer necessary, though certainly makes sense in some situations.

To make this simpler, let’s pretend we’re making an app that lists people (just first and last name) from a database table, that’s it. The code is here https://github.com/grisha/gowebapp.

Directory Layout

It has been my experience that dividing functionality across packages early on is a good idea in Go. Even if it is not completely clear how the final program will be structured, it is good to keep things separate to the extent possible.

For a web app I think something along the lines of the following layout makes sense:

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# github.com/user/foo

foo/            # package main
  |
  +--daemon/    # package daemon
  |
  +--model/     # package model
  |
  +--ui/        # package ui
  |
  +--db/        # package db
  |
  +--assets/    # where we keep JS and other assets

Top level: package main

At the top level we have package main and its code in main.go. The key advantage here is that with this layout go get github.com/user/foo can be the only command required to install the whole application into $GOPATH/bin.

Package main should do as little as possible. The only code that belongs here is to parse the command argument flags. If the app had a config file, I’d stick parsing and checking of that file into yet another package, probably called config. After that main should pass the control to the daemon package.

An essential main.go is:

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package main

import (
    "github.com/user/foo/daemon"
)

var assetsPath string

func processFlags() *daemon.Config {
    cfg := &daemon.Config{}

    flag.StringVar(&cfg.ListenSpec, "listen", "localhost:3000", "HTTP listen spec")
    flag.StringVar(&cfg.Db.ConnectString, "db-connect", "host=/var/run/postgresql dbname=gowebapp sslmode=disable", "DB Connect String")
    flag.StringVar(&assetsPath, "assets-path", "assets", "Path to assets dir")

    flag.Parse()
    return cfg
}

func setupHttpAssets(cfg *daemon.Config) {
    log.Printf("Assets served from %q.", assetsPath)
    cfg.UI.Assets = http.Dir(assetsPath)
}

func main() {
    cfg := processFlags()

    setupHttpAssets(cfg)

    if err := daemon.Run(cfg); err != nil {
        log.Printf("Error in main(): %v", err)
    }
}

The above program accepts three parameters, -listen, -db-connect and -assets-path, nothing earth shattering.

Using structs for clarity

In line cfg := &daemon.Config{} we are creating a daemon.Config object. It’s main purpose is to pass around configuration in a structured and clear format. Every one of our packages defines its own Config type describing the parameters it needs, and packages can include other package configs. We see an example of this in processFlags() above: flag.StringVar(&cfg.Db.ConnectString, .... Here db.Config is included in daemon.Config. I find doing this very useful. These structures also keep open the possibility of serializing configs as JSON, TOML or whatever.

Using http.FileSystem to serve assets

The http.Dir(assetsPath) in setupHttpAssets is in preparation to how we will serve the assets in the ui package. The reason it’s done this way is to leave the door open for cfg.UI.Assets (which is a http.FileSystem interface) to be provided by other implementations, e.g. serving this content from memory. I will describe it in more detail in a later post.

Lastly, main calls daemon.Run(cfg) which is what actually starts our app and where it blocks until it’s terminated.

package daemon

Package daemon contains everything related to running a process. Stuff like which port to listen on, custom logging would belong here, as well anything related to a graceful restart, etc.

Since the job of the daemon package is to initiate the database connection, it will need to import the db package. It’s also responsible for listening on the TCP port and starting the user interface for that listener, therefore it needs to import the ui package, and since the ui package needs to access data, which is done via the model package, it will need to import model as well.

A bare bones daemon might look like this:

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package daemon

import (
    "log"
    "net"
    "os"
    "os/signal"
    "syscall"

    "github.com/grisha/gowebapp/db"
    "github.com/grisha/gowebapp/model"
    "github.com/grisha/gowebapp/ui"
)

type Config struct {
    ListenSpec string

    Db db.Config
    UI ui.Config
}

func Run(cfg *Config) error {
    log.Printf("Starting, HTTP on: %s\n", cfg.ListenSpec)

    db, err := db.InitDb(cfg.Db)
    if err != nil {
        log.Printf("Error initializing database: %v\n", err)
        return err
    }

    m := model.New(db)

    l, err := net.Listen("tcp", cfg.ListenSpec)
    if err != nil {
        log.Printf("Error creating listener: %v\n", err)
        return err
    }

    ui.Start(cfg.UI, m, l)

    waitForSignal()

    return nil
}

func waitForSignal() {
    ch := make(chan os.Signal)
    signal.Notify(ch, syscall.SIGINT, syscall.SIGTERM)
    s := <-ch
    log.Printf("Got signal: %v, exiting.", s)
}

Note how Config includes db.Config and ui.Config as I described earlier.

All the action happens in Run(*Config). We initialize a database connection, create a model.Model instance, and start the ui passing in the config, a pointer to the model and the listener.

package model

The purpose of model is to separate how data is stored in the database from the ui, as well as to contain any business logic an app might have. It’s the brains of the app if you will.

The model package should define a struct (Model seems like an appropriate name) and a pointer to an instance of the struct should be passed to all the ui functions and methods. There should only be one such instance in our app - for extra credit you can enforce that programmatically by making it a singleton, but I don’t think that’s necessary.

Alternatively you could get by without a Model and just use the package model itself. I don’t like this approach, but it’s an option.

The model should also define structs for the data entities we are dealing with. In our example it would be a Person struct. Its members should be exported (capitalized) because other packages will be accessing those. If you use sqlx, this is where you would also specify tags that map elements to db column names, e.g. `db:"first_name"`

Our Person type:

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type Person struct {
    Id          int64
    First, Last string
}

In our case we do not need tags because our column names match the element names, and sqlx conveniently takes care of the capitalization, so Last matches the column named last.

package model should NOT import db

Somewhat counter-intuitive, model cannot import db. This is because db needs to import model, and circular imports are not allowed in Go. This is one case where interfaces come in handy. model needs to define an interface which db should satisfy. For now all we know is we need to list people, so we can start with this definition:

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type db interface {
    SelectPeople() ([]*Person, error)
}

Our app doesn’t really do much, but we know it lists people, so our model should probably have a People() ([]*Person, error) method:

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func (m *Model) People() ([]*Person, error) {
    return m.SelectPeople()
}

To keep things tidy, code should be in separate files, e.g. Person definition should be in person.go, etc. For readability, here is a single file version of our model:

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package model

type db interface {
    SelectPeople() ([]*Person, error)
}

type Model struct {
    db
}

func New(db db) *Model {
    return &Model{
        db: db,
    }
}

func (m *Model) People() ([]*Person, error) {
    return m.SelectPeople()
}

type Person struct {
    Id          int64
    First, Last string
}

package db

db is the actual implementation of the database interaction. This is where the SQL statements are constructed and executed. This package also imports model because it will need to construct those structs from database data.

First, db needs to provide the InitDb function which will establish the database connection, as well as create the necessary tables and prepare the SQL statements.

Our simplistic example doesn’t support migrations, but in theory this is also where they might potentially happen.

We are using PostgreSQL, which means we need to import the pq driver. We are also going to rely on sqlx, and we need our own model. Here is the beginning of our db implementation:

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package db

import (
    "database/sql"

    "github.com/grisha/gowebapp/model"
    "github.com/jmoiron/sqlx"
    _ "github.com/lib/pq"
)

type Config struct {
    ConnectString string
}

func InitDb(cfg Config) (*pgDb, error) {
    if dbConn, err := sqlx.Connect("postgres", cfg.ConnectString); err != nil {
        return nil, err
    } else {
        p := &pgDb{dbConn: dbConn}
        if err := p.dbConn.Ping(); err != nil {
            return nil, err
        }
        if err := p.createTablesIfNotExist(); err != nil {
            return nil, err
        }
        if err := p.prepareSqlStatements(); err != nil {
            return nil, err
        }
        return p, nil
    }
}

Out InitDb() creates an instance of a pgDb, which is our Postgres implementation of the model.db interface. It keeps all that we need to communicate with the database, including the prepared statements, and exports the necessary methods to satisfy the interface.

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type pgDb struct {
    dbConn *sqlx.DB

    sqlSelectPeople *sqlx.Stmt
}

Here is the code to create the tables and the statements. From the SQL perspective this is rather simplistic, it could be a lot more elaborate, of course:

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func (p *pgDb) createTablesIfNotExist() error {
    create_sql := `

       CREATE TABLE IF NOT EXISTS people (
       id SERIAL NOT NULL PRIMARY KEY,
       first TEXT NOT NULL,
       last TEXT NOT NULL);

    `
    if rows, err := p.dbConn.Query(create_sql); err != nil {
        return err
    } else {
        rows.Close()
    }
    return nil
}

func (p *pgDb) prepareSqlStatements() (err error) {

    if p.sqlSelectPeople, err = p.dbConn.Preparex(
        "SELECT id, first, last FROM people",
    ); err != nil {
        return err
    }

    return nil
}

Finally, we need to provide the method to satisfy the interface:

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    people := make([]*model.Person, 0)
    if err := p.sqlSelectPeople.Select(&people); err != nil {
        return nil, err
    }
    return people, nil

Here we’re taking advantage of sqlx to run the query and construct a slice from results with a simple call to Select() (NB: p.sqlSelectPeople is a *sqlx.Stmt). Without sqlx we would have to iterate over the result rows, processing each with Scan, which would be considerably more verbose.

Beware of a very subtle “gotcha” here. people could also be defined as var people []*model.Person and the method would work just the same. However, if the database returned no rows, the method would return nil, not an empty slice. If the result of this method is later encoded as JSON, the former would become null and the latter []. This could cause problems if the client side doesn’t know how to treat null.

That’s it for db.

package ui

Finally, we need to serve all that stuff via HTTP and that’s what the ui package does.

Here is a very simplistic variant:

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package ui

import (
    "fmt"
    "net"
    "net/http"
    "time"

    "github.com/grisha/gowebapp/model"
)

type Config struct {
    Assets http.FileSystem
}

func Start(cfg Config, m *model.Model, listener net.Listener) {

    server := &http.Server{
        ReadTimeout:    60 * time.Second,
        WriteTimeout:   60 * time.Second,
        MaxHeaderBytes: 1 << 16}

    http.Handle("/", indexHandler(m))

    go server.Serve(listener)
}

const indexHTML = `
<!DOCTYPE HTML>
<html>
  <head>
    <meta charset="utf-8">
    <title>Simple Go Web App</title>
  </head>
  <body>
    <div id='root'></div>
  </body>
</html>
`

func indexHandler(m *model.Model) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintf(w, indexHTML)
    })
}

Note how indexHTML contains next to nothing. This is 100% of the HTML that this app will ever serve. It will evolve a little as we get into the client side of the app, but only by a few lines.

Also noteworthy is how the handler is defined. If this idiom is not familiar, it’s worth spending a few minutes (or a day) to internalize it completely as it is very common in Go. indexHandler() is not a handler, it returns a handler function. It is done this way so that we can pass in a *model.Model via closure, since an HTTP handler function definition is fixed and a model pointer is not one of the parameters.

In the case of indexHandler() we’re not actually doing anything with the model pointer, but when we get to implementing an actual list of people we will need it.

Conclusion

Above is essentially all the knowledge required to build a basic Go web app, at least the Go side of it. Next week I’ll get into the client side and we will complete the people listing code.

Continue to part 3.

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