I am building a simple CLI tool in in Go that acts as a wrapper for various password stores (Chef Vault, Ansible Vault, Hashicorp Vault, etc). This is partially as an exercise to get familiar with Go.
In working on this, I came across a situation where I was writing tests and found I needed to create interfaces for many things, just to have the ability to mock dependencies. As such, a fairly simple implementation seems to have a bunch of abstraction, for the sake of the tests.
However, I was recently reading The Go Programming Language and found an example where they mocked their dependencies in the following way.
func Parse() map[string]string {
s := openStore()
// Do something with s to parse into a map…
return s.contents
}
var storeFunc = func openStore() *Store {
// concrete implementation for opening store
}
// and in the testing file…
func TestParse(t *testing.T) {
openStore := func() {
// set contents of mock…
}
parse()
// etc...
}
So for the sake of testing, we store this concrete implementation in a variable, and then we can essentially re-declare the variable in the tests and have it return what we need.
Otherwise, I would have created an interface for this (despite currently only having one implementation) and inject that into the Parse method. This way, we could mock it for the test.
So my question is: What are the advantages and disadvantages of each approach? When is it more appropriate to create an interface for the purposes of a mock, versus storing the concrete function in a variable for re-declaration in the test?
For testing purposes, I tend to use the mocking approach you described instead of creating new interfaces. One of the reasons being, AFAIK, there are no direct ways to identify which structs implement an interface, which is important to me if I wanted to know whether the mocks are doing the right thing.
The main drawback of this approach is that the variable is essentially a package-level global variable (even though it's unexported). So all the drawbacks with declaring global variables apply.
In your tests, you will definitely want to use defer to re-assign storeFunc back to its original concrete implementation once the tests completed.
var storeFunc = func *Store {
// concrete implementation for opening store
}
// and in the testing file…
func TestParse(t *testing.T) {
storeFuncOriginal := storeFunc
defer func() {
storeFunc = storeFuncOriginal
}()
storeFunc := func() {
// set contents of mock…
}
parse()
// etc...
}
By the way, var storeFunc = func openStore() *Store won't compile.
There is no "right way" of answering this.
Having said this, I find the interface approach more general and more clear than defining a function variable and setting it for the test.
Here are some comments on why:
The function variable approach does not scale well if there are several functions you need to mock (in your example it is just one function).
The interface makes more clear which is the behaviour being injected to the function/module as opposed to the function variable which ends up hidden in the implementation.
The interface allows you to inject a type with a state (a struct) which might be useful for configuring the behaviour of the mock.
You can of course rely on the "function variable" approach for simple cases and use the "interface" for more complex functionality, but if you want to be consistent and use just one approach I'd go with the "interface".
I tackle the problem differently. Given
function Parse(s Store) map[string] string{
// Do stuff on the interface Store
}
you have several advantages:
You can use a mock or a stub Store as you see fit.
Imho, the code becomes more transparent. The signature alone makes clear that a Store implementation is required. And the code does not need to be polluted with error handling for opening the Store.
The code documentation can be kept more concise.
However, this makes something pretty obvious: Parse is a function which can be attached to a store, which most likely makes more sense than to parse the store around.
Related
I know this has been asked in various forms many times before but I just can't seem to implement what I'm learning in the way that I need. Any help is appreciated.
I have a series of exchanges which all implement roughly the same APIs. For example, each of them have a GetBalance endpoint. However, some have one or two unique things which need to be accessed within the functions. For example, exchange1 needs to use a client when calling it's balance API, while exchange2 requires both the client variable as well as a clientFutures variable. This is an important note for later.
My background is normal OOP. Obviously Go is different in many ways, hence I'm getting tripped up here.
My current implementation and thinking is as follows:
In exchanges module
type Balance struct {
asset string
available float64
unavailable float64
total float64
}
type Api interface {
GetBalances() []Balance
}
In Binance module
type BinanceApi struct {
key string
secret string
client *binance.Client
clientFutures *futures.Client
Api exchanges.Api
}
func (v *BinanceApi) GetBalance() []exchanges.Balance {
// Requires v.client and v.clientFutures
return []exchanges.Balance{}
}
In Kraken module
type KrakenApi struct {
key string
secret string
client *binance.Client
Api exchanges.Api
}
func (v *KrakenApi) GetBalance() []exchanges.Balance {
// Requires v.client
return []exchanges.Balance{}
}
In main.go
var exchange *Api
Now my thought was I should be able to call something like exchange.GetBalance() and it would use the GetBalance function from above. I would also need some kind of casting? I'm quite lost here. The exchange could either be Binance or Kraken--that gets decided at runtime. Some other code basically calls a GetExchange function which returns an instance of the required API object (already casted in either BinanceApi/KrakenApi)
I'm aware inheritance and polymorphism don't work like other languages, hence my utter confusion. I'm struggling to know what needs to go where here. Go seems to require loads of annoying code necessary for what other languages do on the fly 😓
using *exchanges.Api is quite weird. You're wanting something that implements a given interface. What the underlying type is (whether it's a pointer or a value receiver) is not important, so use exchanges.Api instead.
There is another issue, though. In golang, interfaces are implicit (sometimes referred to as duck-type interfaces). Generally speaking, this means that the interface is not declared in the package that implements it, but rather in the package that depends on a given interface. Some say that you should be liberal in terms of what values you return, but restrictive in terms of what arguments you accept. What this boils down to in your case, is that you'd have something like an api package, that looks somewhat like this:
package api
func NewKraken(args ...any) *KrakenExchange {
// ...
}
func NewBinance(args ...any) *BinanceExchange {
}
then in your other packages, you'd have something like this:
package kraken // or maybe this could be an exchange package
type API interface {
GetBalances() []types.Balance
}
func NewClient(api API, otherArgs ...T) *KrakenClient {
}
So when someone looks at the code for this Kraken package, they can instantly tell what dependencies are required, and what types it works with. The added benefit is that, should binance or kraken need additional API calls that aren't shared, you can go in and change the specific dependencies/interfaces, without ending up with one massive, centralised interface that is being used all over the place, but each time you only end up using a subset of the interface.
Yet another benefit of this approach is when writing tests. There are tools like gomock and mockgen, which allow you to quickly generate mocks for unit tests simply by doing this:
package foo
//go:generate go run github.com/golang/mock/mockgen -destination mocks/dep_mock.go -package mocks your/module/path/to/foo Dependency
type Dependency interface {
// methods here
}
Then run go generate and it'll create a mock object in your/module/path/to/foo/mocks that implements the desired interface. In your unit tests, import he mocks package, and you can do things like:
ctrl := gomock.NewController(t)
dep := mocks.NewDependencyMock(ctrl)
defer ctrl.Finish()
dep.EXPECT().GetBalances().Times(1).Return(data)
k := kraken.NewClient(dep)
bal := k.Balances()
require.EqualValues(t, bal, data)
TL;DR
The gist of it is:
Interfaces are interfaces, don't use pointers to interfaces
Declare interfaces in the package that depends on them (ie the user), not the implementation (provider) side.
Only declare methods in an interface if you are genuinely using them in a given package. Using a central, overarching interface makes this harder to do.
Having the dependency interface declared along side the user makes for self-documenting code
Unit testing and mocking/stubbing is a lot easier to do, and to automate this way
# I want to mock this function
func testCheckPluginFile(fName string){
plugin, _ := plugin.Open(path.Join("/I/expect/folder/","/plugin-lib-test/"+fName))
plugin.Lookup("symbol")
}
# So I put this func like this
func testCheckPluginFile(fName string,pluginOpen func (path string) (*plugin.Plugin, error)){
plugin, _ := pluginOpen(path.Join("/I/expect/folder/","/plugin-lib-test/"+fName))
plugin.Lookup("symbol")
}
But I can't do it beacuse of the plugin.Plugin.lookup
Do u have another way to solve it ?
How to mock [...]plugin funcs in go[...]?
You cannot.
If you want to test that you have to use a real plugin.
Mocking in Golang is a bit tricky, and usually requires to generate some code. You need to write interfaces for the plugin package as well as for plugin.Plugin (if the package does not offer these by itself; something which alas is often the case). Then, use a mock generator (the usual suspect being gomock) to create mocks with all the usual mock functionality (expect, conditional return, etc.). In the production code, provide production implementations which you write yourself; they only consist of simple pass-through methods which call the real thing (Open() and Lookup() in this case). (Remember, you want to test whether testCheckPluginFile works correctly, not whether the plugin package works correctly.) By doing so, you should be able to follow proper TDD workflow.
Why do some packages declare two equal functions the only difference is one is exported and the other is not but the one that is exported just returns the non-exported function like this:
func Foo() {
return foo()
}
func foo() {
log.Println("Hello")
}
Why not just move the log into the exported function and get rid of the extra line? Obviously there is a reason but I don't really see one if you can just use the exported one everywhere. Thanks!
Example here of it being used in Production
You mentioned a couple examples. The first example (https://github.com/yohcop/openid-go/blob/master/verify.go#L11-L13):
func Verify(uri string, cache DiscoveryCache, nonceStore NonceStore) (id string, err error) {
return verify(uri, cache, urlGetter, nonceStore)
}
You can see that the unexported verify function takes an extra urlGetter argument. This may be something that a client of this package cannot or should not provide. The exported function determines how clients of the package can/should use it; the signature of the non-exported function reflects the dependencies required to do whatever business logic verify is doing.
The second example(https://github.com/golang/oauth2/blob/master/oauth2.go#L259-L266):
func StaticTokenSource(t *Token) TokenSource {
return staticTokenSource{t}
}
// staticTokenSource is a TokenSource that always returns the same Token.
type staticTokenSource struct {
t *Token
}
This restricts how clients can construct the staticTokenSource: there is only one way to do it, via the StaticTokenSource constructor, and it cannot be done directly via a struct literal. This can be useful for many reasons, e.g. input validation. In this case, you want the safety of knowing that the client cannot mutate the t field on the object, and in order to do this, you leave the t field unexported. But when it's unexported, the client will not be able to construct the struct literal directly, so you must provide a constructor.
In general, it makes your code much easier to reason about when you can restrict how things are accessed, constructed, or mutated. Golang packages give you a nice mechanism to encapsulate modules of business logic. It's a good idea to think about the conceptual components of your software, and what their interfaces should be. What really needs to be exposed to client code consuming a given component? Only things that really need to be exported should be.
Further reading: Organizing Go code
Suppose that I have a type type T intand I want to define a logic to operate on this type.
What abstraction should I use and When ?
Defining a method on that type:
func (T t) someLogic() {
// ...
}
Defining a function:
func somelogic(T t) {
// ...
}
Some situations where you tend to use methods:
Mutating the receiver: Things that modify fields of the objects are often methods. It's less surprising to your users that x.Foo will modify X than that Foo(x) will.
Side effects through the receiver: Things are often methods on a type if they have side effects on/through the object in subtler ways, like writing to a network connection that's part of the struct, or writing via pointers or slices or so on in the struct.
Accessing private fields: In theory, anything within the same package can see unexported fields of an object, but more commonly, just the object's constructor and methods do. Having other things look at unexported fields is sort of like having C++ friends.
Necessary to satisfy an interface: Only methods can be part of interfaces, so you may need to make something a method to just satisfy an interface. For example, Peter Bourgon's Go intro defines type openWeatherMap as an empty struct with a method, rather than a function, just to satisfy the same weatherProvider interface as other implementations that aren't empty structs.
Test stubbing: As a special case of the above, sometimes interfaces help stub out objects for testing, so your stub implementations might have to be methods even if they have no state.
Some where you tend to use functions:
Constructors: func NewFoo(...) (*Foo) is a function, not a method. Go has no notion of a constructor, so that's how it has to be.
Running on interfaces or basic types: You can't add methods on interfaces or basic types (unless you use type to make them a new type). So, strings.Split and reflect.DeepEqual must be functions. Also, io.Copy has to be a function because it can't just define a method on Reader or Writer. Note that these don't declare a new type (e.g., strings.MyString) to get around the inability to do methods on basic types.
Moving functionality out of oversized types or packages: Sometimes a single type (think User or Page in some Web apps) accumulates a lot of functionality, and that hurts readability or organization or even causes structural problems (like if it becomes harder to avoid cyclic imports). Making a non-method out of a method that isn't mutating the receiver, accessing unexported fields, etc. might be a refactoring step towards moving its code "up" to a higher layer of the app or "over" to another type/package, or the standalone function is just the most natural long-term place for it. (Hat tip Steve Francia for including an example of this from hugo in a talk about his Go mistakes.)
Convenience "just use the defaults" functions: If your users might want a quick way to use "default" object values without explicitly creating an object, you can expose functions that do that, often with the same name as an object method. For instance, http.ListenAndServe() is a package-level function that makes a trivial http.Server and calls ListenAndServe on it.
Functions for passing behavior around: Sometimes you don't need to define a type and interface just to pass functionality around and a bare function is sufficient, as in http.HandleFunc() or template.Funcs() or for registering go vet checks and so on. Don't force it.
Functions if object-orientation would be forced: Say your main() or init() are cleaner if they call out to some helpers, or you have private functions that don't look at any object fields and never will. Again, don't feel like you have to force OO (à la type Application struct{...}) if, in your situation, you don't gain anything by it.
When in doubt, if something is part of your exported API and there's a natural choice of what type to attach it to, make it a method. However, don't warp your design (pulling concerns into your type or package that could be separate) just so something can be a method. Writers don't WriteJSON; it'd be hard to implement one if they did. Instead you have JSON functionality added to Writers via a function elsewhere, json.NewEncoder(w io.Writer).
If you're still unsure, first write so that the documentation reads clearly, then so that code reads naturally (o.Verb() or o.Attrib()), then go with what feels right without sweating over it too much, because often you can rearrange it later.
Use the method if you are manipulating internal secrets of your object
(T *t) func someLogic() {
t.mu.Lock()
...
}
Use the function if you are using the public interface of the object
func somelogic(T *t) {
t.DoThis()
t.DoThat()
}
if you want to change T object, use
func (t *T) someLogic() {
// ...
}
if you donn't change T object and would like a origined-object way , use
func (t T) someLogic() {
// ...
}
but remeber that this will generate a temporay object T to call someLogic
if your like the way c language does, use
func somelogic(t T) {
t.DoThis()
t.DoThat()
}
or
func somelogic(t T) {
t.DoThis()
t.DoThat()
}
one more thing , the type is behide the var in golang.
at the moment I have a package store with following content:
package store
var (
db *Database
)
func Open(url string) error {
// open db connection
}
func FindAll(model interface{}) error {
// return all entries
}
func Close() {
// close db connection
}
This allows me to use store.FindAll from other packages after I have done store.Open in main.go.
However as I saw so far most packages prefer to provide a struct you need to initialize yourself. There are only few cases where this global approach is used.
What are downsides of this approach and should I avoid it?
You can't instantiate connections to 2 storages at once.
You can't easily mock out storage in unit tests of dependent code using convenient tools like gomock.
The standard http package has a ServerMux for generic usecases and but also has one default instance of ServerMux called DefaultServerMux (http://golang.org/pkg/net/http/#pkg-variables) for convenience. So that when you call http.HandleFunc it creates the handler on the default mux. You can find the same approach used in log and many other packages. This is essentially your "singleton" approach.
However, I don't think it's a good idea to follow that pattern in your use case, since the users need to call Open regardless of the default database. And, because of that, using a default instance would not really help and instead would actually make it less convenient:
d := store.Open(...)
defer d.Close()
d.FindAll(...)
is much easier to both write and read than:
store.Open(...)
defer store.Close()
store.FindAll(...)
And, also there are semantic problems: what should happen if someone calls Open twice:
store.Open(...)
defer store.Close()
...
store.Open(...)
store.FindAll(...) // Which db is this referring to?