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drew/sql-i
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# Using Context
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Remember that a common pattern of signaling events or completion is using a blocking channel that receives and empty struct. An empty struct has zero memory footprint. It is typically used to signal completion or stopping.
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The Context.Done() returns a channel that is closed when this Context is canceled `Done() <-chan struct{}`. Contexts have a root and can form a parent child tree structure. A child is "derived" from a parent context and when a Context is cancelled all derived contexts are also canceled. WithCancel and WithTimeout return derived Context values that can be canceled sooner than the parent Context.
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> Incoming requests to a server should create a Context, and outgoing calls to servers should accept a Context. The chain of function calls between them must propagate the Context, optionally replacing it with a derived Context created using WithCancel, WithDeadline, WithTimeout, or WithValue. When a Context is canceled, all Contexts derived from it are also canceled.
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## What about Context.Value
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The problem with context.Values is that it's just an untyped map so you have no type-safety and you have to handle it not actually containing your value. You have to create a coupling of map keys from one module to another and if someone changes something things start breaking.
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In short, if a function needs some values, put them as typed parameters rather than trying to fetch them from
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context.Value
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. This makes it statically checked and documented for everyone to see.
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Context.Value is good for things like "A trace ID". It's information that is not needed for by every function in the call stack.
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> Context.Value should inform not control. The content of context.Value is for maintainers not users. It should never be required input for documented or expected results.
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## Readings
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https://go.dev/blog/context
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https://go.dev/blog/pipelines
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https://faiface.github.io/post/context-should-go-away-go2/
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@ -1,3 +0,0 @@
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module my_context
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go 1.23.1
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@ -1,47 +0,0 @@
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package my_context
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import (
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"context"
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"fmt"
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"net/http"
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)
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type Store interface {
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Fetch(ctx context.Context) (string, error)
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// instead of calling cancel we want to use context above to cross API boundries
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// Cancel()
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}
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func Server(store Store) http.HandlerFunc {
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return func(w http.ResponseWriter, r *http.Request) {
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// THIS WAS WHAT WE DID WHEN WE DIDN"T PROPOGATE CONTEXT
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// // This is the requests ctx not a derived context
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// ctx := r.Context()
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// data := make(chan string, 1)
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// go func() {
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// data <- store.Fetch()
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// }()
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// // Here we are simply waiting on the which channel
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// // will close out first. If the Done() channel on the request
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// // does store.Cancel() is called.
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// // Remember select lets use wait on multiple async operations and execute cases
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// // based on which one completes first as signaled by data being received on that channel.
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// select {
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// case d := <-data:
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// fmt.Fprint(w, d)
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// case <-ctx.Done():
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// store.Cancel()
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// }
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data, err := store.Fetch(r.Context())
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if err != nil {
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return // todo log the error however you like
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}
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fmt.Fprint(w, data)
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}
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}
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@ -1,175 +0,0 @@
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package my_context
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import (
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"context"
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"errors"
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"log"
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"net/http"
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"net/http/httptest"
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"testing"
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"time"
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)
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// THIS WAS WHAT WE DID WHEN WE DIDN"T PROPOGATE CONTEXT
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// type SpyStore struct {
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// response string
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// cancelled bool
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// t *testing.T
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// }
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// func (s *SpyStore) assertWasCancelled() {
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// s.t.Helper()
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// if !s.cancelled {
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// s.t.Error("store was not told to cancel")
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// }
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// }
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// func (s *SpyStore) assertWasNotCancelled() {
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// s.t.Helper()
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// if s.cancelled {
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// s.t.Error("store was told to cancel")
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// }
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// }
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// func (s *SpyStore) Fetch() string {
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// time.Sleep(100 * time.Millisecond)
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// return s.response
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// }
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// func (s *SpyStore) Cancel() {
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// s.cancelled = true
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// }
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// func TestServer(t *testing.T) {
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// data := "Hello, Gopher"
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// t.Run("Happy path", func(t *testing.T) {
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// store := &SpyStore{response: data, t: t}
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// svr := Server(store)
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// request := httptest.NewRequest(http.MethodGet, "/", nil)
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// response := httptest.NewRecorder()
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// svr.ServeHTTP(response, request)
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// if response.Body.String() != data {
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// t.Errorf(`got "%s" want "%s"`, response.Body.String(), data)
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// }
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// store.assertWasNotCancelled()
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// })
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// t.Run("Should cancel", func(t *testing.T) {
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// store := &SpyStore{response: data, t: t}
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// svr := Server(store)
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// request := httptest.NewRequest(http.MethodGet, "/", nil)
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// // we are copying the parent context but with a new Done channel.
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// // The returned context's Done channel is closed when the returned cancel
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// // function is called or when the parent context's Done channel is closed,
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// // whichever happens first.
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// cancellingCtx, cancel := context.WithCancel(request.Context())
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// time.AfterFunc(5*time.Millisecond, cancel)
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// // changing the current request context to our new cancellingCtx
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// request = request.WithContext(cancellingCtx)
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// response := httptest.NewRecorder()
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// svr.ServeHTTP(response, request)
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// store.a
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// })
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// }
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type SpyStore struct {
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response string
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t *testing.T
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}
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func (s *SpyStore) Fetch(ctx context.Context) (string, error) {
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data := make(chan string, 1)
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go func() {
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var result string
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for _, c := range s.response {
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select {
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case <-ctx.Done():
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log.Println("spy store got cancelled")
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return
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default:
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// simulates some network traffic?
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time.Sleep(10 * time.Millisecond)
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result += string(c)
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}
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}
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data <- result
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}()
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select {
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case <-ctx.Done():
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return "", ctx.Err()
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case res := <-data:
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return res, nil
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}
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}
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// Rolling our own Spy for ResponseWriter because httptest.ResponseRecorder can't be
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// used to determine if any response was writing in the error case
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// SpyResponse Writer implements http.ResponseWriter so we can use it in the test.
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type SpyResponseWriter struct {
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written bool
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}
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func (s *SpyResponseWriter) Header() http.Header {
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s.written = true
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return nil
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}
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func (s *SpyResponseWriter) Write([]byte) (int, error) {
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s.written = true
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return 0, errors.New("not implemented")
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}
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func (s *SpyResponseWriter) WriteHeader(statusCode int) {
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s.written = true
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}
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func TestServer(t *testing.T) {
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data := "Hello, Gopher"
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t.Run("Happy Path", func(t *testing.T) {
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store := &SpyStore{response: data, t: t}
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svr := Server(store)
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request := httptest.NewRequest(http.MethodGet, "/", nil)
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response := httptest.NewRecorder()
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svr.ServeHTTP(response, request)
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if response.Body.String() != data {
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t.Errorf(`got "%s", want "%s"`, response.Body.String(), data)
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}
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})
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t.Run("tells store to cancel work if request is cancelled", func(t *testing.T) {
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store := &SpyStore{response: data, t: t}
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svr := Server(store)
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request := httptest.NewRequest(http.MethodGet, "/", nil)
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cancellingCtx, cancel := context.WithCancel(request.Context())
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time.AfterFunc(5*time.Millisecond, cancel)
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request = request.WithContext(cancellingCtx)
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response := &SpyResponseWriter{}
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svr.ServeHTTP(response, request)
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if response.written {
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t.Errorf("A response should not have been written")
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}
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})
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}
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