Context package to support context-aware logging

The new context package supports context-aware logging, integrating with
logrus. Several utilities are provided to associate http requests with a
context, ensuring that one can trace log messages all the way through a
context-aware call stack.

A full description of this functionality is available in doc.go.

Signed-off-by: Stephen J Day <stephen.day@docker.com>
This commit is contained in:
Stephen J Day 2015-02-05 13:56:50 -08:00
parent 13382e45ba
commit 4b14eef595
5 changed files with 675 additions and 0 deletions

76
context/doc.go Normal file
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// Package context provides several utilities for working with
// golang.org/x/net/context in http requests. Primarily, the focus is on
// logging relevent request information but this package is not limited to
// that purpose.
//
// Logging
//
// The most useful aspect of this package is GetLogger. This function takes
// any context.Context interface and returns the current logger from the
// context. Canonical usage looks like this:
//
// GetLogger(ctx).Infof("something interesting happened")
//
// GetLogger also takes optional key arguments. The keys will be looked up in
// the context and reported with the logger. The following example would
// return a logger that prints the version with each log message:
//
// ctx := context.Context(context.Background(), "version", version)
// GetLogger(ctx, "version").Infof("this log message has a version field")
//
// The above would print out a log message like this:
//
// INFO[0000] this log message has a version field version=v2.0.0-alpha.2.m
//
// When used with WithLogger, we gain the ability to decorate the context with
// loggers that have information from disparate parts of the call stack.
// Following from the version example, we can build a new context with the
// configured logger such that we always print the version field:
//
// ctx = WithLogger(ctx, GetLogger(ctx, "version"))
//
// Since the logger has been pushed to the context, we can now get the version
// field for free with our log messages. Future calls to GetLogger on the new
// context will have the version field:
//
// GetLogger(ctx).Infof("this log message has a version field")
//
// This becomes more powerful when we start stacking loggers. Let's say we
// have the version logger from above but also want a request id. Using the
// context above, in our request scoped function, we place another logger in
// the context:
//
// ctx = context.WithValue(ctx, "http.request.id", "unique id") // called when building request context
// ctx = WithLogger(ctx, GetLogger(ctx, "http.request.id"))
//
// When GetLogger is called on the new context, "http.request.id" will be
// included as a logger field, along with the original "version" field:
//
// INFO[0000] this log message has a version field http.request.id=unique id version=v2.0.0-alpha.2.m
//
// Note that this only affects the new context, the previous context, with the
// version field, can be used independently. Put another way, the new logger,
// added to the request context, is unique to that context and can have
// request scoped varaibles.
//
// HTTP Requests
//
// This package also contains several methods for working with http requests.
// The concepts are very similar to those described above. We simply place the
// request in the context using WithRequest. This makes the request variables
// available. GetRequestLogger can then be called to get request specific
// variables in a log line:
//
// ctx = WithRequest(ctx, req)
// GetRequestLogger(ctx).Infof("request variables")
//
// Like above, if we want to include the request data in all log messages in
// the context, we push the logger to a new context and use that one:
//
// ctx = WithLogger(ctx, GetRequestLogger(ctx))
//
// The concept is fairly powerful and ensures that calls throughout the stack
// can be traced in log messages. Using the fields like "http.request.id", one
// can analyze call flow for a particular request with a simple grep of the
// logs.
package context

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package context
import (
"errors"
"net/http"
"strings"
"sync"
"time"
"code.google.com/p/go-uuid/uuid"
"github.com/gorilla/mux"
"golang.org/x/net/context"
)
// Common errors used with this package.
var (
ErrNoRequestContext = errors.New("no http request in context")
)
// WithRequest places the request on the context. The context of the request
// is assigned a unique id, available at "http.request.id". The request itself
// is available at "http.request". Other common attributes are available under
// the prefix "http.request.". If a request is already present on the context,
// this method will panic.
func WithRequest(ctx context.Context, r *http.Request) context.Context {
if ctx.Value("http.request") != nil {
// NOTE(stevvooe): This needs to be considered a programming error. It
// is unlikely that we'd want to have more than one request in
// context.
panic("only one request per context")
}
return &httpRequestContext{
Context: ctx,
startedAt: time.Now(),
id: uuid.New(), // assign the request a unique.
r: r,
}
}
// GetRequest returns the http request in the given context. Returns
// ErrNoRequestContext if the context does not have an http request associated
// with it.
func GetRequest(ctx context.Context) (*http.Request, error) {
if r, ok := ctx.Value("http.request").(*http.Request); r != nil && ok {
return r, nil
}
return nil, ErrNoRequestContext
}
// GetRequestID attempts to resolve the current request id, if possible. An
// error is return if it is not available on the context.
func GetRequestID(ctx context.Context) string {
return GetStringValue(ctx, "http.request.id")
}
// WithResponseWriter returns a new context and response writer that makes
// interesting response statistics available within the context.
func WithResponseWriter(ctx context.Context, w http.ResponseWriter) (context.Context, http.ResponseWriter) {
irw := &instrumentedResponseWriter{
ResponseWriter: w,
Context: ctx,
}
return irw, irw
}
// getVarsFromRequest let's us change request vars implementation for testing
// and maybe future changes.
var getVarsFromRequest = mux.Vars
// WithVars extracts gorilla/mux vars and makes them available on the returned
// context. Variables are available at keys with the prefix "vars.". For
// example, if looking for the variable "name", it can be accessed as
// "vars.name". Implementations that are accessing values need not know that
// the underlying context is implemented with gorilla/mux vars.
func WithVars(ctx context.Context, r *http.Request) context.Context {
return &muxVarsContext{
Context: ctx,
vars: getVarsFromRequest(r),
}
}
// GetRequestLogger returns a logger that contains fields from the request in
// the current context. If the request is not available in the context, no
// fields will display. Request loggers can safely be pushed onto the context.
func GetRequestLogger(ctx context.Context) Logger {
return GetLogger(ctx,
"http.request.id",
"http.request.method",
"http.request.host",
"http.request.uri",
"http.request.referer",
"http.request.useragent",
"http.request.remoteaddr",
"http.request.contenttype")
}
// GetResponseLogger reads the current response stats and builds a logger.
// Because the values are read at call time, pushing a logger returned from
// this function on the context will lead to missing or invalid data. Only
// call this at the end of a request, after the response has been written.
func GetResponseLogger(ctx context.Context) Logger {
l := getLogrusLogger(ctx,
"http.response.written",
"http.response.status",
"http.response.contenttype")
duration := Since(ctx, "http.request.startedat")
if duration > 0 {
l = l.WithField("http.response.duration", duration)
}
return l
}
// httpRequestContext makes information about a request available to context.
type httpRequestContext struct {
context.Context
startedAt time.Time
id string
r *http.Request
}
// Value returns a keyed element of the request for use in the context. To get
// the request itself, query "request". For other components, access them as
// "request.<component>". For example, r.RequestURI
func (ctx *httpRequestContext) Value(key interface{}) interface{} {
if keyStr, ok := key.(string); ok {
if keyStr == "http.request" {
return ctx.r
}
parts := strings.Split(keyStr, ".")
if len(parts) != 3 {
goto fallback
}
switch parts[2] {
case "uri":
return ctx.r.RequestURI
case "remoteaddr":
return ctx.r.RemoteAddr
case "method":
return ctx.r.Method
case "host":
return ctx.r.Host
case "referer":
referer := ctx.r.Referer()
if referer != "" {
return referer
}
case "useragent":
return ctx.r.UserAgent()
case "id":
return ctx.id
case "startedat":
return ctx.startedAt
case "contenttype":
ct := ctx.r.Header.Get("Content-Type")
if ct != "" {
return ct
}
}
}
fallback:
return ctx.Context.Value(key)
}
type muxVarsContext struct {
context.Context
vars map[string]string
}
func (ctx *muxVarsContext) Value(key interface{}) interface{} {
if keyStr, ok := key.(string); ok {
if keyStr == "vars" {
return ctx.vars
}
if strings.HasPrefix(keyStr, "vars.") {
keyStr = strings.TrimPrefix(keyStr, "vars.")
}
if v, ok := ctx.vars[keyStr]; ok {
return v
}
}
return ctx.Context.Value(key)
}
// instrumentedResponseWriter provides response writer information in a
// context.
type instrumentedResponseWriter struct {
http.ResponseWriter
context.Context
mu sync.Mutex
status int
written int64
}
func (irw *instrumentedResponseWriter) Write(p []byte) (n int, err error) {
n, err = irw.ResponseWriter.Write(p)
irw.mu.Lock()
irw.written += int64(n)
// Guess the likely status if not set.
if irw.status == 0 {
irw.status = http.StatusOK
}
irw.mu.Unlock()
return
}
func (irw *instrumentedResponseWriter) WriteHeader(status int) {
irw.ResponseWriter.WriteHeader(status)
irw.mu.Lock()
irw.status = status
irw.mu.Unlock()
}
func (irw *instrumentedResponseWriter) Flush() {
if flusher, ok := irw.ResponseWriter.(http.Flusher); ok {
flusher.Flush()
}
}
func (irw *instrumentedResponseWriter) Value(key interface{}) interface{} {
if keyStr, ok := key.(string); ok {
if keyStr == "http.response" {
return irw.ResponseWriter
}
parts := strings.Split(keyStr, ".")
if len(parts) != 3 {
goto fallback
}
irw.mu.Lock()
defer irw.mu.Unlock()
switch parts[2] {
case "written":
return irw.written
case "status":
if irw.status != 0 {
return irw.status
}
case "contenttype":
contentType := irw.Header().Get("Content-Type")
if contentType != "" {
return contentType
}
}
}
fallback:
return irw.Context.Value(key)
}

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package context
import (
"net/http"
"reflect"
"testing"
"time"
"golang.org/x/net/context"
)
func TestWithRequest(t *testing.T) {
var req http.Request
start := time.Now()
req.Method = "GET"
req.Host = "example.com"
req.RequestURI = "/test-test"
req.Header = make(http.Header)
req.Header.Set("Referer", "foo.com/referer")
req.Header.Set("User-Agent", "test/0.1")
ctx := WithRequest(context.Background(), &req)
for _, testcase := range []struct {
key string
expected interface{}
}{
{
key: "http.request",
expected: &req,
},
{
key: "http.request.id",
},
{
key: "http.request.method",
expected: req.Method,
},
{
key: "http.request.host",
expected: req.Host,
},
{
key: "http.request.uri",
expected: req.RequestURI,
},
{
key: "http.request.referer",
expected: req.Referer(),
},
{
key: "http.request.useragent",
expected: req.UserAgent(),
},
{
key: "http.request.remoteaddr",
expected: req.RemoteAddr,
},
{
key: "http.request.startedat",
},
} {
v := ctx.Value(testcase.key)
if v == nil {
t.Fatalf("value not found for %q", testcase.key)
}
if testcase.expected != nil && v != testcase.expected {
t.Fatalf("%s: %v != %v", testcase.key, v, testcase.expected)
}
// Key specific checks!
switch testcase.key {
case "http.request.id":
if _, ok := v.(string); !ok {
t.Fatalf("request id not a string: %v", v)
}
case "http.request.startedat":
vt, ok := v.(time.Time)
if !ok {
t.Fatalf("value not a time: %v", v)
}
now := time.Now()
if vt.After(now) {
t.Fatalf("time generated too late: %v > %v", vt, now)
}
if vt.Before(start) {
t.Fatalf("time generated too early: %v < %v", vt, start)
}
}
}
}
type testResponseWriter struct {
flushed bool
status int
written int64
header http.Header
}
func (trw *testResponseWriter) Header() http.Header {
if trw.header == nil {
trw.header = make(http.Header)
}
return trw.header
}
func (trw *testResponseWriter) Write(p []byte) (n int, err error) {
if trw.status == 0 {
trw.status = http.StatusOK
}
n = len(p)
trw.written += int64(n)
return
}
func (trw *testResponseWriter) WriteHeader(status int) {
trw.status = status
}
func (trw *testResponseWriter) Flush() {
trw.flushed = true
}
func TestWithResponseWriter(t *testing.T) {
trw := testResponseWriter{}
ctx, rw := WithResponseWriter(context.Background(), &trw)
if ctx.Value("http.response") != &trw {
t.Fatalf("response not available in context: %v != %v", ctx.Value("http.response"), &trw)
}
if n, err := rw.Write(make([]byte, 1024)); err != nil {
t.Fatalf("unexpected error writing: %v", err)
} else if n != 1024 {
t.Fatalf("unexpected number of bytes written: %v != %v", n, 1024)
}
if ctx.Value("http.response.status") != http.StatusOK {
t.Fatalf("unexpected response status in context: %v != %v", ctx.Value("http.response.status"), http.StatusOK)
}
if ctx.Value("http.response.written") != int64(1024) {
t.Fatalf("unexpected number reported bytes written: %v != %v", ctx.Value("http.response.written"), 1024)
}
// Make sure flush propagates
rw.(http.Flusher).Flush()
if !trw.flushed {
t.Fatalf("response writer not flushed")
}
// Write another status and make sure context is correct. This normally
// wouldn't work except for in this contrived testcase.
rw.WriteHeader(http.StatusBadRequest)
if ctx.Value("http.response.status") != http.StatusBadRequest {
t.Fatalf("unexpected response status in context: %v != %v", ctx.Value("http.response.status"), http.StatusBadRequest)
}
}
func TestWithVars(t *testing.T) {
var req http.Request
vars := map[string]string{
"foo": "asdf",
"bar": "qwer",
}
getVarsFromRequest = func(r *http.Request) map[string]string {
if r != &req {
t.Fatalf("unexpected request: %v != %v", r, req)
}
return vars
}
ctx := WithVars(context.Background(), &req)
for _, testcase := range []struct {
key string
expected interface{}
}{
{
key: "vars",
expected: vars,
},
{
key: "vars.foo",
expected: "asdf",
},
{
key: "vars.bar",
expected: "qwer",
},
} {
v := ctx.Value(testcase.key)
if !reflect.DeepEqual(v, testcase.expected) {
t.Fatalf("%q: %v != %v", testcase.key, v, testcase.expected)
}
}
}

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package context
import (
"fmt"
"github.com/Sirupsen/logrus"
"golang.org/x/net/context"
)
// Logger provides a leveled-logging interface.
type Logger interface {
// standard logger methods
Print(args ...interface{})
Printf(format string, args ...interface{})
Println(args ...interface{})
Fatal(args ...interface{})
Fatalf(format string, args ...interface{})
Fatalln(args ...interface{})
Panic(args ...interface{})
Panicf(format string, args ...interface{})
Panicln(args ...interface{})
// Leveled methods, from logrus
Debug(args ...interface{})
Debugf(format string, args ...interface{})
Debugln(args ...interface{})
Error(args ...interface{})
Errorf(format string, args ...interface{})
Errorln(args ...interface{})
Info(args ...interface{})
Infof(format string, args ...interface{})
Infoln(args ...interface{})
Warn(args ...interface{})
Warnf(format string, args ...interface{})
Warnln(args ...interface{})
}
// WithLogger creates a new context with provided logger.
func WithLogger(ctx context.Context, logger Logger) context.Context {
return context.WithValue(ctx, "logger", logger)
}
// GetLogger returns the logger from the current context, if present. If one
// or more keys are provided, they will be resolved on the context and
// included in the logger. While context.Value takes an interface, any key
// argument passed to GetLogger will be passed to fmt.Sprint when expanded as
// a logging key field. If context keys are integer constants, for example,
// its recommended that a String method is implemented.
func GetLogger(ctx context.Context, keys ...interface{}) Logger {
return getLogrusLogger(ctx, keys...)
}
// GetLogrusLogger returns the logrus logger for the context. If one more keys
// are provided, they will be resolved on the context and included in the
// logger. Only use this function if specific logrus functionality is
// required.
func getLogrusLogger(ctx context.Context, keys ...interface{}) *logrus.Entry {
var logger *logrus.Entry
// Get a logger, if it is present.
loggerInterface := ctx.Value("logger")
if loggerInterface != nil {
if lgr, ok := loggerInterface.(*logrus.Entry); ok {
logger = lgr
}
}
if logger == nil {
// If no logger is found, just return the standard logger.
logger = logrus.NewEntry(logrus.StandardLogger())
}
fields := logrus.Fields{}
for _, key := range keys {
v := ctx.Value(key)
if v != nil {
fields[fmt.Sprint(key)] = v
}
}
return logger.WithFields(fields)
}

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package context
import (
"time"
"golang.org/x/net/context"
)
// Since looks up key, which should be a time.Time, and returns the duration
// since that time. If the key is not found, the value returned will be zero.
// This is helpful when inferring metrics related to context execution times.
func Since(ctx context.Context, key interface{}) time.Duration {
startedAtI := ctx.Value(key)
if startedAtI != nil {
if startedAt, ok := startedAtI.(time.Time); ok {
return time.Since(startedAt)
}
}
return 0
}
// GetStringValue returns a string value from the context. The empty string
// will be returned if not found.
func GetStringValue(ctx context.Context, key string) (value string) {
stringi := ctx.Value(key)
if stringi != nil {
if valuev, ok := stringi.(string); ok {
value = valuev
}
}
return value
}