go-logger

go-logger is a logging library based on node-bunyan.

The output is compatible with the bunyan log reader application from that node package.

Usage

You first start by creating a Logger object that will operate on a Stream.

package main

import "github.com/gildas/go-logger"

var Log = logger.Create("myapp")

Then, you can log messages to the same levels from node-bunyan:

Log.Tracef("This is a message at the trace level for %s", myObject)
Log.Debugf("This is a message at the debug level for %s", myObject)
Log.Infof("This is a message at the trace level for %s", myObject)
Log.Warnf("This is a message at the warn level for %s", myObject)
Log.Errorf("This is a message at the error level for %s", myObject, err)
Log.Fatalf("This is a message at the fatal level for %s", myObject, err)

Note the err variable (must implement the error interface) used with the last two log calls. By just adding it to the list of arguments at Error or Fatal level while not mentioning it in the format string will tell the Logger to spit that error in a bunyan Record field.

More generally, Record fields can be logged like this:

Log.Record("myObject", myObject).Infof("Another message about my object")
Log.Record("?myObject", myObject).Infof("Another message about my object, show myObject even if it is nil")
Log.Recordf("myObject", "format %s %+v". myObject.ID(), myObject).Infof("This record uses a formatted value")

log := Log.Record("dynamic", func() any { return myObject.Callme() })

log.Infof("This is here")
log.Infof("That is there")

In the second example, the prefix ? tells the Logger to log the record if the value is nil, empty string, nil UUID, or implements IsNil() bool returning true. By default, the Logger will not log these values.

In the last example, the code myObject.Callme() will be executed each time log is used to write a message. This is used, as an example, to add a timestamp to the log's Record.

In addition to the Bunyan core fields, this library adds a couple of Record Fields:

• topic can be used for stuff like types or general topics (e.g.: "http")
• scope can be used to scope logging within a topic, like a func or a portion of code.

When the Logger is created its topic and scope are set to "main".

Here is a simple example how Record fields can be used with a type:

type Stuff struct {
    Field1 string
    Field2 int
    Logger *logger.Logger // So Stuff carries its own logger
}

func (s *Stuff) SetLogger(l *logger.Logger) {
    s.Logger = l.Topic("stuff").Scope("stuff")
}

func (s Stuff) DoSomething(other *OtherStuff) error {
    log := s.Logger.Scope("dosomething")

    log.Record("other", other).Infof("Need to do something")
    if err := someFunc(s, other); err != nil {
        log.Errorf("Something went wrong with other", err)
        return err
    }
    return nil
}

The call to Record(key, value) creates a new Logger object. So, they are like Russian dolls when it comes down to actually writing the log message to the output stream. In other words, Record objects are collected from their parent's Logger back to the original Logger.

For example:

var Log   = logger.Create("test")
var child = Log.Record("key1", "value1").Record("key2", "value2")

child will actually be something like Logger(Logger(Logger(Stream to stdout))). Though we added only 2 records.

Therefore, to optimize the number of Logger objects that are created, there are some convenience methods that can be used:

func (s stuff) DoSomethingElse(other *OtherStuff) {
    log := s.Logger.Child("new_topic", "new_scope", "id", other.ID(), "key1", "value1")

    log.Infof("I am logging this stuff")

    log.Records("key2", "value2", "key3", 12345).Warnf("Aouch that hurts!")
}

The Child method will create one Logger that has a Record containing a topic, a scope, 2 keys (id and key1) with their values.

The Records method will create one Logger that has 2 keys (key2 and key3) with their values.

For example, with these methods:

var Log    = logger.Create("test")
var child1 = Log.Child("topic", "scope", "key2", "value2", "key3", "value3")
var child2 = child1.Records("key2", "value21", "key4", "value4")

child1 will be something like Logger(Logger(Stream to stdout)). Though we added 2 records.
child2 will be something like Logger(Logger(Logger(Stream to stdout))). Though we added 1 record to the 2 records added previously.

Stream objects

A Stream is where the Logger actually writes its Record data.

When creating a Logger, you can specify the destination it will write to:

var Log = logger.Create("myapp", "file://path/to/myapp.log")
var Log = logger.Create("myapp", "/path/to/myapp.log")
var Log = logger.Create("myapp", "./localpath/to/myapp.log")
var Log = logger.Create("myapp", "stackdriver")
var Log = logger.Create("myapp", "gcp")
var Log = logger.Create("myapp", "/path/to/myapp.log", "stderr")
var Log = logger.Create("myapp", "nil")

The first three Logger objects will write to a file, the fourth to Google Stackdriver, the fifth to Google Cloud Platform (GCP), the sixth to a file and stderr, and the seventh to nowhere (i.e. logs do not get written at all).

By default, when creating the Logger with:

var Log = logger.Create("myapp")

The Logger will write to the standard output or the destination specified in the environment variable LOG_DESTINATION.

You can also create a Logger by passing it a Stream object (these are equivalent to the previous code):

var Log = logger.Create("myapp", &logger.FileStream{Path: "/path/to/myapp.log"})
var Log = logger.Create("myapp", &logger.StackDriverStream{})
var Log = logger.Create("myapp", &logger.NilStream{})
var Log = logger.Create("myapp", &logger.FileStream{Path: "/path/to/myapp.log"}, &logger.StderrStream{})

A few notes:

• the StackDriverStream needs a LogID parameter or the value of the environment variable GOOGLE_PROJECT_ID. (see Google's StackDriver documentation for the description of that parameter).
• NilStream is a Stream that does not write anything, all messages are lost.
• MultiStream is a Stream than can write to several streams.
• StdoutStream and FileStream are buffered by default. Data is written from every LOG_FLUSHFREQUENCY (default 5 minutes) or when the Record's Level is at least ERROR.
• Streams convert the Record to write via a Converter. The converter is set to a default value per Stream.

You can also create a Logger with a combination of destinations and streams, AND you can even add some records right away:

var Log = logger.Create("myapp",
    &logger.FileStream{Path: "/path/to/myapp.log"},
    "stackdriver",
    NewRecord().Set("key", "value"),
)

Setting the LevelSet

All Stream types, except NilStream and MultiStream can use a LevelSet. When set, Record objects that have a Level below the LevelSet are not written to the Stream. This allows to log only stuff above WARN for instance.

A LevelSet is a set of Level objects organized by topic and scope.

The LevelSet can be set via a string or the environment variable LOG_LEVEL:

• LOG_LEVEL=INFO
  will configure the LevelSet to INFO, which is the default if nothing is set;
• LOG_LEVEL=TRACE:{topic1};DEBUG
  will configure the LevelSet to DEBUG for everything and TRACE for the topic topic1 to TRACE (and all the scopes under that topic);
• LOG_LEVEL=INFO;DEBUG:{topic1:scope1,scope2}
  will configure the LevelSet to INFO for everything and to DEBUG for the topic topic1 and scopes scope1, scope2 to DEBUG (all the other scopes under that topic will be set to INFO);
• LOG_LEVEL=INFO;DEBUG:{topic1};TRACE:{topic2}
  will configure the LevelSet to INFO for everything, to DEBUG for the topic topic1, and to TRACE for the topic topic2 (and all the scopes under these topics);
• LOG_LEVEL=INFO;DEBUG:{:scope1}
  will configure the LevelSet to INFO for everything, to DEBUG for the scope topic1 (and all the topics containing that scope);
• The last setting of a topic supersedes the ones set before;
• If the environment variable DEBUG is set to 1, the default Level in the LevelSet is superseded with DEBUG.

At the moment, the LevelSet can be configured only for all Streamer of a Logger.

It is also possible to change the default Level by calling FilterMore()and FilterLess() methods on the Logger or any of its Streamer members. The former will log less data and the latter will log more data. We provide an example of how to use these in the examples folder using Unix signals.

log := logger.Create("myapp", &logger.StdoutStream{})
// We are filtering at INFO
log.FilterLess()
// We are now filtering at DEBUG

StackDriver Stream

If you plan to log to Google's StackDriver from a Google Cloud Kubernetes or a Google Cloud Instance, you do not need the StackDriver Stream and should use the Stdout Stream with the StackDriver Converter, since the standard output of your application will be captured automatically by Google to feed StackDriver:

var Log = logger.Create("myapp", "gcp") // "google" or "googlecloud" are valid aliases
var Log = logger.Create("myapp", &logger.StdoutStream{Converter: &logger.StackDriverConverter{}})

To be able to use the StackDriver Stream from outside Google Cloud, you have some configuration to do first.

On your workstation, you need to get the key filename:

1. Authenticate with Google Cloud

gcloud auth login

2. Create a Service Account (logger-account is just an example of a service account name)

gcloud iam service-acccount create logger-account

3. Associate the Service Account to the Project you want to use

gcloud projects add-iam-policy-binding my-logging-project \
  --member "serviceAccount:[email protected]" \
  --role "roles/logging.logWriter"

4. Retrieve the key filename

gcloud iam service-accounts keys create /path/to/key.json \
  --iam-account [email protected]

You can either set the GOOGLE_APPLICATION_CREDENTIAL and GOOGLE_PROJECT_ID environment variables with the path of the obtained key and Google Project ID or provide them to the StackDriver stream:

var log = logger.Create("myapp", &logger.StackDriverStream{})

var log = logger.Create("myapp", &logger.StackDriverStream{
    Parent:      "my-logging-project",
    KeyFilename: "/path/to/key.json",
})

Writing your own Stream

You can also write your own Stream by implementing the logger.Streamer interface and create the Logger like this:

var log = logger.Create("myapp", &MyStream{})

Logging Source Information

It is possible to log source information such as the source filename and code line, go package, and the caller func.

var Log = logger.Create("myapp", &logger.FileStream{Path: "/path/to/myapp.log", SourceInfo: true})

func MyFunc() {
  Log.Infof("I am Here")
}

Note: Since this feature can be expensive to compute, it is turned of by default.
To turn it on, you need to either specify the option in the Stream object, set the environment variable LOG_SOURCEINFO to true. It is also turned on if the environment variable DEBUG is true.

Timing your funcs

You can automatically log the duration of your func by calling them via the logger:

log.TimeFunc("message shown with the duration", func() {
  log.Info("I am here")
  // ... some stuff that takes time
  time.Sleep(12*time.Second)
})

The duration will logged in the msg record after the given message. It will also be added as a float value in the duration record.

There are 3 more variations for funcs that return an error, a value, an error and a value:

result := log.TimeFuncV("message shown with the duration", func() any {
  log.Info("I am here")
  // ... some stuff that takes time
  time.Sleep(12*time.Second)
  return 12
})

err := log.TimeFuncE("message shown with the duration", func() err {
  log.Info("I am here")
  // ... some stuff that takes time
  time.Sleep(12*time.Second)
  return errors.ArgumentMissing.With("path")
})

result, err := log.TimeFuncVE("message shown with the duration", func() (any, error) {
  log.Info("I am here")
  // ... some stuff that takes time
  time.Sleep(12*time.Second)
  return 12, errors.ArgumentInvalid.With("value", 12)
})

Miscellaneous

The following convenience methods can be used when creating a Logger from another one (received from arguments, for example):

var log = logger.CreateIfNil(OtherLogger, "myapp")

var log = logger.Create("myapp", OtherLogger)

If OtherLogger is nil, the new Logger will write to the NilStream().

var log = logger.Must(logger.FromContext(context))

Must can be used to create a Logger from a method that returns *Logger, error, if there is an error, Must will panic.

FromContext can be used to retrieve a Logger from a GO context. (This is used in the paragraph about HTTP Usage)

log.ToContext will store the Logger to the given GO context.

Redacting

The Logger can redact records as needed by simply implementing the logger.Redactable interface in the data that is logged.

For example:

type Customer {
  ID   uuid.UUID `json:"id"`
  Name string    `json:"name"`
}

// implements logger.Redactable
func (customer Customer) Redact() any {
  return Customer{customer.Name, logger.Redact(customer.ID)}
}

main() {
  // ...
  customer := Customer{uuid, "John Doe"}

  log.Record("customer", customer).Infof("Got a customer")
}

When redacting a field, you can also call logger.RedactWithHash which will redact the value with a string like: "REDACTED-<hash>" where <hash> is a SHA256 hash of the original value. This is useful when you want to redact a value but still want to be able to identify it in the logs. You can also change the prefix with logger.RedactWithPrefixedHash.

For Complex objects, you can also implement the logger.RedactableWithKeys interface:

type Customer {
  ID   uuid.UUID `json:"id"`
  Name string    `json:"name"`
}

// implements logger.RedactableWithKeys
func (customer Customer) Redact(keyToRedact ...string) any {
  redacted := customer
  for _, key := range keyToRedact {
    switch key {
    case "name":
      redacted.Name = logger.RedactWithHash(customer.Name)
    }
  }
}

main() {
  log.RecordWithKeysToRedact("customer", customer, "name").Infof("Got a customer")
}

You can also redact slices and maps of objects:

log.Record("users", logger.RedactSlice(users)).Infof("Got a list of users")
log.Record("users", logger.RedactMap(users)).Infof("Got a map of users")

You can also redact the log messages by providing regular expressions, called redactors. Whenever a redactor matches, its matched content is replaced with "REDACTED".

You can assign several redactors to a single logger:

r1, err := logger.NewRedactor("[0-9]{10}")
r2 := (logger.Redactor)(myregexp)
log := logger.Create("test", r1, r2)

You can also add redactors to a child logger (without modifying the parent logger):

r3 := logger.NewRedactor("[a-z]{8}")
log := parent.Child("topic", "scope", "record1", "value1", r3)

Note: Adding redactors to a logger WILL have a performance impact on your application as each regular expression will be matched against every single message produced by the logger. We advise you to use as few redactors as possible and contain them in child logger, so they have a minimal impact.

Converters

The Converter object is responsible for converting the Record, given to the Stream to write, to match other log viewers.

The default Converter is BunyanConverter so the bunyan log viewer can read the logs.

Here is a list of all the converters:

• BunyanConverter, the default converter (does nothing, actually),
• CloudWatchConverter produces logs that are nicer with AWS CloudWatch log viewer.
• PinoConverter produces logs that can be used by pino,
• StackDriverConverter produces logs that are nicer with Google StackDriver log viewer,

Note: When you use converters, their output will most probably not work anymore with bunyan. That means you cannot have both worlds in the same Streamer. In some situation, you can survive this by using several streamers, one converted, one not.

Writing your own Converter

You can also write your own Converter by implementing the logger.Converter interface:

type MyConverter struct {
  // ...
}

func (converter *MyConverter) Convert(record Record) Record {
    record["newvalue"] = true
    return record
}

var Log = logger.Create("myapp", &logger.StdoutStream{Converter: &MyConverter{}})

Standard Log Compatibility

To use a Logger with the standard go log library, you can simply call the AsStandardLog() method. You can optionally give a Level:

package main

import (
  "net/http"
  "github.com/gildas/go-logger"
)

func main() {
    log := logger.Create("myapp")

    server1 := http.Server{
      // extra http stuff
      ErrorLog: log.AsStandardLog()
    }

    server2 := http.Server{
      // extra http stuff
      ErrorLog: log.AsStandardLog(logger.WARN)
    }
}

You can also give an io.Writer to the standard log constructor:

package main

import (
  "log"
  "net/http"
  "github.com/gildas/go-logger"
)

func main() {
    mylog := logger.Create("myapp")

    server1 := http.Server{
      // extra http stuff
      ErrorLog: log.New(mylog.Writer(), "", 0),
    }

    server2 := http.Server{
      // extra http stuff
      ErrorLog: log.New(mylog.Writer(logger.WARN), "", 0),
    }
}

Since Writer() returns io.Writer, anything that uses that interface could, in theory, write to a Logger.

HTTP Usage

It is possible to pass Logger objects to http.Handler. When doing so, the Logger will automatically write the request identifier ("X-Request-Id" HTTP Header), remote host, user agent, when the request starts and when the request finishes along with its duration.

The request identifier is attached every time the log writes in a Record.

Here is an example:

package main

import (
  "net/http"
  "github.com/gildas/go-logger"
  "github.com/gorilla/mux"
)

func MyHandler() http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
      // Extracts the Logger from the request's context
      //  Note: Use logger.Must only when you know there is a Logger as it will panic otherwise
      log := logger.Must(logger.FromContext(r.Context()))

      log.Infof("Now we are logging inside this http Handler")
    })
}

func main() {
  log := logger.Create("myapp")
  router := mux.NewRouter()
  router.Methods("GET").Path("/").Handler(log.HttpHandler()(MyHandler()))
}

When the http request handler (MyHandler) starts, the following records are logged:

• reqid, contains the request Header X-Request-Id if present, or a random UUID
• path, contains the URL Path of the request
• remote, contains the remote address of the request
• The topic is set to "route" and the scope to the path of the request URL

You can choose another header than "X-Request-Id" to pass the request identifier with the HttpHandlerWithRequestIDHeader(header string) method.

When the http request handler (MyHandler) ends, the following additional records are logged:

• duration, contains the duration in seconds (float64) of the handler execution

Environment Variables

The Logger can be configured completely by environment variables if needed. These are:

• LOG_DESTINATION, default: StdoutStream
  The Streams to write logs to. It can be a comma-separated list (for a MultiStream)
• LOG_LEVEL, default: INFO
  The level to filter by default. If the environment DEBUG is set the default level is DEBUG
• LOG_CONVERTER, default: "bunyan"
  The default Converter to use
• LOG_FLUSHFREQUENCY, default: 5 minutes
  The default Flush Frequency for the streams that will be buffered
• LOG_OBFUSCATION_KEY, default: none
  The SSL public key to use when obfuscating if you want a reversible obfuscation
• GOOGLE_APPLICATION_CREDENTIALS
  The path to the credential file for the StackDriverStream
• GOOGLE_PROJECT_ID
  The Google Cloud Project ID for the StackDriverStream
• DEBUG, default: none
  If set to "1", this will set the default level to filter to DEBUG

You can also use a prefix for the environment variables. When you create a Logger, you can pass a prefix to the Create method. For example:

func main() {
    log := logger.Create("myapp", logger.EnvironmentPrefix("myapp_"))
    log.Infof("This is a message")
}

The logger will look for the environment variables myapp_LOG_DESTINATION, myapp_LOG_LEVEL, etc.

Thanks

Special thanks to @chakrit for his chakrit/go-bunyan that inspired me. In fact earlier versions were wrappers around his library.

Well, we would not be anywhere without the original work of @trentm and the original trentm/node-bunyan. Many, many thanks!