# go-hclog
[][godocs]
[godocs]: https://godoc.org/github.com/hashicorp/go-hclog
`go-hclog` is a package for Go that provides a simple key/value logging
interface for use in development and production environments.
It provides logging levels that provide decreased output based upon the
desired amount of output, unlike the standard library `log` package.
It provides `Printf` style logging of values via `hclog.Fmt()`.
It provides a human readable output mode for use in development as well as
JSON output mode for production.
## Stability Note
This library has reached 1.0 stability. Its API can be considered solidified
and promised through future versions.
## Installation and Docs
Install using `go get github.com/hashicorp/go-hclog`.
Full documentation is available at
http://godoc.org/github.com/hashicorp/go-hclog
## Usage
### Use the global logger
```go
hclog.Default().Info("hello world")
```
```text
2017-07-05T16:15:55.167-0700 [INFO ] hello world
```
(Note timestamps are removed in future examples for brevity.)
### Create a new logger
```go
appLogger := hclog.New(&hclog.LoggerOptions{
Name: "my-app",
Level: hclog.LevelFromString("DEBUG"),
})
```
### Emit an Info level message with 2 key/value pairs
```go
input := "5.5"
_, err := strconv.ParseInt(input, 10, 32)
if err != nil {
appLogger.Info("Invalid input for ParseInt", "input", input, "error", err)
}
```
```text
... [INFO ] my-app: Invalid input for ParseInt: input=5.5 error="strconv.ParseInt: parsing "5.5": invalid syntax"
```
### Create a new Logger for a major subsystem
```go
subsystemLogger := appLogger.Named("transport")
subsystemLogger.Info("we are transporting something")
```
```text
... [INFO ] my-app.transport: we are transporting something
```
Notice that logs emitted by `subsystemLogger` contain `my-app.transport`,
reflecting both the application and subsystem names.
### Create a new Logger with fixed key/value pairs
Using `With()` will include a specific key-value pair in all messages emitted
by that logger.
```go
requestID := "5fb446b6-6eba-821d-df1b-cd7501b6a363"
requestLogger := subsystemLogger.With("request", requestID)
requestLogger.Info("we are transporting a request")
```
```text
... [INFO ] my-app.transport: we are transporting a request: request=5fb446b6-6eba-821d-df1b-cd7501b6a363
```
This allows sub Loggers to be context specific without having to thread that
into all the callers.
### Using `hclog.Fmt()`
```go
totalBandwidth := 200
appLogger.Info("total bandwidth exceeded", "bandwidth", hclog.Fmt("%d GB/s", totalBandwidth))
```
```text
... [INFO ] my-app: total bandwidth exceeded: bandwidth="200 GB/s"
```
### Use this with code that uses the standard library logger
If you want to use the standard library's `log.Logger` interface you can wrap
`hclog.Logger` by calling the `StandardLogger()` method. This allows you to use
it with the familiar `Println()`, `Printf()`, etc. For example:
```go
stdLogger := appLogger.StandardLogger(&hclog.StandardLoggerOptions{
InferLevels: true,
})
// Printf() is provided by stdlib log.Logger interface, not hclog.Logger
stdLogger.Printf("[DEBUG] %+v", stdLogger)
```
```text
... [DEBUG] my-app: &{mu:{state:0 sema:0} prefix: flag:0 out:0xc42000a0a0 buf:[]}
```
Alternatively, you may configure the system-wide logger:
```go
// log the standard logger from 'import "log"'
log.SetOutput(appLogger.StandardWriter(&hclog.StandardLoggerOptions{InferLevels: true}))
log.SetPrefix("")
log.SetFlags(0)
log.Printf("[DEBUG] %d", 42)
```
```text
... [DEBUG] my-app: 42
```
Notice that if `appLogger` is initialized with the `INFO` log level, _and_ you
specify `InferLevels: true`, you will not see any output here. You must change
`appLogger` to `DEBUG` to see output. See the docs for more information.
If the log lines start with a timestamp you can use the
`InferLevelsWithTimestamp` option to try and ignore them. Please note that in order
for `InferLevelsWithTimestamp` to be relevant, `InferLevels` must be set to `true`.