README.Rmd

---
output: github_document
---

<!-- README.md is generated from README.Rmd. Please edit that file. -->

```{r setup, include = FALSE, warning = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.path = "man/figures/README-",
  out.width = "100%"
)

library(log4r)
```

# log4r

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**log4r** is a fast, lightweight, object-oriented approach to logging in R based
on the widely-emulated [Apache Log4j](https://logging.apache.org/log4j/) project.

**log4r** differs from other R logging packages in its focus on performance and
simplicity. As such, it has fewer features -- although it is still quite
extensible, as seen below -- but is much faster. See
`vignette("performance", package = "log4r")` for details.

Unlike other R logging packages, **log4r** also has first-class support for
structured logging. See `vignette("structured-logging", package = "log4r")` for
details.

## Installation

The package is available from CRAN:

```r
install.packages("log4r")
```

If you want to use the development version, you can install the package from
GitHub as follows:

```r
# install.packages("remotes")
remotes::install_github("johnmyleswhite/log4r")
```

## Usage

Logging is configured by passing around `logger` objects created by `logger()`.
By default, this will log to the console and suppress messages below the
`"INFO"` level:

```{r basic-example}
logger <- logger()

log_info(logger, "Located nearest gas station.")
log_warn(logger, "Ez-Gas sensor network is not available.")
log_debug(logger, "Debug messages are suppressed by default.")
```

Logging destinations are controlled by **Appenders**, a few of which are
provided by the package. For instance, if we want to debug-level messages to a
file:

```{r file-example, echo = 1:7}
log_file <- tempfile()
logger <- logger("DEBUG", appenders = file_appender(log_file))

log_info(logger, "Messages are now written to the file instead.")
log_debug(logger, "Debug messages are now visible.")

readLines(log_file)
unlink(log_file)
```

The `appenders` parameter takes a list, so you can log to multiple destinations
transparently.

For local development or simple batch R scripts run manually, writing log
messages to a file for later inspection is convenient. However, for deployed R
applications or automated scripts it is more likely you will need to send logs
to a central location; see
`vignette("logging-beyond-local-files", package = "log4r")`.

To control the format of the messages you can change the **Layout** used by
each appender. Layouts are functions; you can write your own quite easily:

```{r layout-example}
my_layout <- function(level, ...) {
  paste0(format(Sys.time()), " [", level, "] ", ..., collapse = "")
}

logger <- logger(appenders = console_appender(my_layout))
log_info(logger, "Messages should now look a little different.")
```

With an appropriate layout, you can also use *structured logging*, enriching log
messages with contextual fields:

```{r sl-example}
logger <- logger(appenders = console_appender(logfmt_log_layout()))
log_info(
  logger, message = "processed entries", file = "catpics_01.csv",
  entries = 4124, elapsed = 2.311
)
```

## Older APIs

The 0.2 API is still supported, but will issue deprecation warnings when used:

```{r old-api, echo = 1:12}
logger <- create.logger()

logfile(logger) <- log_file
level(logger) <- "INFO"

debug(logger, 'A Debugging Message')
info(logger, 'An Info Message')
warn(logger, 'A Warning Message')
error(logger, 'An Error Message')
fatal(logger, 'A Fatal Error Message')

readLines(log_file)
unlink(log_file)
```

## License

The package is available under the terms of the Artistic License 2.0.