# Logging Beyond Local Files

#### Updated as of 2024-10-12

For local development or simple batch R scripts run manually, writing log messages to a file for later inspection (with `file_appender`) is quite convenient. However, for deployed R applications (like Shiny apps and Plumber APIs) or automated scripts it is more likely that all an organization’s logs will be aggregated in one central place (perhaps with a commercial tool or service1) for searching and monitoring. It can be annoying or impossible to upload log files in these cases.

If your organization’s platform supports reading log messages from regular program output,2 you can just use the default setup, which uses the `console_appender()`. Otherwise, log4r includes three additional appenders to facilitate shipping logs off to an aggregator:

- `syslog_appender`: For writing messages to the system log on Linux, macOS, and other Unix-like operating systems.

- `http_appender`: For sending log messages as HTTP requests.

- `tcp_appender`: For writing log messages to TCP connections.

## Writing to the System Log

The Unix “System log” (syslog) dates to the mid-1980s, and is still widely used. Almost all log aggregation services support ingesting a server’s syslog messages, so often the easiest way to get your logs to these services is to make your R talk to the local syslog.

To use the `syslog_appender`, all you need is an identifier for your R app or script:

    logger <- logger(appenders = syslog_appender("my-R-script"))

Requires the rsyslog package.

## Sending Logs over HTTP

If you’re not already forwarding syslog messages (or need to send logs from Windows), the next most-common approach is to send them over HTTP. Log aggregation services usually provide an HTTP API endpoint to facilitate this:

    logger <- logger(appenders = http_appender("http://logging.example.local"))

Some services use `GET` or `PUT` requests instead of the more intuitive `POST`, which you can opt into as follows:

    logger <- logger(
      appenders = http_appender("http://logging.example.local", method = "GET")
    )

Finally, if you need complete control over the HTTP request (for example, to send a specific header or use authentication), you can pass additional parameters to the underlying httr verb function:

    logger <- logger(
      appenders = http_appender(
        "http://logging.example.local",
        method = "GET",
        layout = default_log_layout(),
        httr::add_headers(`X-Custom-Header` = 1),
        httr::user_agent("my-r-script/1.0.0")
      )
    )

Requires the httr package.

## Writing Directly to TCP Connections

For some workloads, the send-and-receive structure of HTTP requests may be undesirable, so many log aggregators also accept messages directly at a TCP port:

    logger <- logger(
      appenders = tcp_appender("tcp://logging.example.local", port = 9551)
    )

1. Such as Splunk or Loggly. There are also many open-source options, such as the ELK Stack or Graylog.↩︎

2. For example, when running as a Docker container in a cluster with all logs forwarded automatically, or when scripts are wrapped up as SystemD unit files.↩︎