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+journaldriver
+=============
+
+This is a small daemon used to forward logs from `journald` (systemd's
+logging service) to [Stackdriver Logging][].
+
+Many existing log services are written in inefficient dynamic
+languages with error-prone "cover every possible use-case"
+configuration. `journaldriver` instead aims to fit a specific use-case
+very well, instead of covering every possible logging setup.
+
+`journaldriver` can be run on GCP-instances with no additional
+configuration as authentication tokens are retrieved from the
+[metadata server][].
+
+<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-refresh-toc -->
+**Table of Contents**
+
+- [Features](#features)
+- [Usage on Google Cloud Platform](#usage-on-google-cloud-platform)
+- [Usage outside of Google Cloud Platform](#usage-outside-of-google-cloud-platform)
+- [Log levels / severities / priorities](#log-levels--severities--priorities)
+- [NixOS module](#nixos-module)
+- [Stackdriver Error Reporting](#stackdriver-error-reporting)
+
+<!-- markdown-toc end -->
+
+# Features
+
+* `journaldriver` persists the last forwarded position in the journal
+  and will resume forwarding at the same position after a restart
+* `journaldriver` will recognise log entries in JSON format and
+  forward them appropriately to make structured log entries available
+  in Stackdriver
+* `journaldriver` can be used outside of GCP by configuring static
+  credentials
+* `journaldriver` will recognise journald's log priority levels and
+  convert them into equivalent Stackdriver log severity levels
+
+# Usage on Google Cloud Platform
+
+`journaldriver` does not require any configuration when running on GCP
+instances.
+
+1. Install `journaldriver` on the instance from which you wish to
+   forward logs.
+
+2. Ensure that the instance has the appropriate permissions to write
+   to Stackdriver. Google continously changes how IAM is implemented
+   on GCP, so you will have to refer to [Google's documentation][].
+
+   By default instances have the required permissions if Stackdriver
+   Logging support is enabled in the project.
+
+3. Start `journaldriver`, for example via `systemd`.
+
+# Usage outside of Google Cloud Platform
+
+When running outside of GCP, the following extra steps need to be
+performed:
+
+1. Create a Google Cloud Platform service account with the "Log
+   Writer" role and download its private key in JSON-format.
+2. When starting `journaldriver`, configure the following environment
+   variables:
+
+   * `GOOGLE_CLOUD_PROJECT`: Name of the GCP project to which logs
+     should be written.
+   * `GOOGLE_APPLICATION_CREDENTIALS`: Filesystem path to the
+     JSON-file containing the service account's private key.
+   * `LOG_STREAM`: Name of the target log stream in Stackdriver Logging.
+     This will be automatically created if it does not yet exist.
+   * `LOG_NAME`: Name of the target log to write to. This defaults to
+     `journaldriver` if unset, but it is recommended to - for
+     example - set it to the machine hostname.
+
+# Log levels / severities / priorities
+
+`journaldriver` recognises [journald's priorities][] and converts them
+into [equivalent severities][] in Stackdriver. Both sets of values
+correspond to standard `syslog` priorities.
+
+The easiest way to emit log messages with priorites from an
+application is to use [priority prefixes][], which are compatible with
+structured log messages.
+
+For example, to emit a simple warning message (structured and
+unstructured):
+
+```
+$ echo '<4>{"fnord":true, "msg":"structured log (warning)"}' | systemd-cat
+$ echo '<4>unstructured log (warning)' | systemd-cat
+```
+
+# NixOS module
+
+The NixOS package repository [contains a module][] for setting up
+`journaldriver` on NixOS machines. NixOS by default uses `systemd` for
+service management and `journald` for logging, which means that log
+output from most services will be captured automatically.
+
+On a GCP instance the only required option is this:
+
+```nix
+services.journaldriver.enable = true;
+```
+
+When running outside of GCP, the configuration looks as follows:
+
+```nix
+services.journaldriver = {
+  enable                 = true;
+  logStream              = "prod-environment";
+  logName                = "hostname";
+  googleCloudProject     = "gcp-project-name";
+  applicationCredentials = keyFile;
+};
+```
+
+**Note**: The `journaldriver`-module is included in stable releases of
+NixOS since NixOS 18.09.
+
+# Stackdriver Error Reporting
+
+The [Stackdriver Error Reporting][] service of Google's monitoring
+toolbox supports automatically detecting and correlating errors from
+log entries.
+
+To use this functionality log messages must be logged in the expected
+[log format][].
+
+*Note*: Reporting errors from non-GCP instances requires that the
+`LOG_STREAM` environment variable is set to the special value
+`global`.
+
+This value changes the monitored resource descriptor from a log stream
+to the project-global stream. Due to a limitation in Stackdriver Error
+Reporting, this is the only way to correctly ingest errors from
+non-GCP machines. Please see [issue #4][] for more information about
+this.
+
+[Stackdriver Logging]: https://cloud.google.com/logging/
+[metadata server]: https://cloud.google.com/compute/docs/storing-retrieving-metadata
+[Google's documentation]: https://cloud.google.com/logging/docs/access-control
+[NixOS]: https://nixos.org/
+[contains a module]: https://github.com/NixOS/nixpkgs/pull/42134
+[journald's priorities]: http://0pointer.de/public/systemd-man/sd-daemon.html
+[equivalent severities]: https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#logseverity
+[priority prefixes]: http://0pointer.de/public/systemd-man/sd-daemon.html
+[Stackdriver Error Reporting]: https://cloud.google.com/error-reporting/
+[log format]: https://cloud.google.com/error-reporting/docs/formatting-error-messages
+[issue #4]: https://github.com/tazjin/journaldriver/issues/4