simplelog/README.md

# simplelog

## Summary

simplelog is an opinionated logging package designed to facilitate easy and
structured logging in Go applications with an absolute minimum of
boilerplate.

The idea is that you can add a single import line which replaces the
stdlib `log/slog` default handler, and solve the 90% case for logging.

## Current Status

Released v1.0.0 2024-06-14.  Works as intended.  No known bugs.

## Features

-   if output is a tty, outputs pretty color logs
-   if output is not a tty, outputs json
-   supports delivering each log message via a webhook
-   emits every `slog` attribute: those passed to a log call, those
    accumulated with `WithAttrs`, and those qualified by `WithGroup`.
    `slog.Group` values nest, and `slog.LogValuer` values are resolved. In
    json output attributes are object fields (groups become nested objects);
    in console output they are appended as `key=value` pairs, with grouped
    keys written as `group.key=value`. See
    [Attribute output](#attribute-output) for the details worth knowing
-   reports a record that could not be delivered as an error from the
    handler's `Handle` method. See
    [When delivery fails](#when-delivery-fails) for how to receive it

## Planned Features

-   supports delivering logs via tcp RELP (e.g. to remote rsyslog using imrelp)

## Installation

To use simplelog, first ensure your project is set up with Go modules:

```bash
go mod init your_project_name
```

Then, add SimpleLog to your project:

```bash
go get sneak.berlin/go/simplelog
```

## Usage

Below is an example of how to use SimpleLog in a Go application. This
example is provided in the form of a `main.go` file, which demonstrates
logging at various levels using structured logging syntax.

```go
package main

import (
	"log/slog"
	_ "sneak.berlin/go/simplelog"
)

func main() {

    // log structured data with slog as usual:
	slog.Info("User login attempt", slog.String("user", "JohnDoe"), slog.Int("attempt", 3))
	slog.Warn("Configuration mismatch", slog.String("expected", "config.json"), slog.String("found", "config.dev.json"))
	slog.Error("Failed to save data", slog.String("reason", "permission denied"))
}
```

## Attribute output

Attributes reach every handler: the ones passed to the log call, the ones
accumulated with `WithAttrs`, and the ones qualified by the groups open at
the time they were attached. `slog.Group` values nest, and
`slog.LogValuer` values are resolved to the value they stand for. A few
behaviours are worth knowing before you rely on them.

**Your values are never modified.** Whatever you log is read and rendered,
never written to. A map or a slice you pass to `slog.Any` comes back from
the logger exactly as you handed it over, even when a group later uses the
same key.

**Durations are nanoseconds in json, and readable on the console.** The
json and webhook payloads emit a `slog.Duration` as a number of
nanoseconds, matching `slog.NewJSONHandler`, so a consumer can compare and
aggregate the field without parsing it. The console line emits the same
duration as `3s`, matching `slog.NewTextHandler`, because a person reads
it.

**The json payload is an object, with the consequences an object has.**
The record's own fields are named `Time`, `Level`, `Message` and `PC`, and
they own those names: an attribute keyed after one of them is dropped from
the json and webhook output. A key logged more than once keeps its last
value there for the same reason, with one exception: when both are
`slog.Group` values sharing a key, the two groups are merged into one
object holding the members of both, rather than the second replacing the
first. Neither the collision nor the merge applies to the console output,
which is a line of text: every pair appears, in order. If you need a field
called `message`, pick a key that does not collide - the collision is
silent.

**Empty things follow the `slog.Handler` contract.** An empty `Attr` is
ignored, an empty group is elided along with its key, a group with an
empty key is inlined into its parent, and `WithGroup("")` returns the
handler unchanged.

## When delivery fails

Every handler returns an error from `Handle` when it could not deliver
the record:

-   `ConsoleHandler` and `JSONHandler` return the error from their write to
    stdout, wrapped, so `errors.Is` still matches the original
-   `WebhookHandler` returns an error when the request fails, and also when
    the server answers with a status outside 2xx, a redirect included,
    since it does not follow redirects. A request that has not finished
    after 5 seconds fails with a timeout error, so a webhook server that
    never answers holds up a log call for 5 seconds at most
-   `MultiplexHandler`, which simplelog installs as the default, passes the
    record to every handler it holds even after one of them fails, then
    returns all their errors joined with `errors.Join` (nil if none failed)

`slog.Info`, `slog.Error` and the other `slog.Logger` methods throw that
error away. That is how `log/slog` works, and simplelog cannot change it.
To find out whether a record was delivered, build a `slog.Record` and pass
it to the handler yourself, for example
`slog.Default().Handler().Handle(ctx, record)`, then check the error it
returns. Console output takes the file and line from the record's `PC`,
which you set with `runtime.Callers` as the `log/slog` documentation
shows. A record whose `PC` is zero prints `???:0` instead.

## Entrypoints

This repository adheres to the
[Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
standard: normalized scripts in `script/` are the entrypoints for the
development workflow, and the Makefile targets are thin shims that call them.
The scripts are POSIX sh (not bash) so they run in minimal containers such as
alpine. We provide:

- `script/bootstrap` — install all dependencies (go if missing, goimports with
  `go install` at a pinned commit unless the installed one already reports the
  pinned version, then `go mod download`; node through nvm at a pinned version
  if node is missing, yarn at a pinned version if it is missing, then the
  `prettier` that `package.json` and `yarn.lock` pin); golangci-lint is not
  installed, since it runs only in Docker
- `script/setup` — set up the repo for development after a fresh clone: runs
  `script/bootstrap`, then `script/install-precommit`
- `script/projectname` — output the project name (our own extension); used by
  `script/docker` for the image tag
- `script/test` — run the test suite under the race detector in Docker by
  building only the `test` stage of the `Dockerfile`, without the build cache,
  so every run tests; the image is tagged `simplelog-test`
- `script/lint` — run golangci-lint in Docker by building only the `lint`
  stage of the `Dockerfile`, without the build cache, so every run lints; the
  image is tagged `simplelog-lint`
- `script/fmt` — format all files (writes): Go with the goimports that
  `script/bootstrap` installs, then every `*.md` file with `prettier`
- `script/fmt-check` — check formatting (read-only); fails if `gofmt -l`
  reports files, or if `prettier` would change any `*.md` file
- `script/check` — run all checks: `test`, `lint`, `fmt-check` (our own
  extension)
- `script/docker` — build the Docker image without the build cache, tagged
  via `script/projectname` (byte-identical across repos); it also passes the
  output of `git describe` as the `VERSION` build argument, which is ignored
  because this `Dockerfile` does not declare it
- `script/cibuild` — what CI runs: `script/bootstrap`, then `script/check`,
  then the same image build as `script/docker` (byte-identical across repos)
- `script/precommit` — run by the git pre-commit hook (our own extension);
  runs a `go mod tidy` guard, then `script/check`
- `script/install-precommit` — installs the git pre-commit hook (our own
  extension); `make hooks` shims to it

`make hooks` installs the pre-commit hook that runs `script/precommit`.

## License

[WTFPL](./LICENSE)