Go

The justlog3 Go module writes log lines to a file from a background goroutine and, once you set an API token, sends them to JustLog3 Cloud. It uses only the standard library.

Install

go get github.com/OAzizjon/justlog3/go

Go 1.23 or newer. The package name is justlog3. Reference: pkg.go.dev.

Log to a file and the cloud

package main

import (
    "log"
    "os"

    justlog3 "github.com/OAzizjon/justlog3/go"
)

func main() {
    err := justlog3.SetAPIToken(os.Getenv("JUSTLOG3_TOKEN"),
        justlog3.WithURL("https://jl3-cloud.site/api/logs/"))
    if err != nil {
        log.Fatal(err)
    }
    defer justlog3.Shutdown() // flushes files and sends what is left

    logger, err := justlog3.NewLogger("app.log")
    if err != nil {
        log.Fatal(err)
    }
    logger.Log("Hi...")
    logger.Log("There is some error!", justlog3.Red())
    logger.Log("OK", justlog3.Green())
    logger.Log("There is critical error!", justlog3.Red(), justlog3.Prefix("CRITICAL!!"))
}

Always call Shutdown

Go has no exit hook. Without justlog3.Shutdown() (or logger.Flush() / logger.Close()) the last buffered lines are lost. log.Fatal and os.Exit skip deferred calls, so flush before them.

Levels

logger, _ := justlog3.NewBasicLogger("app.log", justlog3.WithConsoleLevel("INFO"))
logger.Debug("only in the file and cloud")
logger.Info("started")
logger.Successf("processed %d items", 42)
logger.Error("payment failed")

WithConsoleLevel only filters the console. Every level has an f variant (Infof, Errorf, ...).

How cloud sending works

• Each logger keeps a cloud buffer of up to 10,000 lines; the oldest lines are dropped on overflow.
• Batches go out when WithCloudCycles(n) lines are waiting (default 75) or every flush interval (default 5 s), at most WithMaxBatch(n) lines (default 100) and 512 KB of text each. Bodies from 1 KB are gzip-compressed.
• No answer or HTTP 5xx: the batch is resent unchanged with the same X-JustLog-Seq, so the server stores it once.
• HTTP 429: nothing is sent until Retry-After. HTTP 413: later batches carry half as many lines.
• HTTP 401 / 403: cloud sending stops and files keep being written. Call SetAPIToken again to resume.

Cloud options: WithURL(url), WithFlushInterval(d), WithTimeout(d), WithMaxBatch(n).

Logger options

Option	Default	Meaning
WithTime(bool)	true	timestamp in every line
WithFilemode("a" or "w")	"a"	"w" truncates the file once on creation
WithCycles(n)	50	lines per disk write
WithCloudCycles(n)	75	waiting lines that wake the cloud sender
WithConsoleLevel(level)	"DEBUG"	BasicLogger only

The file stays open and is reopened if it is renamed or deleted, so log rotation works, on Windows too.