Structured Logging Extension (slog)¶

The ext/slog extension logs every HTTP request and response using Go's standard log/slog package, providing structured, queryable log entries for observability.

Installation¶

go get github.com/jhonsferg/relay/ext/slog

Quick Start¶

import relayslog "github.com/jhonsferg/relay/ext/slog"

client := relay.New(
    relay.WithBaseURL("https://api.example.com"),
    relayslog.Middleware(nil), // uses slog.Default()
)

Custom Logger¶

import (
    "log/slog"
    "os"
    relayslog "github.com/jhonsferg/relay/ext/slog"
)

logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
    Level: slog.LevelDebug,
}))

client := relay.New(
    relay.WithBaseURL("https://api.example.com"),
    relayslog.Middleware(logger),
)

Log Levels¶

Responses are logged at different levels based on HTTP status:

Status Range	Log Level	Example
2xx, 3xx	`INFO`	Successful responses
4xx	`WARN`	Client errors (not found, unauthorized)
5xx	`ERROR`	Server errors
Transport error	`ERROR`	Network failures, timeouts

Log Fields¶

Each log entry contains structured fields:

Field	Type	Description
`method`	string	HTTP method (GET, POST, ...)
`url`	string	Full request URL
`status_code`	int	HTTP response status code
`duration_ms`	float64	Request duration in milliseconds
`attempt`	int	Attempt number (1 for first, 2+ for retries)
`error`	string	Error message (only on transport errors)

Example Log Output¶

{"time":"2026-04-05T10:00:00Z","level":"INFO","msg":"HTTP request","method":"GET","url":"https://api.example.com/users","status_code":200,"duration_ms":45.2,"attempt":1}
{"time":"2026-04-05T10:00:01Z","level":"WARN","msg":"HTTP request","method":"GET","url":"https://api.example.com/users/999","status_code":404,"duration_ms":12.1,"attempt":1}
{"time":"2026-04-05T10:00:02Z","level":"ERROR","msg":"HTTP request","method":"POST","url":"https://api.example.com/data","status_code":500,"duration_ms":230.5,"attempt":3}

Retry Logging¶

When retries are configured, each attempt is logged with its attempt number:

client := relay.New(
    relay.WithBaseURL("https://api.example.com"),
    relay.WithRetry(&relay.RetryConfig{MaxAttempts: 3}),
    relayslog.Middleware(logger),
)

{"level":"ERROR","msg":"HTTP request","method":"GET","url":"/flaky","status_code":503,"attempt":1}
{"level":"ERROR","msg":"HTTP request","method":"GET","url":"/flaky","status_code":503,"attempt":2}
{"level":"INFO", "msg":"HTTP request","method":"GET","url":"/flaky","status_code":200,"attempt":3}

Filtering Log Level¶

Use slog.HandlerOptions to control which levels are emitted:

// Only log warnings and errors (skip successful requests)
logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
    Level: slog.LevelWarn,
}))