Append/Overwrite with Rotation

This example demonstrates how different write modes interact with file rotation in Logly.zig.

Write Modes

Mode	Behavior	Use Case
.append	Append to existing file	Persistent logs, audit trails
.overwrite	Truncate on startup	Session logs, debug output
.append_rotate	Append with rotation trigger	Explicit rotation control

Code Example

const std = @import("std");
const logly = @import("logly");

pub fn main() !void {
    var gpa = std.heap.DebugAllocator(.{}){};
    defer _ = gpa.deinit();
    const allocator = gpa.allocator();

    const logger = try logly.Logger.initWithConfig(allocator, logly.Config.default());
    defer logger.deinit();

    // Append mode with daily rotation
    var sink_append = logly.SinkConfig.file("app.log");
    sink_append.write_mode = .append;
    sink_append.rotation = "daily";
    sink_append.retention = 7;
    _ = try logger.addSink(sink_append);

    // Overwrite mode with size rotation
    var sink_overwrite = logly.SinkConfig.file("session.log");
    sink_overwrite.write_mode = .overwrite;
    sink_overwrite.size_limit = 1024 * 1024; // 1MB
    sink_overwrite.retention = 3;
    _ = try logger.addSink(sink_overwrite);

    try logger.info("Logged to both sinks", @src());
}

How Rotation Works

After rotation, the old file is renamed (e.g., app.log.2026-08-05) and a new fresh file is created. This behavior is consistent across all write modes.

Write Mode	Initial Behavior	After Rotation
.append	Append to existing file	New file starts fresh
.overwrite	Truncate file on startup	New file starts fresh
.append_rotate	Append with rotation trigger	New file starts fresh

Expected Output

app.log              - Append mode, grows over time
app.log.2026-08-05   - Rotated daily, archived
session.log          - Overwrite mode, fresh each run

See Also

• Write Modes Guide - Full write mode documentation
• Rotation Guide - Rotation configuration
• Sink Configuration - All sink options