This stream will output log records to a log file. It has many options.

The log file is specified in the constructor of the file stream:

import { FileStream } from "https://deno.land/x/optic/streams/fileStream/mod.ts";
const fs = new FileStream("/data/logs/logFile.txt");

By default, the file stream will log messages at or above Debug. You may change this by programmatically setting the minimum log level:

const fs = new FileStream("./logFile.txt")
  .withMinLogLevel(Level.Error);

The file stream takes in any of Optic's formatters. See the full documentation on formatting for details.

File system writes are very slow compared to module execution. To improve log write performance, log records are buffered (held in memory) until a threshold is reached at which point the buffered log records are 'flushed' to the log file in one go. This single larger file system access is much more efficient than many smaller writes, resulting in greater module performance.

The default buffer size is 8,192 bytes. This value can be set programmatically if you want a different buffer size:

const fileStream = new FileStream("./logFile.txt")
  .withBufferSize(30000); // in bytes

Buffering can also be disabled if desired, thereby forcing immediate writes of every log record to the log file, by setting the buffer size to 0 bytes.

The buffer is flushed in the following scenarios:

• The new log record, when added to the buffer, would cause the buffer to exceed the max buffer size
• A log record with a log level greater than Error is logged
• The module exits normally (triggering a flush of the buffer via an unload event)
• flush() is called manually. (e.g. fileStream.flush())

You can also setup an auto-flush interval which will flush the buffer on a regular basis. Example:

import { FileStream } from "https://deno.land/x/optic/streams/fileStream/mod.ts";
import { intervalOf } from "https://deno.land/x/optic/utils/timeInterval.ts";

const fileStream = new FileStream("./logFile.txt")
  .withAutoFlushEvery(intervalOf(5).seconds());

which will flush the logs to the filesystem every 5 seconds.

A log file may already exist when your logger initializes. You can control what happens during logging initialization by specifying the desired behavior of the log file initialization mode. There are 3 options:

The log file initialization option is specified via:

const fileStream = new FileStream("./logFile.txt")
  .withLogFileInitMode("append");

By default, the file stream uses a singe log file and will keep appending indefinitely. You may wish to employ a log rotation strategy. This will automatically move the current log file to a backup file and start a fresh one, preventing any individual log file from getting to big. It can also help organize your log files better. There are two rotation strategies:

• File size rotation - When the log file would grow beyond a specified size, then the file is rotated.
• Date/Time rotation - Rotates after a specified time has passed

If a log file write would grow the file bigger than the specified size, then the log file is rotated first. As an example, let's say you have a log file mod.log. Upon its first rotation, mod.log becomes mod.log.1 and the log record(s) are written to a new, empty, mod.log. When mod.log is ready to rotate again, mod.log.1 becomes mod.log.2, mod.log becomes mod.log.1 and the new record(s) are written to a new, empty, mod.log.

Example:

import {
  every,
  FileStream,
} from "https://deno.land/x/optic/streams/fileStream/mod.ts";

const fileStream = new FileStream("./logFile.txt")
  .withLogFileRotation(every(2000000).bytes());
// Or you can specify by kb, mb or gb
// .withLogFileRotation(every(5000).kb());
// .withLogFileRotation(every(200).mb());
// .withLogFileRotation(every(3).gb());

With this rotation strategy, files are rotated after a fixed period of days, hours or minutes. Rotated log files are appended with the date and, for minutes or hours based rotation strategies, the time as well. E.g. mod.log.2020.05.25 or mod.log_2020.05.25_15.33.

Example:

import {
  every,
  FileStream,
} from "https://deno.land/x/optic/streams/fileStream/mod.ts";

const fileStream = new FileStream("./logFile.txt")
  .withLogFileRotation(every(2).days());

By default, a maximum of 7 log files are kept. A custom log file retention policy may also be specified, defining either the maximum number of log files to keep or for how long to keep them. Any log file which falls outside the retention policy is deleted.

As log file retention only makes sense for rotated log files, log file retention policy is an attribute of the log rotation strategy and may be specified as a quantity of files, minutes, hours or days. Retention policy is enforced at stream initialization and on log rotation events.

Examples:

import {
  every,
  FileStream,
  of,
} from "https://deno.land/x/optic/streams/fileStream/mod.ts";

// Retain up to 7 log files maximum
const fixedNumberLogFileRetention = new FileStream("./logFile.txt")
  .withLogFileRotation(
    every(200).mb()
      .withLogFileRetentionPolicy(of(7).files()),
  );

// Retain logs files for maximum of 36 hours
const fixedTimeframeLogFileRetention = new FileStream("./logFile.txt")
  .withLogFileRotation(
    every(500).kb()
      .withLogFileRetentionPolicy(of(36).hours()),
  );