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72 lines
3.6 KiB
Markdown
72 lines
3.6 KiB
Markdown
# File Appender
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The file appender writes log events to a file. It supports an optional maximum file size, and will keep a configurable number of backups. When using the file appender, you should also call `log4js.shutdown` when your application terminates, to ensure that any remaining asynchronous writes have finished. Although the file appender uses the [streamroller](https://github.com/nomiddlename/streamroller) library, this is included as a dependency of log4js so you do not need to include it yourself.
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## Configuration
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* `type` - `"file"`
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* `filename` - `string` - the path of the file where you want your logs written.
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* `maxLogSize` - `integer` (optional) - the maximum size (in bytes) for the log file. If not specified, then no log rolling will happen.
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* `backups` - `integer` (optional, default value = 5) - the number of old log files to keep during log rolling.
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* `layout` - (optional, defaults to basic layout) - see [layouts](layouts.md)
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Any other configuration parameters will be passed to the underlying [streamroller](https://github.com/nomiddlename/streamroller) implementation (see also node.js core file streams):
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* `encoding` - `string` (default "utf-8")
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* `mode`- `integer` (default 0o600 - [node.js file modes](https://nodejs.org/dist/latest-v12.x/docs/api/fs.html#fs_file_modes))
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* `flags` - `string` (default 'a')
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* `compress` - `boolean` (default false) - compress the backup files during rolling (backup files will have `.gz` extension)
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* `keepFileExt` - `boolean` (default false) - preserve the file extension when rotating log files (`file.log` becomes `file.1.log` instead of `file.log.1`)
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Note that, from version 4.x of log4js onwards, the file appender can take any of the options for the [dateFile appender](dateFile.md) as well. So you could roll files by both date and size.
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## Example
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```javascript
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log4js.configure({
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appenders: {
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everything: { type: 'file', filename: 'all-the-logs.log' }
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},
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categories: {
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default: { appenders: [ 'everything' ], level: 'debug' }
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}
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});
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const logger = log4js.getLogger();
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logger.debug('I will be logged in all-the-logs.log');
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```
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This example will result in a single log file (`all-the-logs.log`) containing the log messages.
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## Example with log rolling (and compressed backups)
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```javascript
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log4js.configure({
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appenders: {
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everything: { type: 'file', filename: 'all-the-logs.log', maxLogSize: 10485760, backups: 3, compress: true }
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},
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categories: {
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default: { appenders: [ 'everything' ], level: 'debug'}
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}
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});
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```
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This will result in one current log file (`all-the-logs.log`). When that reaches 10Mb in size, it will be renamed and compressed to `all-the-logs.log.1.gz` and a new file opened called `all-the-logs.log`. When `all-the-logs.log` reaches 10Mb again, then `all-the-logs.log.1.gz` will be renamed to `all-the-logs.log.2.gz`, and so on.
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## Memory usage
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If your application logs a large volume of messages, and find memory usage increasing due to buffering log messages before being written to a file, then you can listen for "log4js:pause" events emitted by the file appenders. Your application should stop logging when it receives one of these events with a value of `true` and resume when it receives an event with a value of `false`.
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```javascript
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log4js.configure({
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appenders: {
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output: { type: 'file', filename: 'out.log' }
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},
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categories: { default: { appenders: ['output'], level: 'debug'}}
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});
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let paused = false;
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process.on("log4js:pause", (value) => paused = value);
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const logger = log4js.getLogger();
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while (!paused) {
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logger.info("I'm logging, but I will stop once we start buffering");
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}
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```
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