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Add Log4j2 JsonTemplateLayout support for JSON log output (#9209)
* Add Log4j2 JsonTemplateLayout support for JSON log output
Add the log4j-layout-template-json dependency and a new log4j2-json.xml
configuration file that uses JsonTemplateLayout to produce structured
JSON logs. Each log event includes @timestamp, level, loggerName,
message, and exception fields suitable for ingestion by ELK stack or
similar log aggregation tools.
The file naming convention (log4j2-json.xml) makes the "JSON" option
automatically available in the Admin Settings log level dropdown via the
existing LoggingApi regex discovery mechanism.
Harvester logs retain their existing PatternLayout as they are consumed
separately.
* Expand logging documentation with configuration details
Document Log4j2 configuration structure (appenders, pattern layout,
log rotation), harvester logging via routing appender, all available
logger modules, JSON logging sample output, custom log configuration
creation, REST API endpoints, and troubleshooting tips.
Also fix the log level preset list to match actual configuration
files (PROD, INDEX, DEV, TEST, JSON — there is no SEARCH preset).
The configuration file configures for each debug level at what severity messages will be logged.
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**`log4j2.xml`** (PROD), **`log4j2-dev.xml`**(DEV),**`log4j2-index.xml`** (INDEX), **`log4j2-json.xml`** (JSON) and **`log4j2-test.xml`** (TEST).
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Each configuration file defines which log levels apply to each logger module.
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The file used is determined by the log level set in the *Admin* → *Settings* page.
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### Configuration structure
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Each Log4j2 configuration file defines three appenders:
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-**Console** — writes to `SYSTEM_OUT` (stdout).
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-**RollingFile** (`File`) — writes to `geonetwork.log` in the log directory.
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-**Harvester** — a routing appender that writes to per-harvester log files (see [Harvester logging](#harvester-logging)).
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The default `PatternLayout` format used by the Console and RollingFile appenders is:
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```
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%date{ISO8601} %-5level [%logger] - %message%n
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```
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Which produces output like:
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```
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2024-06-15T10:23:45,123 INFO [geonetwork.harvester] - Starting harvester run
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```
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### Log file rotation
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The `RollingFile` appender uses size-based rotation:
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- Maximum file size: **10 MB** per log file.
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- Up to **3 backup files** are kept, named `geonetwork.log-1.log`, `geonetwork.log-2.log`, `geonetwork.log-3.log`.
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- When the current log file exceeds 10 MB, it is rolled over and the oldest backup is deleted.
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### Auto-discovery of configuration files
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GeoNetwork scans `/WEB-INF/classes` for files matching the regex `log4j2(-(.*?))?.xml`. Any file that matches this pattern is automatically made available in the *Admin* → *Settings* log level dropdown. The matched group (e.g. `dev` from `log4j2-dev.xml`) is uppercased to form the display name. The base `log4j2.xml` without a suffix is displayed as **PROD**.
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## Harvester logging
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Harvester logs are written to separate files via a **Routing** appender. Each harvester gets its own log file, dynamically named based on the harvester's thread context (`ctx:logfile` and `ctx:harvester`).
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- The log file name is determined at runtime from the harvester context.
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- If no specific context is set, logs go to `harvester_default.log` in the log directory.
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- Harvester log files use the `PatternLayout` with timezone-aware timestamps:
- Even when JSON logging is enabled, harvester logs continue to use plain-text `PatternLayout` since they are consumed separately from the main log file.
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## Logger modules
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The Log4j2 configuration files define logger categories that administrators can adjust. Each log level preset (PROD, INDEX, DEV, etc.) sets these loggers to different levels.
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### GeoNetwork loggers
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The main GeoNetwork logger `geonetwork` controls the base level for all GeoNetwork modules. Submodule loggers inherit from it unless explicitly configured:
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| Logger | Description |
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|--------|-------------|
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| `geonetwork.accessmanager` | Access control and permissions |
Each preset adjusts logger levels differently. As a general guide:
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- **PROD** — most GeoNetwork loggers at `error`, third-party at `error`. Minimal output.
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- **INDEX** — similar to PROD, but `geonetwork.search` at `warn` and `geonetwork.harvest-man` at `info` for indexation monitoring.
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- **DEV** — most GeoNetwork loggers at `debug`, Spring/Hibernate at `debug`. Very verbose.
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- **TEST** — simplified configuration with fewer logger categories, root level at `warn`.
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- **JSON** — same logger levels as PROD, but output format is JSON (see below).
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## JSON logging
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The **JSON** log level uses `log4j2-json.xml` which replaces the default `PatternLayout` with Log4j2's `JsonTemplateLayout`. This produces structured JSON output on both the console and the rolling log file, making it suitable for ingestion by log aggregation tools such as the ELK stack (Elasticsearch, Logstash, Kibana).
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Each log event is output as a JSON object with the following fields:
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- `@timestamp` — event timestamp in ISO 8601 format
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- `level` — log level (e.g. ERROR, INFO, DEBUG)
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- `loggerName` — the logger that produced the event
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- `message` — the log message
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- `exception` — stack trace, if present
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A sample JSON log line:
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```json
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{"@timestamp":"2024-06-15T10:23:45.123+0000","level":"ERROR","loggerName":"geonetwork.index","message":"Failed to index record abc-123","exception":null}
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```
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To activate JSON logging, go to *Admin* → *Settings* and select **JSON** from the log level dropdown.
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The `JsonTemplateLayout` is provided by the `log4j-layout-template-json` library which is included in GeoNetwork's dependencies. The event template is embedded inline in the `log4j2-json.xml` configuration file and can be customized directly to add or modify fields.
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Harvester logs continue to use plain-text `PatternLayout` as they are consumed separately.
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For log ingestion, you can point **Filebeat** or **Logstash** directly at the `geonetwork.log` file when JSON mode is enabled, since each line is a self-contained JSON object.
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## Custom log configurations
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You can create your own log level preset by adding a new configuration file to `/WEB-INF/classes/`:
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1. Copy an existing configuration (e.g. `log4j2.xml`) as a starting point.
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2. Name the new file following the pattern `log4j2-<name>.xml` (e.g. `log4j2-debug-search.xml`).
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3. Adjust the logger levels as needed.
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4. Restart GeoNetwork (or wait for the configuration to be reloaded).
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The new configuration will automatically appear in the *Admin* → *Settings* log level dropdown as **`<NAME>`** (uppercased from the filename suffix, e.g. `DEBUG-SEARCH`).
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## Log REST API
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GeoNetwork provides REST endpoints for managing and viewing logs. All endpoints require **Administrator** role.
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### List available log configurations
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```
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GET /{portal}/api/site/logging
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```
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Returns a JSON array of available log configuration files:
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```json
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[
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{"name": "PROD", "file": "log4j2.xml"},
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{"name": "DEV", "file": "log4j2-dev.xml"},
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{"name": "INDEX", "file": "log4j2-index.xml"},
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{"name": "JSON", "file": "log4j2-json.xml"},
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{"name": "TEST", "file": "log4j2-test.xml"}
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]
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```
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### Get recent log activity
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```
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GET /{portal}/api/site/logging/activity?lines={n}
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```
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Returns the last *n* lines from the log file as plain text. Default is **2000** lines; maximum is **20000**.
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### Download log file as ZIP
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```
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GET /{portal}/api/site/logging/activity/zip
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```
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Downloads the current log file as a ZIP archive, named `catalog-log-<timestamp>.zip`.
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## Troubleshooting
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### Enable Log4j2 internal status logging
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If log output is not appearing as expected, enable Log4j2's internal status logger to diagnose configuration problems. Set the `status` attribute on the `<Configuration>` element:
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```xml
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<Configurationstatus="trace"dest="out">
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```
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Alternatively, set the system property at startup:
This will output Log4j2's internal messages to the console, helping identify issues with appender configuration, missing files, or pattern errors.
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### XSL transformation errors
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XSL transformation errors are not written to `geonetwork.log`. They are written to **`catalina.out`** (when using Tomcat). Check this file if you suspect issues with metadata formatting or schema processing.
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### Temporarily increase logging for a specific module
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To debug a specific subsystem without increasing the global log level, edit the active Log4j2 configuration file and change only the target logger. For example, to debug search issues while keeping everything else at `error`:
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```xml
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<Loggername="geonetwork.search"level="debug"/>
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```
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Then switch to a different log configuration and back in *Admin* → *Settings* to reload, or restart GeoNetwork. Remember to revert the change after debugging.
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