Logger: mongodb_watchdog

The mongodb_watchdog module stores the log entries for your Drupal site in MongoDB collections as mentioned in Settings Configuration

This module uses a separate database, defined by the logger database alias in settings. Do NOT point that alias to the same database as default, because the module Drops the logger database when uninstalling, which would drop all your other data with it.


Below are the configurable items available in mongodb_watchdog.settings

  • mongodb.watchdog.items
    • the maximum item limit on the capped collection used by the module.
    • if not defined, it defaults to 10000.
    • the actual (size-based) limit is derived from this value, assuming 1 kiB per watchdog entry.
  • mongodb.watchdog.limit
    • the maximum severity level (0 to 7, per RFC 5424) to save into the logger storage.
    • events below this level (with a higher numerical level) will be ignored by the module.
    • if not defined, all events will saved.
  • mongodb.watchdog.items_per_page
    • the maximum number of events displayed on the event details page.
  • mongodb_watchdog.request_tracking
    • if true, enable the per-request event tracking.
    • if not defined, it defaults to false because its cost is not entirely negligible.
    • this feature requires the use of mod_unique_id in Apache 2.x
  • mongodb_watchdog.requests
    • if request tracking is enabled, this setting defines the maximum requests limit on the capped collection used by the module.
    • if not defined, it defaults to 100000.
    • the actual (size-based) limit is derived from this variable, assuming 1 kiB per tracker entry.

See Drupal\Core\Logger\RfcLogLevel and Psr\Log\LogLevel for further information about severity levels.

Troubleshooting command

The module provides one single command designed to help troubleshoot issues with logging when this module is enabled. It is available for Drush and Drupal Console indifferently.

The command takes no arguments and returns an analysis of the logger collections which needs to be interpreted.

$ drush mongodb:watchdog:sanitycheck
0: 0
1: 2
1000: 1
2000: 0
3000: 0
4000: 0
5000: 0
6000: 0
7000: 0
8000: 0
9000: 0
9999: 0
10000: 0

What the output of this command represents is the number of watchdog_event_* collections with a document count in the range specified as the key.

In the results, the first and last two entries are specific: they match exact counts, while the others are intervals, so the actual buckets are: 0, 1, 2..1000, 1001..2000, ..., 9001..9998, 9999, 10000. The specific value 10000 is the number of entries allowed in event collections, as these are MongoDB capped collections: whatever its value n, the command will report 0, 1, n-1, n, and 9 ranges in between.

Interpreting the results

As a general rule, on a high-load site, all buckets should have comparable numbers, and the number of events logged grouped by pattern is pseudo-random, except for the 0 bucket, which should be empty.

  • 0 bucket non-empty: unless there was specific manipulation performed by hand, this is a bug, and should be reported: event collections are created when an event is created, by the first insertion, and dropped as needed, but never truncated or created without an insert.
  • 1 bucket has a high value, possibly orders of magnitude higher than other buckets. This denotes an incorrect use of the Drupal logger system, for which the PSR-3 message parameter is a message template, and the variant part of the message is expressed as placeholder values in the options. Look for calls to log operations passing a variable as the message and replace them by a template containing placeholders for the variable content in the message.
  • n-1 bucket has a high value: suspicious situation with the logger, especially if the "n" value is low or 0: please report a possible bug, with accompanying data
  • n bucket has a high value
    • n-1 has a high value too: this is a mostly normal situation, especially if the value in the previous bucket is also high, although it means the data rotation in your site is possibly a bit high and you should increase the size of the capped collections to ensure longer retention of data.
    • n-1 has a low or 0 value: your logs are saturated by the site, and you are losing information, as all the capped collections in that situation are rolling over constantly. Ensure you have enough storage and raise the value capped collection size.
    • The config value to change in these cases is mongodb.watchdog.items.