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# logger_filters (kernel v10.6.3)

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Filters to use with Logger.

All functions exported from this module can be used as primary or handler filters. See `logger:add_primary_filter/2` and `logger:add_handler_filter/3` for more information about how filters are added.

Filters are removed with `logger:remove_primary_filter/1` and `logger:remove_handler_filter/2`.

## See Also

`logger`

## Summary

### Functions

domain(LogEvent, Extra)

This filter provides a way of filtering log events based on a `domain` field in `Metadata`. This field is optional, and the purpose of using it is to group log events from, for example, a specific functional area. This allows filtering or other specialized treatment in a Logger handler.

level(LogEvent, Extra)

This filter provides a way of filtering log events based on the log level. It matches log events by comparing the log level with a specified `MatchLevel`

progress(LogEvent, Extra)

This filter matches all progress reports from `supervisor` and `application_controller`.

remote_gl(LogEvent, Extra)

This filter matches all events originating from a process that has its group leader on a remote node.

## Functions

# domain(LogEvent, Extra)

(since OTP 21.0)

    -spec domain(LogEvent, Extra) -> logger:filter_return()
                    when
                        LogEvent :: logger:log_event(),
                        Extra :: {Action, Compare, MatchDomain},
                        Action :: log | stop,
                        Compare :: super | sub | equal | not_equal | undefined,
                        MatchDomain :: [atom()].

This filter provides a way of filtering log events based on a `domain` field in `Metadata`. This field is optional, and the purpose of using it is to group log events from, for example, a specific functional area. This allows filtering or other specialized treatment in a Logger handler.

A domain field must be a list of atoms, creating smaller and more specialized domains as the list grows longer. The greatest domain is `[]`, which comprises all possible domains.

For example, consider the following domains:

    D1 = [otp]
    D2 = [otp, sasl]

`D1` is the greatest of the two, and is said to be a super-domain of `D2`. `D2` is a sub-domain `D1`. Both `D1` and `D2` are sub-domains of `[]`.

The above domains are used for logs originating from Erlang/OTP. D1 specifies that the log event comes from Erlang/OTP in general, and D2 indicates that the log event is a so called SASL report.

The `Extra` parameter to the `domain/2` function is specified when adding the filter via `logger:add_primary_filter/2` or `logger:add_handler_filter/3`.

The filter compares the value of the `domain` field in the log event's metadata (`Domain`) against `MatchDomain`. The filter matches if the value of `Compare` is:

- `sub` - and `Domain` is equal to or a sub-domain of `MatchDomain`, that is, if `MatchDomain` is a prefix of `Domain`.

- `super` - and `Domain` is equal to or a super-domain of `MatchDomain`, that is, if `Domain` is a prefix of `MatchDomain`.

- `equal` - and `Domain` is equal to `MatchDomain`.

- `not_equal` - and `Domain` differs from `MatchDomain`, or if there is no domain field in metadata.

- `undefined` - and there is no domain field in metadata. In this case `MatchDomain` must be set to `[]`.

If the filter matches and `Action` is `log`, the log event is allowed. If the filter matches and `Action` is `stop`, the log event is stopped.

If the filter does not match, it returns `ignore`, meaning that other filters, or the value of the configuration parameter `filter_default`, decide if the event is allowed or not.

Log events that do not contain any domain field, match only when `Compare` is equal to `undefined` or `not_equal`.

Example: stop all events with domain `[otp, sasl | _]`

    1> logger:set_handler_config(h1, filter_default, log). % this is the default
    ok
    2> Filter = {fun logger_filters:domain/2, {stop, sub, [otp, sasl]}}.
    ...
    3> logger:add_handler_filter(h1, no_sasl, Filter).
    ok

# level(LogEvent, Extra)

(since OTP 21.0)

    -spec level(LogEvent, Extra) -> logger:filter_return()
                   when
                       LogEvent :: logger:log_event(),
                       Extra :: {Action, Operator, MatchLevel},
                       Action :: log | stop,
                       Operator :: neq | eq | lt | gt | lteq | gteq,
                       MatchLevel :: logger:level().

This filter provides a way of filtering log events based on the log level. It matches log events by comparing the log level with a specified `MatchLevel`

The `Extra` parameter is specified when adding the filter via `logger:add_primary_filter/2` or `logger:add_handler_filter/3`.

The filter compares the value of the event's log level (`Level`) to `MatchLevel` by calling `logger:compare_levels(Level, MatchLevel)`. The filter matches if the value of `Operator` is:

- `neq` - and the compare function returns `lt` or `gt`.

- `eq` - and the compare function returns `eq`.

- `lt` - and the compare function returns `lt`.

- `gt` - and the compare function returns `gt`.

- `lteq` - and the compare function returns `lt` or `eq`.

- `gteq` - and the compare function returns `gt` or `eq`.

If the filter matches and `Action` is `log`, the log event is allowed. If the filter matches and `Action` is `stop`, the log event is stopped.

If the filter does not match, it returns `ignore`, meaning that other filters, or the value of the configuration parameter `filter_default`, will decide if the event is allowed or not.

Example: only allow debug level log events

    logger:set_handler_config(h1, filter_default, stop).
    Filter = {fun logger_filters:level/2, {log, eq, debug}}.
    logger:add_handler_filter(h1, debug_only, Filter).
    ok

# progress(LogEvent, Extra)

(since OTP 21.0)

    -spec progress(LogEvent, Extra) -> logger:filter_return()
                      when LogEvent :: logger:log_event(), Extra :: log | stop.

This filter matches all progress reports from `supervisor` and `application_controller`.

If `Extra` is `log`, the progress reports are allowed. If `Extra` is `stop`, the progress reports are stopped.

The filter returns `ignore` for all other log events.

# remote_gl(LogEvent, Extra)

(since OTP 21.0)

    -spec remote_gl(LogEvent, Extra) -> logger:filter_return()
                       when LogEvent :: logger:log_event(), Extra :: log | stop.

This filter matches all events originating from a process that has its group leader on a remote node.

If `Extra` is `log`, the matching events are allowed. If `Extra` is `stop`, the matching events are stopped.

The filter returns `ignore` for all other log events.