A script to reduce the amount of information from MongoDB log files. mlogfilter takes one or several MongoDB log files as input, together with some filter parameters, parses the contained log lines and outputs the lines that match according to the filter parameters.

If several log files are provided, mlogfilter will merge them by timestamp and insert a marker at the beginning of each line, before applying any of the other filters.


mlogfilter [-h] [--version] logfile [logfile ...]
           [--verbose] [--shorten [LENGTH]]
           [--human] [--exclude] [--json]
           [--timestamp-format {ctime-pre2.4, ctime, iso8601-utc, iso8601-local}]
           [--markers MARKERS [MARKERS ...]] [--timezone N [N ...]]
           [--namespace NS] [--operation OP] [--thread THREAD]
           [--slow [SLOW]]  [--fast [FAST]] [--scan]
           [--word WORD [WORD ...]]
           [--from FROM [FROM ...]] [--to TO [TO ...]]

mlogfilter can also be used with shell pipe syntax:

cat logfile | mlogfilter [parameters]


mlogfilter has a number of parameters, some of them define filters for individual lines, others change the behavior or output in general. There are also some parameters specifically for merging files together. All the parameters are described below.

General Parameters


-h, --help
shows the help text and exits.


shows the version number and exits.


shows extra information about the parser and arguments. This is usually only needed for debugging purposes.

Shorten Log Lines

--shorten [LENGTH]
will shorten long lines to at most LENGTH characters, only showing the beginning and end of a line. If LENGTH is not provided, the default is 200. This is useful to ensure that each log line fits into a shell / editor window to compare certain re-occuring values. The line is split in the middle and the excess characters are replaced with ....

Human Readability

makes log lines easier to read for humans. Long numbers will be separated by "thousands" comma separators, and the duration at the end of an operation is augmented with the hours, minutes and seconds for readability.

Exclude Log Lines

This flag will invert the filter, excluding all the lines that match all filters, and only returning the non-matching lines.

JSON Output

If this flag is used, the output will be in JSON format instead of the regular log line string. Use this in conjunction with MongoDB's mongoimport to store the log file data in a MongoDB database.

The values that are extracted and stored as separate fields in the resulting JSON document are (some may not > apply for all lines): line_str (the full line as string), split_tokens (the line as string tokens split on whitespace, stored as array of strings), datetime (the datetime for this line), operation, thread, namespace, nscanned, ntoreturn, nreturned, ninserted, nupdated, duration (the duration in milliseconds), r, w (read or write lock times in microseconds), numYields (the number of yields for this operation)

mlogfilter mongod.log --slow --json | mongoimport -d test -c mycoll

This command would apply the "slow" filter to mongod.log and output the information as JSON, which can then be read my mongoimport. The data is stored in the database test, collection mycoll. Each log line will be stored as a separate document.

Timestamp Format

--timestamp-format FORMAT
Changes the timestamp format to the specified format. Possible formats are:

  • ctime-pre2.4 (the format looks like Fri Jul 26 11:38:37 without milliseconds)
  • ctime (the format looks like Fri Jul 26 11:38:37.712 with milliseconds)
  • iso8601-utc (the format looks like 2013-07-26T11:38:37.712Z)
  • iso8601-local (the format looks like 2013-07-26T11:38:37.712+0000)

Merge Parameters

The following parameters are useful if several log files are provided. mlogfilter will merge the files based on the timestamp of each line, before applying any other filters.

Merge Markers

--markers M [M ...]
Markers help distinguish the source of merged log lines. Each merged line is preceded with a marker that indicates the original file. By default, the marker type is filename. Another marker type can be specified, or custom markers can be provided. These are the available marker types:

  • filename (the original filename in curly braces)
  • enum (numeric markers {1}, {2}, {3}, ...)
  • alpha (letters {a}, {b}, {c}, ...)
  • none (no markers, this is useful if you merge log files form the same host but for different times)

Custom markers are also possible. Make sure the number of specified markers is the same as the number of provided log files.

Timezone Adjustments

--timezone N [N ...]
The timezone parameter adjusts the timestamp of the log lines in hours. If one timezone value is provided, it is applied globally to all log lines. You can also adjust individual log files by providing the same number of timezone parameters as log files. Each logfile is adjusted individually. For negative adjustments, the value has to be quoted, e.g. "-2".

mlogfilter logfile1.log logfile2.log --timezone 4 0

This example would move the time of logfile1.log four hours into the future, while keeping the time of logfile2.log constant.

Filter Parameters

The next set of parameters filter for certain log lines, and lines are only returned if they match all the filters (if --exclude is set, a line is returned if it would normally not be returned, i.e. if it does not match one or more of the filters).


--namespace NS
filter by namespace NS, where NS has to be of the form <database>.<collection>. Only lines matching this namespace are being returned. Note that the $ sign can be matched by escaping it with \$. Currently, only one namespace can be specified.

mlogfilter mongod.log --namespace admin.\$cmd --slow 1000

This will return all admin commands that have been slower than 1 second.


--operation OP
filter by operation OP, where OR can be any of query, insert, update, delete, command, getmore. Only the matching operations are returned. Currently, only one operation can be specified.


--thread THREAD
filter by thread name (marked in square brackets after the timestamp in each line). This is useful to trace a single connection to the database.

mlogfilter mongod.log --thread conn1234

This will return all lines that were issued from connection conn1234. Note that the initial line marking the opening of a connection (containing connection accepted) itself is not on the same thread but on thread [initandlisten].


--pattern P
filter by pattern P, where P has to be a JSON string inside single quotes. A pattern is a transformation of a query into a canonicalized form, where values are replaced by 1 and fields are reordered alphabetically. Only query, update and remove operations matching this query pattern (also called "query shape") are being returned.

mlogfilter mongod.log --pattern '{"_id": 1, "host": 1, "ns": 1}'

This will return all query, update and remove operations that match the pattern {"_id": 1, "host": 1, "ns": 1}.

Note: the field names must be surrounded by double quotes for valid JSON.


--slow MS, --fast MS
returns only operations that are slower/faster than MS milliseconds. Not all lines in a log file have a duration, those without do not match the filter and are not being returned.

Collection Scans

This flag attempts to detect collection scans (not efficiently using an index) and returns only lines that match the detection heuristic. To match, a line has to have an nscanned value of 10000 or larger, and the ratio of n / nscanned must be larger than 100. These values have proven useful to detect collection scans.


--word WORD [WORD ...]
Only lines that contain one or more of the provided words match this filter and are returned.

mlogfilter mongod.log --word assert warning error

The below line matches all lines that contain any of the words assert, warning, error:

Time Slicing

--from FROM [FROM ...], --to TO [TO ...]
These parameters slice the log file by time, by providing either a lower bound (--from) or an upper bound (--to) or both. This feature is implemented using binary search, making time slicing very fast and efficient. It should always be used if appropriate for the task. The arguments that can be passed into these parameters are quite flexible, and are explained below.

Both FROM and TO can accept the same format, which is defined as [DATE] [TIME] [OFFSET] separated by space. The square brackets indicate that any of these parts can also be omitted.

DATE can be any of

  • a 3-letter weekday (e.g., Mon, Wed, Sun, ...)
  • a date as 3-letter month and 1-2 digits day (e.g., Sep 5, Jan 31, Aug 08, ...)
  • the words: today, now, start, end

TIME can be any of

  • hours and minutes (e.g., 20:15, 4:00, 3:25, ...)
  • hours, minutes and seconds (e.g., 13:30:01, 4:55:55, ...)
  • hours, minutes, secondsa and milliseconds (e.g., 13:30:01.123, 4:55:55.700, ...)

OFFSET is again composed of OPERATOR``VALUE``UNIT (not separated by spaces)

OPERATOR is either + or - (the latter requires quotes as it would otherwise be interpreted as another parameter by the argument parser.

VALUE is any number.

UNIT is one of the following:

  • s, sec
  • m, min
  • h, hours
  • d, days
  • w, weeks
  • mo, months
  • y, years

The OFFSET value is added / subtracted from the DATE``TIME value if provided.

There are some rules for ease of use, that allow not to specify DATE and/or TIME. The behavior should be intuitive, and examples are provided below. Here are some of the rules:

if a weekday is provided without a date, the most recent day matching the weekday is assumed.


Assuming the log file begins on Tue Aug 13 00:00:00 and ends on Fri Sep 9 15:31:10

mlogfilter mongod.log --from Wed 19:00

This example would return lines that are later than Wed Sep 6 19:00.

if only a month is specified, the day is assumed to be 1.


The following line matches everything from Sep 1 00:00:00:

mlogfilter mongod.log --from Sep

if the keyword now is specified for, it uses the current date and time.


The following line matches everything from 5 minutes ago:

mlogfilter mongod.log --from "now -5min"

if the keyword today is specified for DATE, it uses the current date and the time 00:00:00


The following line matches everything from today at 00:00:00 to today at 02:00:00:

mlogfilter mongod.log --from today --to +2hours

if the keyword start is specified, it is replaced with the date and time of the beginning of the log file.


The following line matches all but the first day (24h) of the log file:

mlogfilter mongod.log --from start +1day

if the keyword end is specified, it is replaced with the date and time of the end of the log file


The following line returns the last 20 minutes of a log file (note the quotation marks):

mlogfilter mongod.log --from "end -20min"

TIME can be combined with any of the keywords above and has precedence of the keyword time


The following line matches everything from today at 9:30:

mlogfilter mongod.log --from today 9:30

if only an OFFSET is specified without DATE and/or TIME, then it depends if the parameter was --from or --to. For --from, the offset is calculated from the start of the file, and for --to the offset is calculated from the --from time if provided, or otherwise from the end of the file.


The following line matches the hour before the last hour in the log file:

mlogfilter mongod.log --from "end -2h" --to +1h