mdbkit

An offline toolkit for MongoDB structured logs — slow-query analysis, deterministic index advice, incident triage, and diagnostic-data decoding. For MongoDB 4.4 – 8.0, from the terminal, without connecting to anything.

A spiritual successor to mtools' log tools, which never learned to read the JSON log format introduced in 4.4.

namespace    op         count  cumMs  docsEx      scan     plan           shape
shop.events  aggregate  29     3.2m   25,810,000  98889:1  COLLSCAN+SORT  {tenantId:eq, ts:gte} sort:{ts:-1}
shop.orders  find       48     1.4m   6,000,000   2976:1   COLLSCAN+SORT  {status:eq, createdAt:gt}
shop.users   find       31     3.7s   31          1:1      IXSCAN{email}  {email:eq}

Two of those need an index. One is already fine. That distinction is the whole point.

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Try it right now — no MongoDB required

mdbkit demo writes a realistic log containing a real incident, so you can evaluate the tool in about a minute without touching a cluster:

pip install mdbkit          # macOS has no pip by default — see Install below

mdbkit demo --with-extras -o demo.log     # a log + indexes.json, schema.json, explain.json

mdbkit loginfo demo.log                   # what is in this log?
mdbkit queries demo.log                   # which query shapes cost the most?
mdbkit triage demo.log --window 0         # what went wrong, and when?
mdbkit connections demo.log               # who connected, and did anyone fail to?
mdbkit advise demo.log --indexes indexes.json --schema schema.json
mdbkit explain explain.json               # read a saved explain plan

The generated log contains a connection storm from one client, a replica set election, an index build, five failed logins from a service account, and an aggregation burning 25 million document reads to return 261 documents — then advise tells you which index fixes it.

Output is deterministic: the same --seed always produces the same log, so a demo behaves identically every time. Scenarios are incident, healthy (the control case — useful for seeing what "nothing wrong" looks like) and mixed.

Ready for a real server? Jump to the workflows.

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Is it safe to run on a production server?

This is the right question to ask of any tool someone hands you. The honest answer, and how to check it yourself.

What mdbkit never does:

Connect to your database	Analysis commands read files. There is no driver, no URI, no connection.
Send anything anywhere	There is no network code at all. No telemetry, no update check, no crash reporting.
Change anything	It is strictly read-only. Where an action would help, it prints the command for you to review and run.
Execute what it reads	Log lines and explain files are parsed as data with json.loads. Nothing is ever evaluated.
Pull in dependencies	Zero runtime dependencies. Nothing in the supply chain but the Python standard library.

Verify it yourself in 60 seconds — this is a small, dependency-free codebase specifically so that you can:

# 1. No network, no shell-outs, no eval anywhere in the analysis code
pip show -f mdbkit | head -3
grep -rn "socket\|urllib\|requests\|http\|eval(\|exec(" $(python -c "import mdbkit,os;print(os.path.dirname(mdbkit.__file__))")

# 2. Confirm it has no dependencies
pip show mdbkit | grep Requires

# 3. Watch it make no connections while it runs (Linux)
strace -f -e trace=network mdbkit queries mongod.log 2>&1 | grep -c socket

The grep returns nothing for every analysis module. The only file that starts a process is lab.py, which exists to create a throwaway test cluster and is documented as an explicit exception below.

What it does read: the log file you point it at; optionally diagnostic.data (metrics only, never documents); optionally indexes.json and schema.json that you generate with scripts mdbkit prints for you to inspect first. On the database host it also reads /proc and calls statvfs for disk and memory figures — nothing that leaves the machine.

What leaves your machine: nothing. There is no server to send it to.

Still cautious? That is reasonable. Run mdbkit demo first and see what the output looks like on synthetic data, or mdbkit lab to try it against a disposable local cluster before you point it at anything real. Both exist for exactly this reason.

Full detail: SECURITY.md.

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Install

Linux

Most distributions:

pip install mdbkit

Ubuntu 20.04 / Debian / Amazon Linux 2 (Python 3.8 hosts):

sudo apt install pipx        # or: sudo dnf install pipx
pipx install mdbkit
pipx ensurepath && source ~/.bashrc

Modern Ubuntu/Debian complaining about "externally-managed-environment":

pip install mdbkit --break-system-packages

macOS

macOS ships no pip and no pipx, so pip install fails out of the box. Pick whichever of these matches your setup.

With Homebrew (recommended — keeps mdbkit in its own environment):

brew install pipx
pipx ensurepath          # then open a new terminal window
pipx install mdbkit

Without Homebrew, using the Python that comes with macOS:

python3 --version        # accept the Command Line Tools prompt if it appears
python3 -m pip install --user mdbkit

Then put the install location on your PATH — macOS does not do this for you:

echo 'export PATH="$HOME/Library/Python/'"$(python3 -c 'import sys;print(f"{sys.version_info.major}.{sys.version_info.minor}")')"'/bin:$PATH"' >> ~/.zshrc
source ~/.zshrc
mdbkit --version

If you use uv:

uv tool install mdbkit

If pip reports externally-managed-environment, add --break-system-packages, or use the pipx route above, which avoids the problem entirely.

Do not install with sudo. mdbkit is a user-level CLI and needs no elevated privileges — not to install, and not to run.

Windows

py -m pip install mdbkit
py -m mdbkit --version

If mdbkit is not recognised as a command afterwards, the Scripts directory is not on your PATH; py -m mdbkit works regardless.

Air-gapped database hosts

pip download mdbkit -d ./wheels          # on a connected machine
# copy ./wheels across, then:
pip install --no-index --find-links ./wheels mdbkit

Zero runtime dependencies means this is a single wheel — nothing else to resolve.

Upgrading

Use whichever matches how you installed it.

# Linux — installed with pip
pip install --upgrade mdbkit

# Linux — installed with pipx
pipx upgrade mdbkit

# macOS — installed with pipx (Homebrew route)
pipx upgrade mdbkit

# macOS — installed with python3 -m pip --user
python3 -m pip install --user --upgrade mdbkit

# macOS / Linux — installed with uv
uv tool upgrade mdbkit

# Windows
py -m pip install --upgrade mdbkit

Then confirm:

mdbkit --version

If the version has not changed, pip is serving a cached index. Force it:

pipx install --force mdbkit                          # pipx
pip install --upgrade --no-cache-dir mdbkit          # pip
py -m pip install --upgrade --no-cache-dir mdbkit    # Windows

Requires Python 3.8+. mdbkit never updates itself and never checks for updates — upgrades are always explicit.

mdbkit is a Python package on PyPI. It is not in apt/dnf/yum.

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The four questions it answers

Every command reads files or stdin and accepts several files or a glob, so rotated logs work as one stream: mdbkit queries "mongod.log*".

1. Why is my database slow?

mdbkit queries mongod.log                       # shapes ranked by total time
mdbkit queries mongod.log --sort scanRatio      # worst examined:returned first
mdbkit queries mongod.log --shape 1             # full detail on one shape

cumMs is time summed across all occurrences of a shape, not one query. scan is documents examined per document returned — 1:1 is healthy, 98889:1 is a missing index. plan shows what MongoDB actually chose.

2. What index would fix it?

# Optional but much sharper: export what already exists.
mdbkit export-script indexes > export_indexes.js
mdbkit export-script schema  > export_schema.js

mongosh --quiet --host your_db_host --port 27017 \
  --username your_username --password your_password \
  --authenticationDatabase admin \
  --eval "$(cat export_indexes.js)" > indexes.json      # repeat for schema

mdbkit advise mongod.log --indexes indexes.json --schema schema.json --ns shop.orders

Every recommendation states the evidence it reasoned from, a confidence level, the caveats, and how to validate it. It says candidate, not command, and it never tells you to drop an index.

3. What happened at 3am?

mdbkit triage /var/log/mongodb/mongod.log       # last 60 minutes by default
mdbkit triage mongod.log --window 0             # the whole file
mdbkit triage mongod.log --report incident.html # something to attach to a ticket

Cluster health, elections, connection storms, hot collections, index builds, error clusters, slow-query peaks — plus disk, memory, CPU and FTDC metrics when run on the database host. Every finding ends with the next command to run.

4. Did my change actually help?

mdbkit compare before.log --after after.log

slow-query time DOWN 32%  (6.0m -> 4.1m across compared shapes)
shapes: 1 improved, 0 regressed, 0 new, 0 gone, 4 unchanged

IMPROVED
  shop.orders {createdAt:gt, status:eq} sort:{createdAt:-1}
    mean 1.7s -> 33ms (-98%)   scan 2976:1 -> 1:1  [COLLSCAN -> index, in-memory sort gone]

The natural follow-up to advise: you created the index, a day passed, and this tells you whether it worked.

5. Why did it die? (and what does serverStatus say?)

mdbkit oslog /var/log/syslog                  # OOM kills, fd limits, I/O errors
mdbkit triage mongod.log --oslog /var/log/syslog

A mongod log cannot record its own OOM kill — the process is gone before it can write anything. The system log has the answer in one line, and triage will correlate it with the unexplained restart.

mdbkit export-script serverstatus > export_serverstatus.js
mongosh --quiet --host HOST --eval "$(cat export_serverstatus.js)" > status.json
mdbkit serverstatus status.json

[CRIT] Concurrency tickets: Exhausted tickets queue every new operation,
       which looks like slowness with no slow query to blame.
        - read: 1 of 128 free (1%)
[CRIT] WiredTiger cache: Above 80% WiredTiger evicts in the background;
       above 95% application threads are made to evict.
        - 31.0 GiB of 32.0 GiB used (96.9%)

Bonus — who connected?

mdbkit connections mongod.log

Per-IP churn with first/last seen, plus an authenticated-users table showing successful and failed logins per account and when each last authenticated — the question that starts most access incidents.

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Running it on a schedule

mdbkit is deterministic, offline and read-only, which makes it well suited to a cron job. It deliberately cannot send anything anywhere — there is no network code and never will be. So mdbkit produces the verdict and your own script does the talking.

The primitive that makes this work is --exit-code:

Exit code	Meaning
0	Nothing above INFO
1	At least one WARN
2	At least one CRIT

#!/usr/bin/env bash
# /usr/local/bin/mdbkit-watch.sh — hourly health check
set -uo pipefail

LOG=/var/log/mongodb/mongod.log
OUT=$(mktemp)

mdbkit triage "$LOG" --window 60 --oslog /var/log/syslog \
       --only CRIT,WARN --exit-code > "$OUT" 2>&1
STATUS=$?

if [ "$STATUS" -ge 2 ]; then
    # CRIT: wake someone up. Your channel, your call — mdbkit stays offline.
    curl -sf -X POST -H 'Content-type: application/json' \
         --data "{\"text\": \"MongoDB CRIT on $(hostname)\n\`\`\`$(cat "$OUT")\`\`\`\"}" \
         "$SLACK_WEBHOOK_URL"
elif [ "$STATUS" -eq 1 ]; then
    mail -s "MongoDB warnings on $(hostname)" dba@example.com < "$OUT"
fi

rm -f "$OUT"

# hourly triage; only speaks up when something is wrong
0 * * * * /usr/local/bin/mdbkit-watch.sh

# daily slow-query digest, kept for trend comparison
30 6 * * * mdbkit queries /var/log/mongodb/mongod.log \
             --report /var/log/mdbkit/$(date +\%F).html

Because the reports are dated, compare turns them into a trend:

mdbkit compare /var/log/mongodb/mongod.log.1 --after /var/log/mongodb/mongod.log

Two things worth knowing. --only CRIT,WARN keeps the mail short, and a run that finds nothing prints almost nothing — so a silent cron job means a healthy database rather than a broken script. And because every command is read-only and never connects to the database, running this hourly on a production host costs one log read and no risk.

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Trying it against a real cluster

mdbkit lab starts a disposable local MongoDB so you can test against a real server without touching anything that matters.

mdbkit lab start                    # 3-node replica set on 127.0.0.1:28110-28112
mdbkit lab seed                     # 50k documents + a deliberately mixed workload
mdbkit queries $(mdbkit lab logs | head -1)
mdbkit lab destroy --yes            # remove it entirely

It binds to localhost only, uses ports far from 27017 so it can never be confused with a real deployment, and refuses to touch any directory it did not create. It is the one command that starts external processes — see SECURITY.md.

Full options and more examples: mdbkit lab in the reference.

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Command reference

Every command reads files or stdin and writes to stdout. --help works on any command (mdbkit queries --help). Global: mdbkit --version.

All commands that read a log accept one or more paths, a shell glob, a rotated .gz file, or - for stdin. Several files are read as a single stream in filename order, which matches MongoDB's rotation naming:

mdbkit queries mongod.log                       # one file
mdbkit queries mongod.log.1 mongod.log          # explicit list
mdbkit queries "mongod.log*"                    # glob (quote it)
mdbkit queries /var/log/mongodb/mongod.log.*.gz # compressed archives
cat mongod.log | mdbkit queries -               # stdin

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mdbkit loginfo <log>

Overall log summary: server version, host, restarts, connections accepted, slow-query count, warning/error counts, and a per-component line breakdown.

Option	Description
--json	Machine-readable output

mdbkit loginfo /var/log/mongodb/mongod.log
mdbkit loginfo mongod.log.2.gz --json

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mdbkit queries <log>

Slow queries grouped by query shape — literal values stripped, so the same query with different parameters is counted once.

Option	Default	Description
--sort FIELD	totalMs	Order by totalMs, count, mean, max, docsExamined, or scanRatio
--limit N	all	Show only the top N shapes
--min-ms N	0	Ignore operations faster than N milliseconds
--include-system	off	Include internal admin/config/local namespaces (hidden by default — they are server housekeeping, not your workload)
--report FILE		Write a shareable .md or .html report instead (see Shareable reports)
--json		Machine-readable output

Reading the columns:

Column	Meaning
cumMs	Time summed across all occurrences of that shape — not one query
mean / max	Per-occurrence average and worst case
docsEx	Documents examined, summed across all occurrences
scan	Documents examined per document returned. 1:1 is ideal; 3444:1 means a missing or weak index
plan	The plan MongoDB chose: COLLSCAN (no index), IXSCAN{fields} (index used), IDHACK (_id lookup), +SORT (in-memory sort). ? = the plan was not recorded on that line
shape	Fields and operators queried, with the sort

mdbkit queries mongod.log
mdbkit queries mongod.log --sort scanRatio --limit 10
mdbkit queries mongod.log --min-ms 500 --json
mdbkit queries "mongod.log*"              # rotated logs as one stream
mdbkit queries mongod.log --shape 1       # drill into the worst offender

--shape N expands one row of the table:

namespace : shop.events
shape     : {tenantId:eq, ts:gte} sort:{ts:-1}

occurrences   : 29
total time    : 3.2m
mean / max    : 6.6s / 8.9s
docs examined : 25,810,000
docs returned : 261
scan ratio    : 98889 examined per document returned

plans observed
  COLLSCAN                                 29x

flags
  COLLSCAN — no index used for at least one execution
  in-memory SORT — results sorted after retrieval

client applications
  ReportWorker                   15x
  OrderService                   7x

---

mdbkit connections <log>

Connection churn and who authenticated: totals, peak concurrent count, per-source-IP breakdown with first/last seen, the client applications and drivers, and a per-user table.

Option	Description
--json	Machine-readable output

mdbkit connections mongod.log

source ip   accepted  ended  first seen           last seen            appName
----------  --------  -----  -------------------  -------------------  ------------
10.20.9.77  220       0      2026-07-01 08:49:30  2026-07-01 08:49:30  checkout-api
10.20.4.11  4         1      2026-07-01 08:00:15  2026-07-01 09:29:30  OrderService

authenticated users
user          auth db  ok   failed  last authenticated   from
------------  -------  ---  ------  -------------------  -----------
svc_checkout  admin    221  0       2026-07-01 08:49:30  10.20.9.77
etl_batch     admin    0    5       2026-07-01 08:51:54  10.20.11.40

  etl_batch: 5 failed authentication(s) — last error: AuthenticationFailed

This answers the question that starts most access incidents: did that account connect, from where, and when last? If the log shows no authentication events at all, mdbkit says so — either auth is disabled, or the window contains no new logins because clients are reusing connections.

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mdbkit filter <log>

Streams matching raw log lines to stdout. Output stays valid logv2 JSON, so it chains with other tools (including mdbkit itself).

Option	Description
--component NAME	COMMAND, NETWORK, REPL, STORAGE, INDEX, WRITE, QUERY, CONTROL, …
--severity S	I info, W warning, E error, F fatal
--ns NAMESPACE	Exact namespace, e.g. shop.orders
--slow N	Only operations with durationMillis >= N
--from TIMESTAMP	Lower time bound (inclusive)
--to TIMESTAMP	Upper time bound (inclusive)
--msg TEXT	Substring match on the message field
--limit N	Print only the first N matches
--last N	Print only the last N matches — usually what you want during an incident
--as-explain	Rebuild each matching slow query as a runnable mongosh .explain() command instead of printing the raw log line
--explain-script	With --as-explain, wrap in EJSON.stringify() plus usage comments so it can be saved as a .js file

Timestamp formats accepted by --from / --to:

2026-07-01T08:00:00+04:00     with an explicit offset (production logs)
2026-07-01T08:00:00Z          UTC
2026-07-01T08:00:00           no offset — read as the log's own timezone
2026-07-01 08:00:00           space instead of T
2026-07-01T08:00               minute precision
2026-07-01                     whole day

mdbkit filter mongod.log --severity E --last 20    # errors (most recent 20)
mdbkit filter mongod.log --severity F               # fatal — always investigate
mdbkit filter mongod.log --severity W --last 50     # warnings
mdbkit filter mongod.log --component REPL --msg election
mdbkit filter mongod.log --slow 500 --ns shop.orders --limit 50
mdbkit filter mongod.log --from 2026-07-01T14:30:00+04:00 --to 2026-07-01T15:00:00+04:00
mdbkit filter mongod.log --slow 100 | mdbkit queries -

From a slow query in the log to an explain plan, without hand-writing the query — --as-explain rebuilds the command that ran:

# See the actual commands behind your slowest operations
mdbkit filter mongod.log --ns shop.orders --slow 500 --last 3 --as-explain

# Or produce a runnable script, get the plan, and analyze it
mdbkit filter mongod.log --slow 500 --last 1 --as-explain --explain-script > q.js
mongosh --quiet --host your_db_host --username your_username \\
        --password your_password --authenticationDatabase admin \\
        --eval "$(cat q.js)" > explain.json
mdbkit explain explain.json

Rebuilt commands contain the real values from your log (not redacted shapes) — treat them as sensitive.

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mdbkit advise <log>

Deterministic candidate index recommendations from observed slow-query shapes, using the ESR guideline (Equality → Sort → Range). Rules, not AI: the same log always produces the same advice.

Option	Default	Description
--indexes FILE		indexes.json from mdbkit export-script indexes — enables overlap checks against existing indexes
--schema FILE		schema.json from mdbkit export-script schema — enables field-type caveats and confidence adjustment
--ns NAMESPACE	all	Only advise on one namespace (recommended on large logs)
--limit N	10	Show only the top N recommendations (0 = all)
--min-ms N	0	Ignore operations faster than N milliseconds
--min-count N	1	Only advise on shapes seen at least N times
--include-system	off	Include internal admin/config/local namespaces
--json		Machine-readable output

Each recommendation carries a candidate key pattern, the evidence behind it, a confidence level, caveats, and a validation step. mdbkit never advises dropping an index — at most it flags an overlap to investigate.

mdbkit advise mongod.log
mdbkit advise mongod.log --indexes indexes.json --schema schema.json
mdbkit advise mongod.log --ns shop.orders --limit 3

---

mdbkit explain <file>

Analyzes a saved explain("executionStats") document: the plan chain, the examined-vs-returned math, plain-English verdicts, and — when the plan needs help — a candidate index from the same advisor engine.

Option	Description
--indexes FILE	Overlap check against existing indexes
--schema FILE	Field-type caveats
--json	Machine-readable output

Full example. Get a plan for a query and analyze it:

# 1. Capture the plan (adjust host/credentials for your deployment)
mongosh --quiet \\
  --host your_db_host \\
  --port 27017 \\
  --username your_username \\
  --password your_password \\
  --authenticationDatabase admin \\
  --eval 'EJSON.stringify(db.getSiblingDB("shop").orders.find({status:"open"}).sort({ts:-1}).explain("executionStats"))' \\
  > explain.json

# 2. Analyze it
mdbkit explain explain.json

# 3. Sharper, with your existing indexes and sampled schema
mdbkit explain explain.json --indexes indexes.json --schema schema.json

Don't want to write the query by hand? mdbkit filter ... --as-explain rebuilds it from the log for you (see the filter section above).

Legacy mongo shell and Compass output containing NumberLong(...), ISODate(...) or ObjectId(...) is accepted — mdbkit unwraps those automatically, so you do not have to re-export.

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mdbkit triage <log>

"Triage" means: quickly work out what is wrong and what to look at first. Run this when something has gone wrong — or has just gone wrong — and you need one screen that says what happened, how bad it is, and where to look next. Defaults to the last 60 minutes of log time.

Option	Default	Description
--window N	60	Analyze the last N minutes of log time; 0 = the whole file
--dbpath PATH	auto	Override the data directory used for the disk check
--no-sysprobe	off	Skip local disk/memory/CPU probes — use when analyzing a log copied off the host
--ftdc PATH		diagnostic.data directory — adds CPU, memory, cache, queue and connection history from MongoDB's own recorder
--report FILE		Write a shareable .md or .html report instead of terminal output
--json		Machine-readable output

mdbkit triage /var/log/mongodb/mongod.log
mdbkit triage mongod.log --window 30
mdbkit triage mongod.log --ftdc /var/lib/mongodb/diagnostic.data
mdbkit triage mongod.log --report incident.html
mdbkit triage mongod.log --window 0 --no-sysprobe

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mdbkit ftdc {summary|timeline|export} <path>

Decodes diagnostic.data — FTDC (Full-Time Diagnostic Data Capture), the metrics recorder every mongod already runs. It holds CPU, memory, WiredTiger cache, connection, queue and operation history for every node, with no monitoring agent installed and no database connection. It is compressed BSON, not encrypted; mdbkit decodes it offline.

Action	Description
summary	min / avg / max / last per metric, plus per-second rates for counters
timeline	Values bucketed over time — shows when something spiked
export	CSV to stdout, for a spreadsheet or your own tooling

Option	Default	Description
--last DURATION	4h	Analyze only the most recent window — 90m, 4h, 2d
--all	off	Analyze the entire history (see the performance note below)
--metric LABEL	all	Restrict to one metric (repeatable), e.g. --metric conns.current
--step SECONDS	60	Timeline bucket size
--from / --to		Explicit time bounds (same formats as filter)
--json		Machine-readable output

Performance note. diagnostic.data can hold weeks of per-second samples — a few hundred megabytes covering thousands of chunks and several thousand metrics each. Decoding all of it is CPU-bound and takes minutes, so these commands default to the last 4 hours and skip older chunks before decompressing them. On a 250 MB directory that is the difference between about a second and about a minute. Use --last/--from/--to to move the window, and --all when you really do want the whole history.

mdbkit ftdc summary /var/lib/mongodb/diagnostic.data
mdbkit ftdc timeline diagnostic.data --metric conns.current --step 300
mdbkit ftdc export diagnostic.data > metrics.csv

Metric labels include ops.* (insert/query/update/delete/getmore/command), conns.current, conns.available, queue.readers, queue.writers, cache.usedBytes, cache.maxBytes, cache.dirtyBytes, tickets.*, mem.residentMB, and on Linux sys.cpu.* and sys.mem.availableKB.

The data directory can be copied off the host and analyzed elsewhere — it contains metrics only, never document contents.

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Shareable reports — --report FILE

triage and queries can write a self-contained report instead of printing to the terminal — for a ticket, a handover, or a post-incident review.

mdbkit triage mongod.log --report incident.html     # styled, self-contained
mdbkit triage mongod.log --report incident.md       # for tickets and PRs
mdbkit queries mongod.log --limit 20 --report slow-queries.md

The format follows the file extension: .html or .md.

Markdown output looks like this:

# MongoDB incident triage

*window 2026-07-01 08:10 -> 09:10  ·  generated 2026-07-01 09:12*

## Findings

- **[CRIT] Replica set instability** — 3 election/stepdown event(s) at 08:41:02, 08:58:14
    - Starting an election, since we've seen no PRIMARY in election timeout period
    - *next:* `Correlate with connection storms and slow checkpoints below`
- **[WARN] Connection storm** — 2 minute(s) at >= 60 new connections/min; peak 480 at 08:41
    - 10.2.1.7: 312 in the peak minute
    - *next:* `mdbkit connections <log>`
- **[OK] Errors** — No error/fatal severity lines in window.

The HTML version carries the same content with a dark, print-friendly stylesheet. It is fully self-contained: inline CSS, no JavaScript, no external assets or CDN references, so it opens on an air-gapped machine and sends nothing anywhere.

Reports contain the same information as the terminal output — query shapes and metrics, never literal values from your documents.

---

mdbkit demo

Generates a realistic MongoDB structured log so you can evaluate mdbkit — or run a live demo — without a cluster. Output is deterministic for a given seed, so a demo behaves identically every time, including on a projector.

Option	Default	Description
--scenario	mixed	healthy, incident, or mixed
--minutes N	90	How much log time to generate
--seed N	7	Same seed produces byte-identical output
-o, --out FILE	stdout	Write to a file
--with-extras	off	Also write indexes.json, schema.json and explain.json beside the log

mdbkit demo -o demo.log                          # 90 minutes, mixed
mdbkit demo --scenario incident --minutes 30 -o incident.log
mdbkit demo --scenario healthy -o quiet.log      # nothing wrong: the control case
mdbkit demo | mdbkit queries -                   # straight down a pipe

The incident scenario contains an index build, a connection storm from a single client, a replica set election, plan-executor errors, a slow WiredTiger checkpoint, and a burst of unindexed queries afterwards — the shape of a real bad afternoon.

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mdbkit lab

Starts a disposable local MongoDB for testing, reproducing a slow query, or rehearsing a demo. This is the only command that starts external processes; see SECURITY.md for exactly how it is bounded.

Requires mongod on your PATH (and mongosh to initiate the replica set and seed data). Linux and macOS.

Action	What it does
start	Create and start a replica set, print the connection string and log paths
seed	Insert sample data and run a workload with deliberately interesting queries
status	Show ports, pids and whether each node is running
logs	Print the log file paths, ready to pipe into other commands
stop	Stop the nodes, keep the data
destroy	Stop and delete the lab (requires --yes)

Option	Default	Description
--dir PATH	~/.mdbkit-lab	Where the lab lives
--nodes N	3	Replica set size
--port N	28110	Base port — deliberately far from 27017
--slowms N	0	Log every operation, which is what makes the log worth reading
--standalone	off	Single node, no replica set
--docs N	50000	Documents inserted by seed
--yes		Confirm destroy

The full loop:

mdbkit lab start                    # 3-node replica set on 28110-28112
mdbkit lab seed                     # sample data + a mixed workload

mdbkit queries $(mdbkit lab logs | head -1)
mdbkit advise  $(mdbkit lab logs | head -1)

mdbkit lab destroy --yes            # remove everything

mdbkit lab logs prints the log file path of every node, one per line, so it composes with the other commands instead of you hunting for paths:

mdbkit lab logs
# /home/you/.mdbkit-lab/node0/mongod.log
# /home/you/.mdbkit-lab/node1/mongod.log
# /home/you/.mdbkit-lab/node2/mongod.log

mdbkit queries $(mdbkit lab logs | head -1)     # just the primary
mdbkit triage  $(mdbkit lab logs)               # all three as one stream
mdbkit loginfo $(mdbkit lab logs | sed -n 2p)   # a specific secondary

A single node, when you do not need replication — faster to start and it does not require mongosh:

mdbkit lab start --standalone
mdbkit lab seed --docs 5000
mdbkit queries $(mdbkit lab logs)
mdbkit lab destroy --yes

Several labs side by side, for example to compare two MongoDB versions or keep one running while you break another:

mdbkit lab start --dir ~/lab-a --port 28110
mdbkit lab start --dir ~/lab-b --port 28210 --standalone

mdbkit lab status --dir ~/lab-a
mdbkit lab destroy --dir ~/lab-b --yes

Pause without losing data — stop leaves the data directory intact so you can start again later; only destroy deletes anything:

mdbkit lab stop                     # nodes down, data kept
mdbkit lab start                    # back up with the same data
mdbkit lab status                   # ports, pids, running or not

A complete before/after experiment, which is what lab is really for:

mdbkit lab start && mdbkit lab seed
cp $(mdbkit lab logs | head -1) before.log

mongosh --port 28110 --eval \
  'db.getSiblingDB("shop").orders.createIndex({status:1, createdAt:-1})'

mdbkit lab seed                     # run the workload again with the index
cp $(mdbkit lab logs | head -1) after.log

mdbkit compare before.log --after after.log
mdbkit lab destroy --yes

seed runs indexed point lookups alongside deliberately unindexed queries — an equality-plus-range-plus-sort with no supporting index, an aggregation that scans the collection, and updates whose predicate has no index — so the log immediately contains something worth analysing.

Safety. The lab binds to 127.0.0.1 only, refuses to use or delete any directory it did not create, and never touches a MongoDB it did not start. It is a laptop and scratch-VM tool, not a deployment tool.

---

mdbkit oslog [FILE...]

Scans a system log for the things that affect a database process: OOM kills, file-descriptor limits, segmentation faults, filesystem and I/O errors, read-only remounts, conntrack exhaustion, and systemd service exits.

With no argument it reads /var/log/syslog or /var/log/messages if they are readable.

Option	Description
--exit-code	Exit 2 on CRIT, 1 on WARN, else 0
--json	Machine-readable output

mdbkit oslog                                  # whichever system log exists
mdbkit oslog /var/log/messages
mdbkit oslog /var/log/syslog.1 /var/log/syslog

On journald systems there is no text log to read, and mdbkit does not run commands on your behalf. It tells you what to capture instead:

journalctl -k --since '4 hours ago' > kern.log
journalctl -u mongod --since '4 hours ago' >> kern.log
mdbkit oslog kern.log

The same file can be handed to triage --oslog, which correlates it with the mongod log — so an unexplained restart at 09:14 lines up with the OOM kill that caused it.

---

mdbkit serverstatus FILE [--after FILE]

Digests a saved db.adminCommand({serverStatus: 1}) dump. That command returns several hundred fields; this reports the handful that explain a struggling server.

Option	Description
--after FILE	A second dump taken later — turns cumulative counters into true rates
--report FILE	Write a shareable .md or .html report
--exit-code	Exit 2 on CRIT, 1 on WARN, else 0
--json	Machine-readable output

mdbkit export-script serverstatus > export_serverstatus.js

mongosh --quiet --host HOST --port PORT \
  --username USER --password PASS --authenticationDatabase admin \
  --eval "$(cat export_serverstatus.js)" > status.json

mdbkit serverstatus status.json

What it checks: concurrency tickets (exhaustion queues every operation and looks like slowness with no slow query to blame), WiredTiger cache against the 80% background-eviction and 95% application-thread-eviction thresholds, dirty cache, application-thread eviction, connection headroom, queued readers and writers, assertions, flow control, replication role and process memory. Ticket layout is read from either wiredTiger.concurrentTransactions (pre-7.0) or queues.execution (7.0+).

Two dumps give true rates. Almost everything in serverStatus is cumulative since process start, so a single dump only yields lifetime averages:

mongosh ... > before.json ; sleep 60 ; mongosh ... > after.json
mdbkit serverstatus before.json --after after.json

[INFO] Operation counters: Measured over 60 seconds between the two dumps.
        - query    742,000 in 60s  (12366.7/sec)

That is real current load. The same counter read from one dump would have reported 2,127/sec — the average since the server started ten days ago.

---

mdbkit compare BEFORE --after AFTER

Diffs query shapes between two logs and reports what improved, what regressed, and what is new. The natural follow-up to advise: you created an index, a day passed, and this answers whether it worked.

Option	Default	Description
--after FILE...	required	The log(s) from after the change
--ns NAMESPACE	all	Compare only one namespace
--min-count N	3	Ignore shapes seen fewer than N times, so noise in a quiet log does not read as a regression
--min-ms N	0	Ignore operations faster than this
--limit N	15	Shapes to print (0 = all)
--include-system	off	Include internal admin/config/local namespaces
--report FILE		Write a shareable .md or .html report
--json		Machine-readable output

mdbkit compare before.log --after after.log
mdbkit compare before.log --after after.log --ns shop.orders
mdbkit compare "old/mongod.log*" --after "new/mongod.log*" --report change.html

slow-query time DOWN 32%  (6.0m -> 4.1m across compared shapes)
shapes: 1 improved, 0 regressed, 0 new, 0 gone, 4 unchanged

IMPROVED
  shop.orders {createdAt:gt, status:eq} sort:{createdAt:-1}
    mean 1.7s -> 33ms (-98%)   scan 2976:1 -> 1:1  [COLLSCAN -> index, in-memory sort gone]

A shape counts as improved or regressed on a plan change (COLLSCAN becoming an index scan, or the reverse), on an in-memory sort disappearing, or on mean duration moving by more than 20%.

---

mdbkit export-script {schema|indexes}

Prints a small mongosh script to stdout. mdbkit never connects to your database — you run these yourself, so you can read exactly what they do first. Both are read-only and export field names and types only, never document values.

mdbkit export-script indexes > export_indexes.js
mdbkit export-script schema  > export_schema.js

---

Roadmap

Terminal output is and will remain first-class — this tool is built for the Linux box the database actually runs on.

Shipped in v0.4: compare, rotated-log globbing, per-shape drill-down — on top of v0.3's demo and lab, and v0.2's FTDC decoding, incident triage, query reconstruction and shareable reports.

Next up, roughly in order:

• Sharded clusters. mongos logs are a different shape, and the classic sharded failure — a query with no shard key fanning out to every shard — is visible in the log. Also chunk migrations, balancer windows and jumbo chunks. Would come with mdbkit lab --sharded so it can be tested.
• Startup configuration audit. mongod logs warnings at startup about transparent huge pages, readahead, ulimits, NUMA and filesystem choice. These are classic production misconfigurations and they are already in your log — nothing new needs collecting.
• Index usage candidates. Prefix-redundant indexes (an index on {a: 1} when {a: 1, b: 1} exists) are worth examining, but static analysis is not sufficient grounds to drop one — the planner may still be choosing it. So mdbkit will flag candidates and print an $indexStats script to confirm real usage first, never a drop recommendation.
• Confirming the FTDC-based checkpoint, eviction and flow-control detectors against real diagnostic.data — see docs/TESTING-PLAYBOOK.md. Real logs and metrics very welcome.

mdbkit is validated against real-world structured logs (tens of thousands of lines) in addition to its synthetic test fixtures.

Bugs, feature requests, questions

Please use GitHub Issues — it keeps problems and fixes public so the next person can find them. Real-world log lines that parse wrongly are the most valuable bug reports of all (redact literals first!).

Security

mdbkit is offline by design: the codebase contains no network code, never executes or evaluates input, and treats every log line as untrusted data (strict JSON parsing only — shell constructors are never evaluated). See SECURITY.md for the reporting process.

Non-affiliation

mdbkit is an independent community project. It is not affiliated with, endorsed by, or sponsored by MongoDB, Inc. "MongoDB" is a registered trademark of MongoDB, Inc., used here only to describe compatibility.

License

MIT — see LICENSE.