Ever wonder what would happen to an open source database project in case its main developers “got hit by a bus”? Or, less dramatically, if they leave the project completely. That’s what the “bus factor” (also called “truck factor”) measures: how many people would have to disappear before no one left knows how to fix or update specific parts of the code. The Bus Factor Ranking I’ve been messing around with a tool called the Bus Factor Explorer by JetBrains to explore the associated risk in some of the most popular open source databases. I looked at six of the big ones to see where they stand. Here’s the current baseline (March 2026) according to this tool: Bus Factor Explorer Database

Bus Factor (higher is better)



MongoDB

7



MariaDB

5



Redis

5



MySQL

2



PostgreSQL

2



SQLite

2 Database

Bus Factor (higher is better)



MongoDB

7



MariaDB

5



Redis

5



MySQL

2



PostgreSQL

2



SQLite

2 Database

Bus Factor (higher is better) Database Database Bus Factor (higher is better) Bus Factor (higher is better) MongoDB

7 MongoDB MongoDB MongoDB 7 7 7 MariaDB

5 MariaDB MariaDB MariaDB 5 5 5 Redis

5 Redis Redis Redis 5 5 5 MySQL

2 MySQL MySQL MySQL 2 2 2 PostgreSQL

2 PostgreSQL PostgreSQL PostgreSQL 2 2 2 SQLite

2 SQLite SQLite SQLite 2 2 2 For example, for MongoDB to “hit a wall”, 7 devs would have to leave the project. For MySQL, PostgreSQL, and SQLite, if 2 devs leave, the project is at risk of stalling. Of course, this is just one factor; there are other factors that influence the continuation of open source projects, but this is still an interesting insight you can take into consideration when choosing a database. MongoDB MySQL PostgreSQL SQLite The tool has a categorization according to the project’s bus factor (a baseline that can be adjusted in the tool if you want): ✅ OK: 5 or more (MongoDB, MariaDB, Redis)
⚠️ Low: 2 to 4 (MySQL, PostgreSQL, SQLite)
🔴 Dangerous: 0 or 1 (N/A) ✅ OK: 5 or more (MongoDB, MariaDB, Redis) OK ⚠️ Low: 2 to 4 (MySQL, PostgreSQL, SQLite) Low 🔴 Dangerous: 0 or 1 (N/A) Dangerous Simulating Top Contributors Leaving the Projects I also used the tool to see what happens to the root directories (the functional parts of the code) if you start unchecking the top contributors to each project. Specifically, I wanted to see the proportion of directories that would be “lost” if the top one or two contributors left the project. I ignored the individual files at the root level. The results look like this: Database

Directories (total)

Directories lost (1 dev gone)

Directories lost (2 devs gone)



Redis

6

0 (0.0%)

0 (0.0%)



MariaDB

30

2 (6.7%)

5 (16.7%)



MongoDB

28

1 (3.6%)

7 (25.0%)



PostgreSQL

6

0 (0.0%)

5 (83.3%)



MySQL

30

1 (3.3%)

30 (100.0%)



SQLite

11

5 (45.5%)

11 (100.0%) Database

Directories (total)

Directories lost (1 dev gone)

Directories lost (2 devs gone)



Redis

6

0 (0.0%)

0 (0.0%)



MariaDB

30

2 (6.7%)

5 (16.7%)



MongoDB

28

1 (3.6%)

7 (25.0%)



PostgreSQL

6

0 (0.0%)

5 (83.3%)



MySQL

30

1 (3.3%)

30 (100.0%)



SQLite

11

5 (45.5%)

11 (100.0%) Database

Directories (total)

Directories lost (1 dev gone)

Directories lost (2 devs gone) Database Database Directories (total) Directories (total) Directories lost (1 dev gone) Directories lost (1 dev gone) Directories lost (2 devs gone) Directories lost (2 devs gone) Redis

6

0 (0.0%)

0 (0.0%) Redis Redis Redis 6 6 6 0 (0.0%) 0 (0.0%) 0 (0.0%) 0 (0.0%) 0 (0.0%) 0 (0.0%) MariaDB

30

2 (6.7%)

5 (16.7%) MariaDB MariaDB MariaDB 30 30 30 2 (6.7%) 2 (6.7%) 2 (6.7%) 5 (16.7%) 5 (16.7%) 5 (16.7%) MongoDB

28

1 (3.6%)

7 (25.0%) MongoDB MongoDB MongoDB 28 28 28 1 (3.6%) 1 (3.6%) 1 (3.6%) 7 (25.0%) 7 (25.0%) 7 (25.0%) PostgreSQL

6

0 (0.0%)

5 (83.3%) PostgreSQL PostgreSQL PostgreSQL 6 6 6 0 (0.0%) 0 (0.0%) 0 (0.0%) 5 (83.3%) 5 (83.3%) 5 (83.3%) MySQL

30

1 (3.3%)

30 (100.0%) MySQL MySQL MySQL 30 30 30 1 (3.3%) 1 (3.3%) 1 (3.3%) 30 (100.0%) 30 (100.0%) 30 (100.0%) SQLite

11

5 (45.5%)

11 (100.0%) SQLite SQLite SQLite 11 11 11 5 (45.5%) 5 (45.5%) 5 (45.5%) 11 (100.0%) 11 (100.0%) 11 (100.0%) Here, a lower number is better (fewer directories impacted). So, Redis is pretty strong in this simulation, as no directories would take a hit. On the other end, we have MySQL and SQLite, with 100% of their directories potentially left unmaintained if the top two developers leave. PostgreSQL would lose 83% of its directories in a similar situation. These were the big surprises to me, although all this is aligned with the fact that they have a low bus factor (high risk) of 2. Redis MySQL SQLite, PostgreSQL MariaDB did pretty well, especially when compared to MySQL, which supports what I have been trying to say about how MariaDB is much more than a fork of MySQL in my articles and talks (see this and this, if you are curious). MariaDB MySQL MariaDB is much more than a fork of MySQL this this Other Important Factors When Evaluating Open Source Projects You should not rely merely on “bus factor” risk assessments. This is useful in mission-critical situations or when comparing projects that are even in other metrics that you might be using to compare and evaluate them. Here are some other aspects to look at: Corporate Backing: Does the project have large companies behind it? Even if a lead developer leaves, the company is likely to hire someone else to fill the gap, ensuring continuity.
Community Activity: Look at the number of active issues, pull requests, and discussions. A buzzing community can often sustain a project better than a few silent experts.
Documentation Quality: Comprehensive documentation ensures that knowledge isn’t locked in someone’s head, making it easier for new contributors to onboard.
Licensing: Ensure the license fits your use case, as this impacts the project’s long-term viability and adoption. For example, a GPL-licensed project doesn’t allow anybody to distribute a closed-source application based on it, hence the project is future-proof as far as licensing goes. Corporate Backing: Does the project have large companies behind it? Even if a lead developer leaves, the company is likely to hire someone else to fill the gap, ensuring continuity. Corporate Backing: Community Activity: Look at the number of active issues, pull requests, and discussions. A buzzing community can often sustain a project better than a few silent experts. Community Activity: Documentation Quality: Comprehensive documentation ensures that knowledge isn’t locked in someone’s head, making it easier for new contributors to onboard. Documentation Quality: Licensing: Ensure the license fits your use case, as this impacts the project’s long-term viability and adoption. For example, a GPL-licensed project doesn’t allow anybody to distribute a closed-source application based on it, hence the project is future-proof as far as licensing goes. Licensing: Investigate Your Projects and Their Dependencies I investigated the bus factor of other open source projects that I’m a big fan of, like MyBatis, Vaadin, and others, but I’ll let you do your own findings with the tool. Let me know if you find something interesting! tool Appendix: Raw Impact Data This appendix lists every directory and file at the root level that drops to a Bus Factor of 0 in the simulations. and file Bus Factor of 0 1. MongoDB (Baseline: 7 - OK) With 1 dev gone: .bazelrc.local.example, .gitattributes, .prettierignore, MODULE.bazel, distsrc, etc
With 2 devs gone: All of the above, plus bazel, buildscripts, jstests With 1 dev gone: .bazelrc.local.example, .gitattributes, .prettierignore, MODULE.bazel, distsrc, etc With 1 dev gone: .bazelrc.local.example .gitattributes .prettierignore MODULE.bazel distsrc etc With 2 devs gone: All of the above, plus bazel, buildscripts, jstests With 2 devs gone: bazel buildscripts jstests 2. MariaDB (Baseline: 5 - OK) With 1 dev gone: VERSION, config.h.cmake, CMakeLists.txt, libmysqld, debian
With 2 devs gone: VERSION, config.h.cmake, CMakeLists.txt, libmysqld, debian With 1 dev gone: VERSION, config.h.cmake, CMakeLists.txt, libmysqld, debian With 1 dev gone: VERSION config.h.cmake CMakeLists.txt libmysqld debian With 2 devs gone: VERSION, config.h.cmake, CMakeLists.txt, libmysqld, debian With 2 devs gone: VERSION config.h.cmake CMakeLists.txt libmysqld debian 3. Redis (Baseline: 5 - OK) With 1 dev gone: None
With 2 devs gone: None With 1 dev gone: None With 1 dev gone: None With 2 devs gone: None With 2 devs gone: None 4. MySQL (Baseline: 2 - Low) With 1 dev gone: mysql/mysql-server (root meta), iwyu_mappings.imp, config.h.cmake, CMakeLists.txt, libmysql, mysys, client, components, strings, mysql-test
With 2 devs gone: Virtually all project directories (36 total), including storage, sql, router, extra, plugin, utilities, etc. With 1 dev gone: mysql/mysql-server (root meta), iwyu_mappings.imp, config.h.cmake, CMakeLists.txt, libmysql, mysys, client, components, strings, mysql-test With 1 dev gone: mysql/mysql-server iwyu_mappings.imp config.h.cmake CMakeLists.txt libmysql mysys client components strings mysql-test With 2 devs gone: Virtually all project directories (36 total), including storage, sql, router, extra, plugin, utilities, etc. With 2 devs gone: storage sql router extra plugin utilities 5. PostgreSQL (Baseline: 2 - Low) With 1 dev gone: COPYRIGHT, meson_options.txt
With 2 devs gone: Virtually all functional directories (23 total), including src, contrib, doc, config, interfaces, etc. With 1 dev gone: COPYRIGHT, meson_options.txt With 1 dev gone: COPYRIGHT meson_options.txt With 2 devs gone: Virtually all functional directories (23 total), including src, contrib, doc, config, interfaces, etc. With 2 devs gone: src contrib doc config interfaces 6. SQLite (Baseline: 2 - Low) With 1 dev gone: VERSION, manifest.tags, make.bat, manifest.uuid, configure, README.md, mptest, main.mk, Makefile.msc, doc, manifest, tool, src, ext, test
With 2 devs gone: Virtually all project boxes (28 total), including root-level scripts and all core subdirectories. With 1 dev gone: VERSION, manifest.tags, make.bat, manifest.uuid, configure, README.md, mptest, main.mk, Makefile.msc, doc, manifest, tool, src, ext, test With 1 dev gone: VERSION manifest.tags make.bat manifest.uuid configure README.md mptest main.mk Makefile.msc doc manifest tool src ext test With 2 devs gone: Virtually all project boxes (28 total), including root-level scripts and all core subdirectories. With 2 devs gone:

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Investigating the "Bus Factor" Risk in MongoDB, MariaDB, and More 

Investigating the "Bus Factor" Risk in MongoDB, MariaDB, and More

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