How to resolve featureCompatibilityVersion issue between MongoDB 3.6 and 4.2? - mongodb

Question

I recently uninstalled MongoDB 3.6 and then installed MongoDB 4.2. When I navigated to MongoDB/Server/4.2/bin and executed mongo, I got the following error message:
$ mongod
2019-10-22T21:16:36.723-0400 I CONTROL [main] Automatically disabling TLS 1.0, to force-enable TLS 1.0 specify --sslDisabledProtocols 'none'
2019-10-22T21:16:37.217-0400 I CONTROL [initandlisten] MongoDB starting : pid=9376 port=27017 dbpath=C:\data\db\ 64-bit host=DESKTOP-LF34G1M
2019-10-22T21:16:37.217-0400 I CONTROL [initandlisten] targetMinOS: Windows 7/Windows Server 2008 R2
2019-10-22T21:16:37.217-0400 I CONTROL [initandlisten] db version v4.2.1
2019-10-22T21:16:37.218-0400 I CONTROL [initandlisten] git version: edf6d45851c0b9ee15548f0f847df141764a317e
2019-10-22T21:16:37.218-0400 I CONTROL [initandlisten] allocator: tcmalloc
2019-10-22T21:16:37.218-0400 I CONTROL [initandlisten] modules: none
2019-10-22T21:16:37.218-0400 I CONTROL [initandlisten] build environment:
2019-10-22T21:16:37.218-0400 I CONTROL [initandlisten] distmod: 2012plus
2019-10-22T21:16:37.218-0400 I CONTROL [initandlisten] distarch: x86_64
2019-10-22T21:16:37.219-0400 I CONTROL [initandlisten] target_arch: x86_64
2019-10-22T21:16:37.219-0400 I CONTROL [initandlisten] options: {}
2019-10-22T21:16:37.230-0400 I STORAGE [initandlisten] Detected data files in C:\data\db\ created by the 'wiredTiger' storage engine, so setting the active storage engine to 'wire
dTiger'.
2019-10-22T21:16:37.231-0400 I STORAGE [initandlisten] wiredtiger_open config: create,cache_size=1498M,cache_overflow=(file_max=0M),session_max=33000,eviction=(threads_min=4,threa
ds_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000,close_scan_interval=10,close_handle
_minimum=250),statistics_log=(wait=0),verbose=[recovery_progress,checkpoint_progress],
2019-10-22T21:16:37.295-0400 I STORAGE [initandlisten] WiredTiger message [1571793397:294196][9376:140704250879568], txn-recover: Recovering log 22 through 23
2019-10-22T21:16:37.439-0400 I STORAGE [initandlisten] WiredTiger message [1571793397:438812][9376:140704250879568], txn-recover: Recovering log 23 through 23
2019-10-22T21:16:37.561-0400 I STORAGE [initandlisten] WiredTiger message [1571793397:560490][9376:140704250879568], txn-recover: Main recovery loop: starting at 22/896 to 23/256
2019-10-22T21:16:37.712-0400 I STORAGE [initandlisten] WiredTiger message [1571793397:712086][9376:140704250879568], txn-recover: Recovering log 22 through 23
2019-10-22T21:16:37.807-0400 I STORAGE [initandlisten] WiredTiger message [1571793397:806834][9376:140704250879568], txn-recover: Recovering log 23 through 23
2019-10-22T21:16:37.883-0400 I STORAGE [initandlisten] WiredTiger message [1571793397:882664][9376:140704250879568], txn-recover: Set global recovery timestamp: (0,0)
2019-10-22T21:16:37.934-0400 I RECOVERY [initandlisten] WiredTiger recoveryTimestamp. Ts: Timestamp(0, 0)
2019-10-22T21:16:37.943-0400 I STORAGE [initandlisten] Timestamp monitor starting
2019-10-22T21:16:37.947-0400 I CONTROL [initandlisten]
2019-10-22T21:16:37.947-0400 I CONTROL [initandlisten] ** WARNING: Access control is not enabled for the database.
2019-10-22T21:16:37.948-0400 I CONTROL [initandlisten] ** Read and write access to data and configuration is unrestricted.
2019-10-22T21:16:37.948-0400 I CONTROL [initandlisten]
2019-10-22T21:16:37.948-0400 I CONTROL [initandlisten] ** WARNING: This server is bound to localhost.
2019-10-22T21:16:37.948-0400 I CONTROL [initandlisten] ** Remote systems will be unable to connect to this server.
2019-10-22T21:16:37.948-0400 I CONTROL [initandlisten] ** Start the server with --bind_ip <address> to specify which IP
2019-10-22T21:16:37.949-0400 I CONTROL [initandlisten] ** addresses it should serve responses from, or with --bind_ip_all to
2019-10-22T21:16:37.949-0400 I CONTROL [initandlisten] ** bind to all interfaces. If this behavior is desired, start the
2019-10-22T21:16:37.949-0400 I CONTROL [initandlisten] ** server with --bind_ip 127.0.0.1 to disable this warning.
2019-10-22T21:16:37.949-0400 I CONTROL [initandlisten]
2019-10-22T21:16:37.959-0400 I SHARDING [initandlisten] Marking collection local.system.replset as collection version: <unsharded>
2019-10-22T21:16:37.965-0400 F CONTROL [initandlisten] ** IMPORTANT: UPGRADE PROBLEM: Found an invalid featureCompatibilityVersion document (ERROR: BadValue: Invalid value for ver
sion, found 3.6, expected '4.2' or '4.0'. Contents of featureCompatibilityVersion document in admin.system.version: { _id: "featureCompatibilityVersion", version: "3.6" }. See http:
//dochub.mongodb.org/core/4.0-feature-compatibility.). If the current featureCompatibilityVersion is below 4.0, see the documentation on upgrading at http://dochub.mongodb.org/core/
4.0-upgrade-fcv.
2019-10-22T21:16:37.965-0400 I NETWORK [initandlisten] shutdown: going to close listening sockets...
2019-10-22T21:16:37.966-0400 I - [initandlisten] Stopping further Flow Control ticket acquisitions.
2019-10-22T21:16:37.966-0400 I STORAGE [initandlisten] Deregistering all the collections
2019-10-22T21:16:37.966-0400 I STORAGE [initandlisten] Timestamp monitor shutting down
2019-10-22T21:16:37.967-0400 I STORAGE [initandlisten] WiredTigerKVEngine shutting down
2019-10-22T21:16:37.967-0400 I STORAGE [initandlisten] Shutting down session sweeper thread
2019-10-22T21:16:37.968-0400 I STORAGE [initandlisten] Finished shutting down session sweeper thread
2019-10-22T21:16:37.968-0400 I STORAGE [initandlisten] Shutting down journal flusher thread
2019-10-22T21:16:38.037-0400 I STORAGE [initandlisten] Finished shutting down journal flusher thread
2019-10-22T21:16:38.037-0400 I STORAGE [initandlisten] Shutting down checkpoint thread
2019-10-22T21:16:38.038-0400 I STORAGE [initandlisten] Finished shutting down checkpoint thread
2019-10-22T21:16:38.038-0400 I STORAGE [initandlisten] Downgrading WiredTiger datafiles.
2019-10-22T21:16:38.188-0400 I STORAGE [initandlisten] WiredTiger message [1571793398:187822][9376:140704250879568], txn-recover: Recovering log 23 through 24
2019-10-22T21:16:38.273-0400 I STORAGE [initandlisten] WiredTiger message [1571793398:272644][9376:140704250879568], txn-recover: Recovering log 24 through 24
2019-10-22T21:16:38.365-0400 I STORAGE [initandlisten] WiredTiger message [1571793398:365403][9376:140704250879568], txn-recover: Main recovery loop: starting at 23/2048 to 24/256
2019-10-22T21:16:38.533-0400 I STORAGE [initandlisten] WiredTiger message [1571793398:532944][9376:140704250879568], txn-recover: Recovering log 23 through 24
2019-10-22T21:16:38.642-0400 I STORAGE [initandlisten] WiredTiger message [1571793398:642615][9376:140704250879568], txn-recover: Recovering log 24 through 24
2019-10-22T21:16:38.722-0400 I STORAGE [initandlisten] WiredTiger message [1571793398:722405][9376:140704250879568], txn-recover: Set global recovery timestamp: (0,0)
2019-10-22T21:16:39.004-0400 I STORAGE [initandlisten] shutdown: removing fs lock...
2019-10-22T21:16:39.005-0400 I CONTROL [initandlisten] now exiting
2019-10-22T21:16:39.005-0400 I CONTROL [initandlisten] shutting down with code:62
The error which confuses me is:
UPGRADE PROBLEM: Found an invalid featureCompatibilityVersion document (ERROR: BadValue: Invalid value for version, found 3.6, expected '4.2' or '4.0'.
This does not seem to align with what I'm seeing below:
> db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )
{ "featureCompatibilityVersion" : { "version" : "4.2" }, "ok" : 1 }
How am I able to fix this featureCompatibilityVersion issue?

Answer 1

The answer lies in the error message.
If the current featureCompatibilityVersion is below 4.0,
see the documentation on upgrading at
https://docs.mongodb.com/manual/release-notes/4.2/#upgrade-procedures
That page points out that if you have a version 3 installation, first you have to upgrade to 4.0. It doesn't help that on Macs, homebrew offers a mongodb-community installation that does not specify a version number. Currently, mongodb-community is 4.2.
What I did is:
brew services stop mongodb-community
brew uninstall mongodb-community
brew install mongodb-community#4.0
brew install mongodb-community-shell
brew services start mongodb-community#4.0
mongo
db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )
[ it gave: { "featureCompatibilityVersion" : { "version" : "3.6" }, "ok" : 1 } ]
db.adminCommand( { setFeatureCompatibilityVersion: "4.0" } )
ctrl-c
brew services stop mongodb-community#4.0
brew uninstall mongodb-community#4.0
brew uninstall mongodb-community-shell [you get a new one with the next step]
brew install mongodb-community
brew services start mongodb-community
Which doesn't exactly trip off the tongue but did work for me without too much trouble.

Answer 2

My database directory, C:\data\db\, still existed from when 3.6 was running. The directory kept its state even after 3.6 was uninstalled, and was still configured for 3.6.
After clearing the directory, re-running mongod a second time did not produce any errors.

Answer 3

Quick 2023 update:
You can't install mongodb-community#4.0 via brew anymore, so my advice is:
Download the required version (4.0) from https://www.mongodb.com/try/download/community
Extract the files and run the binary bin/mongod directly from the extracted directory (you may need to grant permissions in your OS).
Once it's running, make the adjustments outlined here: https://www.mongodb.com/docs/manual/release-notes/4.2-upgrade-standalone/
Once you have completed the instructions from the link above, shutdown the server.
Now run your newer version of mongod, it should work ok.
Hope this saves time to some people.