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Start mongo with --repair in addition to the dbpath to repair your mongodb instace

@Termi I think you made a typo!

Where you write /usr/lib/unifi/bin/unifi it should be /usr/lib/unifi/bin/mongod

To fix: Do all of this as root.

1) Check logs in /var/log/unifi/ for clear errors
2) stop unifi service (systemctl stop unifi)
3) Potentially recover the crashed mongodb database (was crashed in my case) by starting it once from cmdline with a custom data directory (mongod --dbpath /var/lib/unifi/data/db). Then check the output of it to see if the DB was recovered. If it was, continue. If there was no problem, continue.
4) Connect to the database (mongo mongodb://127.0.0.1:27017( and run the following command:
db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )

That should give you the output
{ "ok" : 1 }

5) Stop the mongodb database
6) Remove the file /usr/lib/unifi/bin/unifi (rm /usr/lib/unifi/bin/unifi)
7) Create the text file /usr/lib/unifi/bin/unifi with the following content

#!/bin/bash  
exec /usr/bin/mongod ${*//--nohttpinterface/}

7) Change owner:group and permissions of /usr/lib/unifi/bin/unifi to root:root and 755
8) Change owner:group of /var/lib/unifi/ to unifi:unifi
9) start unifi again (systemctl start unifi)
10) Check unifi daemon logs in /var/log/unifi/server.log for errors

If that worked, be happy. If it didn't, comment the problem here.

Thanks @treitmayr and @Spider.007

I followed treitmayr comments on 2017-12-31 09:28 with a fresh install and was able to get my UniFi Switch US-48 set up successfully. My MongoDB version is 3.6.1. After creating an account at https://account.ubnt.com, I can now connect/configure the switch at https://myIPaddress:8443

Thanks so much community! I was lost before reading through these comments!

Nevermind, I fixed it. I reinstalled and got a permissions differ error. Fixed them. It works!

OK, so: I downgraded mongodb and wiredtiger to the veresions @Spider.007 specified, then I stopped unifi, started the mongodb using @treitmayr's command, then connected to it and set the featurecompatibilityversion to 3.4 as directed. I then created the shell script at /usr/lib/unifi/bin/mongod which removes the illegal optoin, marked it as executable and chowned to unifi user.

I then upgraded the packages to the newer versions and started Unifi. I now get

[internal] unable to list files under /usr/lib/unifi/data/db/local

Obviously, those files exist. They are currently owned by both unifi and root.

Any ideas what I've missed?

@treitmayr what you suggest probably works for new installs, not existing ones. I fixed mine by following the upgrade procedure; like @abra says - downgrade to mongodb-3.4.9-1 and wiredtiger-2.9.3-1 and upgrade the unifi database instance like this:

$ mongo mongodb://127.0.0.1:27117
> db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )
{ "ok" : 1 }

Now you can upgrade wiredtiger and mongodb to the latest versions - with the fix of a custom wrapper to remove the nohttpinterface parameter it will work

With a clean installation, I kept on getting this error:

[2017-12-31 12:43:54,108] <db-server> ERROR system - [exec] error, rc=2
[2017-12-31 12:43:54,109] <db-server> INFO  db     - DbServer stopped

.. until I tried @treitmayr's suggested fix, and restarted unifi. That seemed to do the trick. Unlike @treitmayr, I did not have to start mongod manually as unifi:unifi user/group, it just worked.

Part of the problem is "net.http" option specified in the config file.

I tried running the DB manually and it worked for a while, after several seconds it crashes and I can't figure out why (the log didn't help as I expected).

From what I can tell the main problem is the mongod parameter "--nohttpinterface" which was deprecated before and was removed with MongoDB 3.6, see link.

In our case the mongod process just exits with an error because unifi still invokes MongoDB as shown below:

bin/mongod --dbpath /usr/lib/unifi/data/db --port 27117 --unixSocketPrefix /usr/lib/unifi/run --logappend --logpath /usr/lib/unifi/logs/mongod.log --nohttpinterface --bind_ip 127.0.0.1

My workaround was to replace the mongod link in /usr/lib/unifi/bin with the following tiny script which just removes the now unsupported parameter:

#!/bin/bash  
exec /usr/bin/mongod ${*//--nohttpinterface/}

Strange enough I still had to restart unifi a couple of times after the above change and had to start the mongod process manually once (as unifi:unifi user/group) before MongoDB was willing to cooperate with unifi. But now it works without problems (until the next unifi update where I will have to redo the change).