The current (4.0.2) install instructions (https://docs.mongodb.com/master/tutorial/install-mongodb-on-red-hat/#install-mongodb-community-edition) do not cover installation on Fedora 28+. Install of RPMs work but service cannot be started.

Also note that RHEL 7 and Fedora use "dnf" vice "yum", even though the config file is still in /etc/yum.repos.d/

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Notes from a Fedora 28 host. Fedora's packaged MongoDB 3.6 works.

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Sep 05 20:37:14 shaphan systemd[1]: Starting MongoDB Database Server...
Sep 05 20:37:14 shaphan mongod[19275]: 2018-09-05T20:37:14.822-0400 I CONTROL [main] Automatically disabling TLS 1.0, to force-enable TLS 1.0 specify --sslDisabledProtocols 'none'
Sep 05 20:37:14 shaphan mongod[19275]: about to fork child process, waiting until server is ready for connections.
Sep 05 20:37:14 shaphan mongod[19275]: forked process: 19277
Sep 05 20:37:14 shaphan mongod[19275]: ERROR: child process failed, exited with error number 1
Sep 05 20:37:14 shaphan mongod[19275]: To see additional information in this output, start without the "--fork" option.
Sep 05 20:37:14 shaphan systemd[1]: mongod.service: Control process exited, code=exited status=1
Sep 05 20:37:14 shaphan systemd[1]: mongod.service: Failed with result 'exit-code'.
Sep 05 20:37:14 shaphan audit[1]: SERVICE_START pid=1 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0 msg='unit=mongod comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=failed'
Sep 05 20:37:14 shaphan systemd[1]: Failed to start MongoDB Database Server.