Another app is currently holding the yum lock

# yum install rdesktop
Loaded plugins: fastestmirror, langpacks
Existing lock /var/run/yum.pid: another copy is running as pid 16020.
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  46 M RSS (1.4 GB VSZ)
    Started: Fri Jul 29 12:08:28 2016 - 01:14 ago
    State  : Running, pid: 16020
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  46 M RSS (1.4 GB VSZ)
    Started: Fri Jul 29 12:08:28 2016 - 01:16 ago
    State  : Sleeping, pid: 16020
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  46 M RSS (1.4 GB VSZ)
    Started: Fri Jul 29 12:08:28 2016 - 01:18 ago
    State  : Sleeping, pid: 16020
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  46 M RSS (1.4 GB VSZ)
    Started: Fri Jul 29 12:08:28 2016 - 01:20 ago
    State  : Sleeping, pid: 16020
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  46 M RSS (1.4 GB VSZ)
    Started: Fri Jul 29 12:08:28 2016 - 01:22 ago
    State  : Sleeping, pid: 16020
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  46 M RSS (1.4 GB VSZ)
    Started: Fri Jul 29 12:08:28 2016 - 01:24 ago
    State  : Sleeping, pid: 16020
^C

Exiting on user cancel.
#

# ps aux | grep yum
root     16020 28.4  2.3 1455028 79512 ?       SN   12:08   1:16 /usr/bin/python /usr/share/PackageKit/helpers/yum/yumBackend.py refresh-cache yes
root     16354  0.0  0.0 112652   952 pts/0    S+   12:12   0:00 grep --color=auto yum
#

When I checked on whether there was a PackageKit package installed on the system, I saw the following information

# rpm -qi PackageKit
Name        : PackageKit
Version     : 1.0.7
Release     : 5.el7.centos
Architecture: x86_64
Install Date: Fri 15 Jul 2016 03:20:47 PM EDT
Group       : Unspecified
Size        : 2706472
License     : GPLv2+ and LGPLv2+
Signature   : RSA/SHA256, Wed 25 Nov 2015 10:24:57 AM EST, Key ID 24c6a8a7f4a80eb5
Source RPM  : PackageKit-1.0.7-5.el7.centos.src.rpm
Build Date  : Sun 22 Nov 2015 12:17:59 PM EST
Build Host  : worker1.bsys.centos.org
Relocations : (not relocatable)
Packager    : CentOS BuildSystem <http://bugs.centos.org>
Vendor      : CentOS
URL         : http://www.freedesktop.org/software/PackageKit/
Summary     : Package management service
Description :
PackageKit is a D-Bus abstraction layer that allows the session user
to manage packages in a secure way using a cross-distro,
cross-architecture API.

I assumed the software hadn't run for awhile due to a recent power outage; network connectivity wasn't restored when the system rebooted and I just restored the network connectivity a moment before running the yum command. So I thought I could just wait a few minutes for it to complete its package update check and try again. And a few minutes later a Software updates available" message appeared at the bottom of the screen. When I checked on whether there were any yum-related proceses running after I saw that message, I found there were none.

And I was able to successfully run the yum install command that had previously produced the "Another app is currently holding the yum lock" message.