<s>
Kubernetes	B-General_Concept
(	O
,	O
commonly	O
abbreviated	O
K8s	B-General_Concept
)	O
is	O
an	O
open-source	B-Application
container	B-Application
orchestration	I-Application
system	O
for	O
automating	O
software	B-Application
deployment	I-Application
,	O
scaling	O
,	O
and	O
management	O
.	O
</s>
<s>
Originally	O
designed	O
by	O
Google	B-Application
,	O
the	O
project	O
is	O
now	O
maintained	O
by	O
the	O
Cloud	B-Application
Native	I-Application
Computing	I-Application
Foundation	I-Application
.	O
</s>
<s>
The	O
name	O
Kubernetes	B-General_Concept
originates	O
from	O
Greek	O
,	O
meaning	O
'	O
helmsman	O
 '	O
or	O
'	O
pilot	O
 '	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
is	O
often	O
abbreviated	O
as	O
K8s	B-General_Concept
,	O
counting	O
the	O
eight	O
letters	O
between	O
the	O
K	O
and	O
the	O
s	O
(	O
a	O
numeronym	O
)	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
works	O
with	O
containerd	O
and	O
CRI-O	O
.	O
</s>
<s>
There	O
are	O
multiple	O
distributions	O
of	O
this	O
platform	O
–	O
from	O
ISVs	B-Application
as	O
well	O
as	O
hosted-on	O
cloud	O
offerings	O
from	O
all	O
the	O
major	O
public	O
cloud	O
vendors	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
( κυβερνήτης	O
kubernḗtēs	O
,	O
Greek	O
for	O
"	O
steersman	O
,	O
navigator	O
"	O
or	O
"	O
guide	O
"	O
,	O
and	O
the	O
etymological	O
root	O
of	O
cybernetics	O
)	O
was	O
announced	O
by	O
Google	B-Application
in	O
mid-2014	O
.	O
</s>
<s>
The	O
project	O
was	O
created	O
by	O
Joe	O
Beda	O
,	O
Brendan	O
Burns	O
,	O
and	O
Craig	O
McLuckie	O
,	O
who	O
were	O
soon	O
joined	O
by	O
other	O
Google	B-Application
engineers	O
,	O
including	O
Brian	O
Grant	O
and	O
Tim	O
Hockin	O
.	O
</s>
<s>
The	O
design	O
and	O
development	O
of	O
Kubernetes	B-General_Concept
was	O
influenced	O
by	O
Google	B-Application
's	I-Application
Borg	B-Operating_System
cluster	O
manager	O
.	O
</s>
<s>
Many	O
of	O
its	O
top	O
contributors	O
had	O
previously	O
worked	O
on	O
Borg	B-Operating_System
;	O
they	O
codenamed	O
Kubernetes	B-General_Concept
""	O
after	O
the	O
Star	O
Trek	O
ex-Borg	O
character	O
Seven	O
of	O
Nine	O
and	O
gave	O
its	O
logo	O
a	O
seven-spoked	O
wheel	O
.	O
</s>
<s>
Unlike	O
Borg	B-Operating_System
,	O
which	O
was	O
written	O
in	O
C++	B-Language
,	O
Kubernetes	B-General_Concept
source	O
code	O
is	O
in	O
the	O
Go	B-Application
language	I-Application
.	O
</s>
<s>
Kubernetes	B-General_Concept
1.0	O
was	O
released	O
on	O
July	O
21	O
,	O
2015	O
.	O
</s>
<s>
Google	B-Application
worked	O
with	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
to	O
form	O
the	O
Cloud	B-Application
Native	I-Application
Computing	I-Application
Foundation	I-Application
(	O
CNCF	O
)	O
and	O
offered	O
Kubernetes	B-General_Concept
as	O
a	O
seed	O
technology	O
.	O
</s>
<s>
In	O
February	O
2016	O
,	O
the	O
Helm	O
package	O
manager	O
for	O
Kubernetes	B-General_Concept
was	O
released	O
.	O
</s>
<s>
Google	B-Application
was	O
already	O
offering	O
managed	O
Kubernetes	B-General_Concept
services	O
,	O
while	O
Red	O
Hat	O
was	O
supporting	O
Kubernetes	B-General_Concept
as	O
part	O
of	O
OpenShift	B-General_Concept
since	O
the	O
inception	O
of	O
the	O
Kubernetes	B-General_Concept
project	O
in	O
2014	O
.	O
</s>
<s>
In	O
2017	O
,	O
the	O
principal	O
competitors	O
rallied	O
around	O
Kubernetes	B-General_Concept
and	O
announced	O
adding	O
native	O
support	O
for	O
it	O
:	O
</s>
<s>
VMWare	O
(	O
proponent	O
of	O
Pivotal	B-General_Concept
Cloud	I-General_Concept
Foundry	I-General_Concept
)	O
in	O
August	O
,	O
</s>
<s>
Mesosphere	B-General_Concept
,	O
Inc	O
.	O
(	O
proponent	O
of	O
Marathon	O
and	O
Mesos	B-General_Concept
)	O
in	O
September	O
,	O
</s>
<s>
Docker	B-Language
,	O
Inc	O
.	O
(	O
proponent	O
of	O
Docker	B-Language
)	O
in	O
October	O
,	O
</s>
<s>
Microsoft	B-Application
Azure	I-Application
also	O
in	O
October	O
,	O
</s>
<s>
AWS	B-Application
announced	O
support	O
for	O
Kubernetes	B-General_Concept
via	O
the	O
Elastic	O
Kubernetes	B-General_Concept
Service	O
(	O
EKS	O
)	O
in	O
November	O
.	O
</s>
<s>
On	O
March	O
6	O
,	O
2018	O
,	O
Kubernetes	B-General_Concept
Project	O
reached	O
ninth	O
place	O
in	O
the	O
list	O
of	O
GitHub	B-Application
projects	O
by	O
the	O
number	O
of	O
commits	B-Protocol
,	O
and	O
second	O
place	O
in	O
authors	O
and	O
issues	O
,	O
after	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Until	O
version	O
1.18	O
,	O
Kubernetes	B-General_Concept
followed	O
an	O
N-2	O
support	O
policy	O
,	O
meaning	O
that	O
the	O
three	O
most	O
recent	O
minor	O
versions	O
receive	O
security	O
updates	O
and	O
bug	O
fixes	O
.	O
</s>
<s>
Starting	O
with	O
version	O
1.19	O
,	O
Kubernetes	B-General_Concept
follows	O
an	O
N-3	O
support	O
policy	O
.	O
</s>
<s>
Originally	O
,	O
it	O
interfaced	O
exclusively	O
with	O
the	O
Docker	B-Language
runtime	B-Device
through	O
a	O
"	O
Dockershim	O
"	O
;	O
however	O
,	O
from	O
November	O
2020	O
up	O
to	O
April	O
2022	O
,	O
Kubernetes	B-General_Concept
has	O
deprecated	O
the	O
shim	B-General_Concept
in	O
favor	O
of	O
directly	O
interfacing	O
with	O
the	O
container	B-Application
through	O
Containerd	O
,	O
or	O
replacing	O
Docker	B-Language
with	O
a	O
runtime	B-Device
that	O
is	O
compliant	O
with	O
the	O
Container	B-Application
Runtime	B-Device
Interface	O
(	O
CRI	O
)	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
defines	O
a	O
set	O
of	O
building	O
blocks	O
(	O
"	O
primitives	O
"	O
)	O
that	O
collectively	O
provide	O
mechanisms	O
that	O
deploy	O
,	O
maintain	O
,	O
and	O
scale	O
applications	O
based	O
on	O
CPU	O
,	O
memory	O
or	O
custom	O
metrics	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
is	O
loosely	B-Application
coupled	I-Application
and	O
extensible	O
to	O
meet	O
different	O
workloads	O
.	O
</s>
<s>
The	O
internal	O
components	O
as	O
well	O
as	O
extensions	O
and	O
containers	B-Application
that	O
run	O
on	O
Kubernetes	B-General_Concept
rely	O
on	O
the	O
Kubernetes	B-General_Concept
API	B-Application
.	O
</s>
<s>
Kubernetes	B-General_Concept
follows	O
the	O
primary/replica	B-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
The	O
components	O
of	O
Kubernetes	B-General_Concept
can	O
be	O
divided	O
into	O
those	O
that	O
manage	O
an	O
individual	O
node	B-Protocol
and	O
those	O
that	O
are	O
part	O
of	O
the	O
control	O
plane	O
.	O
</s>
<s>
The	O
Kubernetes	B-General_Concept
master	O
node	B-Protocol
handles	O
the	O
Kubernetes	B-General_Concept
control	O
plane	O
of	O
the	O
cluster	O
,	O
managing	O
its	O
workload	O
and	O
directing	O
communication	O
across	O
the	O
system	O
.	O
</s>
<s>
The	O
Kubernetes	B-General_Concept
control	O
plane	O
consists	O
of	O
various	O
components	O
,	O
each	O
its	O
own	O
process	O
,	O
that	O
can	O
run	O
both	O
on	O
a	O
single	O
master	O
node	B-Protocol
or	O
on	O
multiple	O
masters	O
supporting	O
high-availability	B-General_Concept
clusters	I-General_Concept
.	O
</s>
<s>
The	O
various	O
components	O
of	O
the	O
Kubernetes	B-General_Concept
control	O
plane	O
are	O
as	O
follows	O
:	O
</s>
<s>
etcd	O
is	O
a	O
persistent	O
,	O
lightweight	O
,	O
distributed	O
,	O
key-value	B-Data_Structure
data	I-Data_Structure
store	I-Data_Structure
that	O
CoreOS	B-Application
has	O
developed	O
.	O
</s>
<s>
etcd	O
favors	O
consistency	O
over	O
availability	O
in	O
the	O
event	O
of	O
a	O
network	O
partition	O
(	O
see	O
CAP	B-Operating_System
theorem	I-Operating_System
)	O
.	O
</s>
<s>
The	O
API	B-Application
server	O
serves	O
the	O
Kubernetes	B-General_Concept
API	B-Application
using	O
JSON	B-General_Concept
over	O
HTTP	B-Protocol
,	O
which	O
provides	O
both	O
the	O
internal	O
and	O
external	O
interface	O
to	O
Kubernetes	B-General_Concept
.	O
</s>
<s>
The	O
API	B-Application
server	O
processes	O
and	O
validates	O
REST	B-Protocol
requests	O
and	O
updates	O
the	O
state	O
of	O
the	O
API	B-Application
objects	O
in	O
etcd	O
,	O
thereby	O
allowing	O
clients	O
to	O
configure	O
workloads	O
and	O
containers	B-Application
across	O
worker	O
nodes	O
.	O
</s>
<s>
The	O
API	B-Application
server	O
uses	O
etcd	O
's	O
watch	O
API	B-Application
to	O
monitor	O
the	O
cluster	O
,	O
roll	O
out	O
critical	O
configuration	O
changes	O
,	O
or	O
restore	O
any	O
divergences	O
of	O
the	O
state	O
of	O
the	O
cluster	O
back	O
to	O
what	O
the	O
deployer	O
declared	O
.	O
</s>
<s>
The	O
scheduler	O
is	O
the	O
extensible	O
component	O
that	O
selects	O
on	O
which	O
node	B-Protocol
an	O
unscheduled	O
pod	O
(	O
the	O
basic	O
entity	O
managed	O
by	O
the	O
scheduler	O
)	O
runs	O
,	O
based	O
on	O
resource	O
availability	O
.	O
</s>
<s>
The	O
scheduler	O
tracks	O
resource	O
use	O
on	O
each	O
node	B-Protocol
to	O
ensure	O
that	O
workload	O
is	O
not	O
scheduled	O
in	O
excess	O
of	O
available	O
resources	O
.	O
</s>
<s>
A	O
controller	O
is	O
a	O
reconciliation	O
loop	O
that	O
drives	O
the	O
actual	O
cluster	O
state	O
toward	O
the	O
desired	O
state	O
,	O
communicating	O
with	O
the	O
API	B-Application
server	O
to	O
create	O
,	O
update	O
,	O
and	O
delete	O
the	O
resources	O
it	O
manages	O
(	O
e.g.	O
,	O
pods	O
or	O
service	O
endpoints	O
)	O
.	O
</s>
<s>
It	O
also	O
handles	O
creating	O
replacement	O
pods	O
if	O
the	O
underlying	O
node	B-Protocol
fails	O
.	O
</s>
<s>
Other	O
controllers	O
that	O
are	O
part	O
of	O
the	O
core	O
Kubernetes	B-General_Concept
system	O
include	O
a	O
DaemonSet	O
Controller	O
for	O
running	O
exactly	O
one	O
pod	O
on	O
every	O
machine	O
(	O
or	O
some	O
subset	O
of	O
machines	O
)	O
,	O
and	O
a	O
Job	O
Controller	O
for	O
running	O
pods	O
that	O
run	O
to	O
completion	O
(	O
e.g.	O
,	O
as	O
part	O
of	O
a	O
batch	O
job	O
)	O
.	O
</s>
<s>
The	O
controller	O
manager	O
is	O
a	O
process	O
that	O
manages	O
a	O
set	O
of	O
core	O
Kubernetes	B-General_Concept
controllers	O
.	O
</s>
<s>
A	O
node	B-Protocol
,	O
also	O
known	O
as	O
a	O
worker	O
or	O
a	O
minion	O
,	O
is	O
a	O
machine	O
where	O
containers	B-Application
(	O
workloads	O
)	O
are	O
deployed	O
.	O
</s>
<s>
Every	O
node	B-Protocol
in	O
the	O
cluster	O
must	O
run	O
a	O
container	B-Application
runtime	B-Device
such	O
as	O
containerd	O
,	O
as	O
well	O
as	O
the	O
below-mentioned	O
components	O
,	O
for	O
communication	O
with	O
the	O
primary	O
for	O
network	O
configuration	O
of	O
these	O
containers	B-Application
.	O
</s>
<s>
Kubelet	O
is	O
responsible	O
for	O
the	O
running	O
state	O
of	O
each	O
node	B-Protocol
,	O
ensuring	O
that	O
all	O
containers	B-Application
on	O
the	O
node	B-Protocol
are	O
healthy	O
.	O
</s>
<s>
It	O
takes	O
care	O
of	O
starting	O
,	O
stopping	O
,	O
and	O
maintaining	O
application	O
containers	B-Application
organized	O
into	O
pods	O
as	O
directed	O
by	O
the	O
control	O
plane	O
.	O
</s>
<s>
Kubelet	O
monitors	O
the	O
state	O
of	O
a	O
pod	O
,	O
and	O
if	O
not	O
in	O
the	O
desired	O
state	O
,	O
the	O
pod	O
re-deploys	O
to	O
the	O
same	O
node	B-Protocol
.	O
</s>
<s>
Node	B-Protocol
status	O
is	O
relayed	O
every	O
few	O
seconds	O
via	O
heartbeat	O
messages	O
to	O
the	O
primary	O
.	O
</s>
<s>
Once	O
the	O
primary	O
detects	O
a	O
node	B-Protocol
failure	O
,	O
the	O
Replication	O
Controller	O
observes	O
this	O
state	O
change	O
and	O
launches	O
pods	O
on	O
other	O
healthy	O
nodes	O
.	O
</s>
<s>
Kube-proxy	O
is	O
an	O
implementation	O
of	O
a	O
network	B-Application
proxy	I-Application
and	O
a	O
load	B-Application
balancer	I-Application
,	O
and	O
it	O
supports	O
the	O
service	O
abstraction	O
along	O
with	O
other	O
networking	O
operation	O
.	O
</s>
<s>
It	O
is	O
responsible	O
for	O
routing	O
traffic	O
to	O
the	O
appropriate	O
container	B-Application
based	O
on	O
IP	O
and	O
port	O
number	O
of	O
the	O
incoming	O
request	O
.	O
</s>
<s>
A	O
container	B-Application
resides	O
inside	O
a	O
pod	O
.	O
</s>
<s>
The	O
container	B-Application
is	O
the	O
lowest	O
level	O
of	O
a	O
micro-service	O
,	O
which	O
holds	O
the	O
running	O
application	O
,	O
libraries	O
,	O
and	O
their	O
dependencies	O
.	O
</s>
<s>
Containers	B-Application
can	O
be	O
exposed	O
to	O
the	O
world	O
through	O
an	O
external	O
IP	O
address	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
has	O
supported	O
Docker	B-Language
containers	B-Application
since	O
its	O
first	O
version	O
.	O
</s>
<s>
In	O
July	O
2016	O
the	O
rkt	O
container	B-Application
engine	O
was	O
added	O
.	O
</s>
<s>
In	O
Kubernetes	B-General_Concept
,	O
namespaces	O
are	O
utilized	O
to	O
segregate	O
the	O
resources	O
it	O
handles	O
into	O
distinct	O
and	O
non-intersecting	O
collections	O
..	O
</s>
<s>
The	O
basic	O
scheduling	O
unit	O
in	O
Kubernetes	B-General_Concept
is	O
a	O
pod	O
,	O
which	O
consists	O
of	O
one	O
or	O
more	O
containers	B-Application
that	O
are	O
guaranteed	O
to	O
be	O
co-located	O
on	O
the	O
same	O
node	B-Protocol
.	O
</s>
<s>
Each	O
pod	O
in	O
Kubernetes	B-General_Concept
is	O
assigned	O
a	O
unique	O
IP	O
address	O
within	O
the	O
cluster	O
,	O
allowing	O
applications	O
to	O
use	O
ports	O
without	O
the	O
risk	O
of	O
conflict	O
.	O
</s>
<s>
Within	O
the	O
pod	O
,	O
all	O
containers	B-Application
can	O
reference	O
each	O
other	O
.	O
</s>
<s>
A	O
volume	O
,	O
such	O
as	O
a	O
local	O
disk	O
directory	O
or	O
a	O
network	O
disk	O
,	O
can	O
be	O
defined	O
within	O
a	O
pod	O
and	O
made	O
accessible	O
to	O
the	O
containers	B-Application
residing	O
in	O
it	O
.	O
</s>
<s>
Pods	O
can	O
be	O
managed	O
manually	O
through	O
the	O
Kubernetes	B-General_Concept
API	B-Application
,	O
or	O
their	O
management	O
can	O
be	O
delegated	O
to	O
a	O
controller	O
.	O
</s>
<s>
Such	O
volumes	O
are	O
also	O
the	O
basis	O
for	O
the	O
Kubernetes	B-General_Concept
features	O
of	O
ConfigMaps	O
(	O
to	O
provide	O
access	O
to	O
configuration	O
through	O
the	O
file	O
system	O
visible	O
to	O
the	O
container	B-Application
)	O
and	O
Secrets	O
(	O
to	O
provide	O
access	O
to	O
credentials	O
needed	O
to	O
access	O
remote	O
resources	O
securely	O
,	O
by	O
providing	O
those	O
credentials	O
on	O
the	O
file	O
system	O
visible	O
only	O
to	O
authorized	O
containers	B-Application
)	O
.	O
</s>
<s>
Typically	O
,	O
the	O
task	O
of	O
determining	O
the	O
node	B-Protocol
on	O
which	O
a	O
pod	O
should	O
run	O
is	O
delegated	O
to	O
the	O
Kubernetes	B-General_Concept
Scheduler	O
.	O
</s>
<s>
However	O
,	O
in	O
certain	O
scenarios	O
,	O
it	O
may	O
be	O
necessary	O
to	O
deploy	O
a	O
pod	O
on	O
every	O
node	B-Protocol
in	O
the	O
cluster	O
,	O
which	O
is	O
particularly	O
helpful	O
for	O
use	O
cases	O
involving	O
log	O
collection	O
,	O
ingress	O
controllers	O
,	O
and	O
storage	O
services	O
.	O
</s>
<s>
The	O
ReplicaSets	O
can	O
also	O
be	O
said	O
to	O
be	O
a	O
grouping	O
mechanism	O
that	O
lets	O
Kubernetes	B-General_Concept
maintain	O
the	O
number	O
of	O
instances	O
that	O
have	O
been	O
declared	O
for	O
a	O
given	O
pod	O
.	O
</s>
<s>
A	O
Kubernetes	B-General_Concept
service	O
is	O
a	O
set	O
of	O
pods	O
that	O
work	O
together	O
,	O
such	O
as	O
one	O
tier	B-Operating_System
of	O
a	O
multi-tier	B-Operating_System
application	I-Operating_System
.	O
</s>
<s>
Kubernetes	B-General_Concept
provides	O
two	O
modes	O
of	O
service	B-Protocol
discovery	I-Protocol
,	O
using	O
environmental	O
variables	O
or	O
using	O
Kubernetes	B-General_Concept
DNS	B-Protocol
.	O
</s>
<s>
Service	B-Protocol
discovery	I-Protocol
assigns	O
a	O
stable	O
IP	O
address	O
and	O
DNS	B-Protocol
name	I-Protocol
to	O
the	O
service	O
,	O
and	O
load	O
balances	O
traffic	O
in	O
a	O
round-robin	B-Protocol
manner	O
to	O
network	O
connections	O
of	O
that	O
IP	O
address	O
among	O
the	O
pods	O
matching	O
the	O
selector	O
(	O
even	O
as	O
failures	O
cause	O
the	O
pods	O
to	O
move	O
from	O
machine	O
to	O
machine	O
)	O
.	O
</s>
<s>
By	O
default	O
a	O
service	O
is	O
exposed	O
inside	O
a	O
cluster	O
(	O
e.g.	O
,	O
back	B-Architecture
end	I-Architecture
pods	O
might	O
be	O
grouped	O
into	O
a	O
service	O
,	O
with	O
requests	O
from	O
the	O
front-end	B-Architecture
pods	O
load-balanced	B-Application
among	O
them	O
)	O
,	O
but	O
a	O
service	O
can	O
also	O
be	O
exposed	O
outside	O
a	O
cluster	O
(	O
e.g.	O
,	O
for	O
clients	O
to	O
reach	O
front-end	B-Architecture
pods	O
)	O
.	O
</s>
<s>
File	O
systems	O
in	O
the	O
Kubernetes	B-General_Concept
container	B-Application
provide	O
ephemeral	B-General_Concept
storage	I-General_Concept
,	O
by	O
default	O
.	O
</s>
<s>
This	O
means	O
that	O
a	O
restart	O
of	O
the	O
pod	O
will	O
wipe	O
out	O
any	O
data	O
on	O
such	O
containers	B-Application
,	O
and	O
therefore	O
,	O
this	O
form	O
of	O
storage	O
is	O
quite	O
limiting	O
in	O
anything	O
but	O
trivial	O
applications	O
.	O
</s>
<s>
A	O
Kubernetes	B-General_Concept
Volume	O
provides	O
persistent	O
storage	O
that	O
exists	O
for	O
the	O
lifetime	O
of	O
the	O
pod	O
itself	O
.	O
</s>
<s>
This	O
storage	O
can	O
also	O
be	O
used	O
as	O
shared	O
disk	O
space	O
for	O
containers	B-Application
within	O
the	O
pod	O
.	O
</s>
<s>
Volumes	O
are	O
mounted	O
at	O
specific	O
mount	O
points	O
within	O
the	O
container	B-Application
,	O
which	O
are	O
defined	O
by	O
the	O
pod	O
configuration	O
,	O
and	O
cannot	O
mount	O
onto	O
other	O
volumes	O
or	O
link	O
to	O
other	O
volumes	O
.	O
</s>
<s>
The	O
same	O
volume	O
can	O
be	O
mounted	O
at	O
different	O
points	O
in	O
the	O
file	O
system	O
tree	O
by	O
different	O
containers	B-Application
.	O
</s>
<s>
Configuration	O
data	O
can	O
be	O
anything	O
as	O
fine-grained	O
as	O
individual	O
properties	O
or	O
coarse-grained	O
information	O
like	O
entire	O
configuration	O
files	O
or	O
JSON	B-General_Concept
/	O
XML	O
documents	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
provides	O
two	O
closely	O
related	O
mechanisms	O
to	O
deal	O
with	O
this	O
need	O
:	O
"	O
configmaps	O
"	O
and	O
"	O
secrets	O
"	O
,	O
both	O
of	O
which	O
allow	O
for	O
configuration	O
changes	O
to	O
be	O
made	O
without	O
requiring	O
an	O
application	O
build	O
.	O
</s>
<s>
A	O
secret	O
and/or	O
a	O
configmap	O
is	O
sent	O
to	O
a	O
node	B-Protocol
only	O
if	O
a	O
pod	O
on	O
that	O
node	B-Protocol
requires	O
it	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
will	O
keep	O
it	O
in	O
memory	O
on	O
that	O
node	B-Protocol
.	O
</s>
<s>
as	O
environment	O
variables	O
(	O
which	O
will	O
be	O
created	O
by	O
Kubernetes	B-General_Concept
when	O
the	O
pod	O
is	O
started	O
)	O
;	O
</s>
<s>
available	O
on	O
the	O
container	B-Application
file	O
system	O
that	O
is	O
visible	O
only	O
from	O
within	O
the	O
pod	O
.	O
</s>
<s>
Recent	O
versions	O
of	O
Kubernetes	B-General_Concept
have	O
introduced	O
support	O
for	O
encryption	O
to	O
be	O
used	O
as	O
well	O
.	O
</s>
<s>
When	O
run	O
in	O
high-availability	B-General_Concept
mode	O
,	O
many	O
databases	O
come	O
with	O
the	O
notion	O
of	O
a	O
primary	O
instance	O
and	O
secondary	O
instances	O
.	O
</s>
<s>
Other	O
applications	O
like	O
Apache	B-Language
Kafka	I-Language
distribute	O
the	O
data	O
amongst	O
their	O
brokers	O
;	O
hence	O
,	O
one	O
broker	O
is	O
not	O
the	O
same	O
as	O
another	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
enables	O
clients	O
(	O
users	O
or	O
internal	O
components	O
)	O
to	O
attach	O
keys	O
called	O
"	O
labels	O
"	O
to	O
any	O
API	B-Application
object	O
in	O
the	O
system	O
,	O
such	O
as	O
pods	O
and	O
nodes	O
.	O
</s>
<s>
Thus	O
,	O
simply	O
changing	O
the	O
labels	O
of	O
the	O
pods	O
or	O
changing	O
the	O
label	O
selectors	O
on	O
the	O
service	O
can	O
be	O
used	O
to	O
control	O
which	O
pods	O
get	O
traffic	O
and	O
which	O
do	O
n't	O
,	O
which	O
can	O
be	O
used	O
to	O
support	O
various	O
deployment	O
patterns	O
like	O
blue-green	B-Application
deployments	I-Application
or	O
A-B	O
testing	O
.	O
</s>
<s>
This	O
capability	O
to	O
dynamically	O
control	O
how	O
services	O
utilize	O
implementing	O
resources	O
provides	O
a	O
loose	B-Application
coupling	I-Application
within	O
the	O
infrastructure	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
an	O
application	O
's	O
pods	O
have	O
labels	O
for	O
a	O
system	O
tier	B-Operating_System
(	O
with	O
values	O
such	O
as	O
frontend	B-Architecture
,	O
backend	B-Architecture
,	O
for	O
example	O
)	O
and	O
a	O
release_track	O
(	O
with	O
values	O
such	O
as	O
canary	O
,	O
production	O
,	O
for	O
example	O
)	O
,	O
then	O
an	O
operation	O
on	O
all	O
of	O
backend	B-Architecture
and	O
canary	O
nodes	O
can	O
use	O
a	O
label	O
selector	O
,	O
such	O
as	O
:	O
</s>
<s>
Just	O
like	O
labels	O
,	O
field	O
selectors	O
also	O
let	O
one	O
select	O
Kubernetes	B-General_Concept
resources	O
.	O
</s>
<s>
metadata.name	O
and	O
metadata.namespace	O
are	O
field	O
selectors	O
that	O
will	O
be	O
present	O
on	O
all	O
Kubernetes	B-General_Concept
objects	O
.	O
</s>
<s>
Add-ons	O
operate	O
just	O
like	O
any	O
other	O
application	O
running	O
within	O
the	O
cluster	O
:	O
they	O
are	O
implemented	O
via	O
pods	O
and	O
services	O
,	O
and	O
are	O
only	O
different	O
in	O
that	O
they	O
implement	O
features	O
of	O
the	O
Kubernetes	B-General_Concept
cluster	O
.	O
</s>
<s>
All	O
Kubernetes	B-General_Concept
clusters	O
should	O
have	O
cluster	O
DNS	B-Protocol
;	O
it	O
is	O
a	O
mandatory	O
feature	O
.	O
</s>
<s>
Cluster	O
DNS	B-Protocol
is	O
a	O
DNS	B-Protocol
server	O
,	O
in	O
addition	O
to	O
the	O
other	O
DNS	B-Protocol
server(s )	O
in	O
your	O
environment	O
,	O
which	O
serves	O
DNS	B-Protocol
records	O
for	O
Kubernetes	B-General_Concept
services	O
.	O
</s>
<s>
Containers	B-Application
started	O
by	O
Kubernetes	B-General_Concept
automatically	O
include	O
this	O
DNS	B-Protocol
server	O
in	O
their	O
DNS	B-Protocol
searches	O
.	O
</s>
<s>
This	O
is	O
a	O
general	O
purpose	O
,	O
web-based	O
UI	O
for	O
Kubernetes	B-General_Concept
clusters	O
.	O
</s>
<s>
Providing	O
a	O
reliable	O
application	O
runtime	B-Device
,	O
and	O
being	O
able	O
to	O
scale	O
it	O
up	O
or	O
down	O
in	O
response	O
to	O
workloads	O
,	O
means	O
being	O
able	O
to	O
continuously	O
and	O
effectively	O
monitor	O
workload	O
performance	O
.	O
</s>
<s>
Container	B-Application
Resource	O
Monitoring	O
provides	O
this	O
capability	O
by	O
recording	O
metrics	O
about	O
containers	B-Application
in	O
a	O
central	O
database	O
,	O
and	O
provides	O
a	O
UI	O
for	O
browsing	O
that	O
data	O
.	O
</s>
<s>
The	O
cAdvisor	O
is	O
a	O
component	O
on	O
a	O
slave	O
node	B-Protocol
that	O
provides	O
a	O
limited	O
metric	O
monitoring	O
capability	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
cost	O
monitoring	O
apps	O
allows	O
to	O
break	O
down	O
costs	O
by	O
pods	O
,	O
nodes	O
,	O
namespaces	O
,	O
and	O
labels	O
.	O
</s>
<s>
Logs	O
should	O
have	O
a	O
separate	O
storage	O
and	O
lifecycle	O
independent	O
of	O
nodes	O
,	O
pods	O
,	O
or	O
containers	B-Application
.	O
</s>
<s>
Otherwise	O
,	O
node	B-Protocol
or	O
pod	O
failures	O
can	O
cause	O
loss	O
of	O
event	O
data	O
.	O
</s>
<s>
The	O
ability	O
to	O
do	O
this	O
is	O
called	O
cluster-level	O
logging	O
,	O
and	O
such	O
mechanisms	O
are	O
responsible	O
for	O
saving	O
container	B-Application
logs	O
to	O
a	O
central	O
log	O
store	O
with	O
search/browsing	O
interface	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
provides	O
no	O
native	O
storage	O
for	O
log	O
data	O
,	O
but	O
one	O
can	O
integrate	O
many	O
existing	O
logging	O
solutions	O
into	O
the	O
Kubernetes	B-General_Concept
cluster	O
.	O
</s>
<s>
Containers	B-Application
emerged	O
as	O
a	O
way	O
to	O
make	O
software	O
portable	O
.	O
</s>
<s>
The	O
container	B-Application
contains	O
all	O
the	O
packages	O
you	O
need	O
to	O
run	O
a	O
service	O
.	O
</s>
<s>
The	O
provided	O
file	O
system	O
makes	O
containers	B-Application
extremely	O
portable	O
and	O
easy	O
to	O
use	O
in	O
development	O
.	O
</s>
<s>
A	O
container	B-Application
can	O
be	O
moved	O
from	O
development	O
to	O
test	O
or	O
production	O
with	O
no	O
or	O
relatively	O
few	O
configuration	O
changes	O
.	O
</s>
<s>
Historically	O
Kubernetes	B-General_Concept
was	O
suitable	O
only	O
for	O
stateless	O
services	O
.	O
</s>
<s>
However	O
,	O
many	O
applications	O
have	O
a	O
database	O
,	O
which	O
requires	O
persistence	O
,	O
which	O
leads	O
to	O
the	O
creation	O
of	O
persistent	O
storage	O
for	O
Kubernetes	B-General_Concept
.	O
</s>
<s>
Implementing	O
persistent	O
storage	O
for	O
containers	B-Application
is	O
one	O
of	O
the	O
top	O
challenges	O
of	O
Kubernetes	B-General_Concept
administrators	O
,	O
DevOps	O
and	O
cloud	O
engineers	O
.	O
</s>
<s>
Containers	B-Application
may	O
be	O
ephemeral	O
,	O
but	O
more	O
and	O
more	O
of	O
their	O
data	O
is	O
not	O
,	O
so	O
one	O
needs	O
to	O
ensure	O
the	O
data	O
's	O
survival	O
in	O
case	O
of	O
container	B-Application
termination	O
or	O
hardware	O
failure	O
.	O
</s>
<s>
When	O
deploying	O
containers	B-Application
with	O
Kubernetes	B-General_Concept
or	O
containerized	O
applications	O
,	O
companies	O
often	O
realize	O
that	O
they	O
need	O
persistent	O
storage	O
.	O
</s>
<s>
They	O
need	O
to	O
provide	O
fast	O
and	O
reliable	O
storage	O
for	O
databases	O
,	O
root	O
images	O
and	O
other	O
data	O
used	O
by	O
the	O
containers	B-Application
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
landscape	O
,	O
the	O
Cloud	B-Application
Native	I-Application
Computing	I-Application
Foundation	I-Application
(	O
CNCF	O
)	O
,	O
has	O
published	O
other	O
information	O
about	O
Kubernetes	B-General_Concept
Persistent	O
Storage	O
including	O
a	O
blog	O
helping	O
to	O
define	O
the	O
container	B-Application
attached	O
storage	O
pattern	O
.	O
</s>
<s>
This	O
pattern	O
can	O
be	O
thought	O
of	O
as	O
one	O
that	O
uses	O
Kubernetes	B-General_Concept
itself	O
as	O
a	O
component	O
of	O
the	O
storage	O
system	O
or	O
service	O
.	O
</s>
<s>
More	O
information	O
about	O
the	O
relative	O
popularity	O
of	O
these	O
and	O
other	O
approaches	O
can	O
be	O
found	O
on	O
the	O
CNCF	O
's	O
landscape	O
survey	O
as	O
well	O
,	O
which	O
showed	O
that	O
OpenEBS	O
from	O
MayaData	O
and	O
Rook	O
a	O
storage	O
orchestration	B-Application
project	O
were	O
the	O
two	O
projects	O
most	O
likely	O
to	O
be	O
in	O
evaluation	O
as	O
of	O
the	O
Fall	O
of	O
2019	O
.	O
</s>
<s>
Container	B-Application
Attached	O
Storage	O
is	O
a	O
type	O
of	O
data	O
storage	O
that	O
emerged	O
as	O
Kubernetes	B-General_Concept
gained	O
prominence	O
.	O
</s>
<s>
The	O
Container	B-Application
Attached	O
Storage	O
approach	O
or	O
pattern	O
relies	O
on	O
Kubernetes	B-General_Concept
itself	O
for	O
certain	O
capabilities	O
while	O
delivering	O
primarily	O
block	O
,	O
file	O
,	O
object	O
and	O
interfaces	O
to	O
workloads	O
running	O
on	O
Kubernetes	B-General_Concept
.	O
</s>
<s>
Common	O
attributes	O
of	O
Container	B-Application
Attached	O
Storage	O
include	O
the	O
use	O
of	O
extensions	O
to	O
Kubernetes	B-General_Concept
,	O
such	O
as	O
custom	O
resource	O
definitions	O
,	O
and	O
the	O
use	O
of	O
Kubernetes	B-General_Concept
itself	O
for	O
functions	O
that	O
otherwise	O
would	O
be	O
separately	O
developed	O
and	O
deployed	O
for	O
storage	O
or	O
data	O
management	O
.	O
</s>
<s>
Examples	O
of	O
functionality	O
delivered	O
by	O
custom	O
resource	O
definitions	O
or	O
by	O
Kubernetes	B-General_Concept
itself	O
include	O
retry	O
logic	O
,	O
delivered	O
by	O
Kubernetes	B-General_Concept
itself	O
,	O
and	O
the	O
creation	O
and	O
maintenance	O
of	O
an	O
inventory	O
of	O
available	O
storage	O
media	O
and	O
volumes	O
,	O
typically	O
delivered	O
via	O
a	O
custom	O
resource	O
definition	O
.	O
</s>
<s>
In	O
Kubernetes	B-General_Concept
version	O
1.9	O
,	O
the	O
initial	O
Alpha	O
release	O
of	O
Container	B-Application
Storage	O
Interface	O
(	O
CSI	O
)	O
was	O
introduced	O
.	O
</s>
<s>
Previously	O
,	O
storage	O
volume	O
plug-ins	O
were	O
included	O
in	O
the	O
Kubernetes	B-General_Concept
distribution	O
.	O
</s>
<s>
By	O
creating	O
a	O
standardized	O
CSI	O
,	O
the	O
code	O
required	O
to	O
interface	O
with	O
external	O
storage	O
systems	O
was	O
separated	O
from	O
the	O
core	O
Kubernetes	B-General_Concept
code	O
base	O
.	O
</s>
<s>
Just	O
one	O
year	O
later	O
,	O
the	O
CSI	O
feature	O
was	O
made	O
Generally	O
Available	O
(	O
GA	O
)	O
in	O
Kubernetes	B-General_Concept
.	O
</s>
<s>
A	O
key	O
component	O
of	O
the	O
Kubernetes	B-General_Concept
control	O
plane	O
is	O
the	O
API	B-Application
Server	O
,	O
which	O
exposes	O
an	O
HTTP	B-Protocol
API	B-Application
that	O
can	O
be	O
invoked	O
by	O
other	O
parts	O
of	O
the	O
cluster	O
as	O
well	O
as	O
end	O
users	O
and	O
external	O
components	O
.	O
</s>
<s>
This	O
API	B-Application
is	O
a	O
REST	B-Protocol
API	I-Protocol
and	O
is	O
declarative	O
in	O
nature	O
.	O
</s>
<s>
There	O
are	O
two	O
kinds	O
of	O
API	B-Application
resources	O
.	O
</s>
<s>
Most	O
of	O
the	O
API	B-Application
resources	O
in	O
the	O
Kubernetes	B-General_Concept
API	B-Application
are	O
objects	O
.	O
</s>
<s>
A	O
small	O
number	O
of	O
API	B-Application
resource	O
types	O
are	O
"	O
virtual	O
"	O
.	O
</s>
<s>
API	B-Application
resources	O
that	O
correspond	O
to	O
objects	O
will	O
be	O
represented	O
in	O
the	O
cluster	O
with	O
unique	O
identifiers	O
for	O
the	O
objects	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
can	O
be	O
extended	O
using	O
Custom	O
Resources	O
.	O
</s>
<s>
These	O
API	B-Application
resources	O
represent	O
objects	O
that	O
are	O
not	O
part	O
of	O
the	O
standard	O
Kubernetes	B-General_Concept
product	O
.	O
</s>
<s>
These	O
interact	O
with	O
Custom	O
Resources	O
,	O
and	O
allow	O
for	O
a	O
true	O
declarative	O
API	B-Application
that	O
allows	O
for	O
the	O
lifecycle	O
management	O
of	O
Custom	O
Resource	O
that	O
is	O
aligned	O
with	O
the	O
way	O
that	O
Kubernetes	B-General_Concept
itself	O
is	O
designed	O
.	O
</s>
<s>
The	O
combination	O
of	O
Custom	O
Resources	O
and	O
Custom	O
Controllers	O
are	O
often	O
referred	O
to	O
as	O
an	O
(	O
Kubernetes	B-General_Concept
)	O
Operator	O
.	O
</s>
<s>
The	O
key	O
use	O
case	O
for	O
Operators	O
are	O
to	O
capture	O
the	O
aim	O
of	O
a	O
human	O
operator	O
who	O
is	O
managing	O
a	O
service	O
or	O
set	O
of	O
services	O
and	O
to	O
implement	O
them	O
using	O
automation	O
,	O
and	O
with	O
a	O
declarative	O
API	B-Application
supporting	O
this	O
automation	O
.	O
</s>
<s>
The	O
same	O
API	B-Application
design	O
principles	O
have	O
been	O
used	O
to	O
define	O
an	O
API	B-Application
to	O
programmatically	O
create	O
,	O
configure	O
,	O
and	O
manage	O
Kubernetes	B-General_Concept
clusters	O
.	O
</s>
<s>
This	O
is	O
called	O
the	O
Cluster	O
API	B-Application
.	O
</s>
<s>
A	O
key	O
concept	O
embodied	O
in	O
the	O
API	B-Application
is	O
using	O
Infrastructure	B-Architecture
as	I-Architecture
Software	I-Architecture
,	O
or	O
the	O
notion	O
that	O
the	O
Kubernetes	B-General_Concept
cluster	O
infrastructure	O
is	O
itself	O
a	O
resource	O
/	O
object	O
that	O
can	O
be	O
managed	O
just	O
like	O
any	O
other	O
Kubernetes	B-General_Concept
resources	O
.	O
</s>
<s>
Similarly	O
,	O
machines	O
that	O
make	O
up	O
the	O
cluster	O
are	O
also	O
treated	O
as	O
a	O
Kubernetes	B-General_Concept
resource	O
.	O
</s>
<s>
The	O
API	B-Application
has	O
two	O
pieces	O
the	O
core	O
API	B-Application
,	O
and	O
a	O
provider	O
implementation	O
.	O
</s>
<s>
The	O
provider	O
implementation	O
consists	O
of	O
cloud-provider	O
specific	O
functions	O
that	O
let	O
Kubernetes	B-General_Concept
provide	O
the	O
cluster	O
API	B-Application
in	O
a	O
fashion	O
that	O
is	O
well-integrated	O
with	O
the	O
cloud-provider	O
'	O
s	O
services	O
and	O
resources	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
is	O
commonly	O
used	O
as	O
a	O
way	O
to	O
host	O
a	O
microservice-based	O
implementation	O
,	O
because	O
it	O
and	O
its	O
associated	O
ecosystem	O
of	O
tools	O
provide	O
all	O
the	O
capabilities	O
needed	O
to	O
address	O
key	O
concerns	O
of	O
any	O
microservice	O
architecture	O
.	O
</s>
<s>
Open	O
source	O
distributions	O
include	O
the	O
original	O
Kubernetes	B-General_Concept
,	O
Amazon	B-Application
EKS-D	O
,	O
Red	B-General_Concept
Hat	I-General_Concept
OpenShift	I-General_Concept
,	O
VMware	O
Tanzu	O
,	O
Mirantis	O
Kubernetes	B-General_Concept
Engine	O
,	O
and	O
D2iQ	O
Kubernetes	B-General_Concept
Platform	O
.	O
</s>
<s>
Managed	O
offerings	O
include	O
GKE	O
,	O
Oracle	B-General_Concept
Container	B-Application
Engine	O
for	O
Kubernetes	B-General_Concept
,	O
Amazon	B-General_Concept
Elastic	I-General_Concept
Kubernetes	B-General_Concept
Service	O
,	O
IBM	O
Kubernetes	B-General_Concept
Service	O
,	O
and	O
Platform9	O
Managed	O
Kubernetes	B-General_Concept
.	O
</s>
<s>
Various	O
vendors	O
offer	O
Kubernetes-based	O
platforms	O
or	O
infrastructure	B-General_Concept
as	I-General_Concept
a	I-General_Concept
service	I-General_Concept
(	O
IaaS	B-General_Concept
)	O
that	O
deploy	O
Kubernetes	B-General_Concept
.	O
</s>
