<s>
A	O
database	B-General_Concept
shard	I-General_Concept
,	O
or	O
simply	O
a	O
shard	B-General_Concept
,	O
is	O
a	O
horizontal	O
partition	B-General_Concept
of	O
data	O
in	O
a	O
database	B-General_Concept
or	O
search	B-Application
engine	I-Application
.	O
</s>
<s>
Each	O
shard	B-General_Concept
is	O
held	O
on	O
a	O
separate	O
database	B-Application
server	I-Application
instance	O
,	O
to	O
spread	O
load	O
.	O
</s>
<s>
Some	O
data	O
within	O
a	O
database	B-General_Concept
remains	O
present	O
in	O
all	O
shards	B-General_Concept
,	O
but	O
some	O
appear	O
only	O
in	O
a	O
single	O
shard	B-General_Concept
.	O
</s>
<s>
Each	O
shard	B-General_Concept
(	O
or	O
server	O
)	O
acts	O
as	O
the	O
single	O
source	O
for	O
this	O
subset	O
of	O
data	O
.	O
</s>
<s>
Horizontal	O
partitioning	O
is	O
a	O
database	B-General_Concept
design	O
principle	O
whereby	O
rows	B-Application
of	O
a	O
database	B-General_Concept
table	O
are	O
held	O
separately	O
,	O
rather	O
than	O
being	O
split	O
into	O
columns	B-Application
(	O
which	O
is	O
what	O
normalization	B-Application
and	O
vertical	B-General_Concept
partitioning	I-General_Concept
do	O
,	O
to	O
differing	O
extents	O
)	O
.	O
</s>
<s>
Each	O
partition	B-General_Concept
forms	O
part	O
of	O
a	O
shard	B-General_Concept
,	O
which	O
may	O
in	O
turn	O
be	O
located	O
on	O
a	O
separate	O
database	B-Application
server	I-Application
or	O
physical	O
location	O
.	O
</s>
<s>
Since	O
the	O
tables	O
are	O
divided	O
and	O
distributed	O
into	O
multiple	O
servers	O
,	O
the	O
total	O
number	O
of	O
rows	B-Application
in	O
each	O
table	O
in	O
each	O
database	B-General_Concept
is	O
reduced	O
.	O
</s>
<s>
This	O
reduces	O
index	B-Data_Structure
size	O
,	O
which	O
generally	O
improves	O
search	O
performance	O
.	O
</s>
<s>
A	O
database	B-General_Concept
shard	I-General_Concept
can	O
be	O
placed	O
on	O
separate	O
hardware	O
,	O
and	O
multiple	O
shards	B-General_Concept
can	O
be	O
placed	O
on	O
multiple	O
machines	O
.	O
</s>
<s>
This	O
enables	O
a	O
distribution	O
of	O
the	O
database	B-General_Concept
over	O
a	O
large	O
number	O
of	O
machines	O
,	O
greatly	O
improving	O
performance	O
.	O
</s>
<s>
In	O
addition	O
,	O
if	O
the	O
database	B-General_Concept
shard	I-General_Concept
is	O
based	O
on	O
some	O
real-world	O
segmentation	O
of	O
the	O
data	O
(	O
e.g.	O
,	O
European	O
customers	O
v	O
.	O
American	O
customers	O
)	O
then	O
it	O
may	O
be	O
possible	O
to	O
infer	O
the	O
appropriate	O
shard	B-General_Concept
membership	O
easily	O
and	O
automatically	O
,	O
and	O
query	O
only	O
the	O
relevant	O
shard	B-General_Concept
.	O
</s>
<s>
In	O
practice	O
,	O
sharding	B-General_Concept
is	O
complex	O
.	O
</s>
<s>
Although	O
it	O
has	O
been	O
done	O
for	O
a	O
long	O
time	O
by	O
hand-coding	O
(	O
especially	O
where	O
rows	B-Application
have	O
an	O
obvious	O
grouping	O
,	O
as	O
per	O
the	O
example	O
above	O
)	O
,	O
this	O
is	O
often	O
inflexible	O
.	O
</s>
<s>
There	O
is	O
a	O
desire	O
to	O
support	O
sharding	B-General_Concept
automatically	O
,	O
both	O
in	O
terms	O
of	O
adding	O
code	O
support	O
for	O
it	O
,	O
and	O
for	O
identifying	O
candidates	O
to	O
be	O
sharded	O
separately	O
.	O
</s>
<s>
Consistent	B-Algorithm
hashing	I-Algorithm
is	O
a	O
technique	O
used	O
in	O
sharding	B-General_Concept
to	O
spread	O
large	O
loads	O
across	O
multiple	O
smaller	O
services	O
and	O
servers	O
.	O
</s>
<s>
Where	O
distributed	B-Architecture
computing	I-Architecture
is	O
used	O
to	O
separate	O
load	O
between	O
multiple	O
servers	O
(	O
either	O
for	O
performance	O
or	O
reliability	O
reasons	O
)	O
,	O
a	O
shard	B-General_Concept
approach	O
may	O
also	O
be	O
useful	O
.	O
</s>
<s>
In	O
the	O
2010s	O
,	O
sharding	B-General_Concept
of	O
execution	B-General_Concept
capacity	O
,	O
as	O
well	O
as	O
the	O
more	O
traditional	O
sharding	B-General_Concept
of	O
data	O
,	O
has	O
emerged	O
as	O
a	O
potential	O
approach	O
to	O
overcome	O
performance	O
and	O
scalability	O
problems	O
in	O
blockchains	B-Application
.	O
</s>
<s>
Horizontal	O
partitioning	O
splits	O
one	O
or	O
more	O
tables	O
by	O
row	O
,	O
usually	O
within	O
a	O
single	O
instance	O
of	O
a	O
schema	B-Application
and	O
a	O
database	B-Application
server	I-Application
.	O
</s>
<s>
It	O
may	O
offer	O
an	O
advantage	O
by	O
reducing	O
index	B-Data_Structure
size	O
(	O
and	O
thus	O
search	O
effort	O
)	O
provided	O
that	O
there	O
is	O
some	O
obvious	O
,	O
robust	O
,	O
implicit	O
way	O
to	O
identify	O
in	O
which	O
partition	B-General_Concept
a	O
particular	O
row	O
will	O
be	O
found	O
,	O
without	O
first	O
needing	O
to	O
search	O
the	O
index	B-Data_Structure
,	O
e.g.	O
,	O
the	O
classic	O
example	O
of	O
the	O
'	O
CustomersEast	O
 '	O
and	O
'	O
CustomersWest	O
 '	O
tables	O
,	O
where	O
their	O
zip	O
code	O
already	O
indicates	O
where	O
they	O
will	O
be	O
found	O
.	O
</s>
<s>
Sharding	B-General_Concept
goes	O
beyond	O
this	O
:	O
it	O
partitions	B-General_Concept
the	O
problematic	O
table(s )	O
in	O
the	O
same	O
way	O
,	O
but	O
it	O
does	O
this	O
across	O
potentially	O
multiple	O
instances	O
of	O
the	O
schema	B-Application
.	O
</s>
<s>
Splitting	O
shards	B-General_Concept
across	O
multiple	O
isolated	O
instances	O
requires	O
more	O
than	O
simple	O
horizontal	O
partitioning	O
.	O
</s>
<s>
The	O
hoped-for	O
gains	O
in	O
efficiency	O
would	O
be	O
lost	O
,	O
if	O
querying	O
the	O
database	B-General_Concept
required	O
multiple	O
instances	O
to	O
be	O
queried	O
,	O
just	O
to	O
retrieve	O
a	O
simple	O
dimension	O
table	O
.	O
</s>
<s>
Beyond	O
partitioning	O
,	O
sharding	B-General_Concept
thus	O
splits	O
large	O
partitionable	O
tables	O
across	O
the	O
servers	O
,	O
while	O
smaller	O
tables	O
are	O
replicated	B-Architecture
as	O
complete	O
units	O
.	O
</s>
<s>
This	O
is	O
also	O
why	O
sharding	B-General_Concept
is	O
related	O
to	O
a	O
shared-nothing	B-Operating_System
architecture	I-Operating_System
—	O
once	O
sharded	O
,	O
each	O
shard	B-General_Concept
can	O
live	O
in	O
a	O
totally	O
separate	O
logical	O
schema	B-Application
instance	O
/	O
physical	O
database	B-Application
server	I-Application
/	O
data	B-Operating_System
center	I-Operating_System
/	O
continent	O
.	O
</s>
<s>
There	O
is	O
no	O
ongoing	O
need	O
to	O
retain	O
shared	O
access	O
(	O
from	O
between	O
shards	B-General_Concept
)	O
to	O
the	O
other	O
unpartitioned	O
tables	O
in	O
other	O
shards	B-General_Concept
.	O
</s>
<s>
This	O
makes	O
replication	B-Architecture
across	O
multiple	O
servers	O
easy	O
(	O
simple	O
horizontal	O
partitioning	O
does	O
not	O
)	O
.	O
</s>
<s>
It	O
is	O
also	O
useful	O
for	O
worldwide	O
distribution	O
of	O
applications	O
,	O
where	O
communications	O
links	O
between	O
data	B-Operating_System
centers	I-Operating_System
would	O
otherwise	O
be	O
a	O
bottleneck	O
.	O
</s>
<s>
There	O
is	O
also	O
a	O
requirement	O
for	O
some	O
notification	O
and	O
replication	B-Architecture
mechanism	O
between	O
schema	B-Application
instances	O
,	O
so	O
that	O
the	O
unpartitioned	O
tables	O
remain	O
as	O
closely	O
synchronized	O
as	O
the	O
application	O
demands	O
.	O
</s>
<s>
This	O
is	O
a	O
complex	O
choice	O
in	O
the	O
architecture	O
of	O
sharded	O
systems	O
:	O
approaches	O
range	O
from	O
making	O
these	O
effectively	O
read-only	O
(	O
updates	O
are	O
rare	O
and	O
batched	O
)	O
,	O
to	O
dynamically	O
replicated	B-Architecture
tables	O
(	O
at	O
the	O
cost	O
of	O
reducing	O
some	O
of	O
the	O
distribution	O
benefits	O
of	O
sharding	B-General_Concept
)	O
and	O
many	O
options	O
in	O
between	O
.	O
</s>
<s>
Altibase	B-Operating_System
provides	O
combined	O
(	O
client-side	O
and	O
server-side	O
)	O
sharding	B-General_Concept
architecture	O
transparent	O
to	O
client	O
applications	O
.	O
</s>
<s>
Apache	B-Language
HBase	I-Language
can	O
shard	B-General_Concept
automatically	O
.	O
</s>
<s>
Azure	O
SQL	O
Database	B-General_Concept
Elastic	O
Database	B-General_Concept
tools	O
shards	B-General_Concept
to	O
scale	O
out	O
and	O
in	O
the	O
data-tier	O
of	O
an	O
application	O
.	O
</s>
<s>
ClickHouse	B-Language
,	O
a	O
fast	O
open-source	O
OLAP	O
database	B-General_Concept
management	O
system	O
,	O
shards	B-General_Concept
.	O
</s>
<s>
Couchbase	B-Operating_System
shards	B-General_Concept
automatically	O
and	O
transparently	O
.	O
</s>
<s>
Db2	O
Data	O
Partitioning	O
Feature	O
(	O
MPP	O
)	O
which	O
is	O
a	O
shared-nothing	B-Operating_System
database	B-General_Concept
partitions	B-General_Concept
running	O
on	O
separate	O
nodes	O
.	O
</s>
<s>
DRDS	O
(	O
Distributed	O
Relational	O
Database	B-General_Concept
Service	O
)	O
of	O
Alibaba	B-General_Concept
Cloud	I-General_Concept
does	O
database/table	O
sharding	B-General_Concept
,	O
and	O
supports	O
Singles	O
 '	O
Day	O
.	O
</s>
<s>
Elasticsearch	B-Application
enterprise	O
search	O
server	O
shards	B-General_Concept
.	O
</s>
<s>
eXtreme	O
Scale	O
is	O
a	O
cross-process	O
in-memory	O
key/value	O
data	O
store	O
(	O
a	O
NoSQL	B-General_Concept
data	O
store	O
)	O
.	O
</s>
<s>
It	O
uses	O
sharding	B-General_Concept
to	O
achieve	O
scalability	O
across	O
processes	O
for	O
both	O
data	O
and	O
MapReduce-style	O
parallel	O
processing	O
.	O
</s>
<s>
Hibernate	B-Library
shards	B-General_Concept
,	O
but	O
has	O
had	O
little	O
development	O
since	O
2007	O
.	O
</s>
<s>
IBM	B-Application
Informix	I-Application
shards	B-General_Concept
since	O
version	O
12.1	O
xC1	O
as	O
part	O
of	O
the	O
MACH11	O
technology	O
.	O
</s>
<s>
Informix	B-Application
12.10	O
xC2	O
added	O
full	O
compatibility	O
with	O
MongoDB	B-Application
drivers	O
,	O
allowing	O
the	O
mix	O
of	O
regular	O
relational	O
tables	O
with	O
NoSQL	B-General_Concept
collections	O
,	O
while	O
still	O
allowing	O
sharding	B-General_Concept
,	O
fail-over	B-General_Concept
and	O
ACID	O
properties	O
.	O
</s>
<s>
Kdb+	B-Application
shards	B-General_Concept
since	O
version	O
2.0	O
.	O
</s>
<s>
MariaDB	B-Application
Spider	O
,	O
an	O
storage	O
engine	O
that	O
supports	O
table	O
federation	O
,	O
table	O
sharding	B-General_Concept
,	O
XA	O
transactions	O
,	O
and	O
ODBC	O
data	O
sources	O
.	O
</s>
<s>
The	O
MariaDB	B-Application
Spider	O
engine	O
is	O
bundled	O
in	O
MariaDB	B-Application
server	O
since	O
version	O
10.0.4	O
.	O
</s>
<s>
MonetDB	B-Application
,	O
an	O
open-source	O
column-store	B-General_Concept
,	O
does	O
read-only	O
sharding	B-General_Concept
in	O
its	O
July	O
2015	O
release	O
.	O
</s>
<s>
MongoDB	B-Application
shards	B-General_Concept
since	O
version	O
1.6	O
.	O
</s>
<s>
MySQL	B-Operating_System
Cluster	I-Operating_System
automatically	O
and	O
transparently	O
shards	B-General_Concept
across	O
low-cost	O
commodity	O
nodes	O
,	O
allowing	O
scale-out	O
of	O
read	O
and	O
write	O
queries	O
,	O
without	O
requiring	O
changes	O
to	O
the	O
application	O
.	O
</s>
<s>
MySQL	B-Application
Fabric	O
(	O
part	O
of	O
MySQL	B-Application
utilities	O
)	O
shards	B-General_Concept
.	O
</s>
<s>
Oracle	O
Database	B-General_Concept
shards	I-General_Concept
since	O
12c	O
Release	O
2	O
and	O
in	O
one	O
liner	O
:	O
Combination	O
of	O
sharding	B-General_Concept
advantages	O
with	O
well-known	O
capabilities	O
of	O
enterprise	O
ready	O
multi-model	O
Oracle	O
Database	B-General_Concept
.	O
</s>
<s>
Oracle	B-Application
NoSQL	I-Application
Database	I-Application
has	O
automatic	O
sharding	B-General_Concept
and	O
elastic	O
,	O
online	O
expansion	O
of	O
the	O
cluster	O
(	O
adding	O
more	O
shards	B-General_Concept
)	O
.	O
</s>
<s>
Solr	B-Language
enterprise	O
search	O
server	O
shards	B-General_Concept
.	O
</s>
<s>
Spanner	B-Operating_System
,	O
Google	O
's	O
global-scale	O
distributed	O
database	B-General_Concept
,	O
shards	B-General_Concept
across	O
multiple	O
Paxos	B-Operating_System
state	O
machines	O
to	O
scale	O
to	O
"	O
millions	O
of	O
machines	O
across	O
hundreds	O
of	O
data	B-Operating_System
centers	I-Operating_System
and	O
trillions	O
of	O
database	B-Application
rows	I-Application
"	O
.	O
</s>
<s>
SQLAlchemy	B-Application
ORM	I-Application
,	O
a	O
data-mapper	O
for	O
the	O
Python	B-Language
programming	I-Language
language	I-Language
shards	B-General_Concept
.	O
</s>
<s>
SQL	O
Server	O
,	O
since	O
SQL	O
Server	O
2005	O
shards	B-General_Concept
with	O
help	O
of	O
3rd	O
party	O
tools	O
.	O
</s>
<s>
Vault	O
,	O
a	O
cryptocurrency	O
,	O
shards	B-General_Concept
to	O
drastically	O
reduce	O
the	O
data	O
that	O
users	O
need	O
to	O
join	O
the	O
network	O
and	O
verify	O
transactions	O
.	O
</s>
<s>
Vitess	B-Application
open-source	O
database	B-General_Concept
clustering	O
system	O
shards	B-General_Concept
MySQL	B-Application
.	O
</s>
<s>
It	O
is	O
a	O
Cloud	B-Application
Native	I-Application
Computing	I-Application
Foundation	I-Application
project	O
.	O
</s>
<s>
ShardingSphere	O
related	O
to	O
a	O
database	B-General_Concept
clustering	O
system	O
providing	O
data	O
sharding	B-General_Concept
,	O
distributed	O
transactions	O
,	O
and	O
distributed	O
database	B-General_Concept
management	O
.	O
</s>
<s>
Sharding	B-General_Concept
a	O
database	B-General_Concept
table	O
before	O
it	O
has	O
been	O
optimized	O
locally	O
causes	O
premature	O
complexity	O
.	O
</s>
<s>
Sharding	B-General_Concept
should	O
be	O
used	O
only	O
when	O
all	O
other	O
options	O
for	O
optimization	O
are	O
inadequate	O
.	O
</s>
<s>
The	O
introduced	O
complexity	O
of	O
database	B-General_Concept
sharding	I-General_Concept
causes	O
the	O
following	O
potential	O
problems	O
:	O
</s>
<s>
Single	B-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
-	O
Corruption	O
of	O
one	O
shard	B-General_Concept
due	O
to	O
network/hardware/systems	O
problems	O
causes	O
failure	O
of	O
the	O
entire	O
table	O
.	O
</s>
<s>
Fail-over	B-General_Concept
server	O
complexity	O
-	O
Fail-over	B-General_Concept
servers	O
must	O
have	O
copies	O
of	O
the	O
fleets	O
of	O
database	B-General_Concept
shards	I-General_Concept
.	O
</s>
<s>
Backups	B-Protocol
complexity	O
-	O
Database	B-General_Concept
backups	B-Protocol
of	O
the	O
individual	O
shards	B-General_Concept
must	O
be	O
coordinated	O
with	O
the	O
backups	B-Protocol
of	O
the	O
other	O
shards	B-General_Concept
.	O
</s>
<s>
Operational	O
complexity	O
-	O
Adding/removing	O
indexes	O
,	O
adding/deleting	O
columns	B-Application
,	O
modifying	O
the	O
schema	B-Application
becomes	O
much	O
more	O
difficult	O
.	O
</s>
<s>
These	O
historical	O
complications	O
of	O
do-it-yourself	O
sharding	B-General_Concept
were	O
addressed	O
by	O
independent	O
software	O
vendors	O
who	O
provided	O
automatic	O
sharding	B-General_Concept
.	O
</s>
<s>
In	O
a	O
database	B-General_Concept
context	O
,	O
most	O
recognize	O
the	O
term	O
"	O
shard	B-General_Concept
"	O
is	O
most	O
likely	O
derived	O
from	O
either	O
one	O
of	O
two	O
sources	O
:	O
Computer	O
Corporation	O
of	O
America	O
's	O
"	O
A	O
System	O
for	O
Highly	O
Available	O
Replicated	B-Architecture
Data	O
"	O
,	O
which	O
utilized	O
redundant	O
hardware	O
to	O
facilitate	O
data	B-Architecture
replication	I-Architecture
(	O
as	O
opposed	O
to	O
horizontal	O
partitioning	O
)	O
;	O
or	O
the	O
critically	O
acclaimed	O
1997	O
MMORPG	B-Protocol
video	O
game	O
Ultima	B-Protocol
Online	I-Protocol
which	O
set	O
8	O
Guinness	O
World	O
Records	O
and	O
was	O
designated	O
by	O
Time	O
as	O
one	O
of	O
the	O
100	O
greatest	O
video	O
games	O
produced	O
of	O
all	O
time	O
.	O
</s>
<s>
Richard	O
Garriott	O
,	O
creator	O
of	O
Ultima	B-Protocol
Online	I-Protocol
,	O
recollects	O
the	O
term	O
being	O
coined	O
during	O
production	O
phase	O
when	O
they	O
attempted	O
to	O
create	O
a	O
self-regulating	O
virtual	O
ecology	O
system	O
,	O
whereby	O
players	O
may	O
leverage	O
new	O
internet	O
access	O
(	O
a	O
revolutionary	O
technology	O
at	O
the	O
time	O
)	O
to	O
interact	O
and	O
harvest	O
in-game	O
resources	O
.	O
</s>
<s>
Garriott	O
's	O
production	O
team	O
attempted	O
to	O
mitigate	O
this	O
issue	O
by	O
separating	O
the	O
global	O
player	O
base	O
into	O
separate	O
sessions	O
,	O
and	O
rewriting	O
part	O
of	O
Ultima	B-Protocol
Online	I-Protocol
fictional	O
connection	O
to	O
the	O
end	O
of	O
Ultima	B-Device
I	I-Device
:	I-Device
The	I-Device
First	I-Device
Age	I-Device
of	I-Device
Darkness	I-Device
,	O
where	O
the	O
defeat	O
of	O
its	O
antagonist	O
Mondain	O
also	O
led	O
to	O
the	O
creation	O
of	O
multiverse	O
"	O
shards	B-General_Concept
"	O
.	O
</s>
<s>
Today	O
,	O
the	O
term	O
"	O
shard	B-General_Concept
"	O
refers	O
to	O
the	O
deployment	O
and	O
use	O
of	O
redundant	O
hardware	O
across	O
database	B-General_Concept
systems	O
.	O
</s>
