Database Reference
In-Depth Information
A reliable configuration service
Going back to the
write()
method in
ActiveKeyValueStore
, recall that it is com-
posed of an
exists
operation followed by either a
create
or a
setData
:
public
void
write
(
String path
,
String value
)
throws
InterruptedException
,
KeeperException
{
Stat stat
=
zk
.
exists
(
path
,
false
);
if
(
stat
==
null
) {
zk
.
create
(
path
,
value
.
getBytes
(
CHARSET
),
Ids
.
OPEN_ACL_UNSAFE
,
CreateMode
.
PERSISTENT
);
}
else
{
zk
.
setData
(
path
,
value
.
getBytes
(
CHARSET
), -
1
);
}
}
Taken as a whole, the
write()
method is idempotent, so we can afford to uncondition-
ally retry it. Here's a modified version of the
write()
method that retries in a loop. It is
set to try a maximum number of retries (
MAX_RETRIES
) and sleeps for
RETRY_PERIOD_SECONDS
between each attempt:
public
void
write
(
String path
,
String value
)
throws
InterruptedException
,
KeeperException
{
int
retries
=
0
;
while
(
true
) {
try
{
Stat stat
=
zk
.
exists
(
path
,
false
);
if
(
stat
==
null
) {
zk
.
create
(
path
,
value
.
getBytes
(
CHARSET
),
Ids
.
OPEN_ACL_UNSAFE
,
CreateMode
.
PERSISTENT
);
}
else
{
zk
.
setData
(
path
,
value
.
getBytes
(
CHARSET
),
stat
.
getVersion
());
}
return
;
}
catch
(
KeeperException
.
SessionExpiredException
e
) {
throw
e
;
}
catch
(
KeeperException e
) {
if
(
retries
++ ==
MAX_RETRIES
) {
throw
e
;
}
// sleep then retry
TimeUnit
.
SECONDS
.
sleep
(
RETRY_PERIOD_SECONDS
);
