Tree Locking
Rather than locking every involved document, as in the previous option, we could lock just part of the directory tree. We will need exclusive access to the file or directory that we want to rename, which can be achieved with an exclusive lock document:
{
"lock_type"
:
"exclusive"
}
And we need shared locks on any parent directories, with a shared lock document:
{
"lock_type"
:
"shared"
,
"lock_count"
:
1
}
- The
lock_count
records the number of processes that hold a shared lock.
A process that wants to rename /clinton/projects/elasticsearch/README.txt
needs an exclusive lock on that file, and a shared lock on /clinton
, /clinton/projects
, and /clinton/projects/elasticsearch
.
A simple create
request will suffice for the exclusive lock, but the shared lock needs a scripted update to implement some extra logic:
if
(
ctx
.
_source
.
lock_type
==
'exclusive'
)
{
assert
false
;
}
ctx
.
_source
.
lock_count
++
- If the
lock_type
isexclusive
, theassert
statement will throw an exception, causing the update request to fail. - Otherwise, we increment the
lock_count
.
This script handles the case where the lock
document already exists, but we will also need an upsert
document to handle the case where it doesn’t exist yet. The full update request is as follows:
POST
/fs/lock/%
2
Fclinton/_update
{
"upsert"
:
{
"lock_type"
:
"shared"
,
"lock_count"
:
1
},
"script"
:
"if (ctx._source.lock_type == 'exclusive')
{ assert false }; ctx._source.lock_count++"
}
- The ID of the document is
/clinton
, which is URL-encoded to%2fclinton
. - The
upsert
document will be inserted if the document does not already exist.
Once we succeed in gaining a shared lock on all of the parent directories, we try to create
an exclusive lock on the file itself:
PUT
/fs/lock/%
2
Fclinton%
2
fprojects%
2
felasticsearch%
2
fREADME.txt/_create
{
"lock_type"
:
"exclusive"
}
Now, if somebody else wants to rename the /clinton
directory, they would have to gain an exclusive lock on that path:
PUT
/fs/lock/%
2
Fclinton/_create
{
"lock_type"
:
"exclusive"
}
This request would fail because a lock
document with the same ID already exists. The other user would have to wait until our operation is done and we have released our locks. The exclusive lock can just be deleted:
DELETE
/fs/lock/%
2
Fclinton%
2
fprojects%
2
felasticsearch%
2
fREADME.txt
The shared locks need another script that decrements the lock_count
and, if the count drops to zero, deletes the lock
document:
if
(--
ctx
.
_source
.
lock_count
==
0
)
{
ctx
.
op
=
'delete'
}
- Once the
lock_count
reaches0
, thectx.op
is changed fromupdate
todelete
.
This update request would need to be run for each parent directory in reverse order, from longest to shortest:
POST
/fs/lock/%
2
Fclinton%
2
fprojects%
2
felasticsearch/_update
{
"script"
:
"if (--ctx._source.lock_count == 0) { ctx.op = 'delete' } "
}
Tree locking gives us fine-grained concurrency control with the minimum of effort. Of course, it is not applicable to every situation—the data model must have some sort of access path like the directory tree for it to work.
Note
None of the three options—global, document, or tree locking—deals with the thorniest problem associated with locking: what happens if the process holding the lock dies?
The unexpected death of a process leaves us with two problems:
- How do we know that we can release the locks held by the dead process?
- How do we clean up the change that the dead process did not manage to complete?
These topics are beyond the scope of this book, but you will need to give them some thought if you decide to use locking.
While denormalization is a good choice for many projects, the need for locking schemes can make for complicated implementations. Instead, Elasticsearch provides two models that help us deal with related entities: nested objects and parent-child relationships.
Comments