We can avoid concurrency issues completely by allowing only one process to make changes at any time. Most changes will involve only a few files and will complete very quickly. A rename of a top-level directory may block all other changes for longer, but these are likely to be much less frequent.

Because document-level changes in Elasticsearch are ACIDic, we can use the existence or absence of a document as a global lock. To request a lock, we try to create the global-lock document:

PUT /fs/lock/global/_create
{}

If this create request fails with a conflict exception, another process has already been granted the global lock and we will have to try again later. If it succeeds, we are now the proud owners of the global lock and we can continue with our changes. Once we are finished, we must release the lock by deleting the global lock document:

DELETE /fs/lock/global

Depending on how frequent changes are, and how long they take, a global lock could restrict the performance of a system significantly. We can increase parallelism by making our locking more fine-grained.

Instead of locking the whole filesystem, we could lock individual documents by using the same technique as previously described. We can use a scrolled search to retrieve all documents that would be affected by the change and create a lock file for each one:

PUT /fs/lock/_bulk
{ "create": { "_id": 1}} 
{ "process_id": 123    } 
{ "create": { "_id": 2}}
{ "process_id": 123    }

If some files are already locked, parts of the bulk request will fail and we will have to try again.

Of course, if we try to lock all of the files again, the create statements that we used previously will fail for any file that is already locked by us! Instead of a simple create statement, we need an update request with an upsert parameter and this script:

if ( ctx._source.process_id != process_id ) { 
  assert false;  
}
ctx.op = 'noop'; 

The full update request looks like this:

POST /fs/lock/1/_update
{
  "upsert": { "process_id": 123 },
  "script": "if ( ctx._source.process_id != process_id )
  { assert false }; ctx.op = 'noop';"
  "params": {
    "process_id": 123
  }
}

If the document doesn’t already exist, the upsert document is inserted—​much the same as the previous create request. However, if the document does exist, the script looks at the process_id stored in the document. If the process_id matches, no update is performed (noop) but the script returns successfully. If it is different, assert false throws an exception and you know that the lock has failed.

Once all locks have been successfully created, you can proceed with your changes.

Afterward, you must release all of the locks, which you can do by retrieving all of the locked documents and performing a bulk delete:

POST /fs/_refresh 

GET /fs/lock/_search?scroll=1m 
{
    "sort" : ["_doc"],
    "query": {
        "match" : {
            "process_id" : 123
        }
    }
}

PUT /fs/lock/_bulk
{ "delete": { "_id": 1}}
{ "delete": { "_id": 2}}

Document-level locking enables fine-grained access control, but creating lock files for millions of documents can be expensive. In some cases, you can achieve fine-grained locking with much less work, as shown in the following directory tree scenario.

Rather than locking every involved document as in the previous example, 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 
}

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++ 

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/%2Fclinton/_update 
{
  "upsert": { 
    "lock_type":  "shared",
    "lock_count": 1
  },
  "script": "if (ctx._source.lock_type == 'exclusive')
  { assert false }; ctx._source.lock_count++"
}

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/%2Fclinton%2fprojects%2felasticsearch%2fREADME.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/%2Fclinton/_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/%2Fclinton%2fprojects%2felasticsearch%2fREADME.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' 
}

This update request would need to be run for each parent directory in reverse order, from longest to shortest:

POST /fs/lock/%2Fclinton%2fprojects%2felasticsearch/_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.

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.