26_Concurrency_solutions.asciidoc

Solving Concurrency Issues

The problem comes when we want to allow more than one person to rename files or directories at the same time. Imagine that you rename the /clinton directory, which contains hundreds of thousands of files. Meanwhile, another user renames the single file /clinton/projects/elasticsearch/README.txt. That user’s change, although it started after yours, will probably finish more quickly.

One of two things will happen:

• You have decided to use version numbers, in which case your mass rename will fail with a version conflict when it hits the renamed README.asciidoc file.

• You didn’t use versioning, and your changes will overwrite the changes from the other user.

The problem is that Elasticsearch does not support ACID transactions. Changes to individual documents are ACIDic, but not changes involving multiple documents.

If your main data store is a relational database, and Elasticsearch is simply being used as a search engine or as a way to improve performance, make your changes in the database first and replicate those changes to Elasticsearch after they have succeeded. This way, you benefit from the ACID transactions available in the database, and all changes to Elasticsearch happen in the right order. Concurrency is dealt with in the relational database.

If you are not using a relational store, these concurrency issues need to be dealt with at the Elasticsearch level. The following are three practical solutions using Elasticsearch, all of which involve some form of locking:

• Global Locking

• Document Locking

• Tree Locking

Tip	The solutions described in this section could also be implemented by applying the same principles while using an external system instead of Elasticsearch.

Global Locking

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.

Document Locking

Instead of locking the whole filesystem, we could lock individual documents by using the same technique as previously described. A process could use a scan-and-scroll request to retrieve the IDs of all documents that would be affected by the change, and would need to create a lock file for each of them:

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

1. The ID of the lock document would be the same as the ID of the file that should be locked.

2. The process_id is a unique ID that represents the process that wants to perform the changes.

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 ) { (1)
  assert false;  (2)
}
ctx.op = 'noop'; (3)

1. process_id is a parameter that we pass into the script.

2. assert false will throw an exception, causing the update to fail.

3. Changing the op from update to noop prevents the update request from making any changes, but still returns success.

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 will be inserted—​much the same as the create request we used previously. However, if the document does exist, the script will look at the process_id stored in the document. If it is the same as ours, it aborts the update (noop) and returns success. If it is different, the assert false throws an exception and we know that the lock has failed.

Once all locks have been successfully created, the rename operation can begin. Afterward, we must release all of the locks, which we can do with a delete-by-query request:

POST /fs/_refresh (1)

DELETE /fs/lock/_query
{
  "query": {
    "term": {
      "process_id": 123
    }
  }
}

1. The refresh call ensures that all lock documents are visible to the delete-by-query request.

Document-level locking enables fine-grained access control, but creating lock files for millions of documents can be expensive. In certain scenarios, such as this example with directory trees, it is possible to achieve fine-grained locking with much less work.

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 (1)
}

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; (1)
}
ctx._source.lock_count++ (2)

1. If the lock_type is exclusive, the assert statement will throw an exception, causing the update request to fail.

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

1. The ID of the document is /clinton, which is URL-encoded to %2fclinton.

2. 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/%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' (1)
}

1. Once the lock_count reaches 0, the ctx.op is changed from update to 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.

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.