25_Concurrency.asciidoc

Denormalization and Concurrency

Of course, data denormalization has downsides too. The first disadvantage is that the index will be bigger because the _source document for every blog post is bigger, and there are more indexed fields. This usually isn’t a huge problem. The data written to disk is highly compressed, and disk space is cheap. Elasticsearch can happily cope with the extra data.

The more important issue is that, if the user were to change his name, all of his blog posts would need to be updated too. Fortunately, users don’t often change names. Even if they did, it is unlikely that a user would have written more than a few thousand blog posts, so updating blog posts with the scroll and bulk APIs would take less than a second.

However, let’s consider a more complex scenario in which changes are common, far reaching, and, most important, concurrent.

In this example, we are going to emulate a filesystem with directory trees in Elasticsearch, much like a filesystem on Linux: the root of the directory is /, and each directory can contain files and subdirectories.

We want to be able to search for files that live in a particular directory, the equivalent of this:

grep "some text" /clinton/projects/elasticsearch/*

This requires us to index the path of the directory where the file lives:

PUT /fs/file/1
{
  "name":     "README.txt", (1)
  "path":     "/clinton/projects/elasticsearch", (2)
  "contents": "Starting a new Elasticsearch project is easy..."
}

1. The filename

2. The full path to the directory holding the file

Note	Really, we should also index directory documents so we can list all files and subdirectories within a directory, but for brevity’s sake, we will ignore that requirement.

We also want to be able to search for files that live anywhere in the directory tree below a particular directory, the equivalent of this:

grep -r "some text" /clinton

To support this, we need to index the path hierarchy:

• /clinton

• /clinton/projects

• /clinton/projects/elasticsearch

This hierarchy can be generated automatically from the path field using the {ref}/analysis-pathhierarchy-tokenizer.html[path_hierarchy tokenizer]:

PUT /fs
{
  "settings": {
    "analysis": {
      "analyzer": {
        "paths": { (1)
          "tokenizer": "path_hierarchy"
        }
      }
    }
  }
}

1. The custom paths analyzer uses the {ref}/analysis-pathhierarchy-tokenizer.html[path_hierarchy tokenizer] with its default settings.

The mapping for the file type would look like this:

PUT /fs/_mapping/file
{
  "properties": {
    "name": { (1)
      "type":  "string",
      "index": "not_analyzed"
    },
    "path": { (2)
      "type":  "string",
      "index": "not_analyzed",
      "fields": {
        "tree": { (2)
          "type":     "string",
          "analyzer": "paths"
        }
      }
    }
  }
}

1. The name field will contain the exact name.

2. The path field will contain the exact directory name, while the path.tree field will contain the path hierarchy.

Once the index is set up and the files have been indexed, we can perform a search for files containing elasticsearch in just the /clinton/projects/elasticsearch directory like this:

GET /fs/file/_search
{
  "query": {
    "filtered": {
      "query": {
        "match": {
          "contents": "elasticsearch"
        }
      },
      "filter": {
        "term": { (1)
          "path": "/clinton/projects/elasticsearch"
        }
      }
    }
  }
}

1. Find files in this directory only.

Every file that lives in any subdirectory under /clinton will include the term /clinton in the path.tree field. So we can search for all files in any subdirectory of /clinton as follows:

GET /fs/file/_search
{
  "query": {
    "filtered": {
      "query": {
        "match": {
          "contents": "elasticsearch"
        }
      },
      "filter": {
        "term": { (1)
          "path.tree": "/clinton"
        }
      }
    }
  }
}

1. Find files in this directory or in any of its subdirectories.

Renaming Files and Directories

So far, so good. Renaming a file is easy—​a simple update or index request is all that is required. You can even use optimistic concurrency control to ensure that your change doesn’t conflict with the changes from another user:

PUT /fs/file/1?version=2 (1)
{
  "name":     "README.asciidoc",
  "path":     "/clinton/projects/elasticsearch",
  "contents": "Starting a new Elasticsearch project is easy..."
}

1. The version number ensures that the change is applied only if the document in the index has this same version number.

We can even rename a directory, but this means updating all of the files that exist anywhere in the path hierarchy beneath that directory. This may be quick or slow, depending on how many files need to be updated. All we would need to do is to use scan-and-scroll to retrieve all the files, and the bulk API to update them. The process isn’t atomic, but all files will quickly move to their new home.