[[search-percolate]]
== Percolate API

The percolator allows to register queries against an index, and then
send `percolate` requests which include a doc, and getting back the
queries that match on that doc out of the set of registered queries.

Think of it as the reverse operation of indexing and then searching.
Instead of sending docs, indexing them, and then running queries. One
sends queries, registers them, and then sends docs and finds out which
queries match that doc.

As an example, a user can register an interest (a query) on all tweets
that contain the word "elasticsearch". For every tweet, one can
percolate the tweet against all registered user queries, and find out
which ones matched.

Here is a quick sample, first, lets create a `test` index:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/test
--------------------------------------------------

Next, we will register a percolator query with a specific name called
`kuku` against the `test` index:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/_percolator/test/kuku -d '{
    "query" : {
        "term" : {
            "field1" : "value1"
        }
    }
}'
--------------------------------------------------

And now, we can percolate a document and see which queries match on it
(note, its not really indexed!):

[source,js]
--------------------------------------------------
curl -XGET localhost:9200/test/type1/_percolate -d '{
    "doc" : {
        "field1" : "value1"
    }
}'
--------------------------------------------------

And the matches are part of the response:

[source,js]
--------------------------------------------------
{"ok":true, "matches":["kuku"]}
--------------------------------------------------

You can unregister the previous percolator query with the same API you
use to delete any document in an index:

[source,js]
--------------------------------------------------
curl -XDELETE localhost:9200/_percolator/test/kuku
--------------------------------------------------

[float]
=== Filtering Executed Queries

Since the registered percolator queries are just docs in an index, one
can filter the queries that will be used to percolate a doc. For
example, we can add a `color` field to the registered query:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/_percolator/test/kuku -d '{
    "color" : "blue",
    "query" : {
        "term" : {
            "field1" : "value1"
        }
    }
}'
--------------------------------------------------

And then, we can percolate a doc that only matches on blue colors:

[source,js]
--------------------------------------------------
curl -XGET localhost:9200/test/type1/_percolate -d '{
    "doc" : {
        "field1" : "value1"
    },
    "query" : {
        "term" : {
            "color" : "blue"
        }
    }
}'
--------------------------------------------------

[float]
=== How it Works

The `_percolator` which holds the repository of registered queries is
just a another index. The query is registered under a concrete index
that exists (or will exist). That index name is represented as the type
in the `_percolator` index (a bit confusing, I know...).

The fact that the queries are stored as docs in another index
(`_percolator`) gives us both the persistency nature of it, and the
ability to filter out queries to execute using another query.

The `_percolator` index uses the `index.auto_expand_replica` setting to
make sure that each data node will have access locally to the registered
queries, allowing for fast query executing to filter out queries to run
against a percolated doc.

The percolate API uses the whole number of shards as percolating
processing "engines", both primaries and replicas. In our above case, if
the `test` index has 2 shards with 1 replica, 4 shards will round-robin
in handling percolate requests. Increasing (dynamically) the number of
replicas will increase the number of percolating processing "engines"
and thus the percolation power.

Note, percolate requests will prefer to be executed locally, and will
not try and round-robin across shards if a shard exists locally on a
node that received a request (for example, from HTTP). It's important to
do some round-robin in the client code among nodes (in any case its
recommended). If this behavior is not desired, the `prefer_local`
parameter can be set to `false` to disable it.

Because the percolator API is processing one document at a time, it
doesn't support queries and filters that run against child and nested
documents such as `has_child`, `has_parent`, `top_children`, and
`nested`.