[glossary] [[glossary]] = Glossary of terms [glossary] [[glossary-analysis]] analysis :: Analysis is the process of converting <> to <>. Depending on which analyzer is used, these phrases: `FOO BAR`, `Foo-Bar`, `foo,bar` will probably all result in the terms `foo` and `bar`. These terms are what is actually stored in the index. + A full text query (not a <> query) for `FoO:bAR` will also be analyzed to the terms `foo`,`bar` and will thus match the terms stored in the index. + It is this process of analysis (both at index time and at search time) that allows Elasticsearch to perform full text queries. + Also see <> and <>. [[glossary-cluster]] cluster :: A cluster consists of one or more <> which share the same cluster name. Each cluster has a single master node which is chosen automatically by the cluster and which can be replaced if the current master node fails. [[glossary-ccr]] {ccr} (CCR):: The {ccr} feature enables you to replicate indices in remote clusters to your local cluster. For more information, see {ref}/xpack-ccr.html[{ccr-cap}]. [[glossary-ccs]] {ccs} (CCS):: The {ccs} feature enables any node to act as a federated client across multiple clusters. See <>. [[glossary-document]] document :: A document is a JSON document which is stored in Elasticsearch. It is like a row in a table in a relational database. Each document is stored in an <> and has a <> and an <>. + A document is a JSON object (also known in other languages as a hash / hashmap / associative array) which contains zero or more <>, or key-value pairs. + The original JSON document that is indexed will be stored in the <>, which is returned by default when getting or searching for a document. [[glossary-field]] field :: A <> contains a list of fields, or key-value pairs. The value can be a simple (scalar) value (eg a string, integer, date), or a nested structure like an array or an object. A field is similar to a column in a table in a relational database. + The <> for each field has a field _type_ (not to be confused with document <>) which indicates the type of data that can be stored in that field, eg `integer`, `string`, `object`. The mapping also allows you to define (amongst other things) how the value for a field should be analyzed. [[glossary-filter]] filter :: A filter is a non-scoring <>, meaning that it does not score documents. It is only concerned about answering the question - "Does this document match?". The answer is always a simple, binary yes or no. This kind of query is said to be made in a <>, hence it is called a filter. Filters are simple checks for set inclusion or exclusion. In most cases, the goal of filtering is to reduce the number of documents that have to be examined. [[glossary-follower-index]] follower index :: Follower indices are the target indices for <>. They exist in your local cluster and replicate <>. [[glossary-force-merge]] force merge :: // tag::force-merge-def[] // tag::force-merge-def-short[] Manually trigger a merge to reduce the number of segments in each shard of an index and free up the space used by deleted documents. // end::force-merge-def-short[] You should not force merge indices that are actively being written to. Merging is normally performed automatically, but you can use force merge after <> to reduce the shards in the old index to a single segment. See the {ref}/indices-forcemerge.html[force merge API]. // end::force-merge-def[] [[glossary-freeze]] freeze :: // tag::freeze-def[] // tag::freeze-def-short[] Make an index read-only and minimize its memory footprint. // end::freeze-def-short[] Frozen indices can be searched without incurring the overhead of of re-opening a closed index, but searches are throttled and might be slower. You can freeze indices to reduce the overhead of keeping older indices searchable before you are ready to archive or delete them. See the {ref}/freeze-index-api.html[freeze API]. // end::freeze-def[] [[glossary-id]] id :: The ID of a <> identifies a document. The `index/id` of a document must be unique. If no ID is provided, then it will be auto-generated. (also see <>) [[glossary-index]] index :: An index is like a _table_ in a relational database. It has a <> which contains a <>, which contains the <> in the index. + An index is a logical namespace which maps to one or more <> and can have zero or more <>. [[glossary-index-alias]] index alias :: + -- // tag::index-alias-def[] // tag::index-alias-desc[] An index alias is a secondary name used to refer to one or more existing indices. Most {es} APIs accept an index alias in place of an index name. // end::index-alias-desc[] See {ref}/indices-add-alias.html[Add index alias]. // end::index-alias-def[] See <>. -- [[glossary-index-template]] index template :: + -- // tag::index-template-def[] // tag::index-template-def-short[] Defines settings and mappings to apply to new indexes that match a simple naming pattern, such as _logs-*_. // end::index-template-def-short[] An index template can also attach a lifecycle policy to the new index. Index templates are used to automatically configure indices created during <>. // end::index-template-def[] -- [[glossary-leader-index]] leader index :: Leader indices are the source indices for <>. They exist on remote clusters and are replicated to <>. [[glossary-mapping]] mapping :: A mapping is like a _schema definition_ in a relational database. Each <> has a mapping, which defines a <>, plus a number of index-wide settings. + A mapping can either be defined explicitly, or it will be generated automatically when a document is indexed. [[glossary-node]] node :: A node is a running instance of Elasticsearch which belongs to a <>. Multiple nodes can be started on a single server for testing purposes, but usually you should have one node per server. + At startup, a node will use unicast to discover an existing cluster with the same cluster name and will try to join that cluster. [[glossary-primary-shard]] primary shard :: Each document is stored in a single primary <>. When you index a document, it is indexed first on the primary shard, then on all <> of the primary shard. + By default, an <> has one primary shard. You can specify more primary shards to scale the number of <> that your index can handle. + You cannot change the number of primary shards in an index, once the index is created. However, an index can be split into a new index using the <>. + See also <> [[glossary-query]] query :: A request for information from {es}. You can think of a query as a question, written in a way {es} understands. A search consists of one or more queries combined. + There are two types of queries: _scoring queries_ and _filters_. For more information about query types, see <>. [[glossary-recovery]] recovery :: + -- Shard recovery is the process of syncing a <> from a <>. Upon completion, the replica shard is available for search. // tag::recovery-triggers[] Recovery automatically occurs during the following processes: * Node startup or failure. This type of recovery is called a *local store recovery*. * <>. * Relocation of a shard to a different node in the same cluster. * {ref}/snapshots-restore-snapshot.html[Snapshot restoration]. // end::recovery-triggers[] -- [[glossary-reindex]] reindex :: // tag::reindex-def[] To cycle through some or all documents in one or more indices, re-writing them into the same or new index in a local or remote cluster. This is most commonly done to update mappings, or to upgrade Elasticsearch between two incompatible index versions. // end::reindex-def[] [[glossary-replica-shard]] replica shard :: Each <> can have zero or more replicas. A replica is a copy of the primary shard, and has two purposes: + 1. increase failover: a replica shard can be promoted to a primary shard if the primary fails 2. increase performance: get and search requests can be handled by primary or replica shards. + By default, each primary shard has one replica, but the number of replicas can be changed dynamically on an existing index. A replica shard will never be started on the same node as its primary shard. [[glossary-rollover]] rollover :: + -- // tag::rollover-def[] // tag::rollover-def-short[] Redirect an alias to begin writing to a new index when the existing index reaches a certain age, number of docs, or size. // end::rollover-def-short[] The new index is automatically configured according to any matching <>. For example, if you're indexing log data, you might use rollover to create daily or weekly indices. See the {ref}/indices-rollover-index.html[rollover index API]. // end::rollover-def[] -- [[glossary-routing]] routing :: When you index a document, it is stored on a single <>. That shard is chosen by hashing the `routing` value. By default, the `routing` value is derived from the ID of the document or, if the document has a specified parent document, from the ID of the parent document (to ensure that child and parent documents are stored on the same shard). + This value can be overridden by specifying a `routing` value at index time, or a <> in the <>. [[glossary-shard]] shard :: + -- // tag::shard-def[] A shard is a single Lucene instance. It is a low-level “worker” unit which is managed automatically by Elasticsearch. An index is a logical namespace which points to <> and <> shards. + Other than defining the number of primary and replica shards that an index should have, you never need to refer to shards directly. Instead, your code should deal only with an index. + Elasticsearch distributes shards amongst all <> in the <>, and can move shards automatically from one node to another in the case of node failure, or the addition of new nodes. // end::shard-def[] -- [[glossary-shrink]] shrink :: // tag::shrink-def[] // tag::shrink-def-short[] Reduce the number of primary shards in an index. // end::shrink-def-short[] You can shrink an index to reduce its overhead when the request volume drops. For example, you might opt to shrink an index once it is no longer the write index. See the {ref}/indices-shrink-index.html[shrink index API]. // end::shrink-def[] [[glossary-source_field]] source field :: By default, the JSON document that you index will be stored in the `_source` field and will be returned by all get and search requests. This allows you access to the original object directly from search results, rather than requiring a second step to retrieve the object from an ID. [[glossary-term]] term :: A term is an exact value that is indexed in Elasticsearch. The terms `foo`, `Foo`, `FOO` are NOT equivalent. Terms (i.e. exact values) can be searched for using _term_ queries. + See also <> and <>. [[glossary-text]] text :: Text (or full text) is ordinary unstructured text, such as this paragraph. By default, text will be <> into <>, which is what is actually stored in the index. + Text <> need to be analyzed at index time in order to be searchable as full text, and keywords in full text queries must be analyzed at search time to produce (and search for) the same terms that were generated at index time. + See also <> and <>. [[glossary-type]] type :: A type used to represent the _type_ of document, e.g. an `email`, a `user`, or a `tweet`. Types are deprecated and are in the process of being removed. See <>.