Detailed overview of ArangoSearch Views

ArangoSearch is a powerful fulltext search component with additional functionality, supported via the text analyzer and tfidf / bm25 scorers, without impact on performance when specifying documents from different collections or filtering on multiple document attributes.

View datasource

Search functionality is exposed to ArangoDB via the view API for views of type arangosearch. The ArangoSearch View is merely an identity transformation applied onto documents stored in linked collections of the same ArangoDB database. In plain terms an ArangoSearch View only allows filtering and sorting of documents located in collections of the same database. The matching documents themselves are returned as-is from their corresponding collections.

Links to ArangoDB collections

A concept of an ArangoDB collection ‘link’ is introduced to allow specifying which ArangoDB collections a given ArangoSearch View should query for documents and how these documents should be queried.

An ArangoSearch Link is a uni-directional connection from an ArangoDB collection to an ArangoSearch View describing how data coming from the said collection should be made available in the given view. Each ArangoSearch Link in an ArangoSearch view is uniquely identified by the name of the ArangoDB collection it links to. An ArangoSearch View may have zero or more links, each to a distinct ArangoDB collection. Similarly an ArangoDB collection may be referenced via links by zero or more distinct ArangoSearch Views. In other words, any given ArangoSearch View may be linked to any given ArangoDB collection of the same database with zero or one link. However, any ArangoSearch View may be linked to multiple distinct ArangoDB collections and similarly any ArangoDB collection may be referenced by multiple ArangoSearch Views.

To configure an ArangoSearch View for consideration of documents from a given ArangoDB collection a link definition must be added to the properties of the said ArangoSearch View defining the link parameters as per the section View definition/modification.

Index

Inverted Index is the heart of ArangoSearch. The index consists of several independent segments and the index segment itself is meant to be treated as a standalone index.

Analyzers

To simplify query syntax ArangoSearch provides a concept of named analyzers which are merely aliases for type+configuration of IResearch analyzers.

View definition/modification

An ArangoSearch View is configured via an object containing a set of view-specific configuration directives and a map of link-specific configuration directives.

During view creation the following directives apply:

• name (required; type: string): the view name
• type (required; type: string): the value "arangosearch"
• any of the directives from the section View properties

During view modification the following directives apply:

• links (optional; type: object): a mapping of collection-name/collection-identifier to one of:
  • link creation - link definition as per the section Link properties
  • link removal - JSON keyword null (i.e. nullify a link if present)
    
• any of the directives from the section View properties

View properties

The following terminology from ArangoSearch architecture is used to understand view properties assignment of its type:

The index consists of several independent segments and the index segment itself is meant to be treated as a standalone index. Commit is meant to be treated as the procedure of accumulating processed data creating new index segments. Consolidation is meant to be treated as the procedure of joining multiple index segments into a bigger one and removing garbage documents (e.g. deleted from a collection). Cleanup is meant to be treated as the procedure of removing unused segments after release of internal resources.

• cleanupIntervalStep (optional; type: integer; default: 10; to disable use: 0)

  ArangoSearch waits at least this many commits between removing unused files in its data directory for the case where the consolidation policies merge segments often (i.e. a lot of commit+consolidate). A lower value will cause a lot of disk space to be wasted for the case where the consolidation policies rarely merge segments (i.e. few inserts/deletes). A higher value will impact performance without any added benefits.

  With every commit or consolidate operation a new state of the view internal data-structures is created on disk. Old states/snapshots are released once there are no longer any users remaining. However, the files for the released states/snapshots are left on disk, and only removed by “cleanup” operation.

• consolidationIntervalMsec (optional; type: integer; default: 60000; to disable use: 0)

  ArangoSearch waits at least this many milliseconds between committing view data store changes and making documents visible to queries. A lower value will cause the view not to account for them, (until commit), and memory usage would continue to grow for the case where there are a few inserts/updates. A higher value will impact performance and waste disk space for each commit call without any added benefits.

  For data retrieval ArangoSearch Views follow the concept of “eventually-consistent”, i.e. eventually all the data in ArangoDB will be matched by corresponding query expressions. The concept of an ArangoSearch View “commit” operation is introduced to control the upper-bound on the time until document addition/removals are actually reflected by corresponding query expressions. Once a commit operation is complete, all documents added/removed prior to the start of the commit operation will be reflected by queries invoked in subsequent ArangoDB transactions, while in-progress ArangoDB transactions will still continue to return a repeatable-read state.

ArangoSearch performs operations in its index based on numerous writer objects that are mapped to processed segments. In order to control memory that is used by these writers (in terms of “writers pool”) one can use writebuffer* properties of a view.

• writebufferIdle (optional; type: integer; default: 64; to disable use: 0)

  Maximum number of writers (segments) cached in the pool.

• writebufferActive (optional; type: integer; default: 0; to disable use: 0)

  Maximum number of concurrent active writers (segments) that perform a transaction. Other writers (segments) wait till current active writers (segments) finish.

• writebufferSizeMax (optional; type: integer; default: 33554432; to disable use: 0)

  Maximum memory byte size per writer (segment) before a writer (segment) flush is triggered. 0 value turns off this limit for any writer (buffer) and data will be flushed periodically based on the value defined for the flush thread (ArangoDB server startup option). 0 value should be used carefully due to high potential memory consumption.

• consolidationPolicy (optional; type: object; default: {})

  The consolidation policy to apply for selecting data store segment merge candidates.

  With each ArangoDB transaction that inserts documents, one or more ArangoSearch internal segments gets created. Similarly, for removed documents the segments containing such documents will have these documents marked as “deleted”. Over time this approach causes a lot of small and sparse segments to be created. A consolidation operation selects one or more segments and copies all of their valid documents into a single new segment, thereby allowing the search algorithm to perform more optimally and for extra file handles to be released once old segments are no longer used.

  • type (optional; type: string; default: "bytes_accum")

    The segment candidates for the “consolidation” operation are selected based upon several possible configurable formulas as defined by their types. The currently supported types are:

    • bytes_accum: Consolidation is performed based on current memory consumption of segments and threshold property value.
    • tier: Consolidate based on segment byte size and live document count as dictated by the customization attributes.

consolidationPolicy properties for bytes_accum type

• threshold (optional; type: float; default: 0.1)

  Defines threshold value of [0.0, 1.0] possible range. Consolidation is performed on segments which accumulated size in bytes is less than all segments’ byte size multiplied by the threshold; i.e. the following formula is applied for each segment: {threshold} > (segment_bytes + sum_of_merge_candidate_segment_bytes) / all_segment_bytes.

consolidationPolicy properties for tier type

• segmentsMin (optional; type: integer; default: 1)

  The minimum number of segments that will be evaluated as candidates for consolidation.

• segmentsMax (optional; type: integer; default: 10)

  The maximum number of segments that will be evaluated as candidates for consolidation.

• segmentsBytesMax (optional; type: integer; default: 5368709120)

  Maximum allowed size of all consolidated segments in bytes.

• segmentsBytesFloor (optional; type: integer; default: 2097152)

  Defines the value (in bytes) to treat all smaller segments as equal for consolidation selection.

• lookahead (optional; type: integer; default: 18446744073709552000)

  The number of additionally searched tiers except initially chosen candidates based on segmentsMin, segmentsMax, segmentsBytesMax, segmentsBytesFloor with respect to defined values. Default value is treated as searching among all existing segments.

Link properties

• analyzers (optional; type: array; subtype: string; default: [ 'identity' ])

  A list of analyzers, by name as defined via the Analyzers, that should be applied to values of processed document attributes.

• fields (optional; type: object; default: {})

  An object {attribute-name: [Link properties]} of fields that should be processed at each level of the document. Each key specifies the document attribute to be processed. Note that the value of includeAllFields is also consulted when selecting fields to be processed. Each value specifies the Link properties directives to be used when processing the specified field, a Link properties value of {} denotes inheritance of all (except fields) directives from the current level.

• includeAllFields (optional; type: boolean; default: false)

  If set to true, then process all document attributes. Otherwise, only consider attributes mentioned in fields. Attributes not explicitly specified in fields will be processed with default link properties, i.e. {}.

• trackListPositions (optional; type: boolean; default: false)

  If set to true, then for array values track the value position in arrays. E.g., when querying for the input { attr: [ 'valueX', 'valueY', 'valueZ' ] }, the user must specify: doc.attr[1] == 'valueY'. Otherwise, all values in an array are treated as equal alternatives. E.g., when querying for the input { attr: [ 'valueX', 'valueY', 'valueZ' ] }, the user must specify: doc.attr == 'valueY'.

• storeValues (optional; type: string; default: "none")

  This property controls how the view should keep track of the attribute values. Valid values are:

  • none: Do not store values with the view.
  • id: Store information about value presence to allow use of the EXISTS() function.