Atlas Search Overview

Atlas Search Indexes

When you configure one or more Atlas Search search indexes, Atlas enables the mongot process on the nodes in the cluster. Each mongot process talks to the mongod on the same node. To create and update indexes, the mongot process performs collection scans on the backend database and opens change streams for each index.

You can specify the fields to index using the following:

• Dynamic mappings, which enables Atlas Search to automatically index all the fields of supported types in each document. This takes disk space and might negatively impact cluster performance.
• Static mappings, which allows you to selectively identify the fields to index.

Atlas Search performs inverted indexing and stores the indexed fields on disk. The data stored on Atlas Search isn't an identical copy of data from the collection on your Atlas cluster, but it still takes some disk space and memory.

Atlas Search provides built-in analyzers for creating indexable terms that correct for differences in punctuation, capitalization, stop words, and more. Analyzers apply parsing and language rules to the query. You can also create a custom analyzer using available built in character filters, tokenizers, and token filters. To learn more about the built-in and custom analyzers, see Process Data with Analyzers.

For text fields, the mongot performs the following to create indexable tokens:

1. Analysis of the text
2. Tokenization, which is breaking up of words in a string to indexable tokens
3. Normalization, such as transforming the text to lower case, folding diacritics, and removing stop words
4. Stemming, such as ignoring plural and other word forms to index the word in the most reduced form

To learn more about Atlas Search support for other data types, see BSON Data Types and Atlas Search Field Types. The mongot process stores the indexed fields and the _id field on disk per index for the collections on the cluster.

If you modify an existing index, Atlas Search performs a no-downtime rebuild of the index. This allows you to continue using the old index for existing and new queries until the index rebuild is complete.

If you make changes to the collection for which you defined Atlas Search indexes, the latest data might not be available immediately for queries. However, mongot monitors the change streams, which allows it to update stored copies of data, and Atlas Search indexes are eventually consistent.

After you set up an Atlas Search index for a collection, you can run queries against the indexed fields.

Atlas Search Queries

Atlas Search queries take the form of an aggregation pipeline stage. Atlas Search provides $search, which must be the first stage in the query pipeline, and $searchMeta stages. These stages can be used in conjunction with other aggregation pipeline stages in your query pipeline. To learn more about these pipeline stages, see Choose an Aggregation Pipeline Stage.

Atlas Search also provides query operators and collectors that you can use inside the Choose an Aggregation Pipeline Stage. The Atlas Search operators provide ways to locate and retrieve matching data from the collection on your Atlas cluster. The collector returns a document representing the search metadata results.

You can use Atlas Search operators to query terms, phrases, geographic shapes and points, numeric values, synonymous terms, and more. You can also search using regex and wildcard expressions. The Atlas Search compound operator allows you to combine multiple operators inside your $search stage to perform a complex search and filter of data based on what must, must not, or should be present in the documents returned by Atlas Search. You can use the compound operator to also match or filter documents in the $search stage itself. Running $match after $search is less performant than running $search with the compound operator.

To learn more about the syntax, options, and usage of the various Atlas Search operators, see Use Operators and Collectors in Queries.

When you run a query, the query first goes to the MongoDB process, which is mongod for a replica set cluster or mongos for a sharded cluster. For sharded clusters, your cluster data is partitioned across mongod instances and each mongot knows about the data on the mongod on the same node only. Therefore, you can't run queries that target a particular shard. mongos directs the queries to all shards, making these scatter gather queries.

The MongoDB process routes the query to the mongot on the same node. Atlas Search performs the search and scoring and returns the document IDs and other search metadata for the matching results to mongod. The mongod then performs a full document lookup implicitly for the matching results and then returns the results to the client.

Atlas Search associates a relevance-based score with every document in the result set. The relevance-based scoring allows the documents to be returned in order from highest score to lowest. Atlas Search scores documents higher if the query term appears frequently in a document and lower if the query term appears across many documents in the collection. Atlas Search also supports customizing the relevance-based default score by boosting, decaying, etc. To learn more about the various ways in which you can customize the resulting scores, see Customize the Score in Results.