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Capped Collections

Query a Capped Collection

On this page

  • About this Task
  • Multiple Concurrent Writes
  • Before you Begin
  • Create a capped collection
  • Insert sample data
  • Steps
  • Return Documents in Insertion Order
  • Return Most Recent Documents
  • Learn More

When you query a capped collection without specifying a sort order, MongoDB returns results in the same order that they were inserted, meaning the oldest documents are returned first.

Use natural ordering to retrieve the most recently inserted elements from the collection efficiently. This is similar to using the tail command on a log file.

About this Task

Generally, TTL (Time To Live) indexes offer better performance and more flexibility than capped collections. TTL indexes expire and remove data from normal collections based on the value of a date-typed field and a TTL value for the index.

Capped collections serialize write operations and therefore have worse concurrent insert, update, and delete performance than non-capped collections. Before you create a capped collection, consider if you can use a TTL index instead.

Multiple Concurrent Writes

If there are concurrent writers to a capped collection, MongoDB does not guarantee that documents are returned in insertion order.

Before you Begin

1

Create a capped collection

db.createCollection("log", { capped: true, size: 100000 } )
2

Insert sample data

db.log.insertMany( [
   {
      message: "system start",
      type: "startup",
      time: 1711403508
   },
   {
      message: "user login attempt",
      type: "info",
      time: 1711403907
   },
   {
      message: "user login fail",
      type: "warning",
      time: 1711404209
   },
   {
      message: "user login success",
      type: "info",
      time: 1711404367
   },
   {
      message: "user logout",
      type: "info",
      time: 1711404555
   }
] )

Steps

The following examples show you how to:

  • Return Documents in Insertion Order

  • Return Most Recent Documents

Return Documents in Insertion Order

Query the log collection for documents where type is info, and use the default sort order:

db.log.find( { type: "info" } )
[
  {
    _id: ObjectId("660204b74cabd75abebadbc2"),
    message: 'user login attempt',
    type: 'info',
    time: 1711403907
  },
  {
    _id: ObjectId("660204b74cabd75abebadbc4"),
    message: 'user login success',
    type: 'info',
    time: 1711404367
  },
  {
    _id: ObjectId("660204b74cabd75abebadbc5"),
    message: 'user logout',
    type: 'info',
    time: 1711404555
  }
]

Documents are returned in the order that they were inserted.

Return Most Recent Documents

To return documents in reverse insertion order (meaning the most recent documents are first), specify the sort() method with the $natural parameter set to -1.

The following query returns the three most recent documents from the log collection, starting with the most recent document:

db.log.find().sort( { $natural: -1 } ).limit(3)
 [
    {
       _id: ObjectId("6601f2484cabd75abebadbbb"),
       message: 'user logout',
       type: 'info',
       time: 1711404555
    },
    {
       _id: ObjectId("6601f2484cabd75abebadbba"),
       message: 'user login success',
       type: 'info',
       time: 1711404367
    },
    {
       _id: ObjectId("6601f2484cabd75abebadbb9"),
       message: 'user login fail',
       type: 'warning',
       time: 1711404209
    }
 ]

Learn More

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