Model Computed Data

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Overview

Computed Pattern
Example

Overview

Often, an application needs to derive a value from source data stored in a database. Computing a new value may require significant CPU resources, especially in the case of large data sets or in cases where multiple documents must be examined.

If a computed value is requested often, it can be more efficient to save that value in the database ahead of time. This way, when the application requests data, only one read operation is required.

Computed Pattern

If your reads significantly outnumber your writes, the computed pattern reduces the frequency of having to perform computations. Instead of attaching the burden of computation to every read, the application stores the computed value and recalculates it as needed. The application can either recompute the value with every write that changes the computed value's source data, or as part of a periodic job.

Note

With periodic updates, the computed value is not guaranteed to be exact in any given read. However, this approach may be worth the performance boost if exact accuracy isn't a requirement.

Example

An application displays movie viewer and revenue information.

Consider the following screenings collection:

db.screenings.insertMany( [
   {
      theater : "Alger Cinema",
      location : "Lakeview, OR",
      movie_title : "Reservoir Dogs",
      num_viewers : 344,
      revenue : 3440
   },
   {
      theater : "City Cinema",
      location : "New York, NY",
      movie_title : "Reservoir Dogs",
      num_viewers : 1496,
      revenue : 22440
   },
   {
      theater : "Overland Park Cinema",
      location : "Boise, ID",
      movie_title : "Reservoir Dogs",
      num_viewers : 760,
      revenue : 7600
   }
] )

Users often want to know how many people saw a certain movie and how much money that movie made. In this example, to total num_viewers and revenue, you must perform a read for theaters that screened a movie with the title "Reservoir Dogs" and sum the values of those fields. To avoid performing that computation every time the information is requested, you can compute the total values and store them in a movies collection with the movie record itself:

db.movies.insertOne( [
   {
      title : "Reservoir Dogs",
      total_viewers : 2600,
      total_revenue : 33480,
      ...
   }
] )

In a low write environment, the computation could be done in conjunction with any update of the screenings data.

In an environment with more regular writes, the computations could be done at defined intervals - every hour for example. The source data in screenings isn't affected by writes to the movies collection, so you can run calculations at any time.

This is a common design pattern that reduces CPU workload and increases application performance. Whenever you are performing the same calculations repeatedly and you have a high read to write ratio, consider the Computed Pattern.

Other Sample Use Cases

In addition to cases where summing is requested frequently, such as getting total revenue or viewers in the movie database example, the computed pattern is a good fit wherever calculations need to be run against data. For example:

A car company that runs massive aggregation queries on vehicle data, storing results to show for the next few hours until the data is recomputed.
A consumer reporting company that compiles data from several different sources to create rank-ordered lists like the "100 Best-Reviewed Gadgets". The lists can be regenerated periodically while the underlying data is updated independently.

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