Reactive Scala Driver for MongoDB

Asynchronous & Non-Blocking

Failover Strategy

A FailoverStrategy defines if and how many times ReactiveMongo should retry the operations (DB resolution, query, insertion, command, etc.) that could fail for the following reasons.

• The entire node set is not available (probably because of a network failure).
• The primary is not available, preventing to run write operations and consistent reads.
• The operation could not be done because of a credentials problem (e.g. the application is not yet logged on to the database).

The other causes (business errors, normal database errors, fatal errors, etc.) are not handled.

The default FailoverStrategy retries 5 times, with 500 ms between each attempt.

Let’s say that we want to define a FailoverStrategy that waits more time before a new attempt.

import scala.concurrent.duration._

import reactivemongo.api.FailoverStrategy

val strategy =
  FailoverStrategy(
    initialDelay = 500 milliseconds,
    retries = 5,
    delayFactor =
      attemptNumber => 1 + attemptNumber * 0.5
  )

This strategy retries at most 5 times, waiting for initialDelay * ( 1 + attemptNumber * 0.5 ) between each attempt (attemptNumber starting from 1). Here is the way the attempts will be run:

• #1: 750 milliseconds (500 * (1 + 1 * 0.5)) = 500 * 1.5 = 750)
• #2: 1000 milliseconds (500 * (1 + 2 * 0.5)) = 500 * 2 = 1000)
• #3: 1250 milliseconds (500 * (1 + 3 * 0.5)) = 500 * 2.5 = 1250)
• #4: 1500 milliseconds (500 * (1 + 4 * 0.5)) = 500 * 3 = 1500)
• #5: 1750 milliseconds (500 * (1 + 5 * 0.5)) = 500 * 3.5 = 1750)

You can specify a strategy by giving it as a parameter to connection.database or database.collection:

import scala.concurrent.duration._
import scala.concurrent.ExecutionContext.Implicits.global

import reactivemongo.api.FailoverStrategy

val defaultStrategy = FailoverStrategy()

val customStrategy =
  FailoverStrategy(
    initialDelay = 500 milliseconds,
    retries = 5,
    delayFactor =
      attemptNumber => 1 + attemptNumber * 0.5
  )

def connection1: reactivemongo.api.MongoConnection = ???

// database-wide strategy
val db1 = connection1.db("dbname", customStrategy)

// collection-wide strategy
val db2 = connection1.db("dbname", defaultStrategy)
val collection = db2.collection("collname", customStrategy)

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