Retryable Reads

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Retryable reads allow MongoDB drivers to automatically retry certain read operations a single time if they encounter certain network or server errors.

Prerequisites

Minimum Driver Version

Official MongoDB drivers compatible with MongoDB Server 4.2 and later support retryable reads.

For more information on official MongoDB drivers, see MongoDB Drivers.

Minimum Server Version
Drivers can only retry read operations if connected to MongoDB Server 3.6 or later.

Enabling Retryable Reads

Official MongoDB drivers compatible with MongoDB Server 4.2 and later enable retryable reads by default. To explicitly disable retryable reads, specify retryReads=false in the connection string for the deployment.

The mongo shell does not support retryable reads.

Retryable Read Operations

MongoDB drivers support retrying the following read operations. The list references a generic description of each method. For specific syntax and usage, defer to the driver documentation for that method.

MethodsDescriptions
Collection.aggregate
Collection.count
Collection.countDocuments
Collection.distinct
Collection.estimatedDocumentCount
Collection.find
Database.aggregate

For Collection.aggregate and Database.aggregate, drivers can only retry aggregation pipelines which do not include write stages, such as $out or $merge.

 CRUD API Read Operations
Collection.watch
Database.watch
MongoClient.watch
 Change Stream Operations
MongoClient.listDatabases
Database.listCollections
Collection.listIndexes
 Enumeration Operations
GridFS Operations backed by Collection.find (e.g. GridFSBucket.openDownloadStream) GridFS File Download Operations

MongoDB drivers may include retryable support for other operations, such as helper methods or methods that wrap a retryable read operation. Defer to the driver documentation to determine whether a method explicitly supports retryable reads.

See also参阅

Retryable Read Specification: Supported Read Operations

Unsupported Read Operations

The following operations do not support retryable reads:

Behavior行为

Persistent Network Errors

MongoDB retryable reads make only one retry attempt. This helps address transient network errors or replica set elections, but not persistent network errors.

Failover Period

The driver performs server selection using the read command’s original read preference before retrying the read operation. If the driver cannot select a server for the retry attempt using the original read preference, the driver returns the original error.

The drivers wait serverSelectionTimeoutMS milliseconds before performing server selection. Retryable reads do not address instances where no eligible servers exist after waiting serverSelectionTimeoutMS.