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Read Preferences, Read Concerns, and Write Concerns

RESTHeart provides flexible ways to configure read and write operations for MongoDB, allowing you to specify these concerns at both the connection and request levels.

Configuration Options

You can specify read and write concerns in two primary ways:

1. MongoDB Connection String Configuration

In the MongoDB client configuration, you can set default read and write concerns using the connection string:

mongodb://localhost:27017/mydb?readPreference=primaryPreferred&readConcern=majority&writeConcern=majority

2. Request-Level Query Parameters

RESTHeart extends the MongoDB write API by allowing you to specify read and write concerns directly in the request query parameters.

Read Preferences

You can specify the read preference using the readPreference query parameter:

GET /mycollection?readPreference=primary
GET /mycollection?readPreference=primaryPreferred
GET /mycollection?readPreference=secondary
GET /mycollection?readPreference=secondaryPreferred
GET /mycollection?readPreference=nearest

Read Concerns

Set read concerns with the readConcern query parameter:

GET /mycollection?readConcern=local
GET /mycollection?readConcern=majority
GET /mycollection?readConcern=linearizable
GET /mycollection?readConcern=snapshot

Write Concerns

Specify write concerns using the writeConcern query parameter:

POST /mycollection?writeConcern=majority
PUT /mycollection?writeConcern=w:2
DELETE /mycollection?writeConcern=w:1

Supported Values

Read Preferences

  • primary: Reads only from the primary node

  • primaryPreferred: Reads from primary, falls back to secondary

  • secondary: Reads only from secondary nodes

  • secondaryPreferred: Reads from secondary, falls back to primary

  • nearest: Reads from the node with the lowest network latency

Read Concerns

  • local: Returns the most recent data available

  • majority: Returns data that has been acknowledged by a majority of replica set members

  • linearizable: Ensures read operations reflect the most recent write operations

  • snapshot: Returns data from a snapshot of the data at the time of the read operation

Write Concerns

  • w:0: No acknowledgment

  • w:1: Acknowledgment from primary

  • w:majority: Acknowledgment from a majority of replica set members

  • Numeric values (w:2, w:3) specify the number of nodes that must acknowledge the write

Example Use Cases

Reading from Nearest Node

GET /users?readPreference=nearest

Writing with Majority Acknowledgment

POST /transactions?writeConcern=majority

Consistent Read After Write

GET /orders?readConcern=majority&readPreference=primary

Best Practices

  • Use majority write concern for important transactions to ensure data durability

  • Choose read preferences based on your application’s consistency and latency requirements

  • Consider network topology and replica set configuration when selecting read and write concerns

Notes

  • Request-level query parameters override connection string configurations

  • Not all combinations of read and write concerns are appropriate for every use case

  • Carefully evaluate performance implications of your chosen read and write concerns