Read-Replicas

In scenarios where the Ed-Fi ODS / API is experiencing both heavy read and write loads, or where there is a need for a deployment that supports high availability, the ODS databases can be deployed such that there is a primary writable database, with one or more read replicas which are kept up to date in real-time. The API can be configured to use this secondary database for all GET requests to reduce the load on the primary database server.

info

For SQL Server, this functionality is provided through SQL Server Always On Availability Groups (see What is an Always On availability group?). For PostgreSQL, the options available will depend on your cloud host and/or environment (see High Availability, Load Balancing, and Replication in the PostgreSQL documentation).

API Configuration

The API identifies the presence of a read-replica ODS by the existence of a record in the OdsInstanceDerivative table in the EdFi_Admin database with a DerivativeType value of "ReadReplica":

This connection string can also be overridden through configuration, as the example below for SQL Server demonstrates (note the addition of "ApplicationIntent" to the connection string):

{
  "OdsInstances": {
    "2": {
      "ConnectionString": "Server=AGListener; Database=EdFi_Ods; Encrypt=False; Trusted_Connection=True; Application Name=EdFi.Ods.WebApi;",
      "ConnectionStringByDerivativeType": {
        "ReadReplica": "Server=AGListener; Database=EdFi_Ods; Encrypt=False; Trusted_Connection=True; Application Name=EdFi.Ods.WebApi; ApplicationIntent=ReadOnly"
      }
    }
  }
}

info

The API will always use the read/write connection to load data for cache initialization purposes to ensure that the latest data is used, even if the cache initialization is triggered during a GET request which is being served using a read-replica.