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Sizing and Scaling


Flagsmith has a very simple architecture, making it well understood when it comes to serving high loads.

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Generally this component is not put under any sort of significant load. It can be load balanced if required. It does not require sticky-sessions.


The API is completely stateless. This means it can scale out behind a load balancer almost perfectly. As an example, when running on AWS ECS/Fargate we run with:

  • cpu=1024
  • memory=2048

In terms of auto scaling, we recommend basing the autoscaling off the ECSServiceAverageCPUUtilization metric, with a target_value of 50 and a 30 second cool-down timeout.


Our recommendation is to first scale the database up with a more powerful single server.


Once the database connections have been saturated by the API cluster, adding read replicas to the database solves the next bottleneck of database connections.

Flagsmith can be set up to handle as many read replicas as needed. To add replicas, you'll need to set the REPLICA_DATABASE_URLS environment variable with a comma separated list of database urls.


REPLICA_DATABASE_URLS: postgres://,postgres://

Use the REPLICA_DATABASE_URLS_DELIMITER environment variable if you are using any , characters in your passwords.

In addition to typical read replicas, which usually exist locally in the same data centre to the application. There is also support for replicas across regions via the CROSS_REGION_REPLICA_DATABASE_URLS environment variable which is set in the same way as the REPLICA_DATABASE_URLS with cross region replicas having their own matching CROSS_REGION_REPLICA_DATABASE_URLS_DELIMITER which also defaults to , as above.

Cross region replicas are only used once all typical replicas have gone offline, since the performance characteristics wouldn't be favorable to spread replica load at longer latencies. Both REPLICA_DATABASE_URLS and CROSS_REGION_REPLICA_DATABASE_URLS can be used alone or simultaneously.

To support different configurations there are two different replication strategies available. By setting REPLICA_READ_STRATEGY to DISTRIBUTED (the default option) the load to the replicas is distributed evenly. If your use-case, on the otherhand, is to utilize fallback replicas (primary, secondary, etc) the REPLICA_READ_STRATEGY should be set to SEQUENTIAL so a replica is only used if all the other replica's preceding it have gone offline. This strategy is applicable to both typical replicas as well as to cross region replicas.

We would also recommend testing pgBouncer in your environment as it generally optimises database connections and reduces the load on the database.