flippercloud/flipper · error · ArgumentError

The MONGO_URL environment variable must be set. For example:

Error message

The MONGO_URL environment variable must be set. For example: mongodb://127.0.0.1:27017/flipper

What it means

Error "The MONGO_URL environment variable must be set. For example: mongodb://127.0.0.1:27017/flipper" thrown in flippercloud/flipper.

Source

Thrown at lib/flipper/adapters/mongo.rb:195

              value = doc[gate.key.to_s]
              Typecast.from_json(value)
            else
              unsupported_data_type gate.data_type
            end
        end
        result
      end
    end
  end
end

Flipper.configure do |config|
  config.adapter do
    url = ENV["FLIPPER_MONGO_URL"] || ENV["MONGO_URL"]
    collection = ENV["FLIPPER_MONGO_COLLECTION"] || "flipper"

    unless url
      raise ArgumentError, "The MONGO_URL environment variable must be set. For example: mongodb://127.0.0.1:27017/flipper"
    end

    Flipper::Adapters::Mongo.new(Mongo::Client.new(url)[collection])
  end
end

View on GitHub (pinned to 1f86de3ec9)

Solutions

1. Set the MONGO_URL environment variable before booting the app, e.g. export MONGO_URL=mongodb://127.0.0.1:27017/flipper
2. Add MONGO_URL to your .env file or deployment config so every environment provides it
3. Pass a MongoDB connection string explicitly when configuring the adapter instead of relying on the environment variable

When it happens

Trigger: Thrown at lib/flipper/adapters/mongo.rb:195 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of flippercloud/flipper@1f86de3ec9 (2026-08-23). Data as JSON: /api/errors/bb306a7e1f225363. Report an issue: GitHub.