Active Operation Limiter

GraphQL::Enterprise::ActiveOperationLimiter prevents clients from running too many GraphQL operations at the same time. It uses Redis to track currently-running operations.

Why?

Some clients may suddently swamp a server with tons of requests, occupying all available Ruby processes and therefore interrupting service for other clients. This limiter aims to prevent that at the GraphQL level by halting queries when a client already has lots of queries running. That way, server processes will remain available for other clients’ requests.

Setup

To use this limiter, update the schema configuration and include context[:limiter_key] in your queries.

Schema Setup

To setup the schema, add use GraphQL::Enterprise::ActiveOperationLimiter with a default limit: value:

class MySchema < GraphQL::Schema
  # ...
  use GraphQL::Enterprise::ActiveOperationLimiter,
    redis: Redis.new(...),
    limit: 5
end

limit: false may also be given, which defaults to no limit for this limiter.

It also accepts a stale_request_seconds: option. The limiter uses that value to clean up request data in case of a crash or other unexpected scenario.

Before requests will actually be halted, soft mode must be disabled.

Query Setup

In order to limit clients, the limiter needs a client identifier for each GraphQL operation. By default, it checks context[:limiter_key] to find it:

context = {
  viewer: current_user,
  # for example:
  limiter_key: logged_in? ? "user:#{current_user.id}" : "anon-ip:#{request.remote_ip}",
  # ...
}

result = MySchema.execute(query_str, context: context)

Operations with the same context[:limiter_key] will rate limited in the same buckets. A limiter key is required; if a query is run without one, the limiter will raise an error.

To provide a client identifier another way, see Customization.

Customization

GraphQL::Enterprise::ActiveOperationLimiter provides several hooks for customizing its behavior. To use these, make a subclass of the limiter and override methods as described:

# app/graphql/limiters/active_operations.rb
class Limiters::ActiveOperations < GraphQL::Enterprise::ActiveOperationsLimiter
  # override methods here
end

The hooks are:

• def limiter_key(query) should return a string which identifies the current client for query.
• def limit_for(key, query) should return an integer or nil. If an integer is returned, that limit is applied for the current query. If nil is returned, no limit is applied to the current query.
• def soft_limit?(key, query) can be implemented to customize the application of “soft mode”. By default, it checks a setting in redis.
• def handle_redis_error(err) is called when the limit rescues an error from Redis. By default, it’s passed to warn and the query is not halted.

Instrumentation

While the limiter is installed, it adds some information to the query context about its operation. It can be accessed at context[:active_operation_limiter]:

result = MySchema.execute(...)

pp result.context[:active_operation_limiter]
# {:key=>"user:123", :limit=>2, :soft=>false, :limited=>true}

It returns a Hash containing:

• key: [String], the limiter key used for this query
• limit: [Integer, nil], the limit applied to this query
• soft: [Boolean], true if the query was run in “soft mode”
• limited: [Boolean], true if the query exceeded the rate limit (but if soft: was also true, then the query was not halted)

You could use this to add detailed metrics to your application monitoring system, for example:

MyMetrics.increment("graphql.active_operation_limiter", tags: result.context[:active_operation_limiter])