OWLer (Open Web Crawler) is a distributed and collaborative crawling system developed as part of a common effort to build an Open Web Index. OWLer comprises three layers: the Crawler, the URL Frontier, and the Persistence layer. Among these, the URL Frontier plays a central role by managing the crawl space and distributing URLs to crawlers. The URL Frontier is implemented as a RESTful service with a gRPC API, facilitating communication with crawlers. It uses a distributed storage system to persist URLs discovered or visited during crawls. Designed for modularity, scalability, and fault tolerance, the URL Frontier can be deployed in a distributed environment. This repository originates from a fork of the URLFrontier framework and its OpenSearch-based implementation by Presearch. The fork enhances performance, scalability, and integrates seamlessly with the OWLer system. The software is implemented in Java and licensed under the Apache License 2.0.