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OCaml Infrastructure: How the opam-repository Works

The opam repository serves as the central package registry for the OCaml ecosystem, hosting over 4,500 packages and processing nearly 200 new packages and releases each month. Understanding how this critical infrastructure works is essential for any OCaml developer looking to contribute packages or understand the ecosystem's inner workings.

Architecture and Infrastructure

The opam repository operates as a curated Git repository containing package metadata rather than the packages themselves. Each package is described by an opam file that specifies dependencies, build instructions, and other metadata. The package also includes a URL pointing to the source of the actual package. The actual source code remains in the original repositories maintained by package authors.

For example, the yojson.3.0.0 package provides a simple illustration of the package format. For more details, see the opam package format documentation.

The CI Pipeline

At the heart of the repository's reliability is an extensive continuous integration (CI) system built around OCurrent, which powers the opam-repo-ci service. When you submit a pull request to add or update a package, this system:

  • Validates package metadata using an extensive set of linting rules
  • Tests compilation across multiple OCaml compiler versions (currently supporting back to OCaml 4.08)
  • Verifies installation on various platforms including Linux distributions, macOS, FreeBSD, Windows, and multiple CPU architectures (x86_64, ARM64, ARM32, PowerPC, s390x, RISC-V)
  • Runs package tests to ensure functionality
  • Checks reverse dependencies to ensure your changes don't break existing packages that depend on your package

This comprehensive testing matrix helps maintain the repository's high quality standards. Windows support was added in 2024, marking a significant expansion of the platform coverage.

Supporting Infrastructure

The opam repository integrates with several other OCaml infrastructure services:

Infrastructure Funding

As of 2025, the OCaml.org infrastructure is primarily funded and supported by the Cambridge Computer Lab and Tarides. Additional cloud hosting costs are covered by community contributions. For architecture-specific testing, Oregon State University Open Source Lab provides essential PowerPC infrastructure support, IBM LinuxONE at Marist University provides S390 machines for the CI pipelines, and Jane Street has generously donated x86 machines to Cambridge University. This combination of institutional support, corporate sponsorship, and community backing enables the comprehensive multi-platform testing that ensures package quality across the entire OCaml ecosystem.

Governance and Curation Policies

The opam package repository is a commons rather than a publishing platform: it is manually curated, so not all packages submitted for publication are accepted; it is maintained communally, so anyone can suggest changes to any package.

Core Principles

The repository maintains quality through several key policies, documented in the repository policies. The policies are derived from some core principles, including:

Package Utility: Packages must provide substantial utility to the OCaml community. Single-function modules or packages with minimal functionality are generally rejected to preserve namespace and repository efficiency.

Naming and Security: Package names are reviewed to prevent confusion with existing packages and potential security risks. Name squatting and packages with deceptively similar names to established libraries are not permitted.

Dependency Constraints: The repository discourages overly strict version constraints (using = or <=) as they make package combination difficult and worsen the user experience. Instead, packages should use reasonable upper bounds and allow opam's solver to find compatible combinations.

Maintenance Requirements: Packages must be actively maintained for modern OCaml versions. There is a compiler cutoff threshold, which will be increased as newer releases become available. Packages that don't support recent compiler versions may be archived.

Maintenance Intent Declarations

One distinctive feature of the opam repository is the ability for package maintainers to explicitly declare their maintenance intentions using the x-maintenance-intent field in package metadata. This allows maintainers to express their commitment level upfront—you don't need to be stuck maintaining something forever. This is fairly unusual in package ecosystems and provides clarity for both maintainers and users.

When maintainers indicate they're no longer supporting certain versions or stepping back entirely, the community is notified. If no new maintainer volunteers, packages that no longer meet the compiler cutoff threshold or are marked as unmaintained get moved to the opam-repository-archive rather than being removed entirely. This archiving process helps manage repository growth while preserving historical packages.

The Human Element

Review Process

Every package submission undergoes human review in addition to automated testing. Maintainers check for compliance with repository policies, verify licensing information, assess package utility, and ensure proper dependency declarations. This human oversight is crucial for maintaining the repository's curation standards.

Maintainer Team and Acknowledgments

The repository is maintained by a dedicated team of volunteers who review every package submission. The current active maintainers are listed in the governance documentation.

The opam-repository has benefited from the contributions of many dedicated individuals over the years. Since its inception, Anil Madhavapeddy, Thomas Gazagnaire, and Jeremy Yallop established the foundations of the repository and its processes. Kate Deplaix (kit-ty-kate) served as the primary maintainer from 2017 to 2024, during which time she established many of the current processes, comprehensive documentation, and quality standards. She now contributes to the opam tool itself.

Today's active maintenance team includes Marcello Seri, Shon Feder, Raphaël Proust, Hannes Mehnert, and Jan Midtgaard, who review submissions and ensure the repository's continued quality. The CI infrastructure is maintained by Mark Elvers, whose work keeps the testing systems running smoothly — a laborious but essential task.

Beyond package review, Hannes Mehnert has been working on initiatives like Conex to improve package security since 2015.

We extend our heartfelt gratitude to all these maintainers, contributors, and everyone else who has helped make the opam-repository a reliable foundation for the OCaml ecosystem. Their tireless volunteer work reviewing submissions, upholding quality standards, and providing guidance to contributors is essential to the health and continued growth of our community.

Community Involvement

The maintainer team actively seeks new volunteers to help manage the growing submission volume. The role offers excellent opportunities to:

  • Connect with and support contributors from across the OCaml ecosystem
  • Learn advanced packaging techniques and build system intricacies
  • Contribute to ecosystem sustainability and health
  • Gain experience with large-scale open source project management

For information about the onboarding process, see the Onboarding documentation. If you're interested in contributing to this essential piece of OCaml infrastructure, check out the call for volunteers or reach out to the current maintainer team.

Publishing Workflow

For package authors, the typical publishing workflow involves:

  1. Creating a package source archive (commonly done by pushing a git tag to your online forge of choice: GitHub, GitLab, Codeberg, or other)
  2. Opening a pull request to opam-repository (often automated with tools like dune-release or opam-publish)
  3. Monitoring the pull request's CI results and addressing any build failures or policy violations
  4. Responding to maintainer feedback during the review process
  5. Package acceptance and automatic propagation to opam users worldwide

The process is streamlined by extensive documentation including the CONTRIBUTING.md guide and the comprehensive wiki that covers everything from basic submission guidelines to detailed policy explanations.

Looking Forward

The opam repository continues to evolve as the OCaml ecosystem grows. Recent initiatives include the introduction of an archiving policy, improving maintainer documentation, and scaling the review process to handle increasing submission volumes.

A Thriving Ecosystem

The opam repository's design as a shared metadata format has enabled a diverse ecosystem of tools and repositories. Multiple package management clients can consume opam metadata, including the traditional opam client, Dune's package management, esy, and even Nix. This flexibility demonstrates the health and maturity of the OCaml package ecosystem.

Beyond the main repository, there's a rich ecosystem of specialized package repositories that complement it. These include cross-compilation overlays (notably for Windows and iOS), organization-specific repositories (such as Jane Street's packages and OxCaml), and domain-specific collections like the Rocq proof assistant packages. By design, these repositories can be stacked and composed together, allowing users to mix and match packages from different sources while maintaining dependency resolution.

For developers interested in contributing to this critical infrastructure, whether as package authors or potential maintainers, the repository represents one of the most impactful ways to support the broader OCaml community. The combination of technical infrastructure, human oversight, and community governance makes the opam repository a model for how modern package ecosystems can balance growth with quality.