# Dependency Deprecation Policy - Synapse

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# Deprecation Policy

Synapse has a number of platform dependencies (Python, Rust, PostgreSQL, and SQLite) and application dependencies (Python and Rust packages). This document outlines the policy towards which versions we support, and when we drop support for versions in the future.

## Platform Dependencies

Synapse follows the upstream support life cycles for Python and PostgreSQL, i.e. when a version reaches End of Life Synapse will withdraw support for that version in future releases.

Details on the upstream support life cycles for Python and PostgreSQL are documented at https://endoflife.date/python and https://endoflife.date/postgresql.

A Rust compiler is required to build Synapse from source. For any given release the minimum required version may be bumped up to a recent Rust version, and so people building from source should ensure they can fetch recent versions of Rust (e.g. by using rustup).

The oldest supported version of SQLite is the version provided by Debian oldstable.

### Context

It is important for system admins to have a clear understanding of the platform requirements of Synapse and its deprecation policies so that they can effectively plan upgrading their infrastructure ahead of time. This is especially important in contexts where upgrading the infrastructure requires auditing and approval from a security team, or where otherwise upgrading is a long process.

By following the upstream support life cycles Synapse can ensure that its dependencies continue to get security patches, while not requiring system admins to constantly update their platform dependencies to the latest versions.

For Rust, the situation is a bit different given that a) the Rust foundation does not generally support older Rust versions, and b) the library ecosystem generally bump their minimum support Rust versions frequently. In general, the Synapse team will try to avoid updating the dependency on Rust to the absolute latest version, but introducing a formal policy is hard given the constraints of the ecosystem.

On a similar note, SQLite does not generally have a concept of “supported release”; bugfixes are published for the latest minor release only. We chose to track Debian’s oldstable as this is relatively conservative, predictably updated and is consistent with the `.deb` packages released by Matrix.org.

## Application dependencies

For application-level Python dependencies, we often specify loose version constraints (ex. `>=X.Y.Z`) to be forwards compatible with any new versions. Upper bounds (`<A.B.C`) are only added when necessary to prevent known incompatibilities.

When selecting a minimum version, while we are mindful of the impact on downstream package maintainers, our primary focus is on the maintainability and progress of Synapse itself.

For developers, a Python dependency version can be considered a “no-brainer” upgrade once it is available in both the latest Debian Stable and Ubuntu LTS repositories. No need to burden yourself with extra scrutiny or consideration at this point.

We aggressively update Rust dependencies. Since these are statically linked and managed entirely by `cargo` during build, they can pose no ongoing maintenance burden on others. This allows us to freely upgrade to leverage the latest ecosystem advancements assuming they don’t have their own system-level dependencies.

### Context

Because Python dependencies can easily be managed in a virtual environment, we are less concerned about the criteria for selecting minimum versions. The only thing of concern is making sure we’re not making it unnecessarily difficult for downstream package maintainers. Generally, this just means avoiding the bleeding edge for a few months.

The situation for Rust dependencies is fundamentally different. For packagers, the concerns around Python dependency versions do not apply. The `cargo` tool handles downloading and building all libraries to satisfy dependencies, and these libraries are statically linked into the final binary. This means that from a packager’s perspective, the Rust dependency versions are an internal build detail, not a runtime dependency to be managed on the target system. Consequently, we have even greater flexibility to upgrade Rust dependencies as needed for the project. Some distros (e.g. Fedora) do package Rust libraries, but this appears to be the outlier rather than the norm.