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rfc-158-port-content-store-to-postgresql.md

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status implementation status_last_reviewed
accepted
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2024-03-04

Port Content Store to PostgreSQL on RDS

Summary

GOV.UK Publishing should port Content Store from its current legacy self-hosted MongoDB, to PostgreSQL running on AWS' RDS.

Problem

Content Store is a critical component of GOV.UK. All front-end apps rely on it for serving requests, it serves the GOV.UK content API directly, and is the authoritative source of all content published on GOV.UK. However, it stores this content in a self-hosted legacy database (MongoDB v2.6) which is poorly understood, difficult to support, and has been marked end-of-life since 2016.

There are many consequences of this long-standing tech debt:

  • we do not receive security updates for MongoDB
  • support for this version is hard to find in tooling and client libraries
  • when incidents occur with it we do not have the expertise in-house to fully understand and resolve issues
  • when GOV.UK replatformed onto Kubernetes in March 2023, the MongoDB cluster was not migrated over. It still runs on EC2 instances configured via GOV.UK Puppet, and is a blocker for deprecating that large legacy codebase
  • we cannot run the same version locally as in production (it is not available for the standard developer laptops), meaning compatibility and testing is largely based on hope rather than rigour
  • GOV.UK developers (and the wider industry pool of developers from which we hire) tend to be significantly less familiar with Mongo than PostgreSQL, which most of the rest of GOV.UK runs on
  • The Mongoid ORM adapter which we use to connect to Mongo from our Ruby on Rails applications does not support many of the features of the more standard ActiveRecord, which is used throughout GOV.UK - meaning the Content Store application is harder to work on and creates more toil for developers

Architectural Decision Record 0038 recommended in 2019 that in general MongoDB should be replaced with Amazon's proprietary DocumentDB. However this has not proved to be a workable decision in practise for several reasons, including an inability to run DocumentDB locally, its "Mongo compatibility mode" not being fully compatible, and increasing dependence on single-vendor proprietary software being against general government policy of choosing open-source by default.

Proposal

A previous options paper written by Ryan Brooks (previous Lead Technical Architect for GOV.UK Publishing) recommended migrating to RDS PostgreSQL as the most practical option for Content Store.

GOV.UK Publishing Platform team will implement this recommendation - we will port Content Store to run on PostgreSQL, using Amazon's RDS managed service in integration, staging and production. This will bring Content Store in line with most of GOV.UK, and externalise responsibility for the mechanics of running and updating a highly-available datastore under load, to Amazon. It will also allow us to use the exact same version of PostgreSQL for local development as in production.

We have already performed tech 'spikes' to prove the concept of a) porting the application, and b) migrating the full dataset over to PostgreSQL (overall Trello 'epic' card, forked application running on PostgreSQL).

We have also completed a round of performance testing and optimisation, verifying that for the most typical and critical use case of Content Store - find and return the document corresponding to a given URL path - performance on PostgreSQL is at least as fast as for Mongo on the same hardware.

Whilst there are several possible ways to manage the migration, this RFC is focussed on the target end-state, not how to get there. We can, however, state that we are confident we can achieve this migration with :

  • zero or near-zero downtime
  • no significant changes to the HTTP Content Store APIs
  • comparable performance for most queries as a result of the move

Consequences

While the majority of applications correctly use the API to interact with Content Store data, there are some downstream processes which depend on the nightly database backups, and therefore will need to be changed. These include:

  • The overnight environment sync job, which dumps live data to S3 and imports it into integration & staging environments
  • Data Services' GCP Storage Transfer job to upload the S3 backup to Google Cloud Platform for subsequent analysis and processing
  • Data Services' MongoDB Content tool which allows the user to explore a local copy of the database

Possible mitigations

All of the above are solvable problems. Publishing Platform team will make best efforts to submit the necessary changes themselves where practical (this is most likely on the environment sync job) or collaborate with the owning team on changes where we don't have the skills or context to do it well ourselves.

We've already spoken to Data Services about this change and the impact on their downstream services - the provisional plan is to introduce an additional step on their import side which stands up a PostgreSQL database in GCP and then exports to Mongo from there, as they have a significant number of queries/analyses based on Mongo. We plan to have a period of dual-running on both databases to make a smoother switchover, and allow time for backups to accumulate and for import jobs to be ported over before switching the Mongo DBs off entirely - based on the approach which The Guardian blogged about when they did a similar migration of their CMS. But these are all implementation questions to be tackled in detail once we achieve consensus on the end state, as outlined in this RFC.