Requests for comment/Standards for external services

This document is an attempt to formalize the output of the "Architecting Core: Standalone Services" session from the 2018 Wikimedia Technical Conference.

Proposal
The following proposals aim to set architectural principles and standards for when, how and why to to extend MediaWiki via an external service for use in the WMF production environment.

Definition
Standalone services are applications that extended MediaWiki's functionality, but that are operationally distinct in some meaningful way. The mechanism of extension is not specified, beyond saying that it's necessarily interprocess as opposed to intraprocess; queues, API calls and other types of RPC mechanisms, XHR from a client, are all valid examples, while shelling out from MediaWiki itself is not. It is important that the core business logic is implemented in the service, rather than within MediaWiki itself.

Selection Criteria: Deciding whether an external service is appropriate
The properties listed here are intended to be a guide as to whether a given feature can be provided externally to MediaWiki or not. They are intended to be necessary, but not sufficient. If a proposed feature has one or more properties that appear in the left column, but no properties that appear in the right column, then that feature could be implemented as a standalone service. If the proposed feature has one or more properties that appear in the right column, then a wide consensus must be reached before implementing it as a standalone service.

If any of the following are true, then the feature absolutely should be implemented as an external service, with appropriate architectural changes made elsewhere to eliminate disqualifying properties.

Given MediaWiki is not just part of the Wikimedia production infrastructure but also a software used my many third-parties, we can't just delegate some of its fundamental functions to an external service completely. Whenever an external service provides a functionality, we also need to ask ourselves if said functionality is fundamental or optional to MediaWiki.If the functionality replaced by the external service is fundamental, a fallback solution must be present within MediaWiki that substitutes what is being implemented in the service. As an example: for Wikimedia purposes, async processing for MediaWiki is implemented via a series of external services; for simple installations, a MySql-backed version of the same mecahnism exists and cannot be dropped from MediaWiki. What constitutes a core functionality of MediaWiki and what doesn't will need to be further defined elsewhere, and is beyond the scope of this document.

Architectural Guidelines for external services
This document only deals with principles (e.g. an application needs to be observable and expose appropriate metrics) and not with implementation guidelines. Practical implementation guidelines will be written to detail how the principles enumerated in this document are to be applied in technical terms (e.g. the application must expose RED metrics from a  endpoint in prometheus format, with a precise naming convention). The reason to split the two parts is that while we don't expect the principles to change much across time, but we do expect the implementation guidelines to change quicker due to technical evolution.

There are various aspects of the development and usage cycle of a new service, and several of those need to be as standardized as possible across the board in order not to make the complexity of our ecosystem become unmaintanable. In general, adopting a non-monolithic architecture has its costs, and unless standards are maintained regarding how different applications need to interoperate and how they're developed.

There are several aspects of the development of a service that need to be taken into account:


 * Development policies
 * Security/privacy requirements
 * Production deployment

In the next few sections we analyze the requirements a new service must fullfill in each of those categories.

Development policies
Everything we develop should be free, open to collaboration and useful in itself. So, a new service must:


 * Actually do something
 * There is no existing FLOSS software that provides the same functionality
 * Avoid needlessly duplicating features or functionality provided in other services
 * Be licensed under an OSI-approved license
 * Provide a configuration mechanism that does not involve changing the distributed code
 * Use a language and toolset that have been approved by TechCom

While some of our services will be only useful in the WMF context, in other cases the standalone service is intended to be distributed for general use. In that case, it must have the following properties:


 * Have a documented installation and uninstallation process that conform to our implementation guidelines
 * Have a documented upgrade process that conform with our implementation guidelines
 * Be versioned using semver
 * Indicate versions of MediaWiki with which it's compatible
 * Provide a mechanism by which support (community or otherwise) can be requested
 * Provide a mechanism by which patches can be proposed
 * Provide a mechanism by which public security advisories are issued

Security and privacy
All features implemented as standalone services must have the following properties:


 * Minimize data collection for any type of PII
 * Provide for compliance with the WMF privacy/data retention policies.
 * Implement privacy controls that are at least equivalent to those of any calling service. For example, if the privacy controls of the calling service specify that IP addresses will not be stored for more than 90 days, the external service may not store IP addresses for longer than that time.
 * Have a privacy policy and privacy practices that are compatible with the WMF/Wikimedia properties
 * Have passed review by WMF Security
 * Have resources allocated so that a prompt response to any security incident is possible

Production deployment
If the standalone service is intended to be used in the Wikimedia production environment, it should comply with the guidelines above, and in addition must


 * Be deployable with standard WMF tooling (as specified in the implementation guidelines)
 * Have an owner, and a plan for onging maintenance. If the owner of a service is missing (because the team is disbanded/has a different focus), a new owner must be found via the code stewardship process
 * Have logging that conforms to the WMF standards - specified in the service implementation guidelines
 * Collect RED metrics; be able to export those metrics according to WMF standards specified in the implementation guidelines
 * Have a runbook for operational purposes
 * Support a multi-datacenter active-active (or active-passive) deployment


 * Service Level Iindicators must be defined for the service, and Service Level Objectives should be agreed upon. Failure to meet said service level objectives SHOULD result action to bring the service back into operational agreed upon Service Level Objectives. The Service Level Objectives can of course be reevaluated and changed, but preferably not as a result of a violation but rather an informed process
 * Have pinned / pinnable dependencies that don't need to be downloaded at runtime and/or from untrusted source
 * Have backups if the service stores any data
 * Have users, or a plan to acquire users

Service - Service interaction
Services will likely interact with each other; if that is the case, measures must be taken not to make the whole system dependent on the failure of a single component. Also, increased observability in the flow of requests is needed. So any new service that needs to be deployed in production should:


 * Degrade gracefully its functionality if it can't access another service. If that's not possible, maybe the new service should be logically tied to the other. An exception is explicitly made for the MediaWiki API, given quite a few services might depend on its availability to be useful.
 * Implement circuit breaking and backoff in case the server responds with an error of some kind.
 * Include concurrency limiting policies on all requests (server-side)
 * Add the appropriate tracing headers to the request, according to the WMF standards specified in the implementation guidelines
 * Log all requests received via the production logging facilities
 * Be able to perform requests via TLS to a specific hostname/ip provided via configuration.
 * Provide telemetry information about requests performed to other services, following the implementation guidelines.