Mirroring infrastructure in PyPI

PEP: 374
Title: Mirroring infrastructure in PyPI
Author: Tarek Ziadé
Discussions-To: Catalog SIG
Status: Draft
Python-Version: 2.6

Abstract

This PEP describes a mirroring infrastructure for PyPI.

Motivation

PyPI is hosting over 4000 package and is used on a daily-basis by people to build applications. Systems like easy_install and zc.buildout makes a intensive usage of PyPI.

In a sense, PyPI became a single point of failure, and people are starting to set up some mirrors, wether they are private or public. Those mirrors are active mirrors, which means that they are browsing PyPI to get synced.

The motivation of this PEP is to set up a registering mechanism in PyPI in order to list all the public PyPI mirrors and to provide an event based system where all mirrors get informed via RPC when a package has been uploaded, modified, or removed, so they can eventually sync themselves.

This PEP describes:

mirror listing and registering
ping mechanism

Mirror listing and registering

A new HTML page will be added at http://pypi.python.org/mirrors that can be browsed like the simple index. This page just gives a list of the mirrors through a list of links.

These links are the url of the simple index of each mirrors. The page will look like this:

<html>
  <head><title>PyPI mirrors</title></head>
  <body>
    <p>PyPI mirrors
    
    If you want to register a new mirror, send a mail
    to catalog-SIG@python.org with:
        
        - the url of your mirror.
        - the name and email of the maintainer.
        - the url to ping when PyPI is updated.

    The registering is done manually and to become a
    mirror, you need to strictly follow the package index
    API defined here: 
    http://peak.telecommunity.com/DevCenter/EasyInstall#package-index-api

    </p> 
    <a href="http://example.com/pypi">Mirror #1</a>
    <a href="http://example2.com/pypi">Mirror #2</a>
  </body>
</html>

When a mirror is proposed on the mailing list, it is manually added in a dedicated SQL table in the PyPI application.

The mirror list page is a simple html page that can be browsed by the tool out there that wants to get a list of registered mirrors.

Ping mechanism

Everytime a package is uploaded, removed or modified at PyPI, the server will ping via XML-RPC all the mirrors that are registered in order to tell them that something has changed.

The RPC request will use the following pseudo-code

    >>> import xmlrpclib
    >>> from socket import setdefaulttimeout
    >>> MIRROR_TIMEOUT = 2
    >>> setdefaulttimeout(MIRROR_TIMEOUT)
    >>> for mirror_url in mirror_urls:
    ...     mirror = ServerProxy(mirror_url)
    ...     try:
    ...         mirror.removed_packages(removed_names)
    ...         mirror.modified_packages(modified_names)
    ...     except (timeout, xmlrpclib.ProtocolError):
    ...         log_failure(mirror_url)

removed_names and modified_names are the name of the packages (the distutils ids).
the MIRROR_TIMEOUT will prevent PyPI to slow down everytime it calls the mirrors. The mirrors are responsible to do the synchronisation job and must do it asynchronously from that call, which has to remain a ping for the sake of performances. The mirror decides what to do with the information it gets from PyPI.
everytime a mirror fails, it is log in the SQL database where we keep a "reliability ratio". It start with 1.0 and decreases after each failure. It is reset to 1.0 every 100 calls. If the number of failures during these 100 calls goes down to 0.75, the mirror is declared unreliable, and a mail is sent to the distutils mailing list. The mirror will then be eventually removed if the problem persists.

Costs

Someone has to manage the list of mirrors. This work should not take too much time. I am willing to be that maintainer if the people that maintain the server don't have the time, or don't trust me.

Implementation

XXX

Third-party applications

A sample module will be written and published to demonstrate how a mirror can implement the RPC APIs in Python.

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