= SkyTools - tools for PostgreSQL =

This is a package of tools in use in Skype for replication and failover. It also includes a generic queuing mechanism called PgQ and a utility library for Python scripts, as well as a script for setting up and managing WAL based standby servers.

== Overview ==

It contains the following modules:

=== PgQ ===

PgQ is a queuing system written in PL/pgSQL, Python and C code. It is based on snapshot-based event handling ideas from Slony-I, and is written for general usage.

PgQ provides an efficient, transactional, queueing system with multi-node support (including work sharing and splitting, failover and switchover, for queues and for consumers).

Rules:

• There can be several queues in a database.
• There can be several producers than can insert into any queue.
• There can be several consumers on one queue.
• There can be several subconsumers on a consumer.

PgQ is split into 3 layers: Producers, Ticker and Consumers.

Producers and Consumers respectively push and read events into a queue. Producers just need to call PostgreSQL stored procedures (like a trigger on a table or a PostgreSQL call from the application). Consumers are frequently written in Python (the preferred language as it has a powerful Skytools Framework), but are not limited to Python; any language able to run PostgreSQL stored procedures can be used.

Ticker is a daemon which splits the queues into batchs of events and handle the maintenance of the system. The Ticker is provided with Skytools.

Documentation:

• PgQ ticker daemon (pgqd) usage: link:doc/pgqd.html[]
• PgQ admin tool (qadm) usage: link:doc/qadmin.html[]
• PgQ SQL API overview: link:doc/pgq-sql.html[]
• PgQ SQL reference: link:pgq/[]

=== Londiste ===

Replication tool written in Python, using PgQ as event transport.

Features:

• Tables can be added one-by-one into set.
• Initial COPY for one table does not block event replay for other tables.
• Can compare tables on both sides.

Documentation:

• Londiste script usage: doc/londiste3.txt (also available as man 1 londiste)

• Londiste HOWTOs: doc/howto/

=== walmgr ===

This script will setup WAL archiving, does the initial backup, and runtime WAL archive and restore.

It can also be used for up-to-last-second partial file copying, so that less than the whole file is lost in case of loss of the master database server.

== Source tree contents ==

doc/:: Documentation in AsciiDoc format. Source for both html and man pages.

python/:: Python modules and primary executables - walmgr, londiste, qadmin, pgqadm.

python/pgq/:: Python framework for PgQ.

python/londiste/:: Londiste replication.

python/skytools/:: Low-level utilities for writing database scripts in Python.

sql/:: Database modules.

sql/pgq/:: Table definitions and functions for PgQ queueing.

sql/pgq_node/:: Framework for cascaded consuming.

sql/pgq_coop/:: Functions for cooperative consuming.

sql/londiste/:: Table definitions and functions for Londiste replication.

sql/ticker/:: PgQ ticker written in C.

scripts/:: Python scripts with lesser priority.

lib/:: libusual C library, for pgqd.

debian/:: Debian packaging. This is for creating private packages, official Debian packages use their own packaging code.

misc/:: Random scripts used for building.

== Upgrade from 2.1 ==

Assuming PgQ + Londiste setup. This will upgrade PgQ to 3.0 and install Londiste 3 in parallel with Londiste 2.

1. Install Postgres modules. They are backwards compatible with 2.1.
2. Stop pgqadm.py ticker processes.
3. Apply pgq.upgrade_2.1_to_3.0.sql
4. Apply pgq.upgrade.sql
5. Apply pgq_node.sql
6. Apply londiste.sql - this will throw error on CREATE SCHEMA, but should otherwise apply fine.
7. Start pgqd.

The files mentioned above are installed under $PREFIX/share/skytools3/ directory.