= 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 libary, for pgqd.

debian/::
    Debian packaging.  This is for creating private packages,
    official Debian packages uses it's own packagin 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
3. Apply pgq.upgrade.sql
4. Apply pgq_node.sql
5. Apply londiste.sql - this will throw error on CREATE SCHEMA, but should otherwise apply fine.
6. Start pgqd.

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