= 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 PgQ and utility library for
Python scripts, as well as a script for setting up and managing WAL based
standby servers.

== Overview ==

It contains following tools:

=== 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, written for
general usage.

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

Rules:

- There can be several queues in 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 splited 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). And
consumers are frequently written in Python, the prefered language as it
has a powerful SKytools Framework but it is not limitative and any
language able to run PostgreSQL stored procedures can be used.

*Ticker* is a daemon which splits the queues in batch of events and
handle the maintenance of the system. The Ticker is provided with the
Skytools.

Documentation:

- PgQ ticker daemon (pgqd) usage: link:pgqd.html[]
- PgQ admin tool (qadm) usage: link:qadmin.html[]
- PgQ SQL API overview: link:pgq-sql.html[]
- PgQ SQL reference: http://skytools.projects.postgresql.org/skytools-3.0/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/londiste.cmdline.txt
  (also available as `man 1 londiste`)
- Londiste configuration: doc/londiste.config.txt
  (also available as `man 5 londiste`)
- Londiste reference: doc/londiste.ref.txt

=== walmgr ===

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

It can also be used for up-to-last-second partial file copying,
so less than whole file is lost in case of loss of 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 ./upgrade/final/v3.0_pgq_core.sql
4. Apply installed pgq_node.sql
5. Apply installed londiste.sql - this will throw error on CREATE SCHEMA, but should otherwise apply fine.
6. Start pgqd.