NAME
londiste - PostgreSQL replication engine written in python
SYNOPSIS
londiste.py [option] config.ini command [arguments]
DESCRIPTION
Londiste is the PostgreSQL replication engine portion of the SkyTools
suite, by Skype. This suite includes packages implementing specific
replication tasks and/or solutions in layers, building upon each other.
PgQ is a generic queue implementation based on ideas from Slony-I´s
snapshot based event batching. Londiste uses PgQ as its transport
mechanism to implement a robust and easy to use replication solution.
Londiste is an asynchronous master-slave(s) replication system.
Asynchronous means that a transaction commited on the master is not
guaranteed to have made it to any slave at the master´s commit time;
and master-slave means that data changes on slaves are not reported
back to the master, it´s the other way around only.
The replication is trigger based, and you choose a set of tables to
replicate from the provider to the subscriber(s). Any data changes
occuring on the provider (in a replicated table) will fire the londiste
trigger, which fills a queue of events for any subscriber(s) to care
about.
A replay process consumes the queue in batches, and applies all given
changes to any subscriber(s). The initial replication step involves
using the PostgreSQL´s COPY command for efficient data loading.
QUICK-START
Basic londiste setup and usage can be summarized by the following
steps:
1. create the subscriber database, with tables to replicate
2. edit a londiste configuration file, say conf.ini, and a PgQ ticker
configuration file, say ticker.ini
3. install londiste on the provider and subscriber nodes. This step
requires admin privileges on both provider and subscriber sides,
and both install commands can be run remotely:
$ londiste.py conf.ini provider install
$ londiste.py conf.ini subscriber install
4. launch the PgQ ticker on the provider machine:
$ pgqadm.py -d ticker.ini ticker
5. launch the londiste replay process:
$ londiste.py -d conf.ini replay
6. add tables to replicate from the provider database:
$ londiste.py conf.ini provider add table1 table2 ...
7. add tables to replicate to the subscriber database:
$ londiste.py conf.ini subscriber add table1 table2 ...
To replicate to more than one subscriber database just repeat each of
the described subscriber steps for each subscriber.
COMMANDS
The londiste command is parsed globally, and has both options and
subcommands. Some options are reserved to a subset of the commands, and
others should be used without any command at all.
GENERAL OPTIONS
This section presents options available to all and any londiste
command.
-h, --help
show this help message and exit
-q, --quiet
make program silent
-v, --verbose
make program more verbose
PROVIDER COMMANDS
$ londiste.py config.ini provider <command>
Where command is one of:
provider install
Installs code into provider and subscriber database and creates queue.
Equivalent to doing following by hand:
CREATE LANGUAGE plpgsql;
CREATE LANGUAGE plpython;
\i .../contrib/txid.sql
\i .../contrib/pgq.sql
\i .../contrib/londiste.sql
select pgq.create_queue(queue name);
provider add <table name> ...
Registers table(s) on the provider database and adds the londiste
trigger to the table(s) which will send events to the queue. Table
names can be schema qualified with the schema name defaulting to public
if not supplied.
--all
Register all tables in provider database, except those that are
under schemas pgq, londiste, information_schema or pg_*.
provider remove <table name> ...
Unregisters table(s) on the provider side and removes the londiste
triggers from the table(s). The table removal event is also sent to the
queue, so all subscribers unregister the table(s) on their end as well.
Table names can be schema qualified with the schema name defaulting to
public if not supplied.
provider add-seq <sequence name> ...
Registers a sequence on provider.
provider remove-seq <sequence name> ...
Unregisters a sequence on provider.
provider tables
Shows registered tables on provider side.
provider seqs
Shows registered sequences on provider side.
SUBSCRIBER COMMANDS
londiste.py config.ini subscriber <command>
Where command is one of:
subscriber install
Installs code into subscriber database. Equivalent to doing following
by hand:
CREATE LANGUAGE plpgsql;
\i .../contrib/londiste.sql
This will be done under the Postgres Londiste user, if the tables
should be owned by someone else, it needs to be done by hand.
subscriber add <table name> ...
Registers table(s) on subscriber side. Table names can be schema
qualified with the schema name defaulting to public if not supplied.
Switches (optional):
--all
Add all tables that are registered on provider to subscriber
database
--force
Ignore table structure differences.
--excect-sync
Table is already synced by external means so initial COPY is
unnecessary.
--skip-truncate
When doing initial COPY, don´t remove old data.
subscriber remove <table name> ...
Unregisters table(s) from subscriber. No events will be applied to the
table anymore. Actual table will not be touched. Table names can be
schema qualified with the schema name defaulting to public if not
supplied.
subscriber add-seq <sequence name> ...
Registers a sequence on subscriber.
subscriber remove-seq <sequence name> ...
Unregisters a sequence on subscriber.
subscriber resync <table name> ...
Tags table(s) as "not synced". Later the replay process will notice
this and launch copy process(es) to sync the table(s) again.
subscriber tables
Shows registered tables on the subscriber side, and the current state
of each table. Possible state values are:
NEW
the table has not yet been considered by londiste.
in-copy
Full-table copy is in progress.
catching-up
Table is copied, missing events are replayed on to it.
wanna-sync:<tick-id>
The "copy" process catched up, wants to hand the table over to
"replay".
do-sync:<tick_id>
"replay" process is ready to accept it.
ok
table is in sync.
subscriber fkeys
Show pending and active foreign keys on tables. Takes optional type
argument - pending or active. If no argument is given, both types are
shown.
Pending foreign keys are those that were removed during COPY time but
have not restored yet, The restore happens autmatically if both tables
are synced.
subscriber triggers
Show pending and active triggers on tables. Takes optional type
argument - pending or active. If no argument is given, both types are
shown.
Pending triggers keys are those that were removed during COPY time but
have not restored yet, The restore of triggers does not happen
autmatically, it needs to be done manually with restore-triggers
command.
subscriber restore-triggers <table name>
Restores all pending triggers for single table. Optionally trigger name
can be given as extra argument, then only that trigger is restored.
subscriber register
Register consumer on queue. This usually happens automatically when
replay is launched, but
subscriber unregister
Unregister consumer from provider´s queue. This should be done if you
want to shut replication down.
REPLICATION COMMANDS
replay
The actual replication process. Should be run as daemon with -d switch,
because it needs to be always running.
It´s main task is to get batches of events from PgQ and apply them to
subscriber database.
Switches:
-d, --daemon
go background
-r, --reload
reload config (send SIGHUP)
-s, --stop
stop program safely (send SIGINT)
-k, --kill
kill program immidiately (send SIGTERM)
UTILITY COMMAND
repair <table name> ...
Attempts to achieve a state where the table(s) is/are in sync, compares
them, and writes out SQL statements that would fix differences.
Syncing happens by locking provider tables against updates and then
waiting until the replay process has applied all pending changes to
subscriber database. As this is dangerous operation, it has a hardwired
limit of 10 seconds for locking. If the replay process does not catch
up in that time, the locks are released and the repair operation is
cancelled.
Comparing happens by dumping out the table contents of both sides,
sorting them and then comparing line-by-line. As this is a CPU and
memory-hungry operation, good practice is to run the repair command on
a third machine to avoid consuming resources on either the provider or
the subscriber.
compare <table name> ...
Syncs tables like repair, but just runs SELECT count(*) on both sides
to get a little bit cheaper, but also less precise, way of checking if
the tables are in sync.
CONFIGURATION
Londiste and PgQ both use INI configuration files, your distribution of
skytools include examples. You often just have to edit the database
connection strings, namely db in PgQ ticker.ini and provider_db and
subscriber_db in londiste conf.ini as well as logfile and pidfile to
adapt to you system paths.
See londiste(5).
SEE ALSO
londiste(5)
https://developer.skype.com/SkypeGarage/DbProjects/SkyTools/
Reference guide[1]
NOTES
1. Reference guide
http://skytools.projects.postgresql.org/doc/londiste.ref.html
09/22/2008