pgqadm.py [option] config.ini command [arguments]
PgQ is Postgres based event processing system. It is part of SkyTools package that contains several useful implementations on this engine. Main function of PgQadm is to maintain and keep healthy both pgq internal tables and tables that store events.
SkyTools is scripting framework for Postgres databases written in Python that provides several utilities and implements common database handling logic.
Event - atomic piece of data created by Producers. In PgQ event is one record in one of tables that services that queue. Event record contains some system fields for PgQ and several data fileds filled by Producers. PgQ is neither checking nor enforcing event type. Event type is someting that consumer and produser must agree on. PgQ guarantees that each event is seen at least once but it is up to consumer to make sure that event is processed no more than once if that is needed.
Batch - PgQ is designed for efficiency and high throughput so events are grouped into batches for bulk processing. Creating these batches is one of main tasks of PgQadm and there are several parameters for each queue that can be use to tune size and frequency of batches. Consumerss receive events in these batches and depending on business requirements process events separately or also in batches.
Queue - Event are stored in queue tables i.e queues. Several producers can write into same queeu and several consumers can read from the queue. Events are kept in queue until all the consumers have seen them. We use table rotation to decrease hard disk io. Queue can contain any number of event types it is up to Producer and Consumer to agree on what types of events are passed and how they are encoded For example Londiste producer side can produce events for more tables tan consumer side needs so consumer subscribes only to those tables it needs and events for other tables are ignores.
Producer - applicatione that pushes event into queue. Prodecer can be written in any langaage that is able to run stored procedures in Postgres.
Consumer - application that reads events from queue. Consumers can be written in any language that can interact with Postgres. SkyTools package contains several useful consumers written in Python that can be used as they are or as good starting points to write more complex consumers.
Basic PgQ setup and usage can be summarized by the following steps:
- 1. create the database
- 2. edit a PgQ ticker configuration file, say ticker.ini
install PgQ internal tables
$ pgqadm.py ticker.ini install
launch the PgQ ticker on databse machine as daemon
$ pgqadm.py -d ticker.ini ticker
$ pgqadm.py ticker.ini create <queue>
register or run consumer to register it automatically
$ pgqadm.py ticker.ini register <queue> <consumer>
- 7. start producing events
[pgqadm] job_name = pgqadm_somedb
db = dbname=somedb
# how often to run maintenance [seconds] maint_delay = 600
# how often to check for activity [seconds] loop_delay = 0.1
logfile = ~/log/%(job_name)s.log pidfile = ~/pid/%(job_name)s.pid
Start ticking & maintenance process. Usually run as daemon with -d option. Must be running for PgQ to be functional and for consumers to see any events.
Show overview of registered queues and consumers and queue health. This command is used when you want to know what is happening inside PgQ.
Installs PgQ schema into database from config file.
Create queue tables into pgq schema. As soon as queue is created producers can start inserting events into it. But you must be aware that if there are no consumers on the queue the events are lost until consumer is registered.
Drop queue and all it's consumers from PgQ. Queue tables are dropped and all the contents are lost forever so use with care as with most drop commands.
register <queue> <consumer>
Register given consumer to listen to given queue. First batch seen by this consumer is the one completed after registration. Registration happens automatically when consumer is run first time so using this command is optional but may be needed when producers start producing events before consumer can be run.
unregister <queue> <consumer>
Removes consumer from given queue. Note consumer must be stopped before issuing this command otherwise it automatically registers again.
config [<queue> [<variable>=<value> ... ]]
Show or change queue config. There are several parameters that can be set for each queue shown here with default values:
- If no tick has happend during given number of seconds then one is generated just to keep queue lag in control. It may be increased if there is no need to deliver events fast. Not much room to decrease it :)
- Threshold number of events in filling batch that triggers tick. Can be increased to encourage PgQ to create larger batches or decreased to encourage faster ticking with smaller batches.
- Number of seconds that can pass without ticking if no events are coming to queue. These empty ticks are used as keep alive signals for batch jobs and monitoring.
queue_rotation_period (2 hours)
- Interval of time that may pass before PgQ tries to rotate tables to free up space. Not PgQ can not rotate tables if there are long transactions in database like VACUUM or pg_dump. May be decreased if low on disk space or increased to keep longer history of old events. To small values might affect performance badly because postgres tends to do seq scans on small tables. Too big values may waste disk space.
Looking at queue config.
$ pgqadm.py mydb.ini config testqueue queue_ticker_max_lag = 3 queue_ticker_max_count = 500 queue_ticker_idle_period = 60 queue_rotation_period = 7200 $ pgqadm.py conf/pgqadm_myprovider.ini config testqueue queue_ticker_max_lag=10 queue_ticker_max_count=300 Change queue bazqueue config to: queue_ticker_max_lag='10', queue_ticker_max_count='300' $
- show help message
- make program silent
- make program verbose
- go background
- reload config (send SIGHUP)
- stop program safely (send SIGINT)
- kill program immidiately (send SIGTERM)