Hi pgsql-hackers,
The LISTEN / NOTIFY feature (along with the pg_notify() function) is a
unique feature that differentiates Postgresql from nearly all other
relational database systems. With the exception of SQL Server, I know
of no other RDBMSs that allow a client to be asynchronously notified by
the database server.
This feature embodies the modern "push" approach and allows delivering
timely data to the user as it changes, instead of the more traditional
"pull" approach which requires the user to request the data at specific
intervals. Vendors are rolling out "push" technologies to meet market
demand. Microsoft recently introduced SignalR - which is a framework
for pushing content to ASP.NET Web pages. Similarly Complex Event
Processing systems "push" information to users' dashboards in real-time.
In contrast with RDBMS's where asynchronous notification is a special
feature, message broker software implementations live and breathe
asynchronous notification. So I feel that the LISTEN / NOTIFY feature
is trying to deliver some of the asynchronous notification features of a
message broker but it lacks some of the flexibility.
One particular shortcoming of LISTEN / NOTIFY is the fact that the
channel specified on the LISTEN must _exactly _match the channel
specified on the NOTIFY. Here is an example of the problem:
I have two listeners:
1. Interested in all stock quote updates
2. Interested in stock quote updates for IBM only
There is a table that contains stock prices with a trigger proc that
issues a NOTIFY using pg_notify() upon update. There isn't a single
channel that I can use that will deliver the message to both listeners.
To get around the problem I could publish a message on channel "PRICE"
and another message on channel "PRICE.IBM" but sending two notifications
is far from optimal.
Message brokers have implemented a neat way to get around this issue.
It is accomplished by allowing wildcards in message topic subscriptions.
Here is an example
implementation:http://activemq.apache.org/nms/activemq-wildcards.html
* is used to separate names in a path
* * is used to match any name in a path
* > is used to recursively match any destination starting from this name
For example using the example above, these subscriptions are possible
Subscription Meaning
PRICE.> Any price for any product on any exchange
PRICE.STOCK.> Any price for a stock on any exchange
PRICE.STOCK.NASDAQ.* Any stock price on NASDAQ
PRICE.STOCK.*.IBM Any IBM stock price on any exchange
My request is to implement the same or similar feature in Postgresql.
Thank you.
-Sev
