commit a2a77f62d0f1cbdbe733d45d0191a608ef2f6d09 Author: David G. Johnston Date: Thu Oct 1 23:13:46 2020 +0000 Document NOTIFY/LISTEN interchange as queue interactions Rework the NOTIFY documentation to remove some inconsistent terminology usage as well as some redundancy and reframe the discussion in terms more closely aligned to the implementation that it used - a shared FIFO queue where consumers track their location on the queue (as opposed to having multiple queues and duplicating messages which are then removed immediately after being processed by individual sessions.) Remove specifying limited deduplication from three different perspectives and just say when it is performed. Remove the usage of the term "signal". This is probably a bit excessive but as the backends also "signal" each other it seemed clearer to use indiacte/communicate or just notification event to refer to the application-level behavior. diff --git a/doc/src/sgml/ref/notify.sgml b/doc/src/sgml/ref/notify.sgml index d7dcbea02d..82067ae69c 100644 --- a/doc/src/sgml/ref/notify.sgml +++ b/doc/src/sgml/ref/notify.sgml @@ -39,11 +39,14 @@ NOTIFY channel [ , NOTIFY provides a simple - interprocess communication mechanism for a collection of processes + interprocess communication mechanism, + in essentially the form of a first-in-first-out (FIFO) queue, + for a collection of processes accessing the same PostgreSQL database. - A payload string can be sent along with the notification, and - higher-level mechanisms for passing structured data can be built by using - tables in the database to pass additional data from notifier to listener(s). + While a payload string can be sent along with the notification, + higher-level mechanisms for passing structured data + from notifier to listener(s) must be built using + tables in the database. @@ -61,12 +64,12 @@ NOTIFY channel [ , . But no such association is enforced by the NOTIFY and LISTEN commands. For example, a database designer could use several different channel names - to signal different sorts of changes to a single table. Alternatively, + to indicate different sorts of changes to a single table. Alternatively, the payload string could be used to differentiate various cases. - When NOTIFY is used to signal the occurrence of changes + When NOTIFY is used to communicate the occurrence of changes to a particular table, a useful programming technique is to put the NOTIFY in a statement trigger that is triggered by table updates. In this way, notification happens automatically when the table is changed, @@ -76,40 +79,38 @@ NOTIFY channel [ , NOTIFY interacts with SQL transactions in some important ways. Firstly, if a NOTIFY is executed inside a - transaction, the notify events are not delivered until and unless the + transaction, the notification events are not enqueued until and unless the transaction is committed. This is appropriate, since if the transaction is aborted, all the commands within it have had no effect, including NOTIFY. But it can be disconcerting if one - is expecting the notification events to be delivered immediately. Secondly, if - a listening session receives a notification signal while it is within a transaction, - the notification event will not be delivered to its connected client until just - after the transaction is completed (either committed or aborted). Again, the + is expecting the notification events to be delivered to client connections immediately. + Secondly, a listening session never processes the notification queue events + while it is within a transaction. Again, the reasoning is that if a notification were delivered within a transaction that was later aborted, one would want the notification to be undone somehow — but - the server cannot take back a notification once it has sent it to the client. + the server cannot take back a notification once it has delivered + it to the client. So notification events are only delivered between transactions. The upshot of this - is that applications using NOTIFY for real-time signaling - should try to keep their transactions short. + is that applications using NOTIFY should try to keep their + transactions short. + + + + If, within the same transaction, a channel name is notified multiple times, + with identical payload strings, only the first the notification is enqueued. - If the same channel name is signaled multiple times with identical - payload strings within the same transaction, only one instance of the - notification event is delivered to listeners. - On the other hand, notifications with distinct payload strings will - always be delivered as distinct notifications. Similarly, notifications from - different transactions will never get folded into one notification. - Except for dropping later instances of duplicate notifications, NOTIFY guarantees that notifications from the same - transaction get delivered in the order they were sent. It is also - guaranteed that messages from different transactions are delivered in + transaction get delivered in the order they were issued. It also + guarantees that messages from different transactions are delivered in the order in which the transactions committed. It is common for a client that executes NOTIFY - to be listening on the same notification channel itself. In that case + to be listening on the same notification channel. In that case it will get back a notification event, just like all the other listening sessions. Depending on the application logic, this could result in useless work, for example, reading a database table to @@ -131,7 +132,7 @@ NOTIFY channel [ , channel - Name of the notification channel to be signaled (any identifier). + Name of the notification channel (any identifier). @@ -154,16 +155,19 @@ NOTIFY channel [ , Notes - There is a queue that holds notifications that have been sent but not - yet processed by all listening sessions. If this queue becomes full, - transactions calling NOTIFY will fail at commit. + The notification FIFO queue contains a single entry per notification event + and all listening sessions publish which events they have processed. Events + which have been processed by all sessions are removed. If a listening session + holds a transaction open for a very long time the queue could become full, + at which point transactions calling NOTIFY will fail at commit. + + The queue is quite large (8GB in a standard installation) and should be - sufficiently sized for almost every use case. However, no cleanup can take - place if a session executes LISTEN and then enters a - transaction for a very long time. Once the queue is half full you will see + sufficiently sized for almost every use case. Once the queue is half + full you will see warnings in the log file pointing you to the session that is preventing cleanup. In this case you should make sure that this session ends its - current transaction so that cleanup can proceed. + current transaction so that events can be removed. The function pg_notification_queue_usage returns the