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LISTEN/NOTIFY doc improvements.

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Tom Lane 27 years ago
parent 30debec6e5
commit 55a5639e93
2 changed files with 194 additions and 92 deletions
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  1. 105
      doc/src/sgml/ref/listen.sgml
  2. 179
      doc/src/sgml/ref/notify.sgml

105
doc/src/sgml/ref/listen.sgml
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@ -10,20 +10,20 @@ LISTEN
LISTEN
</REFNAME>
<REFPURPOSE>
Listen for notification on a relation
Listen for notification on a notify condition
</REFPURPOSE>
<REFSYNOPSISDIV>
<REFSYNOPSISDIVINFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSYNOPSISDIVINFO>
<SYNOPSIS>
LISTEN <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
LISTEN <REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
</SYNOPSIS>
<REFSECT2 ID="R2-SQL-LISTEN-1">
<REFSECT2INFO>
<DATE>1998-09-01</DATE>
<DATE>1998-10-07</DATE>
</REFSECT2INFO>
<TITLE>
Inputs
@ -33,11 +33,11 @@ Inputs
<VARIABLELIST>
<VARLISTENTRY>
<TERM>
<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
</TERM>
<LISTITEM>
<PARA>
Table object used for notification.
Name of notify condition.
</VARIABLELIST>
@ -62,63 +62,95 @@ Outputs
<VARIABLELIST>
<VARLISTENTRY>
<TERM>
LISTEN
<returnvalue>LISTEN</returnvalue>
</TERM>
<LISTITEM>
<PARA>
Message returned upon successful completion of registration.
</VARIABLELIST>
</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>
<returnvalue>NOTICE Async_Listen: We are already listening on notifyname</returnvalue>
</TERM>
<LISTITEM>
<PARA>
If this backend is already registered for that notify condition.
</PARA>
</LISTITEM>
</VARLISTENTRY>
</variablelist>
</LISTITEM>
</VARLISTENTRY>
</VARIABLELIST>
</REFSECT2>
</REFSYNOPSISDIV>
<REFSECT1 ID="R1-SQL-LISTEN-1">
<REFSECT1INFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSECT1INFO>
<TITLE>
Description
</TITLE>
<PARA>
LISTEN is used to register the current backend as a listener on the relation
<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>.
When the command
<command>NOTIFY <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE></command>
is called either from within a rule or at the query level, the
frontend applications corresponding to the listening backends
are notified. When the backend process exits, this registration
is cleared.
LISTEN registers the current <productname>Postgres</productname> backend as a
listener on the notify condition
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>.
<para>
Whenever the command
<command>NOTIFY <REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE></command>
is invoked, either by this backend or another one connected to
the same database, all the backends currently listening on that notify
condition are notified, and each will in turn notify its connected
frontend application. See the discussion of <command>NOTIFY</command>
for more information.
<para>
This event notification is performed through the libpq protocol
and frontend application interface. The application program
must call the routine
<function>PQnotifies</function>
in order to find out the name of the class to which a given
notification corresponds. If this code is not included in
the application, the event notification will be queued and
never be processed.
A backend can be deregistered for a given notify condition with the
<command>UNLISTEN</command> command. Also, a backend's listen registrations
are automatically cleared when the backend process exits.
<para>
The method a frontend application must use to detect notify events depends on
which <productname>Postgres</productname> application programming interface it
uses. With the basic libpq library, the application issues
<command>LISTEN</command> as an ordinary SQL command, and then must
periodically call the routine <function>PQnotifies</function> to find out
whether any notify events have been received. Other interfaces such as
libpgtcl provide higher-level methods for handling notify events; indeed,
with libpgtcl the application programmer should not even issue
<command>LISTEN</command> or <command>UNLISTEN</command> directly. See the
documentation for the library you are using for more details.
<para>
The reference page for <command>NOTIFY</command> contains a more extensive
discussion of the use of <command>LISTEN</command> and
<command>NOTIFY</command>.
<REFSECT2 ID="R2-SQL-LISTEN-3">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSECT2INFO>
<TITLE>
Notes
</TITLE>
<para>
Note that <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
needs not to be a valid class name but can be any string valid
as a name up to 32 characters long.
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
can be any string valid as a name;
it need not correspond to the name of any actual table. If
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
is enclosed in double-quotes, it need not even be a syntactically
valid name, but can be any string up to 31 characters long.
<para>
A restriction in some previous releases of
<productname>Postgres</productname> that a
<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
which does not correspond to an actual table must be enclosed in double-quotes
is no longer present.
In some previous releases of
<productname>Postgres</productname>,
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
had to be enclosed in double-quotes when it did not correspond to any existing
table name, even if syntactically valid as a name. That is no longer required.
</REFSECT2>
@ -128,6 +160,7 @@ Usage
</TITLE>
<PARA>
<ProgramListing>
-- Configure and execute a listen/notify sequence from psql
postgres=> listen virtual;
LISTEN
postgres=> notify virtual;

179
doc/src/sgml/ref/notify.sgml
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@ -10,22 +10,22 @@ NOTIFY
NOTIFY
</REFNAME>
<REFPURPOSE>
Signals all frontends and backends listening on a class
Signals all frontends and backends listening on a notify condition
</REFPURPOSE>
<REFSYNOPSISDIV>
<REFSYNOPSISDIVINFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSYNOPSISDIVINFO>
<SYNOPSIS>
<REPLACEABLE CLASS="PARAMETER">
</REPLACEABLE>
NOTIFY <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
NOTIFY <REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
</SYNOPSIS>
<REFSECT2 ID="R2-SQL-NOTIFY-1">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSECT2INFO>
<TITLE>
Inputs
@ -35,11 +35,11 @@ Inputs
<VARIABLELIST>
<VARLISTENTRY>
<TERM>
<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
</TERM>
<LISTITEM>
<PARA>
Table or arbitrary relation class used for notification.
Notify condition to be signaled.
</VARIABLELIST>
@ -47,7 +47,7 @@ Table or arbitrary relation class used for notification.
<REFSECT2 ID="R2-SQL-NOTIFY-2">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSECT2INFO>
<TITLE>
Outputs
@ -69,66 +69,141 @@ NOTIFY
</TERM>
<LISTITEM>
<PARA>
Notification message from backend.
</VARIABLELIST>
Acknowledgement that notify command has executed.
</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>
Notify events
</TERM>
<LISTITEM>
<PARA>
Events are delivered to listening frontends; whether and how each frontend
application reacts depends on its programming.
</PARA>
</LISTITEM>
</VARLISTENTRY>
</variablelist>
</LISTITEM>
</VARLISTENTRY>
</VARIABLELIST>
</REFSECT2>
</REFSYNOPSISDIV>
<REFSECT1 ID="R1-SQL-NOTIFY-1">
<REFSECT1INFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSECT1INFO>
<TITLE>
Description
</TITLE>
<PARA>
<command>NOTIFY</command> is used to awaken all sessions which have
previously executed
<command>LISTEN <replaceable class="parameter">classname</replaceable></command>.
This can be used either within an instance-level rule
as part of the action body or from a normal query.
The <command>NOTIFY</command> command sends a notify event to each
frontend application that has previously executed
<command>LISTEN <replaceable class="parameter">notifyname</replaceable></command>
for the specified notify condition in the current database.
<para>
The information passed to the frontend for a notify event includes the notify
condition name and the notifying backend process's PID. It is up to the
database designer to define the condition names that will be used in a given
database and what each one means.
<para>
When used from within a normal query,
this can be thought of as interprocess communication (IPC). When
used from within a rule, this can be thought of as an alert mechanism.
Commonly, the notify condition name is the same as the name of some table in
the database, and the notify event essentially means "I changed this table,
take a look at it to see what's new". But no such association is enforced by
the <command>NOTIFY</command> and <command>LISTEN</command> commands. For
example, a database designer could use several different condition names
to signal different sorts of changes to a single table.
<para>
<command>NOTIFY</command> provides a simple form of signal or
IPC (interprocess communication) mechanism for a collection of processes
accessing the same <productname>Postgres</productname> database.
Higher-level mechanisms can be built by using tables in the database to
pass additional data (beyond a mere condition name) from notifier to
listener(s).
<para>
When <command>NOTIFY</command> is used to signal the occurrence of changes
to a particular table, a useful programming technique is to put the
<command>NOTIFY</command> in a rule that is triggered by table updates.
In this way, notification happens automatically when the table is changed,
and the application programmer can't accidentally forget to do it.
<para>
Note that the mere fact that a <command>NOTIFY</command> has been
executed does not imply anything in particular about the state
of the class (e.g., that it has been updated),
nor does the notification protocol transmit any useful information
other than the class name.
Therefore, all <command>NOTIFY</command> does is indicate that some backend
wishes its peers to
examine <replaceable class="parameter">classname</replaceable>
in some application-specific way.
<command>NOTIFY</command> interacts with SQL transactions in some important
ways. Firstly, if a <command>NOTIFY</command> is executed inside a
transaction, the notify events are not delivered until and unless the
transaction is committed. This is appropriate, since if the transaction
is aborted we would like all the commands within it to have had no effect
--- including <command>NOTIFY</command>. But it can be disconcerting if one
is expecting the notify events to be delivered immediately. Secondly, if
a listening backend receives a notify signal while it is within a transaction,
the notify event will not be delivered to its connected frontend until just
after the transaction is completed (either committed or aborted). Again, the
reasoning is that if a notify were delivered within a transaction that was
later aborted, one would want the notification to be undone somehow --- but
the backend cannot "take back" a notify once it has sent it to the frontend.
So notify events are delivered only between transactions. The upshot of this
is that applications using <command>NOTIFY</command> for real-time signaling
should try to keep their transactions short.
<para>
In fact, <replaceable class="parameter">classname</replaceable>
need not be the name of an SQL class at all.
It is best thought of as a condition name
that the application programmer selects.
<command>NOTIFY</command> behaves rather like Unix signals in one important
respect: if the same notify name is signaled multiple times in quick
succession, recipients may get only one notify event for several executions
of <command>NOTIFY</command>. So it is a bad idea to depend on the number
of notifies received; instead use <command>NOTIFY</command> to wake up
applications that need to pay attention to something, and use a database
object (such as a sequence) to keep track of what happened or how many times
it happened.
<para>
This event notification is performed through the libpq protocol
and frontend application interface. The application program
must call the routine <function>PQnotifies</function>
in order to find out the name of the class to which a given
notification corresponds.
If this code is not included in the application,
the event notification will be
queued and never be processed.
It is common for a frontend that sends <command>NOTIFY</command> to be
listening on the same notify name itself. In that case it will get back a
notify event, just like all the other listening frontends. Depending on the
application logic, this could result in useless work --- for example,
re-reading a database table to find the same updates that that frontend just
wrote out. In <productname>Postgres</productname> 6.4 and later, it is
possible to avoid such extra work by noticing whether the notifying backend
process's PID (supplied in the notify event message) is the same as one's own
backend's PID (available from libpq). When they are the same, the notify
event is one's own work bouncing back, and can be ignored. (Despite what was
said in the preceding paragraph, this is a safe technique.
<productname>Postgres</productname> keeps self-notifies separate from notifies
arriving from other backends, so you cannot miss an outside notify by ignoring
your own notifies.)
<REFSECT2 ID="R2-SQL-NOTIFY-3">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
<DATE>1998-10-07</DATE>
</REFSECT2INFO>
<TITLE>
Notes
</TITLE>
<PARA>
</PARA>
<para>
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
can be any string valid as a name;
it need not correspond to the name of any actual table. If
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
is enclosed in double-quotes, it need not even be a syntactically
valid name, but can be any string up to 31 characters long.
<para>
In some previous releases of
<productname>Postgres</productname>,
<REPLACEABLE CLASS="PARAMETER">notifyname</REPLACEABLE>
had to be enclosed in double-quotes when it did not correspond to any existing
table name, even if syntactically valid as a name. That is no longer required.
<para>
In <productname>Postgres</productname> releases prior to 6.4, the backend
PID delivered in a notify message is always the PID of the frontend's own
backend. So it is not possible to distinguish one's own notifies from other
clients' notifies in those earlier releases.
</REFSECT2>
@ -137,19 +212,13 @@ Notes
Usage
</TITLE>
<PARA>
</PARA>
<ProgramListing>
-- Configure and execute a listen/notify sequence
-- from psql
CREATE TABLE t (i int4);
<computeroutput>
LISTEN t;
</computeroutput>
NOTIFY t;
<computeroutput>
-- Configure and execute a listen/notify sequence from psql
postgres=> listen virtual;
LISTEN
postgres=> notify virtual;
NOTIFY
ASYNC NOTIFY of 't' from backend pid '10949' received
</computeroutput>
ASYNC NOTIFY of 'virtual' from backend pid '11239' received
</ProgramListing>
</REFSECT1>

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