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Generic Messages for Logical Decoding

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API and mechanism to allow generic messages to be inserted into WAL that are
intended to be read by logical decoding plugins. This commit adds an optional
new callback to the logical decoding API.

Messages are either text or bytea. Messages can be transactional, or not, and
are identified by a prefix to allow multiple concurrent decoding plugins.

(Not to be confused with Generic WAL records, which are intended to allow crash
recovery of extensible objects.)

Author: Petr Jelinek and Andres Freund
Reviewers: Artur Zakirov, Tomas Vondra, Simon Riggs
Discussion: 5685F999.6010202@2ndquadrant.com
This commit is contained in:
Simon Riggs
2016-04-06 10:05:41 +01:00
parent 989be0810d
commit 3fe3511d05
27 changed files with 693 additions and 33 deletions
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38
doc/src/sgml/logicaldecoding.sgml
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@ -363,6 +363,7 @@ typedef struct OutputPluginCallbacks
LogicalDecodeBeginCB begin_cb;
LogicalDecodeChangeCB change_cb;
LogicalDecodeCommitCB commit_cb;
LogicalDecodeMessageCB message_cb;
LogicalDecodeFilterByOriginCB filter_by_origin_cb;
LogicalDecodeShutdownCB shutdown_cb;
} OutputPluginCallbacks;
@ -602,6 +603,43 @@ typedef bool (*LogicalDecodeFilterByOriginCB) (
more efficient.
</para>
</sect3>
<sect3 id="logicaldecoding-output-plugin-message">
<title>Generic Message Callback</title>
<para>
The optional <function>message_cb</function> callback is called whenever
a logical decoding message has been decoded.
<programlisting>
typedef void (*LogicalDecodeMessageCB) (
struct LogicalDecodingContext *,
ReorderBufferTXN *txn,
XLogRecPtr message_lsn,
bool transactional,
const char *prefix,
Size message_size,
const char *message
);
</programlisting>
The <parameter>txn</parameter> parameter contains meta information about
the transaction, like the time stamp at which it has been committed and
its XID. Note however that it can be NULL when the message is
non-transactional and the XID was not assigned yet in the transaction
which logged the message. The <parameter>lsn</parameter> has WAL
position of the message. The <parameter>transactional</parameter> says
if the message was sent as transactional or not.
The <parameter>prefix</parameter> is arbitrary null-terminated prefix
which can be used for identifying interesting messages for the current
plugin. And finally the <parameter>message</parameter> parameter holds
the actual message of <parameter>message_size</parameter> size.
</para>
<para>
Extra care should be taken to ensure that the prefix the output plugin
considers interesting is unique. Using name of the extension or the
output plugin itself is often a good choice.
</para>
</sect3>
</sect2>
<sect2 id="logicaldecoding-output-plugin-output">