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><H1
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><A
NAME="PROTOCOL-REPLICATION"
>46.3. Streaming Replication Protocol</A
></H1
><P
>To initiate streaming replication, the frontend sends the
<TT
CLASS="LITERAL"
>replication</TT
> parameter in the startup message. This tells the
backend to go into walsender mode, wherein a small set of replication commands
can be issued instead of SQL statements. Only the simple query protocol can be
used in walsender mode.
The commands accepted in walsender mode are:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>IDENTIFY_SYSTEM</DT
><DD
><P
> Requests the server to identify itself. Server replies with a result
set of a single row, containing three fields:
</P
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>systemid</DT
><DD
><P
> The unique system identifier identifying the cluster. This
can be used to check that the base backup used to initialize the
standby came from the same cluster.
</P
></DD
><DT
>timeline</DT
><DD
><P
> Current TimelineID. Also useful to check that the standby is
consistent with the master.
</P
></DD
><DT
>xlogpos</DT
><DD
><P
> Current xlog flush location. Useful to get a known location in the
transaction log where streaming can start.
</P
></DD
></DL
></DIV
><P>
</P
></DD
><DT
>START_REPLICATION <TT
CLASS="REPLACEABLE"
><I
>XXX</I
></TT
>/<TT
CLASS="REPLACEABLE"
><I
>XXX</I
></TT
></DT
><DD
><P
> Instructs server to start streaming WAL, starting at
WAL position <TT
CLASS="REPLACEABLE"
><I
>XXX</I
></TT
>/<TT
CLASS="REPLACEABLE"
><I
>XXX</I
></TT
>.
The server can reply with an error, e.g. if the requested section of WAL
has already been recycled. On success, server responds with a
CopyBothResponse message, and then starts to stream WAL to the frontend.
WAL will continue to be streamed until the connection is broken;
no further commands will be accepted.
</P
><P
> WAL data is sent as a series of CopyData messages. (This allows
other information to be intermixed; in particular the server can send
an ErrorResponse message if it encounters a failure after beginning
to stream.) The payload in each CopyData message follows this format:
</P
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>XLogData (B)</DT
><DD
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Byte1('w')</DT
><DD
><P
> Identifies the message as WAL data.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The starting point of the WAL data in this message, given in
XLogRecPtr format.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The current end of WAL on the server, given in
XLogRecPtr format.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The server's system clock at the time of transmission,
given in TimestampTz format.
</P
></DD
><DT
>Byte<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></DT
><DD
><P
> A section of the WAL data stream.
</P
></DD
></DL
></DIV
><P>
</P
></DD
></DL
></DIV
><P>
</P
><P
> A single WAL record is never split across two CopyData messages.
When a WAL record crosses a WAL page boundary, and is therefore
already split using continuation records, it can be split at the page
boundary. In other words, the first main WAL record and its
continuation records can be sent in different CopyData messages.
</P
><P
> Note that all fields within the WAL data and the above-described header
will be in the sending server's native format. Endianness, and the
format for the timestamp, are unpredictable unless the receiver has
verified that the sender's system identifier matches its own
<TT
CLASS="FILENAME"
>pg_control</TT
> contents.
</P
><P
> If the WAL sender process is terminated normally (during postmaster
shutdown), it will send a CommandComplete message before exiting.
This might not happen during an abnormal shutdown, of course.
</P
><P
> The receiving process can send replies back to the sender at any time,
using one of the following message formats (also in the payload of a
CopyData message):
</P
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Primary keepalive message (B)</DT
><DD
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Byte1('k')</DT
><DD
><P
> Identifies the message as a sender keepalive.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The current end of WAL on the server, given in
XLogRecPtr format.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The server's system clock at the time of transmission,
given in TimestampTz format.
</P
></DD
></DL
></DIV
><P>
</P
></DD
></DL
></DIV
><P>
</P
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Standby status update (F)</DT
><DD
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Byte1('r')</DT
><DD
><P
> Identifies the message as a receiver status update.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The location of the last WAL byte + 1 received and written to disk
in the standby, in XLogRecPtr format.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The location of the last WAL byte + 1 flushed to disk in
the standby, in XLogRecPtr format.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The location of the last WAL byte + 1 applied in the standby, in
XLogRecPtr format.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The server's system clock at the time of transmission,
given in TimestampTz format.
</P
></DD
></DL
></DIV
><P>
</P
></DD
></DL
></DIV
><P>
</P
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Hot Standby feedback message (F)</DT
><DD
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Byte1('h')</DT
><DD
><P
> Identifies the message as a Hot Standby feedback message.
</P
></DD
><DT
>Byte8</DT
><DD
><P
> The server's system clock at the time of transmission,
given in TimestampTz format.
</P
></DD
><DT
>Byte4</DT
><DD
><P
> The standby's current xmin. This may be 0, if the standby is
sending notification that Hot Standby feedback will no
longer be sent on this connection. Later non-zero messages may
reinitiate the feedback mechanism.
</P
></DD
><DT
>Byte4</DT
><DD
><P
> The standby's current epoch.
</P
></DD
></DL
></DIV
><P>
</P
></DD
></DL
></DIV
><P>
</P
></DD
><DT
>BASE_BACKUP [<TT
CLASS="LITERAL"
>LABEL</TT
> <TT
CLASS="REPLACEABLE"
><I
>'label'</I
></TT
>] [<TT
CLASS="LITERAL"
>PROGRESS</TT
>] [<TT
CLASS="LITERAL"
>FAST</TT
>] [<TT
CLASS="LITERAL"
>WAL</TT
>] [<TT
CLASS="LITERAL"
>NOWAIT</TT
>]</DT
><DD
><P
> Instructs the server to start streaming a base backup.
The system will automatically be put in backup mode before the backup
is started, and taken out of it when the backup is complete. The
following options are accepted:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>LABEL</TT
> <TT
CLASS="REPLACEABLE"
><I
>'label'</I
></TT
></DT
><DD
><P
> Sets the label of the backup. If none is specified, a backup label
of <TT
CLASS="LITERAL"
>base backup</TT
> will be used. The quoting rules
for the label are the same as a standard SQL string with
<A
HREF="runtime-config-compatible.html#GUC-STANDARD-CONFORMING-STRINGS"
>standard_conforming_strings</A
> turned on.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>PROGRESS</TT
></DT
><DD
><P
> Request information required to generate a progress report. This will
send back an approximate size in the header of each tablespace, which
can be used to calculate how far along the stream is done. This is
calculated by enumerating all the file sizes once before the transfer
is even started, and may as such have a negative impact on the
performance - in particular it may take longer before the first data
is streamed. Since the database files can change during the backup,
the size is only approximate and may both grow and shrink between
the time of approximation and the sending of the actual files.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>FAST</TT
></DT
><DD
><P
> Request a fast checkpoint.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>WAL</TT
></DT
><DD
><P
> Include the necessary WAL segments in the backup. This will include
all the files between start and stop backup in the
<TT
CLASS="FILENAME"
>pg_xlog</TT
> directory of the base directory tar
file.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>NOWAIT</TT
></DT
><DD
><P
> By default, the backup will wait until the last required xlog
segment has been archived, or emit a warning if log archiving is
not enabled. Specifying <TT
CLASS="LITERAL"
>NOWAIT</TT
> disables both
the waiting and the warning, leaving the client responsible for
ensuring the required log is available.
</P
></DD
></DL
></DIV
><P>
</P
><P
> When the backup is started, the server will first send two
ordinary result sets, followed by one or more CopyResponse
results.
</P
><P
> The first ordinary result set contains the starting position of the
backup, given in XLogRecPtr format as a single column in a single row.
</P
><P
> The second ordinary result set has one row for each tablespace.
The fields in this row are:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>spcoid</DT
><DD
><P
> The oid of the tablespace, or <TT
CLASS="LITERAL"
>NULL</TT
> if it's the base
directory.
</P
></DD
><DT
>spclocation</DT
><DD
><P
> The full path of the tablespace directory, or <TT
CLASS="LITERAL"
>NULL</TT
>
if it's the base directory.
</P
></DD
><DT
>size</DT
><DD
><P
> The approximate size of the tablespace, if progress report has
been requested; otherwise it's <TT
CLASS="LITERAL"
>NULL</TT
>.
</P
></DD
></DL
></DIV
><P>
</P
><P
> After the second regular result set, one or more CopyResponse results
will be sent, one for PGDATA and one for each additional tablespace other
than <TT
CLASS="LITERAL"
>pg_default</TT
> and <TT
CLASS="LITERAL"
>pg_global</TT
>. The data in
the CopyResponse results will be a tar format (following the
<SPAN
CLASS="QUOTE"
>"ustar interchange format"</SPAN
> specified in the POSIX 1003.1-2008
standard) dump of the tablespace contents, except that the two trailing
blocks of zeroes specified in the standard are omitted.
After the tar data is complete, a final ordinary result set will be sent.
</P
><P
> The tar archive for the data directory and each tablespace will contain
all files in the directories, regardless of whether they are
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> files or other files added to the same
directory. The only excluded files are:
<P
></P
></P><UL
COMPACT="COMPACT"
><LI
STYLE="list-style-type: disc"
><P
> <TT
CLASS="FILENAME"
>postmaster.pid</TT
>
</P
></LI
><LI
STYLE="list-style-type: disc"
><P
> <TT
CLASS="FILENAME"
>postmaster.opts</TT
>
</P
></LI
><LI
STYLE="list-style-type: disc"
><P
> <TT
CLASS="FILENAME"
>pg_xlog</TT
>, including subdirectories. If the backup is run
with WAL files included, a synthesized version of <TT
CLASS="FILENAME"
>pg_xlog</TT
> will be
included, but it will only contain the files necessary for the
backup to work, not the rest of the contents.
</P
></LI
></UL
><P>
Owner, group and file mode are set if the underlying file system on
the server supports it.
</P
><P
> Once all tablespaces have been sent, a final regular result set will
be sent. This result set contains the end position of the
backup, given in XLogRecPtr format as a single column in a single row.
</P
></DD
></DL
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