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Replication README updates.

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Fujii Masao
This commit is contained in:
Robert Haas
2011-03-10 08:59:59 -05:00
parent 74a09d9210
commit fcb99609b6
1 changed files with 18 additions and 13 deletions
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31
src/backend/replication/README
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@@ -4,11 +4,11 @@ Walreceiver - libpqwalreceiver API
---------------------------------- ----------------------------------
The transport-specific part of walreceiver, responsible for connecting to The transport-specific part of walreceiver, responsible for connecting to
the primary server and receiving WAL files, is loaded dynamically to avoid the primary server, receiving WAL files and sending messages, is loaded
having to link the main server binary with libpq. The dynamically loaded dynamically to avoid having to link the main server binary with libpq.
module is in libpqwalreceiver subdirectory. The dynamically loaded module is in libpqwalreceiver subdirectory.
The dynamically loaded module implements three functions: The dynamically loaded module implements four functions:
bool walrcv_connect(char *conninfo, XLogRecPtr startpoint) bool walrcv_connect(char *conninfo, XLogRecPtr startpoint)
@@ -16,7 +16,6 @@ bool walrcv_connect(char *conninfo, XLogRecPtr startpoint)
Establish connection to the primary, and starts streaming from 'startpoint'. Establish connection to the primary, and starts streaming from 'startpoint'.
Returns true on success. Returns true on success.
bool walrcv_receive(int timeout, unsigned char *type, char **buffer, int *len) bool walrcv_receive(int timeout, unsigned char *type, char **buffer, int *len)
Retrieve any message available through the connection, blocking for Retrieve any message available through the connection, blocking for
@@ -26,6 +25,10 @@ otherwise false. On success, a pointer to the message payload is stored in
returned buffer is valid until the next call to walrcv_* functions, the returned buffer is valid until the next call to walrcv_* functions, the
caller should not attempt freeing it. caller should not attempt freeing it.
void walrcv_send(const char *buffer, int nbytes)
Send a message to XLOG stream.
void walrcv_disconnect(void); void walrcv_disconnect(void);
Disconnect. Disconnect.
@@ -45,11 +48,15 @@ to fetch more WAL (if streaming replication is configured).
Walreceiver is a postmaster subprocess, so the startup process can't fork it Walreceiver is a postmaster subprocess, so the startup process can't fork it
directly. Instead, it sends a signal to postmaster, asking postmaster to launch directly. Instead, it sends a signal to postmaster, asking postmaster to launch
it. Before that, however, startup process fills in WalRcvData->conninfo, it. Before that, however, startup process fills in WalRcvData->conninfo,
and initializes the starting point in WalRcvData->receivedUpto. and initializes the starting point in WalRcvData->receiveStart.
As walreceiver receives WAL from the master server, and writes and flushes As walreceiver receives WAL from the master server, and writes and flushes
it to disk (in pg_xlog), it updates WalRcvData->receivedUpto. Startup process it to disk (in pg_xlog), it updates WalRcvData->receivedUpto and signals
polls that to know how far it can proceed with WAL replay. the startup process to know how far WAL replay can advance.
Walreceiver sends information about replication progress to the master server
whenever either it writes or flushes new WAL, or the specified interval elapses.
This is used for reporting purpose.
Walsender IPC Walsender IPC
------------- -------------
@@ -80,11 +87,9 @@ phase. A walsenders will look like a regular backends until it's done with the
initialization and has marked itself in PMSignal array, and at process initialization and has marked itself in PMSignal array, and at process
termination, after unmarking the PMSignal slot. termination, after unmarking the PMSignal slot.
Each walsender allocates an entry from the WalSndCtl array, and advertises Each walsender allocates an entry from the WalSndCtl array, and tracks
there how far it has streamed WAL already. This is used at checkpoints, to information about replication progress. User can monitor them via
avoid recycling WAL that hasn't been streamed to a slave yet. However, statistics views.
that doesn't stop such WAL from being recycled when the connection is not
established.
Walsender - walreceiver protocol Walsender - walreceiver protocol