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Add option to control snapshot export to CREATE_REPLICATION_SLOT 

We used to export snapshots unconditionally in CREATE_REPLICATION_SLOT
in the replication protocol, but several upcoming patches want more
control over what happens.

Suppress snapshot export in pg_recvlogical, which neither needs nor can
use the exported snapshot.  Since snapshot exporting can fail this
improves reliability.

This also paves the way for allowing the creation of replication slots
on standbys, which cannot export snapshots because they cannot allocate
new XIDs.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
This commit is contained in:
Peter Eisentraut 2017-03-14 17:13:56 -04:00
parent 7150402655
commit eb4da3e380
10 changed files with 141 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
doc/src/sgml/logicaldecoding.sgml
View File

@ -271,8 +271,9 @@ $ pg_recvlogical -d postgres --slot test --drop-slot
<sect2>
<title>Exported Snapshots</title>
<para>
When a new replication slot is created using the streaming replication interface,
a snapshot is exported
When a new replication slot is created using the streaming replication
interface (see <xref linkend="protocol-replication-create-slot">), a
snapshot is exported
(see <xref linkend="functions-snapshot-synchronization">), which will show
exactly the state of the database after which all changes will be
included in the change stream. This can be used to create a new replica by
@ -282,6 +283,12 @@ $ pg_recvlogical -d postgres --slot test --drop-slot
database's state at that point in time, which afterwards can be updated
using the slot's contents without losing any changes.
</para>
<para>
Creation of a snapshot is not always possible. In particular, it will
fail when connected to a hot standby. Applications that do not require
snapshot export may suppress it with the <literal>NOEXPORT_SNAPSHOT</>
option.
</para>
</sect2>
</sect1>

19
doc/src/sgml/protocol.sgml
View File

@ -1486,8 +1486,8 @@ The commands accepted in walsender mode are:
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CREATE_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</> [ <literal>TEMPORARY</> ] { <literal>PHYSICAL</> [ <literal>RESERVE_WAL</> ] | <literal>LOGICAL</> <replaceable class="parameter">output_plugin</> }
<varlistentry id="protocol-replication-create-slot" xreflabel="CREATE_REPLICATION_SLOT">
<term><literal>CREATE_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</> [ <literal>TEMPORARY</> ] { <literal>PHYSICAL</> [ <literal>RESERVE_WAL</> ] | <literal>LOGICAL</> <replaceable class="parameter">output_plugin</> [ <literal>EXPORT_SNAPSHOT</> | <literal>NOEXPORT_SNAPSHOT</> ] }
<indexterm><primary>CREATE_REPLICATION_SLOT</primary></indexterm>
</term>
<listitem>
@ -1538,6 +1538,21 @@ The commands accepted in walsender mode are:
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>EXPORT_SNAPSHOT</></term>
<term><literal>NOEXPORT_SNAPSHOT</></term>
<listitem>
<para>
Decides what to do with the snapshot created during logical slot
initialization. <literal>EXPORT_SNAPSHOT</>, which is the default,
will export the snapshot for use in other sessions. This option can't
be used inside a transaction. <literal>NOEXPORT_SNAPSHOT</> will
just use the snapshot for logical decoding as normal but won't do
anything else with it.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>

6
src/backend/commands/subscriptioncmds.c
View File

@ -314,7 +314,11 @@ CreateSubscription(CreateSubscriptionStmt *stmt, bool isTopLevel)
PG_TRY();
{
walrcv_create_slot(wrconn, slotname, false, &lsn);
/*
* Create permanent slot for the subscription. We won't use the
* initial snapshot for anything, so no need to export it.
*/
walrcv_create_slot(wrconn, slotname, false, false, &lsn);
ereport(NOTICE,
(errmsg("created replication slot \"%s\" on publisher",
slotname)));

15
src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
View File

@ -68,6 +68,7 @@ static void libpqrcv_send(WalReceiverConn *conn, const char *buffer,
static char *libpqrcv_create_slot(WalReceiverConn *conn,
const char *slotname,
bool temporary,
bool export_snapshot,
XLogRecPtr *lsn);
static bool libpqrcv_command(WalReceiverConn *conn,
const char *cmd, char **err);
@ -720,7 +721,7 @@ libpqrcv_send(WalReceiverConn *conn, const char *buffer, int nbytes)
*/
static char *
libpqrcv_create_slot(WalReceiverConn *conn, const char *slotname,
bool temporary, XLogRecPtr *lsn)
bool temporary, bool export_snapshot, XLogRecPtr *lsn)
{
PGresult *res;
StringInfoData cmd;
@ -728,13 +729,19 @@ libpqrcv_create_slot(WalReceiverConn *conn, const char *slotname,
initStringInfo(&cmd);
appendStringInfo(&cmd, "CREATE_REPLICATION_SLOT \"%s\" ", slotname);
appendStringInfo(&cmd, "CREATE_REPLICATION_SLOT \"%s\"", slotname);
if (temporary)
appendStringInfo(&cmd, "TEMPORARY ");
appendStringInfo(&cmd, " TEMPORARY");
if (conn->logical)
appendStringInfo(&cmd, "LOGICAL pgoutput");
{
appendStringInfo(&cmd, " LOGICAL pgoutput");
if (export_snapshot)
appendStringInfo(&cmd, " EXPORT_SNAPSHOT");
else
appendStringInfo(&cmd, " NOEXPORT_SNAPSHOT");
}
res = libpqrcv_PQexec(conn->streamConn, cmd.data);
pfree(cmd.data);

43
src/backend/replication/repl_gram.y
View File

@ -79,6 +79,8 @@ Node *replication_parse_result;
%token K_SLOT
%token K_RESERVE_WAL
%token K_TEMPORARY
%token K_EXPORT_SNAPSHOT
%token K_NOEXPORT_SNAPSHOT
%type <node> command
%type <node> base_backup start_replication start_logical_replication
@ -91,7 +93,9 @@ Node *replication_parse_result;
%type <defelt> plugin_opt_elem
%type <node> plugin_opt_arg
%type <str> opt_slot var_name
%type <boolval> opt_reserve_wal opt_temporary
%type <boolval> opt_temporary
%type <list> create_slot_opt_list
%type <defelt> create_slot_opt
%%
@ -202,18 +206,18 @@ base_backup_opt:
create_replication_slot:
/* CREATE_REPLICATION_SLOT slot TEMPORARY PHYSICAL RESERVE_WAL */
K_CREATE_REPLICATION_SLOT IDENT opt_temporary K_PHYSICAL opt_reserve_wal
K_CREATE_REPLICATION_SLOT IDENT opt_temporary K_PHYSICAL create_slot_opt_list
{
CreateReplicationSlotCmd *cmd;
cmd = makeNode(CreateReplicationSlotCmd);
cmd->kind = REPLICATION_KIND_PHYSICAL;
cmd->slotname = $2;
cmd->temporary = $3;
cmd->reserve_wal = $5;
cmd->options = $5;
$$ = (Node *) cmd;
}
/* CREATE_REPLICATION_SLOT slot TEMPORARY LOGICAL plugin */
| K_CREATE_REPLICATION_SLOT IDENT opt_temporary K_LOGICAL IDENT
| K_CREATE_REPLICATION_SLOT IDENT opt_temporary K_LOGICAL IDENT create_slot_opt_list
{
CreateReplicationSlotCmd *cmd;
cmd = makeNode(CreateReplicationSlotCmd);
@ -221,10 +225,36 @@ create_replication_slot:
cmd->slotname = $2;
cmd->temporary = $3;
cmd->plugin = $5;
cmd->options = $6;
$$ = (Node *) cmd;
}
;
create_slot_opt_list:
create_slot_opt_list create_slot_opt
{ $$ = lappend($1, $2); }
| /* EMPTY */
{ $$ = NIL; }
;
create_slot_opt:
K_EXPORT_SNAPSHOT
{
$$ = makeDefElem("export_snapshot",
(Node *)makeInteger(TRUE), -1);
}
| K_NOEXPORT_SNAPSHOT
{
$$ = makeDefElem("export_snapshot",
(Node *)makeInteger(FALSE), -1);
}
| K_RESERVE_WAL
{
$$ = makeDefElem("reserve_wal",
(Node *)makeInteger(TRUE), -1);
}
;
/* DROP_REPLICATION_SLOT slot */
drop_replication_slot:
K_DROP_REPLICATION_SLOT IDENT
@ -291,11 +321,6 @@ opt_physical:
| /* EMPTY */
;
opt_reserve_wal:
K_RESERVE_WAL { $$ = true; }
| /* EMPTY */ { $$ = false; }
;
opt_temporary:
K_TEMPORARY { $$ = true; }
| /* EMPTY */ { $$ = false; }

2
src/backend/replication/repl_scanner.l
View File

@ -100,6 +100,8 @@ RESERVE_WAL { return K_RESERVE_WAL; }
LOGICAL { return K_LOGICAL; }
SLOT { return K_SLOT; }
TEMPORARY { return K_TEMPORARY; }
EXPORT_SNAPSHOT { return K_EXPORT_SNAPSHOT; }
NOEXPORT_SNAPSHOT { return K_NOEXPORT_SNAPSHOT; }
"," { return ','; }
";" { return ';'; }

58
src/backend/replication/walsender.c
View File

@ -51,6 +51,7 @@
#include "catalog/pg_type.h"
#include "commands/dbcommands.h"
#include "commands/defrem.h"
#include "funcapi.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
@ -737,6 +738,48 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
return count;
}
/*
* Process extra options given to CREATE_REPLICATION_SLOT.
*/
static void
parseCreateReplSlotOptions(CreateReplicationSlotCmd *cmd,
bool *reserve_wal,
bool *export_snapshot)
{
ListCell *lc;
bool snapshot_action_given = false;
bool reserve_wal_given = false;
/* Parse options */
foreach (lc, cmd->options)
{
DefElem *defel = (DefElem *) lfirst(lc);
if (strcmp(defel->defname, "export_snapshot") == 0)
{
if (snapshot_action_given || cmd->kind != REPLICATION_KIND_LOGICAL)
ereport(ERROR,
(errcode(ERRCODE_SYNTAX_ERROR),
errmsg("conflicting or redundant options")));
snapshot_action_given = true;
*export_snapshot = defGetBoolean(defel);
}
else if (strcmp(defel->defname, "reserve_wal") == 0)
{
if (reserve_wal_given || cmd->kind != REPLICATION_KIND_PHYSICAL)
ereport(ERROR,
(errcode(ERRCODE_SYNTAX_ERROR),
errmsg("conflicting or redundant options")));
reserve_wal_given = true;
*reserve_wal = true;
}
else
elog(ERROR, "unrecognized option: %s", defel->defname);
}
}
/*
* Create a new replication slot.
*/
@ -746,6 +789,8 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
const char *snapshot_name = NULL;
char xpos[MAXFNAMELEN];
char *slot_name;
bool reserve_wal = false;
bool export_snapshot = true;
DestReceiver *dest;
TupOutputState *tstate;
TupleDesc tupdesc;
@ -754,6 +799,8 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
Assert(!MyReplicationSlot);
parseCreateReplSlotOptions(cmd, &reserve_wal, &export_snapshot);
/* setup state for XLogReadPage */
sendTimeLineIsHistoric = false;
sendTimeLine = ThisTimeLineID;
@ -799,10 +846,13 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
DecodingContextFindStartpoint(ctx);
/*
* Export a plain (not of the snapbuild.c type) snapshot to the user
* that can be imported into another session.
* Export the snapshot if we've been asked to do so.
*
* NB. We will convert the snapbuild.c kind of snapshot to normal
* snapshot when doing this.
*/
snapshot_name = SnapBuildExportSnapshot(ctx->snapshot_builder);
if (export_snapshot)
snapshot_name = SnapBuildExportSnapshot(ctx->snapshot_builder);
/* don't need the decoding context anymore */
FreeDecodingContext(ctx);
@ -810,7 +860,7 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
if (!cmd->temporary)
ReplicationSlotPersist();
}
else if (cmd->kind == REPLICATION_KIND_PHYSICAL && cmd->reserve_wal)
else if (cmd->kind == REPLICATION_KIND_PHYSICAL && reserve_wal)
{
ReplicationSlotReserveWal();

5
src/bin/pg_basebackup/streamutil.c
View File

@ -338,8 +338,13 @@ CreateReplicationSlot(PGconn *conn, const char *slot_name, const char *plugin,
appendPQExpBuffer(query, "CREATE_REPLICATION_SLOT \"%s\" PHYSICAL",
slot_name);
else
{
appendPQExpBuffer(query, "CREATE_REPLICATION_SLOT \"%s\" LOGICAL \"%s\"",
slot_name, plugin);
if (PQserverVersion(conn) >= 100000)
/* pg_recvlogical doesn't use an exported snapshot, so suppress */
appendPQExpBuffer(query, " NOEXPORT_SNAPSHOT");
}
res = PQexec(conn, query->data);
if (PQresultStatus(res) != PGRES_TUPLES_OK)

2
src/include/nodes/replnodes.h
View File

@ -56,7 +56,7 @@ typedef struct CreateReplicationSlotCmd
ReplicationKind kind;
char *plugin;
bool temporary;
bool reserve_wal;
List *options;
} CreateReplicationSlotCmd;

6
src/include/replication/walreceiver.h
View File

@ -183,7 +183,7 @@ typedef void (*walrcv_send_fn) (WalReceiverConn *conn, const char *buffer,
int nbytes);
typedef char *(*walrcv_create_slot_fn) (WalReceiverConn *conn,
const char *slotname, bool temporary,
XLogRecPtr *lsn);
bool export_snapshot, XLogRecPtr *lsn);
typedef bool (*walrcv_command_fn) (WalReceiverConn *conn, const char *cmd,
char **err);
typedef void (*walrcv_disconnect_fn) (WalReceiverConn *conn);
@ -224,8 +224,8 @@ extern PGDLLIMPORT WalReceiverFunctionsType *WalReceiverFunctions;
WalReceiverFunctions->walrcv_receive(conn, buffer, wait_fd)
#define walrcv_send(conn, buffer, nbytes) \
WalReceiverFunctions->walrcv_send(conn, buffer, nbytes)
#define walrcv_create_slot(conn, slotname, temporary, lsn) \
WalReceiverFunctions->walrcv_create_slot(conn, slotname, temporary, lsn)
#define walrcv_create_slot(conn, slotname, temporary, export_snapshot, lsn) \
WalReceiverFunctions->walrcv_create_slot(conn, slotname, temporary, export_snapshot, lsn)
#define walrcv_command(conn, cmd, err) \
WalReceiverFunctions->walrcv_command(conn, cmd, err)
#define walrcv_disconnect(conn) \