27.2. Log-Shipping Standby Servers
Continuous archiving can be used to create a high availability (HA) cluster configuration with one or more standby servers ready to take over operations if the primary server fails. This capability is widely referred to as warm standby or log shipping .
The primary and standby server work together to provide this capability, though the servers are only loosely coupled. The primary server operates in continuous archiving mode, while each standby server operates in continuous recovery mode, reading the WAL files from the primary. No changes to the database tables are required to enable this capability, so it offers low administration overhead compared to some other replication solutions. This configuration also has relatively low performance impact on the primary server.
Directly moving WAL records from one database server to another is typically described as log shipping. PostgreSQL implements file-based log shipping by transferring WAL records one file (WAL segment) at a time. WAL files (16MB) can be shipped easily and cheaply over any distance, whether it be to an adjacent system, another system at the same site, or another system on the far side of the globe. The bandwidth required for this technique varies according to the transaction rate of the primary server. Record-based log shipping is more granular and streams WAL changes incrementally over a network connection (see Section 27.2.5 ).
It should be noted that log shipping is asynchronous, i.e., the WAL
records are shipped after transaction commit. As a result, there is a
window for data loss should the primary server suffer a catastrophic
failure; transactions not yet shipped will be lost. The size of the
data loss window in file-based log shipping can be limited by use of the
archive_timeout
parameter, which can be set as low
as a few seconds. However such a low setting will
substantially increase the bandwidth required for file shipping.
Streaming replication (see
Section 27.2.5
)
allows a much smaller window of data loss.
Recovery performance is sufficiently good that the standby will typically be only moments away from full availability once it has been activated. As a result, this is called a warm standby configuration which offers high availability. Restoring a server from an archived base backup and rollforward will take considerably longer, so that technique only offers a solution for disaster recovery, not high availability. A standby server can also be used for read-only queries, in which case it is called a hot standby server. See Section 27.4 for more information.
27.2.1. Planning
It is usually wise to create the primary and standby servers so that they are as similar as possible, at least from the perspective of the database server. In particular, the path names associated with tablespaces will be passed across unmodified, so both primary and standby servers must have the same mount paths for tablespaces if that feature is used. Keep in mind that if CREATE TABLESPACE is executed on the primary, any new mount point needed for it must be created on the primary and all standby servers before the command is executed. Hardware need not be exactly the same, but experience shows that maintaining two identical systems is easier than maintaining two dissimilar ones over the lifetime of the application and system. In any case the hardware architecture must be the same - shipping from, say, a 32-bit to a 64-bit system will not work.
In general, log shipping between servers running different major PostgreSQL release levels is not possible. It is the policy of the PostgreSQL Global Development Group not to make changes to disk formats during minor release upgrades, so it is likely that running different minor release levels on primary and standby servers will work successfully. However, no formal support for that is offered and you are advised to keep primary and standby servers at the same release level as much as possible. When updating to a new minor release, the safest policy is to update the standby servers first - a new minor release is more likely to be able to read WAL files from a previous minor release than vice versa.
27.2.2. Standby Server Operation
A server enters standby mode if a
standby.signal
file exists in the data directory when the server is started.
In standby mode, the server continuously applies WAL received from the
primary server. The standby server can read WAL from a WAL archive
(see
restore_command
) or directly from the primary
over a TCP connection (streaming replication). The standby server will
also attempt to restore any WAL found in the standby cluster's
pg_wal
directory. That typically happens after a server
restart, when the standby replays again WAL that was streamed from the
primary before the restart, but you can also manually copy files to
pg_wal
at any time to have them replayed.
At startup, the standby begins by restoring all WAL available in the
archive location, calling
restore_command
. Once it
reaches the end of WAL available there and
restore_command
fails, it tries to restore any WAL available in the
pg_wal
directory.
If that fails, and streaming replication has been configured, the
standby tries to connect to the primary server and start streaming WAL
from the last valid record found in archive or
pg_wal
. If that fails
or streaming replication is not configured, or if the connection is
later disconnected, the standby goes back to step 1 and tries to
restore the file from the archive again. This loop of retries from the
archive,
pg_wal
, and via streaming replication goes on until the server
is stopped or failover is triggered by a trigger file.
Standby mode is exited and the server switches to normal operation
when
pg_ctl promote
is run,
pg_promote()
is called, or a trigger file is found
(
promote_trigger_file
). Before failover,
any WAL immediately available in the archive or in
pg_wal
will be
restored, but no attempt is made to connect to the primary.
27.2.3. Preparing the Primary for Standby Servers
Set up continuous archiving on the primary to an archive directory accessible from the standby, as described in Section 26.3 . The archive location should be accessible from the standby even when the primary is down, i.e., it should reside on the standby server itself or another trusted server, not on the primary server.
If you want to use streaming replication, set up authentication on the
primary server to allow replication connections from the standby
server(s); that is, create a role and provide a suitable entry or
entries in
pg_hba.conf
with the database field set to
replication
. Also ensure
max_wal_senders
is set
to a sufficiently large value in the configuration file of the primary
server. If replication slots will be used,
ensure that
max_replication_slots
is set sufficiently
high as well.
Take a base backup as described in Section 26.3.2 to bootstrap the standby server.
27.2.4. Setting Up a Standby Server
To set up the standby server, restore the base backup taken from primary
server (see
Section 26.3.4
). Create a file
standby.signal
in the standby's cluster data
directory. Set
restore_command
to a simple command to copy files from
the WAL archive. If you plan to have multiple standby servers for high
availability purposes, make sure that
recovery_target_timeline
is set to
latest
(the default), to make the standby server follow the timeline change
that occurs at failover to another standby.
Note
restore_command should return immediately if the file does not exist; the server will retry the command again if necessary.
If you want to use streaming replication, fill in primary_conninfo with a libpq connection string, including the host name (or IP address) and any additional details needed to connect to the primary server. If the primary needs a password for authentication, the password needs to be specified in primary_conninfo as well.
If you're setting up the standby server for high availability purposes, set up WAL archiving, connections and authentication like the primary server, because the standby server will work as a primary server after failover.
If you're using a WAL archive, its size can be minimized using the
archive_cleanup_command
parameter to remove files that are no
longer required by the standby server.
The
pg_archivecleanup
utility is designed specifically to
be used with
archive_cleanup_command
in typical single-standby
configurations, see
pg_archivecleanup
.
Note however, that if you're using the archive for backup purposes, you
need to retain files needed to recover from at least the latest base
backup, even if they're no longer needed by the standby.
A simple example of configuration is:
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass options=''-c wal_sender_timeout=5000''' restore_command = 'cp /path/to/archive/%f %p' archive_cleanup_command = 'pg_archivecleanup /path/to/archive %r'
You can have any number of standby servers, but if you use streaming
replication, make sure you set
max_wal_senders
high enough in
the primary to allow them to be connected simultaneously.
27.2.5. Streaming Replication
Streaming replication allows a standby server to stay more up-to-date than is possible with file-based log shipping. The standby connects to the primary, which streams WAL records to the standby as they're generated, without waiting for the WAL file to be filled.
Streaming replication is asynchronous by default
(see
Section 27.2.8
), in which case there is
a small delay between committing a transaction in the primary and the
changes becoming visible in the standby. This delay is however much
smaller than with file-based log shipping, typically under one second
assuming the standby is powerful enough to keep up with the load. With
streaming replication,
archive_timeout
is not required to
reduce the data loss window.
If you use streaming replication without file-based continuous
archiving, the server might recycle old WAL segments before the standby
has received them. If this occurs, the standby will need to be
reinitialized from a new base backup. You can avoid this by setting
wal_keep_size
to a value large enough to ensure that
WAL segments are not recycled too early, or by configuring a replication
slot for the standby. If you set up a WAL archive that's accessible from
the standby, these solutions are not required, since the standby can
always use the archive to catch up provided it retains enough segments.
To use streaming replication, set up a file-based log-shipping standby
server as described in
Section 27.2
. The step that
turns a file-based log-shipping standby into streaming replication
standby is setting the
primary_conninfo
setting
to point to the primary server. Set
listen_addresses
and authentication options
(see
pg_hba.conf
) on the primary so that the standby server
can connect to the
replication
pseudo-database on the primary
server (see
Section 27.2.5.1
).
On systems that support the keepalive socket option, setting tcp_keepalives_idle , tcp_keepalives_interval and tcp_keepalives_count helps the primary promptly notice a broken connection.
Set the maximum number of concurrent connections from the standby servers (see max_wal_senders for details).
When the standby is started and
primary_conninfo
is set
correctly, the standby will connect to the primary after replaying all
WAL files available in the archive. If the connection is established
successfully, you will see a
walreceiver
in the standby, and
a corresponding
walsender
process in the primary.
27.2.5.1. Authentication
It is very important that the access privileges for replication be set up
so that only trusted users can read the WAL stream, because it is
easy to extract privileged information from it. Standby servers must
authenticate to the primary as an account that has the
REPLICATION
privilege or a superuser. It is
recommended to create a dedicated user account with
REPLICATION
and
LOGIN
privileges for replication. While
REPLICATION
privilege gives very high permissions, it does not allow the user to
modify any data on the primary system, which the
SUPERUSER
privilege does.
Client authentication for replication is controlled by a
pg_hba.conf
record specifying
replication
in the
database
field. For example, if the standby is running on
host IP
192.168.1.100
and the account name for replication
is
foo
, the administrator can add the following line to the
pg_hba.conf
file on the primary:
# Allow the user "foo" from host 192.168.1.100 to connect to the primary # as a replication standby if the user's password is correctly supplied. # # TYPE DATABASE USER ADDRESS METHOD host replication foo 192.168.1.100/32 md5
The host name and port number of the primary, connection user name,
and password are specified in the
primary_conninfo
.
The password can also be set in the
~/.pgpass
file on the
standby (specify
replication
in the
database
field).
For example, if the primary is running on host IP
192.168.1.50
,
port
5432
, the account name for replication is
foo
, and the password is
foopass
, the administrator
can add the following line to the
postgresql.conf
file on the
standby:
# The standby connects to the primary that is running on host 192.168.1.50 # and port 5432 as the user "foo" whose password is "foopass". primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
27.2.5.2. Monitoring
An important health indicator of streaming replication is the amount
of WAL records generated in the primary, but not yet applied in the
standby. You can calculate this lag by comparing the current WAL write
location on the primary with the last WAL location received by the
standby. These locations can be retrieved using
pg_current_wal_lsn
on the primary and
pg_last_wal_receive_lsn
on the standby,
respectively (see
Table 9.88
and
Table 9.89
for details).
The last WAL receive location in the standby is also displayed in the
process status of the WAL receiver process, displayed using the
ps
command (see
Section 28.1
for details).
You can retrieve a list of WAL sender processes via the
pg_stat_replication
view. Large differences between
pg_current_wal_lsn
and the view's
sent_lsn
field
might indicate that the primary server is under heavy load, while
differences between
sent_lsn
and
pg_last_wal_receive_lsn
on the standby might indicate
network delay, or that the standby is under heavy load.
On a hot standby, the status of the WAL receiver process can be retrieved
via the
pg_stat_wal_receiver
view. A large
difference between
pg_last_wal_replay_lsn
and the
view's
flushed_lsn
indicates that WAL is being
received faster than it can be replayed.
27.2.6. Replication Slots
Replication slots provide an automated way to ensure that the primary does not remove WAL segments until they have been received by all standbys, and that the primary does not remove rows which could cause a recovery conflict even when the standby is disconnected.
In lieu of using replication slots, it is possible to prevent the removal
of old WAL segments using
wal_keep_size
, or by
storing the segments in an archive using
archive_command
or
archive_library
.
However, these methods often result in retaining more WAL segments than
required, whereas replication slots retain only the number of segments
known to be needed. On the other hand, replication slots can retain so
many WAL segments that they fill up the space allocated
for
pg_wal
;
max_slot_wal_keep_size
limits the size of WAL files
retained by replication slots.
Similarly, hot_standby_feedback and vacuum_defer_cleanup_age provide protection against relevant rows being removed by vacuum, but the former provides no protection during any time period when the standby is not connected, and the latter often needs to be set to a high value to provide adequate protection. Replication slots overcome these disadvantages.
27.2.6.1. Querying and Manipulating Replication Slots
Each replication slot has a name, which can contain lower-case letters, numbers, and the underscore character.
Existing replication slots and their state can be seen in the
pg_replication_slots
view.
Slots can be created and dropped either via the streaming replication protocol (see Section 55.4 ) or via SQL functions (see Section 9.27.6 ).
27.2.6.2. Configuration Example
You can create a replication slot like this:
postgres=# SELECT * FROM pg_create_physical_replication_slot('node_a_slot');
slot_name | lsn
-------------+-----
node_a_slot |
postgres=# SELECT slot_name, slot_type, active FROM pg_replication_slots;
slot_name | slot_type | active
-------------+-----------+--------
node_a_slot | physical | f
(1 row)
To configure the standby to use this slot,
primary_slot_name
should be configured on the standby. Here is a simple example:
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' primary_slot_name = 'node_a_slot'
27.2.7. Cascading Replication
The cascading replication feature allows a standby server to accept replication connections and stream WAL records to other standbys, acting as a relay. This can be used to reduce the number of direct connections to the primary and also to minimize inter-site bandwidth overheads.
A standby acting as both a receiver and a sender is known as a cascading standby. Standbys that are more directly connected to the primary are known as upstream servers, while those standby servers further away are downstream servers. Cascading replication does not place limits on the number or arrangement of downstream servers, though each standby connects to only one upstream server which eventually links to a single primary server.
A cascading standby sends not only WAL records received from the primary but also those restored from the archive. So even if the replication connection in some upstream connection is terminated, streaming replication continues downstream for as long as new WAL records are available.
Cascading replication is currently asynchronous. Synchronous replication (see Section 27.2.8 ) settings have no effect on cascading replication at present.
Hot standby feedback propagates upstream, whatever the cascaded arrangement.
If an upstream standby server is promoted to become the new primary, downstream
servers will continue to stream from the new primary if
recovery_target_timeline
is set to
'latest'
(the default).
To use cascading replication, set up the cascading standby so that it can
accept replication connections (that is, set
max_wal_senders
and
hot_standby
,
and configure
host-based authentication
).
You will also need to set
primary_conninfo
in the downstream
standby to point to the cascading standby.
27.2.8. Synchronous Replication
PostgreSQL streaming replication is asynchronous by default. If the primary server crashes then some transactions that were committed may not have been replicated to the standby server, causing data loss. The amount of data loss is proportional to the replication delay at the time of failover.
Synchronous replication offers the ability to confirm that all changes
made by a transaction have been transferred to one or more synchronous
standby servers. This extends that standard level of durability
offered by a transaction commit. This level of protection is referred
to as 2-safe replication in computer science theory, and group-1-safe
(group-safe and 1-safe) when
synchronous_commit
is set to
remote_write
.
When requesting synchronous replication, each commit of a write transaction will wait until confirmation is received that the commit has been written to the write-ahead log on disk of both the primary and standby server. The only possibility that data can be lost is if both the primary and the standby suffer crashes at the same time. This can provide a much higher level of durability, though only if the sysadmin is cautious about the placement and management of the two servers. Waiting for confirmation increases the user's confidence that the changes will not be lost in the event of server crashes but it also necessarily increases the response time for the requesting transaction. The minimum wait time is the round-trip time between primary and standby.
Read-only transactions and transaction rollbacks need not wait for replies from standby servers. Subtransaction commits do not wait for responses from standby servers, only top-level commits. Long running actions such as data loading or index building do not wait until the very final commit message. All two-phase commit actions require commit waits, including both prepare and commit.
A synchronous standby can be a physical replication standby or a logical
replication subscriber. It can also be any other physical or logical WAL
replication stream consumer that knows how to send the appropriate
feedback messages. Besides the built-in physical and logical replication
systems, this includes special programs such
as
pg_receivewal
and
pg_recvlogical
as well as some third-party replication systems and custom programs.
Check the respective documentation for details on synchronous replication
support.
27.2.8.1. Basic Configuration
Once streaming replication has been configured, configuring synchronous
replication requires only one additional configuration step:
synchronous_standby_names
must be set to
a non-empty value.
synchronous_commit
must also be set to
on
, but since this is the default value, typically no change is
required. (See
Section 20.5.1
and
Section 20.6.2
.)
This configuration will cause each commit to wait for
confirmation that the standby has written the commit record to durable
storage.
synchronous_commit
can be set by individual
users, so it can be configured in the configuration file, for particular
users or databases, or dynamically by applications, in order to control
the durability guarantee on a per-transaction basis.
After a commit record has been written to disk on the primary, the
WAL record is then sent to the standby. The standby sends reply
messages each time a new batch of WAL data is written to disk, unless
wal_receiver_status_interval
is set to zero on the standby.
In the case that
synchronous_commit
is set to
remote_apply
, the standby sends reply messages when the commit
record is replayed, making the transaction visible.
If the standby is chosen as a synchronous standby, according to the setting
of
synchronous_standby_names
on the primary, the reply
messages from that standby will be considered along with those from other
synchronous standbys to decide when to release transactions waiting for
confirmation that the commit record has been received. These parameters
allow the administrator to specify which standby servers should be
synchronous standbys. Note that the configuration of synchronous
replication is mainly on the primary. Named standbys must be directly
connected to the primary; the primary knows nothing about downstream
standby servers using cascaded replication.
Setting
synchronous_commit
to
remote_write
will
cause each commit to wait for confirmation that the standby has received
the commit record and written it out to its own operating system, but not
for the data to be flushed to disk on the standby. This
setting provides a weaker guarantee of durability than
on
does: the standby could lose the data in the event of an operating system
crash, though not a
PostgreSQL
crash.
However, it's a useful setting in practice
because it can decrease the response time for the transaction.
Data loss could only occur if both the primary and the standby crash and
the database of the primary gets corrupted at the same time.
Setting
synchronous_commit
to
remote_apply
will
cause each commit to wait until the current synchronous standbys report
that they have replayed the transaction, making it visible to user
queries. In simple cases, this allows for load balancing with causal
consistency.
Users will stop waiting if a fast shutdown is requested. However, as when using asynchronous replication, the server will not fully shutdown until all outstanding WAL records are transferred to the currently connected standby servers.
27.2.8.2. Multiple Synchronous Standbys
Synchronous replication supports one or more synchronous standby servers;
transactions will wait until all the standby servers which are considered
as synchronous confirm receipt of their data. The number of synchronous
standbys that transactions must wait for replies from is specified in
synchronous_standby_names
. This parameter also specifies
a list of standby names and the method (
FIRST
and
ANY
) to choose synchronous standbys from the listed ones.
The method
FIRST
specifies a priority-based synchronous
replication and makes transaction commits wait until their WAL records are
replicated to the requested number of synchronous standbys chosen based on
their priorities. The standbys whose names appear earlier in the list are
given higher priority and will be considered as synchronous. Other standby
servers appearing later in this list represent potential synchronous
standbys. If any of the current synchronous standbys disconnects for
whatever reason, it will be replaced immediately with the
next-highest-priority standby.
An example of
synchronous_standby_names
for
a priority-based multiple synchronous standbys is:
synchronous_standby_names = 'FIRST 2 (s1, s2, s3)'
In this example, if four standby servers
s1
,
s2
,
s3
and
s4
are running, the two standbys
s1
and
s2
will be chosen as synchronous standbys
because their names appear early in the list of standby names.
s3
is a potential synchronous standby and will take over
the role of synchronous standby when either of
s1
or
s2
fails.
s4
is an asynchronous standby since
its name is not in the list.
The method
ANY
specifies a quorum-based synchronous
replication and makes transaction commits wait until their WAL records
are replicated to
at least
the requested number of
synchronous standbys in the list.
An example of
synchronous_standby_names
for
a quorum-based multiple synchronous standbys is:
synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
In this example, if four standby servers
s1
,
s2
,
s3
and
s4
are running, transaction commits will
wait for replies from at least any two standbys of
s1
,
s2
and
s3
.
s4
is an asynchronous
standby since its name is not in the list.
The synchronous states of standby servers can be viewed using
the
pg_stat_replication
view.
27.2.8.3. Planning for Performance
Synchronous replication usually requires carefully planned and placed standby servers to ensure applications perform acceptably. Waiting doesn't utilize system resources, but transaction locks continue to be held until the transfer is confirmed. As a result, incautious use of synchronous replication will reduce performance for database applications because of increased response times and higher contention.
PostgreSQL allows the application developer to specify the durability level required via replication. This can be specified for the system overall, though it can also be specified for specific users or connections, or even individual transactions.
For example, an application workload might consist of: 10% of changes are important customer details, while 90% of changes are less important data that the business can more easily survive if it is lost, such as chat messages between users.
With synchronous replication options specified at the application level (on the primary) we can offer synchronous replication for the most important changes, without slowing down the bulk of the total workload. Application level options are an important and practical tool for allowing the benefits of synchronous replication for high performance applications.
You should consider that the network bandwidth must be higher than the rate of generation of WAL data.
27.2.8.4. Planning for High Availability
synchronous_standby_names
specifies the number and
names of synchronous standbys that transaction commits made when
synchronous_commit
is set to
on
,
remote_apply
or
remote_write
will wait for
responses from. Such transaction commits may never be completed
if any one of synchronous standbys should crash.
The best solution for high availability is to ensure you keep as many
synchronous standbys as requested. This can be achieved by naming multiple
potential synchronous standbys using
synchronous_standby_names
.
In a priority-based synchronous replication, the standbys whose names appear earlier in the list will be used as synchronous standbys. Standbys listed after these will take over the role of synchronous standby if one of current ones should fail.
In a quorum-based synchronous replication, all the standbys appearing in the list will be used as candidates for synchronous standbys. Even if one of them should fail, the other standbys will keep performing the role of candidates of synchronous standby.
When a standby first attaches to the primary, it will not yet be properly
synchronized. This is described as
catchup
mode. Once
the lag between standby and primary reaches zero for the first time
we move to real-time
streaming
state.
The catch-up duration may be long immediately after the standby has
been created. If the standby is shut down, then the catch-up period
will increase according to the length of time the standby has been down.
The standby is only able to become a synchronous standby
once it has reached
streaming
state.
This state can be viewed using
the
pg_stat_replication
view.
If primary restarts while commits are waiting for acknowledgment, those waiting transactions will be marked fully committed once the primary database recovers. There is no way to be certain that all standbys have received all outstanding WAL data at time of the crash of the primary. Some transactions may not show as committed on the standby, even though they show as committed on the primary. The guarantee we offer is that the application will not receive explicit acknowledgment of the successful commit of a transaction until the WAL data is known to be safely received by all the synchronous standbys.
If you really cannot keep as many synchronous standbys as requested
then you should decrease the number of synchronous standbys that
transaction commits must wait for responses from
in
synchronous_standby_names
(or disable it) and
reload the configuration file on the primary server.
If the primary is isolated from remaining standby servers you should fail over to the best candidate of those other remaining standby servers.
If you need to re-create a standby server while transactions are
waiting, make sure that the commands pg_backup_start() and
pg_backup_stop() are run in a session with
synchronous_commit
=
off
, otherwise those
requests will wait forever for the standby to appear.
27.2.9. Continuous Archiving in Standby
When continuous WAL archiving is used in a standby, there are two
different scenarios: the WAL archive can be shared between the primary
and the standby, or the standby can have its own WAL archive. When
the standby has its own WAL archive, set
archive_mode
to
always
, and the standby will call the archive
command for every WAL segment it receives, whether it's by restoring
from the archive or by streaming replication. The shared archive can
be handled similarly, but the
archive_command
or
archive_library
must
test if the file being archived exists already, and if the existing file
has identical contents. This requires more care in the
archive_command
or
archive_library
, as it must
be careful to not overwrite an existing file with different contents,
but return success if the exactly same file is archived twice. And
all that must be done free of race conditions, if two servers attempt
to archive the same file at the same time.
If
archive_mode
is set to
on
, the
archiver is not enabled during recovery or standby mode. If the standby
server is promoted, it will start archiving after the promotion, but
will not archive any WAL or timeline history files that
it did not generate itself. To get a complete
series of WAL files in the archive, you must ensure that all WAL is
archived, before it reaches the standby. This is inherently true with
file-based log shipping, as the standby can only restore files that
are found in the archive, but not if streaming replication is enabled.
When a server is not in recovery mode, there is no difference between
on
and
always
modes.