Backup and recovery
===================
PGD is designed to be a distributed, highly available system. If one or
more nodes of a cluster are lost, the best way to replace them is to
clone new nodes directly from the remaining nodes.
The role of backup and recovery in PGD is to provide for disaster
recovery (DR), such as in the following situations:
- Loss of all nodes in the cluster
- Significant, uncorrectable data corruption across multiple nodes as a
result of data corruption, application error, or security breach
Backup
------
pg_dump
^^^^^^^
You can use pg_dump, sometimes referred to as *logical backup*, normally
with PGD.
pg_dump dumps both local and global sequences as if they were local
sequences. This behavior is intentional, to allow a PGD schema to be
dumped and ported to other PostgreSQL databases. This means that
sequence-kind metadata is lost at the time of dump, so a restore
effectively resets all sequence kinds to the value of
``bdr.default_sequence_kind`` at time of restore.
To create a post-restore script to reset the precise sequence kind for
each sequence, you might want to use a SQL script like this:
.. code:: sql
SELECT SELECT bdr.alter_sequence_set_kind(||
nspname||.||relname||,||seqkind||);
FROM bdr.sequences
WHERE seqkind != local;
If you run pg_dump using ``bdr.crdt_raw_value = on`` , then you can
reload the dump only with ``bdr.crdt_raw_value = on`` .
Technical Support recommends the use of physical backup techniques for
backup and recovery of PGD.
Physical backup
^^^^^^^^^^^^^^^
You can take physical backups of a node in an EDB Postgres Distributed
cluster using standard PostgreSQL software, such as `Barman `_ .
You can perform a physical backup of a PGD node using the same procedure
that applies to any PostgreSQL node. A PGD node is just a PostgreSQL
node running the BDR extension.
Consider these specific points when applying PostgreSQL backup
techniques to PGD:
- PGD operates at the level of a single database, while a physical
backup includes all the databases in the instance. Plan your databases
to allow them to be easily backed up and restored.
- Backups make a copy of just one node. In the simplest case, every node
has a copy of all data, so you need to back up only one node to
capture all data. However, the goal of PGD isn’t met if the site
containing that single copy goes down, so the minimum is at least one
node backup per site (with many copies, and so on).
- However, each node might have unreplicated local data, or the
definition of replication sets might be complex so that all nodes
don’t subscribe to all replication sets. In these cases, backup
planning must also include plans for how to back up any unreplicated
local data and a backup of at least one node that subscribes to each
replication set.
Eventual consistency
^^^^^^^^^^^^^^^^^^^^
The nodes in an EDB Postgres Distributed cluster are *eventually
consistent* but not *entirely consistent*. A physical backup of a given
node provides point-in-time recovery capabilities limited to the states
actually assumed by that node.
The following example shows how two nodes in the same EDB Postgres
Distributed cluster might not (and usually don’t) go through the same
sequence of states.
Consider a cluster with two nodes, ``N1`` and ``N2`` , that’s initially
in state ``S`` . If transaction ``W1`` is applied to node ``N1`` , and
at the same time a non-conflicting transaction ``W2`` is applied to node
``N2`` , then node ``N1`` goes through the following states:
::
(N1) S --> S + W1 --> S + W1 + W2
Node ``N2`` goes through the following states:
::
(N2) S --> S + W2 --> S + W1 + W2
That is, node ``N1`` *never* assumes state ``S + W2`` , and node ``N2``
likewise never assumes state ``S + W1`` . However, both nodes end up in
the same state ``S + W1 + W2`` . Considering this situation might affect
how you decide on your backup strategy.
Point-in-time recovery (PITR)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The previous example showed that the changes are also inconsistent in
time. ``W1`` and ``W2`` both occur at time ``T1`` , but the change
``W1`` isn’t applied to ``N2`` until ``T2`` .
PostgreSQL PITR is designed around the assumption of changes arriving
from a single master in COMMIT order. Thus, PITR is possible by scanning
through changes until one particular point in time (PIT) is reached.
With this scheme, you can restore one node to a single PIT from its
viewpoint, for example, ``T1`` . However, that state doesn’t include
other data from other nodes that committed near that time but had not
yet arrived on the node. As a result, the recovery might be considered
to be partially inconsistent, or at least consistent for only one
replication origin.
With PostgreSQL PITR, you can use the standard syntax:
::
recovery_target_time = T1
PGD allows for changes from multiple masters, all recorded in the WAL
log for one node, separately identified using replication origin
identifiers.
PGD allows PITR of all or some replication origins to a specific point
in time, providing a fully consistent viewpoint across all subsets of
nodes.
Thus for multi-origins, you can view the WAL stream as containing
multiple streams all mixed up into one larger stream. There’s still just
one PIT, but that’s reached as different points for each origin
separately.
The WAL stream is read until requested origins have found their PIT. All
changes are applied up until that point, except that any transaction
records aren’t marked as committed for an origin after the PIT on that
origin is reached.
You end up with one LSN “stopping point” in WAL, but you also have one
single timestamp applied consistently, just as you do with single-origin
PITR.
Once you reach the defined PIT, a later one might also be set to allow
the recovery to continue, as needed.
After the desired stopping point is reached, if the recovered server
will be promoted, shut it down first. Move the LSN forward to an LSN
value higher than used on any timeline on this server using
``pg_resetwal`` . This approach ensures that there are no duplicate LSNs
produced by logical decoding.
In the specific example shown, ``N1`` is restored to ``T1`` . It also
includes changes from other nodes that were committed by ``T1`` , even
though they weren’t applied on ``N1`` until later.
To request multi-origin PITR, use the standard syntax in the
``postgresql.conf`` file:
::
recovery_target_time = T1
You need to specify the list of replication origins that are restored to
``T1`` in one of two ways. You can use a separate
``multi_recovery.conf`` file by way of a new parameter,
``recovery_target_origins`` :
::
recovery_target_origins = *
Or you can specify the origin subset as a list in
``recovery_target_origins`` :
::
recovery_target_origins = 1,3
The local WAL activity recovery to the specified
``recovery_target_time`` is always performed implicitly. For origins
that aren’t specified in ``recovery_target_origins`` , recovery can stop
at any point, depending on when the target for the list mentioned in
``recovery_target_origins`` is achieved.
In the absence of the ``multi_recovery.conf`` file, the recovery
defaults to the original PostgreSQL PITR behavior that’s designed around
the assumption of changes arriving from a single master in COMMIT order.
.. Note::
This feature is available only with EDB Postgres Extended. Barman doesn't create a `multi_recovery.conf` file.
Restore
-------
While you can take a physical backup with the same procedure as a
standard PostgreSQL node, it’s slightly more complex to restore the
physical backup of a PGD node.
EDB Postgres Distributed cluster failure or seeding a new cluster from a backup
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The most common use case for restoring a physical backup involves the
failure or replacement of all the PGD nodes in a cluster, for instance
in the event of a data center failure.
You might also want to perform this procedure to clone the current
contents of a EDB Postgres Distributed cluster to seed a QA or
development instance.
In that case, you can restore PGD capabilities based on a physical
backup of a single PGD node, optionally plus WAL archives:
- If you still have some PGD nodes live and running, fence off the host
you restored the PGD node to, so it can’t connect to any surviving PGD
nodes. This practice ensures that the new node doesn’t confuse the
existing cluster.
- Restore a single PostgreSQL node from a physical backup of one of the
PGD nodes.
- If you have WAL archives associated with the backup, create a suitable
``postgresql.conf`` , and start PostgreSQL in recovery to replay up to
the latest state. You can specify an alternative ``recovery_target``
here if needed.
- Start the restored node, or promote it to read/write if it was in
standby recovery. Keep it fenced from any surviving nodes!
- Clean up any leftover PGD metadata that was included in the physical
backup.
- Fully stop and restart the PostgreSQL instance.
- Add further PGD nodes with the standard procedure based on the
``bdr.join_node_group()`` function call.
Cleanup of PGD metadata
^^^^^^^^^^^^^^^^^^^^^^^
To clean up leftover PGD metadata:
1. Drop the PGD node using `bdr.drop_node `_ .
2. Fully stop and restart PostgreSQL (important!).
Cleanup of replication origins
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You must explicitly remove replication origins with a separate step
because they’re recorded persistently in a system catalog. They’re
therefore included in the backup and in the restored instance. They
aren’t removed automatically when dropping the BDR extension because
they aren’t explicitly recorded as its dependencies.
To track progress of incoming replication in a crash-safe way, PGD
creates one replication origin for each remote master node. Therefore,
for each node in the previous cluster run this once:
::
SELECT pg_replication_origin_drop(bdr_dbname_grpname_nodename);
You can list replication origins as follows:
::
SELECT * FROM pg_replication_origin;
Those created by PGD are easily recognized by their name.
Cleanup of replication slots
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If a physical backup was created with ``pg_basebackup`` , replication
slots are omitted from the backup.
Some other backup methods might preserve replications slots, likely in
outdated or invalid states. Once you restore the backup, use these
commands to drop all replication slots:
::
SELECT pg_drop_replication_slot(slot_name)
FROM pg_replication_slots;
If you have a reason to preserve some slots, you can add a
``WHERE slot_name LIKE 'bdr%'`` clause, but this is rarely useful.
.. Warning::
Never use these commands to drop replication slots on a live PGD node