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Initial BDR failover documentation
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Ian Barwick
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doc/bdr-failover.md
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doc/bdr-failover.md
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BDR failover with repmgrd
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=========================
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`repmgr 4` provides support for monitoring BDR nodes and taking action in case
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one of the nodes fails.
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*NOTE* Due to the nature of BDR, it's only safe to use this solution for
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a two-node scenario. Introducing additional nodes will create an inherent
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risk of node desynchronisation if a node goes down without being cleanly
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removed from the cluster.
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In contrast to streaming replication, there's no concept of "promoting" a new
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primary node with BDR. Instead, "failover" involves monitoring both nodes
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with `repmgrd` and redirecting queries from the failed node to the remaining
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active node. This can be done by using the event notification script generated by
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`repmgrd` to dynamically reconfigure a proxy server/connection pooler such
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as PgBouncer.
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Prerequisites
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-------------
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`repmgr 4` requires PostgreSQL 9.6 with the BDR 2 extension enabled and
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configured for a two-node BDR network. `repmgr 4` packages
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must be installed on each node before attempting to configure repmgr.
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*NOTE* `repmgr 4` will refuse to install if it detects more than two
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BDR nodes.
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Application database connections *must* be passed through a proxy server/
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connection pooler such as PgBouncer, and it must be possible to dynamically
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reconfigure that from `repmgrd`. The example demonstrated in this document
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will use PgBouncer.
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The proxy server / connection poolers must not be installed on the database
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servers.
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Configuration
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-------------
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Sample configuration for `repmgr.conf`:
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node_id=1
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node_name='node1'
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conninfo='host=node1 dbname=bdrtest user=repmgr connect_timeout=2'
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replication_type='bdr'
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event_notifications=bdr_failover
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event_notification_command='/path/to/bdr-pgbouncer.sh %n %e %s "%c" "%a" >> /tmp/bdr-failover.log 2>&1'
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# repmgrd options
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reconnect_attempts=5
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reconnect_interval=6
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Adjust settings as appropriate; copy and adjust for the second node (particularly
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the values `node_id`, `node_name` and `conninfo`).
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Note that the values provided for the `conninfo` string must be valid for
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connections from *both* nodes in the cluster.
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If defined, `event_notifications` will restrict execution of `event_notification_command`
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to the specified events.
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`event_notification_command` is the script which does the actual "heavy lifting"
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of reconfiguring the proxy server/ connection pooler. It is fully user-definable;
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a sample implementation is documented below.
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repmgr setup
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------------
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Register both nodes:
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$ repmgr -f /etc/repmgr.conf bdr register
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NOTICE: attempting to install extension "repmgr"
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NOTICE: "repmgr" extension successfully installed
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NOTICE: node record created for node 'node1' (ID: 1)
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NOTICE: BDR node 1 registered (conninfo: host=localhost dbname=bdrtest user=repmgr port=5501)
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$ repmgr -f /etc/repmgr.conf bdr register
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NOTICE: node record created for node 'node2' (ID: 2)
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NOTICE: BDR node 2 registered (conninfo: host=localhost dbname=bdrtest user=repmgr port=5502)
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The `repmgr` extension will be automatically created when the first
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node is registered, and will be propagated to the second node.
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*IMPORTANT* ensure the repmgr package is available on both nodes before
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attempting to register the first node
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At this point the meta data for both nodes has been created; executing
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`repmgr cluster show` (on either node) should produce output like this:
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$ repmgr -f /etc/repmgr.conf cluster show
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ID | Name | Role | Status | Upstream | Connection string
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----+-------+------+-----------+----------+-----------------------------------------------------
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1 | node1 | bdr | * running | | host=node1 dbname=bdrtest user=repmgr
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2 | node2 | bdr | * running | | host=node2 dbname=bdrtest user=repmgr
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Additionally it's possible to see a log of significant events; so far
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this will only record the two node registrations (in reverse chronological order):
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Node ID | Event | OK | Timestamp | Details
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---------+--------------+----+---------------------+----------------------------------------------
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2 | bdr_register | t | 2017-07-27 17:51:48 | node record created for node 'node2' (ID: 2)
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1 | bdr_register | t | 2017-07-27 17:51:00 | node record created for node 'node1' (ID: 1)
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Defining the "event_notification_command"
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-----------------------------------------
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Key to "failover" execution is the `event_notification_command`, which is a
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user-definable script which should reconfigure the proxy server/
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connection pooler.
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Each time `repmgr` (or `repmgrd`) records an event, it can optionally
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execute the script defined in `event_notification_command` to
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take further action; details of the event will be passed as parameters.
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Following placeholders are available to the script:
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%n - node ID
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%e - event type
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%s - success (1 or 0)
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%t - timestamp
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%d - details
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%c - conninfo string of the next available node
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%a - name of the next available node
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Note that `%c` and `%a` will only be provided during `bdr_failover`
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events, which is what is of interest here.
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The provided sample script (`scripts/bdr-pgbouncer.sh`) is configured like
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this:
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event_notification_command='/path/to/bdr-pgbouncer.sh %n %e %s "%c" "%a"'
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and parses the configures parameters like this:
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NODE_ID=$1
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EVENT_TYPE=$2
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SUCCESS=$3
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NEXT_CONNINFO=$4
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NEXT_NODE_NAME=$5
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It also contains some hard-coded values about the PgBouncer configuration for
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both nodes; these will need to be adjusted for your local environment of course
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(ideally the scripts would be maintained as templates and generated by some
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kind of provisioning system).
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repmgrd
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-------
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Node failover
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-------------
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Node recovery
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-------------
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Following failure of a BDR node, if the node subsequently becomes available again,
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a `bdr_recovery` event will be generated. This could potentially be used to
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reconfigure PgBouncer automatically to bring the node back into the available pool,
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however it would be prudent to manually verify the node's status before
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exposing it to the application.
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