Creating and joining PGD groups v5.6

Creating and joining PGD groups

For PGD, every node must connect to every other node. To make configuration easy, when a new node joins, it configures all existing nodes to connect to it. For this reason, every node, including the first PGD node created, must know the PostgreSQL connection string that other nodes can use to connect to it. This connection string is sometimes referred to as a data source name (DSN).

Both formats of connection string are supported. So you can use either key-value format, like host=myhost port=5432 dbname=mydb, or URI format, like postgresql://myhost:5432/mydb.

The SQL function bdr.create_node_group() creates the PGD group from the local node. Doing so activates PGD on that node and allows other nodes to join the PGD group, which consists of only one node at that point. At the time of creation, you must specify the connection string for other nodes to use to connect to this node.

Once the node group is created, every further node can join the PGD group using the bdr.join_node_group() function.

Alternatively, use the command line utility bdr_init_physical to create a new node, using pg_basebackup. If using pg_basebackup, the bdr_init_physical utility can optionally specify the base backup of only the target database. The earlier behavior was to back up the entire database cluster. With this utility, the activity completes faster and also uses less space because it excludes unwanted databases. If you specify only the target database, then the excluded databases get cleaned up and removed on the new node.

When a new PGD node is joined to an existing PGD group or a node subscribes to an upstream peer, before replication can begin the system must copy the existing data from the peer nodes to the local node. This copy must be carefully coordinated so that the local and remote data starts out identical. It's not enough to use pg_dump yourself. The BDR extension provides built-in facilities for making this initial copy.

During the join process, the BDR extension synchronizes existing data using the provided source node as the basis and creates all metadata information needed for establishing itself in the mesh topology in the PGD group. If the connection between the source and the new node disconnects during this initial copy, restart the join process from the beginning.

The node that's joining the cluster must not contain any schema or data that already exists on databases in the PGD group. We recommend that the newly joining database be empty except for the BDR extension. However, it's important that all required database users and roles are created.

Optionally, you can skip the schema synchronization using the synchronize_structure parameter of the bdr.join_node_group function. In this case, the schema must already exist on the newly joining node.

We recommend that you select the source node that has the best connection (logically close, ideally with low latency and high bandwidth) as the source node for joining. Doing so lowers the time needed for the join to finish.

Coordinate the join procedure using the Raft consensus algorithm, which requires most existing nodes to be online and reachable.

The logical join procedure (which uses the bdr.join_node_group function) performs data sync doing COPY operations and uses multiple writers (parallel apply) if those are enabled.

Node join can execute concurrently with other node joins for the majority of the time taken to join. However, only one regular node at a time can be in either of the states PROMOTE or PROMOTING. These states are typically fairly short if all other nodes are up and running. Otherwise the join is serialized at this stage. The subscriber-only nodes are an exception to this rule, and they can be concurrently in PROMOTE and PROMOTING states as well, so their join process is fully concurrent.

The join process uses only one node as the source, so it can be executed when nodes are down if a majority of nodes are available. This approach can cause a complexity when running logical join. During logical join, the commit timestamp of rows copied from the source node is set to the latest commit timestamp on the source node. Committed changes on nodes that have a commit timestamp earlier than this (because nodes are down or have significant lag) can conflict with changes from other nodes. In this case, the newly joined node can be resolved differently to other nodes, causing a divergence. As a result, we recommend not running a node join when significant replication lag exists between nodes. If this is necessary, run LiveCompare on the newly joined node to correct any data divergence once all nodes are available and caught up.

pg_dump can fail when there's concurrent DDL activity on the source node because of cache-lookup failures. Since bdr.join_node_group uses pg_dump internally, it might fail if there's concurrent DDL activity on the source node. Retrying the join works in that case.

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