Choosing your architecture v5

Always On architectures reflect EDB’s Trusted Postgres architectures. They encapsulate practices and help you to achieve the highest possible service availability in multiple configurations. These configurations range from single-location architectures to complex distributed systems that protect from hardware failures and data center failures. The architectures leverage EDB Postgres Distributed’s multi-master capability and its ability to achieve 99.999% availability, even during maintenance operations.

You can use EDB Postgres Distributed for architectures beyond the examples described here. Use-case-specific variations have been successfully deployed in production. However, these variations must undergo rigorous architecture review first. Also, EDB’s standard deployment tool for Always On architectures, you must enable Trusted Postgres Architect (TPA) to support the variations before they can be supported in production environments.

Standard EDB Always On architectures

EDB has identified a set of standardized architectures to support single- or multi-location deployments with varying levels of redundancy, depending on your recovery point objective (RPO) and recovery time objective (RTO) requirements.

The Always ON architecture uses three database node groups as a basic building block. You can also use a five-node group for extra redundancy.

EDB Postgres Distributed consists of the following major building blocks:

• Bi-Directional Replication (BDR) — A Postgres extension that creates the multi-master mesh network
• PGD Proxy — A connection router that makes sure the application is connected to the right data nodes.

All Always On architectures protect an increasing range of failure situations. For example, a single active location with two data nodes protects against local hardware failure but doesn't provide protection from location (data center or availability zone) failure. Extending that architecture with a backup at a different location ensures some protection in case of the catastrophic loss of a location. However, you still must restore the database from backup first, which might violate RTO requirements. Adding a second active location connected in a multi-master mesh network ensures that service remains available even if a location goes offline. Finally, adding a third location (this can be a witness-only location) allows global Raft functionality to work even if one location goes offline. The global Raft is primarily needed to run administrative commands. Also, some features like DDL or sequence allocation might not work without it, while DML replication can continue to work even without global Raft.

Each architecture can provide zero RPO, as data can be streamed synchronously to at least one local master, guaranteeing zero data loss in case of local hardware failure.

Increasing the availability guarantee always drives added cost for hardware and licenses, networking requirements, and operational complexity. It's important to carefully consider the availability and compliance requirements before choosing an architecture.

Architecture details

By default, application transactions don't require cluster-wide consensus for DML (selects, inserts, updates, and deletes), allowing for lower latency and better performance. However, for certain operations, such as generating new global sequences or performing distributed DDL, EDB Postgres Distributed requires an odd number of nodes to make decisions using a [Raft] (https://raft.github.io)-based consensus model. Thus, even the simpler architectures always have three nodes, even if not all of them are storing data.

Applications connect to the standard Always On architectures by way of multi-host connection strings, where each PGD Proxy server is a distinct entry in the multi-host connection string. You must always have at least two proxy nodes in each location to ensure high availability. You can colocate the proxy with the database instance, in which case we recommend putting the proxy on every data node.

Other connection mechanisms have been successfully deployed in production. However, they aren't part of the standard Always On architectures.

Always On Single Location

• Additional replication between data nodes 1 and 3 isn't shown but occurs as part of the replication mesh
• Redundant hardware to quickly restore from local failures
  • 3 PGD nodes
    • Can be 3 data nodes (recommended)
    • Can be 2 data nodes and 1 witness that doesn't hold data (not depicted)
  • A PGD Proxy for each data node with affinity to the applications
    • Can be colocated with data node
• Barman for backup and recovery (not depicted)
  • Offsite is optional but recommended
  • Can be shared by multiple clusters
• Postgres Enterprise Manager (PEM) for monitoring (not depicted)
  • Can be shared by multiple clusters

Always On multi-location

• Application can be Active/Active in each location or can be Active/Passive or Active DR with only one location taking writes.
• Additional replication between data nodes 1 and 3 isn't shown but occurs as part of the replication mesh.
• Redundant hardware to quickly restore from local failures.
  • 6 PGD nodes total, 3 in each location
    • Can be 3 data nodes (recommended)
    • Can be 2 data nodes and 1 witness which does not hold data (not depicted)
  • A PGD-Proxy for each data node with affinity to the applications
    • can be co-located with data node
• Barman for backup and recovery (not depicted).
  • Can be shared by multiple clusters
• Postgres Enterprise Manager (PEM) for monitoring (not depicted).
  • Can be shared by multiple clusters
• An optional witness node must be placed in a third region to increase tolerance for location failure.
  • Otherwise, when a location fails, actions requiring global consensus are blocked, such as adding new nodes and distributed DDL.

Choosing your architecture

All architectures provide the following:

• Hardware failure protection
• Zero downtime upgrades
• Support for availability zones in public/private cloud

Use these criteria to help you to select the appropriate Always On architecture.

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• Architecture details
• Choosing your architecture