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Scaling TimescaleDB

TimescaleDB supports three main deployment options: as an single database server, in a tradition primary/replica deployment, or in a multi-node deployment with horizontal scalability.

Single instance (node)

A single instance of PostgreSQL with TimescaleDB installed can often support the needs of very large datasets and application querying. In a regular PostgreSQL instance without TimescaleDB, a common problem with scaling database performance on a single machine is the significant cost/performance trade-off between memory and disk. Eventually, our entire dataset will not fit in memory, and you will need to write your data and indexes to disk.

Once the data is sufficiently large that we can’t fit all pages of our indexes (e.g., B-trees) in memory, then updating a random part of the tree can involve swapping in data from disk. And databases like PostgreSQL keep a B-tree (or other data structure) for each table index, in order for values in that index to be found efficiently. So, the problem compounds as you index more columns.

But because each of the chunks created by TimescaleDB is itself stored as a separate database table, all of its indexes are built only across these much smaller tables rather than a single table representing the entire dataset. So if we size these chunks properly, we can fit the latest tables (and their B-trees) completely in memory, and avoid this swap-to-disk problem, while maintaining support for multiple indexes.

For more on the motivation and design of TimescaleDB, please see our technical blog post.

Primary and backup replication

TimescaleDB supports streaming replication from a "primary" database server to separate "replica" servers, using the standard PostgreSQL physical replication protocol. The protocol works by streaming records of database modifications from the primary server to one or more replicas, which can then be used as read-only nodes (to scale queries) or as failover servers (for high availability).

PostgreSQL streaming replication leverages the Write Ahead Log (WAL), which is an append-only series of instructions that captures every atomic database change. The replication works by continuously shipping segments of the WAL from the primary to any connected replicas. Each replica then applies the WAL changes and makes them available for querying, ensuring that operations to the primary are applied atomically and in an identical order to each replica.

TimescaleDB (from underlying PostgreSQL functionality) supports various options for physical replication that trade off performance and consistency.

Replication can occur synchronously. When an insert or schema modification operation is executed on the primary, the operation is replicated (or even applied) to replicas before the primary returns any result. This ensures that replicas always stay perfectly up-to-date.

Replication can also occur asynchronously. Operations to the primary return to a client as soon as they are executed, but any changes are queued for asynchronous transmission to any replicas. Asynchronous replicas are often used for ad-hoc data exploration, to avoid heavy query load on replicas from interfering with a production primary server.

In fact, a single TimescaleDB primary can have both synchronous and asynchronous replicas, for a mix of HA failover and read scaling. The main limitation of primary/backup replication is that each server stores a full copy of the database.

Multi-node TimescaleDB and distributed hypertables

TimescaleDB 2.0 also supports horizontally scaling across many servers. Instead of a primary node (and each replica) which stores the full copy of the data, a distributed hypertable can be spread across multiple nodes, such that each node only stores a portion of the distributed hypertable (namely, a subset of the chunks). This allows TimescaleDB to scale storage, inserts, and queries beyond the capabilities of a single TimescaleDB instance. Distributed hypertables and regular hypertables look very similar, with the main difference being that distributed chunks are not stored locally (and some other current limitations).

In a multi-node deployment of TimescaleDB, a database can assume the role of either an access node or a data node; both run the identical TimescaleDB software for operational simplicity.

A client connects to an access node database. The client can create a distributed hypertable on the access node, which stores cluster-wide information about the different data nodes as well as how chunks belonging to distributed hypertables are spread across those data nodes. Access nodes can also store non-distributed hypertables, as well as regular PostgreSQL tables.

Data nodes do not store cluster-wide information, and otherwise look just as if they were stand-alone TimescaleDB instances.

Clients interact with the distributed hypertable all through the access node, performing schema operations, inserting data, or querying the data as if it's a standard table. When receiving a query, the access node uses local information about chunks to determine to which data nodes to push down queries. Query optimizations across the cluster attempt to minimize data transferred between data nodes and the access node, such that aggregates are performed on data nodes whenever possible, and only aggregated or filtered results are passed back to the access node for merging and returning to a client.

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