> ## Documentation Index > Fetch the complete documentation index at: https://planetscale.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Neki terminology > Definitions for the Postgres, PlanetScale, and Neki concepts used throughout the documentation. ## Database technologies ### Postgres Postgres is an open-source relational database. Applications connect to a Neki database using the Postgres wire protocol and standard Postgres clients. ### Neki Neki is PlanetScale's distributed Postgres system. It places routers in front of Postgres shards and provides query routing, topology management, failover, sharding, and online data-movement workflows. ### PlanetScale PlanetScale is a managed database platform focused on reliability, scalability, and developer productivity. PlanetScale operates the infrastructure around Neki and provides the application, APIs, and workflows used to create and manage databases. ## PlanetScale concepts ### Organization An organization contains PlanetScale databases and the people who can access them. Organization settings control membership, billing, and access to shared resources. ### Database A database is the top-level resource an application connects to. It belongs to an organization and contains one or more branches. ### Branch A branch is an isolated database environment. A database has a default branch, often named `main`, and can have additional branches for development and testing. Production branches are intended for production workloads and use a highly available configuration. Development branches are intended for development and testing. ### Configuration profile A configuration profile is a set of infrastructure settings shared by one or more shards. It defines the cluster size, replica count, Postgres version, parameters, and enabled extensions. Changing a profile applies the new settings to every shard assigned to it. ### Cluster size A cluster size defines the CPU, memory, and storage resources assigned to a Postgres instance. Every shard assigned to a configuration profile uses the profile's selected cluster size. ## Neki concepts ### Router A router accepts Postgres connections from applications. It plans each query, uses the data topology to find its destination shards, sends work to those shards, and combines results when a query reaches more than one shard. Routers do not store durable application data and are interchangeable. ### Admin The admin monitors the Postgres instances in a Neki cluster through their sidecars. It analyzes cluster health and coordinates repairs, including replica recovery and primary failover. This control-plane service is different from a database administrator or Postgres role. Its selected size establishes a capacity baseline, and PlanetScale automatically increases its memory when needed. ### Orchestration layer The orchestration layer, implemented by the Neki operator, reconciles the configuration requested through PlanetScale into running routers, Admin services, shards, and Postgres instances. It coordinates rolling changes, resizing, backups, restores, and replacement instances and reports their readiness and progress. It is outside the SQL query path and does not determine data placement. ### Postgres instance A Postgres instance is one Postgres server within a shard. A serving instance can be the primary or a replica. ### Primary The primary is the writable Postgres instance for a shard. Each shard has one primary at a time. ### Replica A replica is a read-only Postgres instance that copies changes from the primary through Postgres physical replication. Replicas provide read capacity and can be promoted during failover. ### Shard A shard is a set of Postgres instances that replicate the same data. It has one primary and zero or more replicas. Provisioning a shard creates database capacity. It does not assign tables or rows to that shard; the [data topology](/docs/neki/data-topology) controls data placement. ### Shard group A shard group is a routing layout that maps key ranges to physical shards. Tables in the same group can keep related rows together when they use the same shard index. A shard group in the data topology is different from a PlanetScale configuration profile. A configuration profile controls infrastructure settings; a shard group controls data placement and query routing. ### Data topology The data topology is Neki's routing map. It defines shard groups, shard indexes, key ranges, table bindings, and the shards that own each range. ### Shard index A shard index turns values from one or more table columns into routing values. Neki compares those routing values with a shard group's key ranges to choose a destination shard. ### Key range A key range assigns part of a shard group's routing space to a physical shard. Together, a shard group's key ranges determine where its rows belong. ### Authoritative shard group The authoritative shard group provides the canonical Postgres object identifiers (OIDs) and schema events for a database. Neki rewrites result sets so clients always see those OIDs. Sequence metadata operations also use this group by default. It contains exactly one key range that spans the entire keyspace. ### Reference table A reference table stores a copy of the same logical rows in multiple shard groups. This allows local reads and joins without routing every lookup back to one owner shard. ### Global secondary index A global secondary index, or GSI, uses a lookup table to map another key to the shard key of an owner row. Neki can read this mapping before routing a query to the owner table. Applications cannot query, write to, or change the lookup table directly through a router. ### Replicator Replicator is the Neki component that copies existing rows and then streams changes from a source to a target. It runs the workflows used to move tables, reshard data, and apply online schema changes. ### Workflow A workflow is the multi-shard unit of a Replicator migration. It tracks an online data movement operation while the database continues serving traffic. ## Need help? Get help from [the PlanetScale Support team](https://planetscale.com/contact?initial=support), or join our [Discord community](https://pscale.link/community) to see how others are using PlanetScale.