Overview

Module khepri_projection

• Description
• Data Types
• Function Index
• Function Details

Description

Khepri projections

Projections build a replicated ETS table using tree nodes from the store which match a khepri_path:pattern(). When a tree node matching a projection's pattern is changed in the store, the tree node is passed through the projection's projection_fun() to create record(s). These records are then stored in the projection's ETS table for all members in a Khepri cluster.

Projections provide a way to query the store as fast as possible and are appropriate for lookups which require low latency and/or high throughput. Projection tables contain all records matching the pattern, though, so the memory footprint of a projection table grows with the number of tree nodes in the store matching the pattern.

Projection ETS tables are owned by the Khepri cluster and are deleted when the cluster stops.

Data Types

extended_projection_fun()

extended_projection_fun() = fun((Table::ets:tid(), Path::khepri_path:native_path(), OldPayload::khepri:node_props(), NewPayload::khepri:node_props()) -> any())

An extended projection function.

In some cases, a tree node in the store might correspond to many objects in a projection table. Extended projection functions are allowed to call ETS functions directly in order to build the projection table.

OldPayload or NewPayload are empty maps if there is no tree node. For example, a newly created tree node will have an empty map for OldPayload and a khepri:node_props() map with values for NewPayload.

This function is compiled like a transaction function except that calls to the ets module are allowed.

name()

name() = atom()

The name of a projection.

options()

options() = #{type => ets:table_type(), keypos => pos_integer(), read_concurrency => boolean(), write_concurrency => boolean() | auto, compressed => boolean(), standalone_fun_options => horus:options()}

Options which control the created ETS table.

If provided, standalone_fun_options are merged with defaults and passed to horus:to_standalone_fun/2. The remaining options are a subset of the options available to ets:new/2. Refer to the ets:new/2 documentation for a reference on each type and available values.

projection()

projection() = #khepri_projection{name = atom(), projection_fun = copy | horus:horus_fun(), ets_options = [atom() | tuple()]}

A projection resource.

projection_fun()

projection_fun() = copy | simple_projection_fun() | extended_projection_fun()

A function that formats an entry in the tree into a record to be stored in a projection.

• "simple" - a function that takes the path within the tree for a tree node and the payload and returns a record. See simple_projection_fun().
• "copy" - a projection that ignores the path and use the tree node's payload as the record directly. This is a special case that avoids the overhead of executing standalone functions for the sake of performance.
• "extended" - a function that takes the ETS table identifier, path, and old and new tree node properties and executes ETS functions directly. See extended_projection_fun().

simple_projection_fun()

simple_projection_fun() = fun((Path::khepri_path:native_path(), Payload::khepri:data()) -> Record::tuple())

A simple projection function.

Simple projection functions only take the path and payload for a tree node in the store. The record produced by the function is used to create and delete objects in the ETS table.

Function Index

Function Details

new/2

See also: khepri:register_projection/4, khepri_projection:new/3, khepri_projection:new/3.

new/3

Creates a new projection data structure.

name/1

Returns the name of the given Projection.

Overview

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