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https://github.com/lilactown/pyramid

A library for storing and querying graph data in Clojure
https://github.com/lilactown/pyramid

clojure clojurescript eql graph-database memory-database pathom

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A library for storing and querying graph data in Clojure

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# pyramid



A library for storing and querying graph data in Clojure.



Features:

* Graph query engine that works on any in-memory data store

* Algorithm for taking trees of data and storing them in normal form in Clojure

  data.



## Install & docs



[![Clojars Project](https://img.shields.io/clojars/v/town.lilac/pyramid.svg)](https://clojars.org/town.lilac/pyramid) [![cljdoc badge](https://cljdoc.org/badge/town.lilac/pyramid)](https://cljdoc.org/d/town.lilac/pyramid/CURRENT)



## Why



Clojure is well known for its graph databases like datomic and datascript which

implement a datalog-like query language. There are contexts where the power vs

performance tradeoffs of these query languages don't make sense, which is where

pyramid can shine.



Pyramid focuses on doing essential things like selecting data from and

traversing relationships between entities, while eschewing arbitrary logic like

what SQL and datalog provide. What it lacks in features it makes up for in read

performance when combined with a data store that has fast in-memory look ups of

entities by key, such as Clojure maps. It can also be extended to databases like

Datomic, DataScript and Asami.



## What



Pyramid can be useful at each evolutionary stage of a program where one needs

to traverse and make selections out of graphs of data.



### Selection



Pyramid starts by working with Clojure data structures. A simple program that

uses pyramid can use a query to select specific data out of a large, deeply

nested tree, like a supercharged `select-keys`.



```clojure

(def data

  {:people [{:given-name "Bob" :surname "Smith" :age 29}

            {:given-name "Alice" :surname "Meyer" :age 43}]

   :items {}})



(def query [{:people [:given-name]}])



(pyramid.core/pull data query)

;; => {:people [{:given-name "Bob"} {:given-name "Alice"}]}

```



### Transformation



Pyramid combines querying with the [Visitor pattern](https://en.wikipedia.org/wiki/Visitor_pattern)

in a powerful way, allowing one to easily perform transformations of selections

of data. Simply annotate parts of your query with metadata

`{:visitor (fn visit [data selection] ,,,)}` and the `visit` function will be

used to transform the data in a depth-first, post-order traversal (just like

`clojure.walk/postwalk`).



```clojure

(def data

  {:people [{:given-name "Bob" :surname "Smith" :age 29}

            {:given-name "Alice" :surname "Meyer" :age 43}]

   :items {}})



(defn fullname

  [{:keys [given-name surname] :as person}]

  (str given-name " " surname))



(def query [{:people ^{:visitor fullname} [:given-name :surname]}])



(pyramid.core/pull data query)

;; => {:people ["Bob Smith" "Alice Meyer"]}

 ```



### Accretion



A more complex program may need to keep track of that data over time, or query

data that contains cycles, which can be done by creating a `pyramid.core/db`.

"Pyramid dbs" are maps that have a particular structure:



```clojure

;; for any entity identified by `[key id]`, it follows the shape:

{key {id {,,,}}

```



Adding data to a db will [normalize](https://en.wikipedia.org/wiki/Database_normalization)

the data into a flat structure allowing for easy updating of entities as new

data is obtained and allow relationships that are hard to represent in trees.

Queries can traverse the references inside this data.



See [docs/GUIDE.md](docs/GUIDE.md).



### Durability



A program may grow to need durable storage and other features that more full

featured in-memory databases provide. Pyramid provides a protocol, `IPullable`,

which can be extended to allow queries to run over any store that data can be

looked up by a tuple, `[primary-key value]`. This is generalizable to most

databases like Datomic, DataScript, Asami and SQLite.



### Full stack



The above shows the evolution of a single program, but many programs never grow

beyond the accretion stage. Pyramid has been used primarily in user interfaces

where data is stored in a data structure and queried over time to show different

views on a large graph. Through its protocols, it can now be extended to be used

with durable storage on the server as well.



## Concepts



**Query**: A query is written using [EQL](https://edn-query-language.org/eql/1.0.0/what-is-eql.html),

a query language implemented inside Clojure. It provides the ability to select

data in a nested, recursive way, making it ideal for traversing graphs of data.

It does not provide arbitrary logic like SQL or Datalog.



**Entity map**: a Clojure map which contains information that uniquely identifies

the domain entity it is about. E.g. `{:person/id 1234 :person/name "Bill"

:person/age 67}` could be uniquely identified by it's `:person/id` key. By 

default, any map which contains a key which `(= "id" (name key))` is true, is an

entity map and can be normalized using `pyramid.core/db`.



**Ident function**: a function that takes a map and returns a tuple `[:key val]`

that uniquely identifies the entity the map describes.



**Lookup ref**: a 2-element Clojure vector that has a keyword and a value which

together act as a pointer to a domain entity. E.g. `[:person/id 1234]`.

`pyramid.core/pull` will attempt to look them up in the db value if they appear

in the result at a location where the query expects to do a join.



## Usage



See [docs/GUIDE.md](docs/GUIDE.md)



## Prior art



- [Fulcro](https://fulcro.fulcrologic.com/)

- @souenzzo's POC [eql-refdb](https://github.com/souenzzo/eql-refdb)

- [MapGraph](https://github.com/stuartsierra/mapgraph/blob/master/test/com/stuartsierra/mapgraph/compare.clj)

- [EntityDB](https://keechma.com/guides/entitydb/)

- [DataScript](https://github.com/tonsky/datascript/) and derivatives

- [Pathom](https://github.com/wilkerlucio/pathom)

- [juxt/pull](https://github.com/juxt/pull)

- [ribelo/doxa](https://github.com/ribelo/doxa)



## Copyright



Copyright © 2023 Will Acton. Distributed under the EPL 2.0.