# Chapter 1. Introduction

This chapter provides an introduction to the available graph algorithms, and instructions for installation and use.

This library provides efficiently implemented, parallel versions of common graph algorithms for Neo4j 3.x, exposed as Cypher procedures.

## 1.1. Graph Algorithms Book

Amy Hodler and Mark Needham recently finished writing the O’Reilly Graph Algorithms Book. For a limited time only, you can download your free copy from neo4j.com/graph-algorithms-book

## 1.2. Algorithms

Graph algorithms are used to compute metrics for graphs, nodes, or relationships.

They can provide insights on relevant entities in the graph (centralities, ranking), or inherent structures like communities (community-detection, graph-partitioning, clustering).

Many graph algorithms are iterative approaches that frequently traverse the graph for the computation using random walks, breadth-first or depth-first searches, or pattern matching.

Due to the exponential growth of possible paths with increasing distance, many of the approaches also have high algorithmic complexity.

Fortunately, optimized algorithms exist that utilize certain structures of the graph, memoize already explored parts, and parallelize operations. Whenever possible, we’ve applied these optimizations.

### 1.2.1. Centrality algorithms

The following centrality algorithms determine the importance of distinct nodes in a network:

### 1.2.2. Community detection algorithms

The following community detection algorithms evaluate how a group is clustered or partitioned, as well as its tendency to strengthen or break apart:

### 1.2.3. Path finding algorithms

The following path finding algorithms help find the shortest path or evaluate the availability and quality of routes:

### 1.2.4. Similarity algorithms

These algorithms help calculate the similarity of nodes:

### 1.2.6. Preprocessing functions and procedures

The following feature and procedure can be used as part of the data preparation process:

## 1.3. Installation

### 1.3.1. Neo4j Desktop

If we are using the Neo4j Desktop, the library can be installed from the 'Plugins' tab of a database.

The installer will download a copy of the graph algorithms library and place it in the 'plugins' directory of the database. It will also add the following entry to the settings file:

``dbms.security.procedures.unrestricted=algo.*``

### 1.3.2. Neo4j Server

If we are using a standalone Neo4j Server, the library will need to be installed and configured manually.

1. Download `graph-algorithms-algo-[version].jar` from the matching release and copy it into the `\$NEO4J_HOME/plugins` directory. We can work out which release to download by referring to the versions file.
2. Add the following to your `\$NEO4J_HOME/conf/neo4j.conf` file:

``dbms.security.procedures.unrestricted=algo.*``

We need to give the library unrestricted access because the algorithms use the lower level Kernel API to read from, and to write to Neo4j.

3. Restart Neo4j

### 1.3.3. Verifying installation

Once we’ve installed the library, to see a list of all the algorithms, run the following query:

``CALL algo.list()``

## 1.4. Usage

These algorithms are exposed as Neo4j procedures. They can be called directly from Cypher in your Neo4j Browser, from cypher-shell, or from your client code.

For most algorithms there are two procedures:

• `algo.<name>` - this procedure writes results back to the graph as node-properties, and reports statistics.
• `algo.<name>.stream` - this procedure returns a stream of data. For example, node-ids and computed values.

For large graphs, the streaming procedure might return millions, or even billions of results. In this case it may be more convenient to store the results of the algorithm, and then use them with later queries.