|
| 1 | +--- |
| 2 | +title: "BFS" |
| 3 | +description: "Breadth-First Search (BFS) explores a graph level by level, visiting all neighbors of a node before moving to the next depth." |
| 4 | +parent: "Algorithms" |
| 5 | +--- |
| 6 | + |
| 7 | +# BFS |
| 8 | + |
| 9 | +## Overview |
| 10 | + |
| 11 | +The Breadth-First Search (BFS) procedure allows you to perform a breadth-first traversal of a graph starting from a specific node. |
| 12 | +BFS explores all the nodes at the present depth before moving on to nodes at the next depth level. |
| 13 | +This is particularly useful for finding the shortest path between two nodes or exploring a graph layer by layer. |
| 14 | + |
| 15 | +## Syntax |
| 16 | + |
| 17 | +``` |
| 18 | +CALL algo.bfs(start_node, max_depth, relationship) |
| 19 | +YIELD nodes, edges |
| 20 | +``` |
| 21 | + |
| 22 | +## Arguments |
| 23 | + |
| 24 | +| Name | Type | Description | Default | |
| 25 | +|--------------|----------------|-----------------------------------------------------------------------------|------------| |
| 26 | +| start_node | Node | Starting node for the BFS traversal | (Required) | |
| 27 | +| max_depth | Integer | Maximum depth to traverse | (Required) | |
| 28 | +| relationship | String or null | The relationship type to traverse. If null, all relationship types are used | null | |
| 29 | + |
| 30 | +## Returns |
| 31 | + |
| 32 | +| Name | Type | Description | |
| 33 | +|-------|------|----------------------------------------------| |
| 34 | +| nodes | List | List of visited nodes in breadth-first order | |
| 35 | +| edges | List | List of edges traversed during the BFS | |
| 36 | + |
| 37 | +## Examples |
| 38 | + |
| 39 | +### Social Network Friend Recommendations |
| 40 | + |
| 41 | +This example demonstrates how to use BFS to find potential friend recommendations in a social network. |
| 42 | +By exploring friends of friends, BFS uncovers second-degree connections—people you may know through mutual friends—which are often strong candidates for relevant and meaningful recommendations. |
| 43 | + |
| 44 | +#### Create the Graph |
| 45 | + |
| 46 | +```cypher |
| 47 | +CREATE |
| 48 | + (alice:Person {name: 'Alice', age: 28, city: 'New York'}), |
| 49 | + (bob:Person {name: 'Bob', age: 32, city: 'Boston'}), |
| 50 | + (charlie:Person {name: 'Charlie', age: 35, city: 'Chicago'}), |
| 51 | + (david:Person {name: 'David', age: 29, city: 'Denver'}), |
| 52 | + (eve:Person {name: 'Eve', age: 31, city: 'San Francisco'}), |
| 53 | + (frank:Person {name: 'Frank', age: 27, city: 'Miami'}), |
| 54 | +
|
| 55 | + (alice)-[:FRIEND]->(bob), |
| 56 | + (alice)-[:FRIEND]->(charlie), |
| 57 | + (bob)-[:FRIEND]->(david), |
| 58 | + (charlie)-[:FRIEND]->(eve), |
| 59 | + (david)-[:FRIEND]->(frank), |
| 60 | + (eve)-[:FRIEND]->(frank) |
| 61 | +``` |
| 62 | + |
| 63 | + |
| 64 | + |
| 65 | +#### Find Friends of Friends (Potential Recommendations) |
| 66 | + |
| 67 | +``` |
| 68 | +// Find Alice's friends-of-friends (potential recommendations) |
| 69 | +MATCH (alice:Person {name: 'Alice'}) |
| 70 | +CALL algo.bfs(alice, 2, 'FRIEND') |
| 71 | +YIELD nodes |
| 72 | +
|
| 73 | +// Process results to get only depth 2 connections (friends of friends) |
| 74 | +WHERE size(nodes) >= 3 |
| 75 | +WITH alice, nodes[2] AS potential_friend |
| 76 | +WHERE NOT (alice)-[:FRIEND]->(potential_friend) |
| 77 | +RETURN potential_friend |
| 78 | +``` |
| 79 | + |
| 80 | +In this social network example, the BFS algorithm helps find potential friend recommendations by identifying people who are connected to Alice's existing friends but not directly connected to Alice yet. |
| 81 | + |
| 82 | + |
| 83 | +## Performance Considerations |
| 84 | + |
| 85 | +- **Indexing:** Ensure properties used for finding your starting node are indexed for optimal performance |
| 86 | +- **Maximum Depth:** Choose an appropriate max_depth value based on your graph's connectivity; large depths in highly connected graphs can result in exponential growth of traversed nodes |
| 87 | +- **Relationship Filtering:** When applicable, specify the relationship type to limit the traversal scope |
| 88 | +- **Memory Management:** Be aware that the procedure stores visited nodes in memory to avoid cycles, which may require significant resources in large, densely connected graphs |
| 89 | + |
| 90 | +## Error Handling |
| 91 | + |
| 92 | +Common errors that may occur: |
| 93 | + |
| 94 | +- **Null Starting Node:** If the start_node parameter is null, the procedure will raise an error; ensure your MATCH clause successfully finds the starting node |
| 95 | +- **Invalid Relationship Type:** If you specify a relationship type that doesn't exist in your graph, the traversal will only include the starting node |
| 96 | +- **Memory Limitations:** For large graphs with high connectivity, an out-of-memory error may occur if too many nodes are visited |
| 97 | +- **Result Size:** If the BFS traversal returns too many nodes, query execution may be slow or time out; in such cases, try reducing the max_depth or filtering by relationship types |
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