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10. Graph Databases

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Introduction
The Lack of relationship for RDBMS and NoSQL
Graph Databases: Features
Relations
Query Language
Data Modeling with Graphs
Conclusions

Published in: Data & Analytics
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10. Graph Databases

  1. 1. Graph Oriented Databases Ciao ciao Vai a fare ciao ciao Dr. Fabio Fumarola
  2. 2. Outline • Introduction • The Lack of relationship for RDBMS and NoSQL • Graph Databases: Features • Relations • Query Language • Data Modeling with Graphs • Conclusions 2
  3. 3. Introduction • We live in a connected world • Everything is connected: Social Network, Biology, Bioinformatics • The NoSQL databases analyzed store data using aggregate • Here we compare graph databases with relational databases and aggregate NOSQL in storing graph data 3
  4. 4. Three Facts 1. Relational Databases Lack Relationships 1. NoSQL Databases also Lack Relationships 2. Graph Databases Embrace Relationships 4
  5. 5. Relational Databases Lack Relationships • For decades we tried to accommodate connected, semi-structured datasets inside relational databases. • But: – relational databases are designed to codify tabular structures – They struggle when modeling ad hoc exceptional relationships that are in real world. 5
  6. 6. Relational Databases Lack Relationships • Relationships in relational database only mean joining tables • But we want to model the semantic of relationships that connect real world • As outlier data multiplies: 1. The structure of the dataset becomes more complex and less uniform 2. The relational data becomes more complex and less uniform (large join tables, sparsely populated rows, a lot o null values) 6
  7. 7. Example of customer-centric orders • Complex joins • Foreign key constraints • Sparse table with null values • Reciprocal queries are costly “What products did a customer buy?” 7
  8. 8. NoSQL Databases Also Lack Relationships • key-value, document, or column-oriented store sets of disconnected documents/values/columns • One well-known strategy for adding relationships is to embed an aggregate’s identifier inside the field belonging to another aggregate • But this require joins at the application level • Some NoSQL have some concept of navigability but it is expensive for complex joins 8
  9. 9. Example of aggregate oriented orders • Some properties are references to foreign aggregates • This relationship are not first-class citizens • Are not intended as real realtionships 9
  10. 10. Example of a Small Social Network • it’s easy to find a user’s immediate friends • friendship isn’t always reflexive • We can have brute-force scan across the whole dataset looking for friends entries 10
  11. 11. Graph Databases Embrace Relationships • The previous examples have dealt with implicitly connected data • We infer semantic dependencies between entities • We model the data based on this connections • Our application have to navigate on this flat and disconnected data, and deal with slow queries. • In contrast, in the graph world, connected data is stored as connected data 11
  12. 12. Example Social Network 12 • The node user:Bob is a Vertex with a property Bob • We also see relations which are Edges: • Boss_of • Friend_of • Married_to
  13. 13. GRaph DB Features 13
  14. 14. CAP Theorem 14
  15. 15. Consistency • Since Graph DBs operate on connected nodes, they could not scale well distributing nodes across servers. • There are solutions supporting distribution: – Neo4j uses one master and several slaves – OrientDB uses MVCC for distributed eventual data structures – TitanDB partition data by using HBase or Cassandra 15
  16. 16. Transactions • Most of the Graph DB are ACID-compliant • Before doing an operation we have to start a transaction. • Without wrapping operations in a transaction we will get an Exception. 16
  17. 17. Availability • Neo4j from version 1.8 achieves availability by providing for replicated slaves. • Infinity Graph, FlockDB and TitanDB provides for distribute storage of the nodes. • Neo4J uses Zookeeper to keep track of the last transaction Ids persisted on each slave node and the current master node 17
  18. 18. Going Into Relations 18
  19. 19. Relations • Relations in a graph naturally forms paths. • Querying or traversing the graph involves following a path. • A query on the graph is also known as traversing the graph • As advantage we can change the traversing requirements without changing nodes and edges 19
  20. 20. Relations • In graph databases traversal operation are highly efficient. • In the book Neo4j in Action, Partner and Vukotic perform an experiment comparing relational store and Neo4j 20
  21. 21. Relations • In a depth two (friend-of-friend), both relational db and graph db perform well enough • But when we do the depth three it clear that relational db can no longer deal 21
  22. 22. Relations • Both aggregate store and relational databases perform poorly because of the index lookups. • Graphs, on the other hand, use index-free adjacency list to ensure that traversing connected data is extremely fast. 22
  23. 23. Relations another case study • Let us consider the purchase history of a user as connected data. • If we notice that users who buy strawberry ice cream also buy espresso beans, we can start to recommend those beans to users who normally only buy the ice cream. 23
  24. 24. Relations and Recommendations • The previous was a one dimensional recommendation • We can join our graph with graph from other domains. • For example, we can ask to fine – “all the flavors of ice cream liked by people who live near a user, and enjoy espresso, but dislike Brussels sprouts.” 24
  25. 25. Relations and Patterns • We can use relations to query graph-patterns • Such pattern-matching queries are: – extremely difficult to write in SQL – And are laborious to write against aggregate stores • In both cases they tend to perform very poorly • In the other hand, graph databases are optimized for such kind of queries 25
  26. 26. Query Language 26
  27. 27. Query Language • Graph DBs support query languages such as Gremlin, Cypher and SPARQL • Gremlin is a DSL for traversing graphs; • It can traverse all the graph databases implementing the Blueprints 27
  28. 28. 1. Indexing: Nodes and Edges • Indexes are necessary to find the starting node to being traversal. • How Indexes works: – Can index properties of nodes and edges. – Adds are done in transactions • Nodes retrieved can be used to raise queries 28
  29. 29. 2. Querying In- Out- Relationships • Having a node we can query both for Incoming and Outgoing relationships. • We can apply directional filters on the queries when querying for relations 29
  30. 30. 3. Querying Breadth- Depth- • Graph databases are really powerful to query for incoming and outgoing relationships. • Moreover, we can make the traverser go top-down or sideways on the graph by using: – BREADTH_FIRST or – DEPTH_FIRST 30
  31. 31. 4. Querying Paths • An other good feature of graph databases is – finding paths between two nodes. – Determining if there are multiple paths – finding the shortest path • Many Graph DBs use algorithms such as the Dijkstra’s algorithm for finding shortest paths. 31
  32. 32. 5. Querying Paths • Finally, with Graph DBs it is possible to use Match operator • The MATCH is used for matching patterns in relationships • The WHERE filters the properties on a node or relationship • The RETURN specifies what to get in the result set. 32
  33. 33. Data Modeling with Graphs Intro 33
  34. 34. Data Modeling with Graphs how do we model the world in graph terms? 34
  35. 35. how do we model the world in graph terms? • Formalization of the base model • Enrich the model • Testing the model 35
  36. 36. Formalization of the base model • Modeling is an abstracting activity motivated by a particular need or goal • We model in order to define structures that can manipulated. • There are no natural representations of the world the way it “really is,” • There are just many purposeful selections, abstractions, and simplifications that useful for satisfying a particular goal 36
  37. 37. Formalization of the base model • Graph data modeling is different from many other techniques. • There is a close affinity between logical and physical models. • In relational databases we start from a logical model to arrive to the physical model. • With graph databases, this gap shrinks considerably. 37
  38. 38. The Graph Model • A property graph is made up of nodes, relationships, and properties. 38
  39. 39. Publishing Messages • We organize messages in order 39
  40. 40. Nodes Nodes contain properties •Think of nodes as documents that store properties in the form of arbitrary key-value pairs. •The keys are strings and the values are arbitrary data types. 40
  41. 41. Relationships Relationships connect and structure nodes. •A relationship always has a direction, a label, and a start node and an end node—there are no dangling relationships. •Together, a relationship’s direction and label add semantic clarity to the structuring of nodes. 41
  42. 42. Relationships: Attributes Like nodes, relationships can also have properties. •The ability to add properties to relationships is particularly useful for: – Providing additional metadata for graph algorithms – Adding additional semantics to relationships (including quality and weight), – and for constraining queries at runtime. 42
  43. 43. Modeling Steps: Outline • The initial stage of modeling is similar to the first stage of many other data modeling techniques, that is: – to understand and agree on the entities in the domain – how they interrelate – and the rules that govern their state transitions 43
  44. 44. Describe the Model in Terms of the Application’s Needs • Agile user stories provide a concise means for expressing an outside-in, user-centered view of the application needs. • Here’s an example of a user story for a book review web application: – AS A reader who likes a book, – I WANT to know which books other readers who like the same book have liked, – SO THAT I can find other books to read. 44
  45. 45. Describe the Model in Terms of the Application’s Needs • This story expresses a user need, which motivates the shape and content of our data model. • From a data modeling point of view: – the AS A clause establishes a context comprising two entities—a reader and a book—plus the LIKES relationship that connects them. – The I WANT clause exposes more LIKES relationships, and more entities: other readers and other books. 45
  46. 46. Describe the Model in Terms of the Application’s Needs • The entities and relationships in analyzing the user story quickly translate into a simple data model 46
  47. 47. Modeling Rationale • Use nodes to represent entities • Use relationships both: – to express the connections between entities and – to establish semantic context for each entity • Use relationship direction to further clarify relationship semantics 47
  48. 48. Describe the Model: Guidelines • Use node properties – to represent entity attributes, plus any necessary entity metadata, such as timestamps, version numbers, etc • Use relationship properties – to express the strength, weight, or quality of a relationship, plus any necessary relationship metadata, such as timestamps, version numbers, etc. 48
  49. 49. Modeling Temporal Relations as Nodes • When two or more domain entities interact for a period of time, a fact emerges • We represent these facts as separate nodes • In the following examples we show how we might model facts and actions using intermediate nodes. 49
  50. 50. Example: Employment Ian was employed as an engineer at Neo Technology 50
  51. 51. Example: Performance William Hartnell played the Doctor in the story The Sensorites 51
  52. 52. Example: Emailing Ian emailed Jim, and copied in Alistair 52
  53. 53. Example: Timeline Tree 53 A timeline tree showing the broadcast dates for four episodes of a TV program
  54. 54. Example: Linked-List A doubly linked list representing a time-ordered series of events 54
  55. 55. Iterative and Incremental • We develop the data model feature by feature, user story by user story • This will ensure we identify the relationships our application will use to query the graph • With the iterative and incremental delivery of application features we will be a corrected model that provides the right abstraction 55
  56. 56. Data Modeling: Enrich • The next steps diverges from the relational data methodology • Instead of transforming a domain model’s graph-like representation into tables, we enrich it. • That is, for each entity in our domain, “we ensure that we’ve captured both the properties and the connections to neighboring entities necessary to support our application goals”. 56
  57. 57. Data Modeling: Enrich • Remember, the domain model is not totally aligned to reality. • it is a purposeful abstraction of those aspects of our domain relevant to our application goals. • By enriching our domain graph with additional properties and relationships, we effectively produce a graph model aligned to our application’s data needs 57
  58. 58. Data Modeling: Enrich In graph terms, we are ensuring that: •each node has the appropriate properties •every node is in the correct semantic context. we do this by creating named and directed (and often attributed) relationships between the nodes to capture the structural aspects of the domain. 58
  59. 59. Data Modeling: Test • The next step is to test how suitable it is for answering realistic queries • Also if Graph DB are great in supporting evolving structures there are some design decisions to consider • By reviewing the domain model and the resulting graph model at this early stage, we can avoid these pitfalls. 59
  60. 60. Data Modeling: Test • In practice there are two techniques that we can apply here • The first, and simplest, is just to check that the graph reads well. • We pick a start node, and then follow relationships to other nodes, reading each node’s role and each relationship’s name as we go • Doing so should create sensible sentences 60
  61. 61. Data Modeling: Test • The second one is to consider queries we’ll run on the graph. • To validate that the graph supports the kinds of queries we expect to run on it, we must describe those queries. • Given a described query if we can easily write the query in Cypher or Gremlin we can be more certain that the graph meets the needs of our domain. 61
  62. 62. Conclusions 62
  63. 63. Avoid Anti-Patterns • In the general case, don’t encode entities into relationships. • It’s also important to realize that graphs are a naturally additive structure • It’s quite natural to add facts in terms of domain entities and how they interrelate adding nodes and relationships • If we model in accordance with the questions we want to ask of our data, an accurate representation of the domain will emerge. 63
  64. 64. When to Use • Connected Data • Routing, Dispatch, and Location-based Services • Recommendation Engines 64
  65. 65. When Not to Use • When you need to update all or a subset of entities, for example in analytics • In situation when you need to apply operations that work on the global graph • When you don’t know the starting point of your query 65

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