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Document-oriented data access for
domain-driven .NET systems with
PostgreSQL & Marten
Bojan Veljanovski, CTO at HASELT
October, 2017 @ Macedonian .NET User Group
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What we are going to talk about?
● Limitations in relational-only DBs for storing true domain
aggregates
● How document-oriented DBs come into play
● Marten as document DB for .NET with PostgreSQL
● SQL + NoSQL => best of both worlds
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Setting context
● How I got interested in document-oriented DBs?
Why it makes so much sense?
● Why PostgreSQL?
Why not MongoDb, CouchDb, RavenDb?
What about SQL Server, MySQL?
● Who else uses this approach?
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Sample domain model
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DDD Aggregate
Cluster of domain objects that can be treated as a
single unit.
● Consists of
○ Aggregate Root (or Root Entity)
○ Entities
○ Value Objects
○ Services
○ Repositories 5

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Basic characteristics of an Aggregate Root
● Global identity
● Ensures invariants within aggregate
● Guarantees consistency
● Can reference other ARs only by identity
● Cannot reference non ARs
● Transactional consistency
● Stored and loaded in DB as a whole. 6

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Relational-only model
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Sample domain model in code
Let’s see the sample codebase in Visual Studio.
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Limitations when working with
domain models in relational-only DBs...
● Cannot store an array of values
○ Needs new table with 1-m relation
● ...an array of objects
○ Needs new table with 1-m relation
● …a nested object
○ Needs new table with 1-1 relation
● …an AR in a single table
○ AR is spanned across many tables, and needs joinsThese limitations often lead to...
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These limitations often lead to...
● Complex O/R mapping code
● Sacrificing encapsulation for easier O/R mapping
● Leak of domain logic to upper layers
● Anemic domain models - only data, no behavior
● Dev teams focusing on data-relations instead of
system behavior
● Implicit “SELECT N+1” problems
● Unit tests full with mocks & stubs 10

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Moving out of the relational-only mindset
From thinking in terms of relations and data
To thinking in terms of behavior and data
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Document model
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How document-oriented DBs come into play
● Single document per [whole] aggregate
● Can store complex object graphs
● Can store arrays, collections, nested objects
● No need for ORM
● No limitations for creating rich domain models
● Dev team focus is in on behavior and logic
● Models are easy to unit test
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Introducing PostgreSQL
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Introducing PostgreSQL
● Object-relational database
● ACID
● Transactional
● Highly Extendable
● Open Source
● Free
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PostgreSQL NoSQL data types
● Key/Value store
○ hstore
● Document store
○ json and jsonb
● Spatial and Geographic objects
○ postgis 16

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PostgreSQL as document store:
Creating table with JSONB column
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Inserting documents
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Querying documents (1)
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Querying documents (2)
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Indexing documents
Various indexes are supported. 21

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Marten as Document DB for
.NET with PostgreSQL
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Introducing Marten
● OSS .NET library that provides polyglot persistence
● Document store
● Event store
● Uses PostgreSQL’s powerful JSON support
● Initial pre-alpha release on 08.12.2015
● Current version 2.3.2
● Community chat: gitter.im/JasperFx/marten
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Getting started
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Persisting documents
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How our data looks like? -> User
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Loading document by Id
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Querying documents with LINQ
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Filtering in arrays
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Paging and sorting
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Querying with plain SQL through Dapper
(yes, querying NoSQL documents with plain-old SQL)
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Adding Foreign Key to Order document
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How our data looks like with FK? -> Order
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Querying multiple documents
by using SQL JOIN
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Other notable features in Marten as Document Db
● Foreign Keys
● Indexes
● Duplicate columns
● Batched queries
● Compiled queries
● Bulk insert
● Multitenancy
● Optimistic Concurrency 35

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Closing
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Summary
● Relational-only DBs limit the ability to craft good
domain models
● Document DBs are natural fit for DDD Aggregates
● SQL + NoSQL in Postgres = best of both worlds
in a single DB provider
● Marten helps .NET devs leverage PostgreSQL as
polyglot database
● Polyglot data access mindset - there is no single
approach
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Resources
● Marten
○ https://jasperfx.github.io/marten
○ https://gitter.im/JasperFx/marten
● PostgreSQL JSON Functions and Operators
○ https://www.postgresql.org
● This presentation’s code samples
○ https://github.com/bojanv91/MartenPostgreSamples
● Other useful articles
○ https://martinfowler.com/bliki/AggregateOrientedDatabase.html
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