ravendb

Document databases in practice

Luigi Berrettini

Nicola Baldihttp://it.linkedin.com/in/nicolabaldi

http://it.linkedin.com/in/luigiberrettini

Overview

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Unbounded result sets problem

Unbounded number of requests problem

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They favor denormalization overcomposition and joins

Relations are different than in RDBMSs

They are schema-less, but attention should be paid in designing documents

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« a conceptual model should be drawn with little or no regard for the software that might implement it » (Martin Fowler, UML Distilled)

A domain model should be independent from implementation details like persistence

In RavenDB this is somewhat true

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RDBMS are schema-full• tuples = sets of key-value pairs ⇒ flat structure

• more complex data structures are stored as relations

Document databases are schema-less• object graphs stored as docs ⇒ no flat structure

• each document is treated as a single entity

RavenDB suggested approach is to follow the aggregate pattern from the DDD book

ENTITY

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Some objects are not defined primarily by their attributes

They represent a thread of identity that runs through time and often across distinct representations

Mistaken identity can lead to data corruption

VALUE OBJECT

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When you care only about the attributes of an element of the model, classify it as a value object

Make it express the meaning of the attributes it conveys and give it related functionality

Treat the value object as immutable

Don't give it any identity and avoid the design complexities necessary to maintain entities

AGGREGATE

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Invariants are consistency rules that must be maintained whenever data changes

They’ll involve relationships within an aggregate(relations & foreign keys: order / orderlines)

Invariants applied within an aggregate will be enforced with the completion of each transaction

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Cluster entities and value objects into aggregates and define boundaries around each

Choose one entity to be the root of each aggregate and control all access to the objects inside the boundary through the root

Allow external objects to hold references to the root only

Transient references to internal members can be passed out for use within a single operation only

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Because the root controls access, it cannot be blindsided by changes to the internals

This arrangement makes it practical to enforce all invariants for objects in the aggregate and for the aggregate as a whole in any state change

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Nested child document

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Document referenced by ID

Denormalized reference

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we clone properties that we care about when displaying or processing a containing document

avoids many cross document lookups and results in only the necessary data being transmitted over the network

it makes other scenarios more difficult: if we add frequently changing data, keeping details in synch could become very demanding on the server

use only for rarely changing data or for data that can be dereferenced by out-of-sync data

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Order contains denormalized data

from Customer

and Product

Full data are

saved elsewhere

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Querying

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DocumentStore• used to connect to a RavenDB data store

• thread-safe

• one instance per database per application

Session• used to perform operations on the database

• not thread-safe

• implements the Unit of Work pattern

in a single session, a single document (identified by its key) always resolves to the same instance

change tracking

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Sequential GUID key• when document key is not relevant (e.g. log entries)

• entity Id = sequential GUID (sorts well for indexing)

• Id property missing / not set ⇒ server generates a key

Identity key• entity Id = prefix + next available integer Id for it

• Id property set to a prefix = value ending with slash

• new DocumentStore ⇒ server sends a range of HiLo keys

Assign a key yourself• for documents which already have native id (e.g. users)

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soft-limit = 128no Take() replaced by Take(128)

hard-limit = 1024if x > 1024 Take(x) returns 1024 documents

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RavenDB can skip over some results internally ⇒ TotalResults value invalidated

For proper paging use SkippedResults:

Skip(currentPage * pageSize + SkippedResults)

Assuming a page size of 10…

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RavenDB supports Count and Distinct

SelectMany, GroupBy and Join are not supported

The let keyword is not supported

For such operations an index is needed

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All queries use an index to return results

Dynamic = created automatically by the server

Static = created explicitly by the user

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no matching static index to query ⇒ RavenDB automatically creates a dynamic index on the fly (on first user query)

based on requests coming in, RavenDB can decide to promote a temporary index to a permanent one

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permanent

expose much more functionality

low latency: on first run dynamic indexes have performance issues

map / reduce

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Advanced topics

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an index is made of documents

document• atomic unit of indexing and searching

• flat ⇒ recursion and joins must be denormalized

• flexible schema

• made of fields

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field• a name-value pair with associated info

• can be indexed if you're going to search on it⇒ tokenization by analysis

• can be stored in order to preserve original untokenized value within document

example of physical index structure{“__document_id”: “docs/1”, “tag”: “NoSQL”}

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One to one

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One to many ⇒ SELECT N+1

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Value type

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indexing: thread executed on creation or update

server responds quickly BUT you may query stale indexes (better stale than offline)

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documentStore.Conventions.DefaultQueryingConsistency

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ConsistencyOptions.QueryYourWritessame behavior ofWaitForNonStaleResultsAsOfLastWrite

ConsistencyOptions.MonotonicReadyou never go back in time and read older data than what you have already seen

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