Document databases in practice Luigi Berrettini Nicola Baldi http://it.linkedin.com/in/nicolabaldi http://it.linkedin.com/in/luigiberrettini
Jan 27, 2015
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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