At StampedeCon 2014, Alex Miller (Cognitect) presented "Datomic – A Modern Database." Datomic is a distributed database designed to run on next-generation cloud architectures. Datomic stores facts and retractions using a flexible schema, consistent transactions, and a logic-based query language. The focus on facts over time gives you the ability to look at the state of the database at any point in time and traverse your transactional data in many ways. We’ll take a tour of the Datomic data model, transactions, query language, and architecture to highlight some of the unique attributes of Datomic and why it is an ideal modern database.

1. Datomic
A Modern Database
Alex Miller

2. Overview
• Rationale
• Data model - facts
• Schema - entities, attributes
• Transactions - assert, retract, excise
• Architecture - peer, transactor, storage
• Queries - datalog, rules

3. Rationale

4. A database that
reconsiders...
• Immutable data model
• Flexible, extensible schemas
• Importance of time in our data
• Transactions and queries as data
• Deployment for the cloud
• Storage as a service

5. Data Model

6. Fact
• One piece of information
• About one thing
• At a specific point in time
• An immutable value

7. Fact = Datom

8. Fact = Datom
Entity Attribute Value Tx op
21005 :name "Stuart" 1000 assert
21005 :likes tea 1000 assert
21005 :likes coffee 1022 assert
21005 :likes tea 1022 retract

9. Fact = Datom
Entity Attribute Value Tx op
21005 :name "Stuart" 1000 assert
21005 :likes tea 1000 assert
21005 :likes coffee 1022 assert
21005 :likes tea 1022 retract

10. Fact = Datom
Entity Attribute Value Tx op
21005 :name "Stuart" 1000 assert
21005 :likes tea 1000 assert
21005 :likes coffee 1022 assert
21005 :likes tea 1022 retract

11. Datoms are Values
Entity Attribute Value Tx op
21005 :name "Stuart" 1000 assert
21005 :likes tea 1000 assert
21005 :likes coffee 1022 assert
21005 :likes tea 1022 retract

12. Datoms are Efficient
Entity Attribute Value Tx op
21005 43 "Stuart" 1000 true
21005 75 3299 1000 true
21005 75 1730 1022 true
21005 75 3299 1022 false

13. Database
• A collection of facts
• At a specific point in time
• An immutable value

14. Database is a Value
Entity Attribute Value Tx op
21005 :name "Stuart" 1000 assert
21005 :likes tea 1000 assert
21005 :likes coffee 1022 assert
21005 :likes tea 1022 retract

15. Schema

16. Entities + Attributes
• Entities - identity (no "type")
• Similar to: nodes in a graph, rows in rdbms
• Attributes
• Similar to: edges in a graph, cols in rdbms
• Relation from entities to values, or
• Relation from entities to entities

17. Value Attributes
“Alex Miller” “puredanger”
:handle
Entity Value
Attribute

18. Ref Attributes
“Alex Miller”
:follows
Entity Entity
Attribute
“Mario Aquino”

19. Attribute Definition
• Entities, just like your data
• Required attribute attributes:
• ident (keyword) - attribute identifier
• valueType (keyword) - string, boolean, long,
bigint, float, double, bigdec, instant, uuid,
uri, bytes, ref
• cardinality (keyword) - one or many

20. Optional Attributes
• Optional attribute attributes:
• doc (string) - documentation string
• unique (keyword) - value or identity
• index (boolean) - index this attribute
• fulltext (boolean) - searchable
• isComponent (boolean) - composite values
• noHistory (boolean) - whether to retain history
(rarely used, for things like counters)

21. Attribute Definition
{:db/id #db/id[:db.part/db]
:db/ident :comment/body
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}

22. ERD Modeling
Legend
Quote Useruser
text
timestamp
handle
email
first-name
last-name
follows
Entity Type attribute relationship
attribute

23. Alternative Views
Structure View
row datoms sharing a common E
column datoms sharing a common A
document traversal of attributes
graph traversal of reference attributes

24. Transactions

25. Transactions
• Set of facts (assertions or retractions)
• Applied at a point in time

26. Transactions as Data
• Defined as data (not INSERT/UPDATE/
DELETE strings)
• Transactions are *also* entities!
• You can query them - for when they
happened and what datoms they include
• And add new facts about them!

27. Datoms in a
Transaction
;; E A V Tx op
[21005 :name "Stuart" 1000 true]
[21005 :likes 3299 1000 true]
[21005 :likes 1730 1022 true]
[21005 :likes 3299 1022 false]

28. Integrity
• ACID transactions
• Equivalent to isolation level SERIALIZED

29. Constraints
• Schema constraints enforced on attributes
• Transaction functions
• From old db value to new db value
• Enforce arbitrary constraints
• Can reject transactions

30. Uses for Tx Fns
• Atomic update
• Maintaining integrity constraints (composite
keys)
• Strict validation
• Constructing entities
• Annotating transactions

31. Architecture

32. Server
Indexing
Trans-
actions
Query
App Process
I/O
App
Strings
DDL + DML
Result Sets
Storage
cache
monolithic server

33. Server
Indexing
Trans-
actions
Query
App Process
I/O
App
Strings
DDL + DML
Result Sets
Storage
cache
monolithic server
Storage Service
App Process
D Peer Lib
b,c,ea,d,e a,b,d
D Transactor
Indexing
Trans-
actions
Query
Cache
App
Data
Data
Data
segments
Live
Index
Data
Segments
Data Segments
peer, transactor, storage

34. Peer
Library
Storage Service
Transactor
Datomic Components

35. Your Application
Peer
Library
Storage Service
Transactor
Peer Library
• Included in your app
• Executes queries locally

36. Your Application
Peer
Library
Storage Service
Transactor
Peer Library
• Reads data from storage
• Caches locally
cache

37. Your Application
Peer
Library
Storage Service
Transactor
Scale Horizontally
cache
Your Application
Peer
Library
Your Application
Peer
Library
cache cache

38. Your Application
Peer
Library
Storage Service
Transactor
Transactor
• Standalone system
• Scales vertically

39. Your Application
Peer
Library
Storage Service
Transactor
Transactor
• Standalone system
• Hot standby for failover

40. Your Application
Peer
Library
Storage Service
Transactor
Transactor
• Coordinates writes
• Guarantees ACID transactions

41. Your Application
Peer
Library
Storage Service
Transactor
Transactor
•Writes transaction log to storage
• Generates indexes

42. Storage Service
Transactor
Transactor
• Broadcasts live updates
Your Application
Peer
Library
cache
Your Application
Peer
Library
Your Application
Peer
Library
cache cache

43. Your Application
Peer
Library
Storage Service
Transactor
Storage
• Provided as a service
• Many different back-ends

44. Your Application
Peer
Library
Storage Service
Transactor
Local Storage
• Memory
• Filesystem

45. PostgreSQL, Oracle, …
Your Application
Peer
Library
Storage Service
Transactor
Nearby Storage
• SQL database

46. Your Application
Peer
Library
Storage Service
Transactor
Distributed Storage
• DynamoDB
• Riak
• Couchbase
• Infinispan
• Cassandra

47. Your Application
Peer
Library
Storage Service
Your Application
Peer
Library
Your Application
Peer
Library
Datomic Components
Transactor

48. Your Application
Peer
Library
Storage Service
Transactions
Your Application
Peer
Library
Your Application
Peer
Library
ACID Writes
Transactor
Transactions
& Indexes

49. Your Application
Peer
Library
Storage Service
Live updates
Your Application
Peer
Library
Your Application
Peer
Library
Live Updates
Transactor

50. Your Application
Peer
Library
Storage Service
Your Application
Peer
Library
Your Application
Peer
Library
Distributed Reads
Transactor
Reads

51. Your Application
Peer
Library
Storage Service
Your Application
Peer
Library
Your Application
Peer
Library
Local Caches
cache cache cache
Transactor
Reads

52. Your Application
Peer
Library
Storage Service
Your Application
Peer
Library
Your Application
Peer
Library
Shared Memcached
Transactor
Reads
Memcached
cache cache cache

53. Queries

54. Queries as Data
• Datalog
• Queries defined as data, not as strings

55. Find User’s
Comments
(d/q
'[:find ?comment
:in $ ?email
:where [?user :user/email ?email]
[?comment :comment/author ?user]]
db
"editor@example.com")

56. (d/q
'[:find ?comment
:in $ ?email
:where [?user :user/email ?email]
[?comment :comment/author ?user]]
db
"editor@example.com")
Data Pattern
?user
:user/email
"editor@example.com"
?comment :comment/author

57. [:find ?customer ?email
:in $cust $emp
:where [$cust ?customer :email ?email]
[$emp _ :email ?email]]
“Find me the customers who are
also employees.”
Join across dbs
(d/q query custDb empDb)
implicit
join

58. [:find ?customer ?email
:in $cust $emp
:where [$cust ?customer :email ?email]
[$emp _ :email ?email]]
“Find me the customers who are
also employees.”
Join across dbs
(d/q query custDb empDb)
data patterns can be led
by database names

59. Travel Through Time
• Database *now*
• Database *last week*
• Database *if I added some transactions*

60. Views of a database
name semantics supports
(default) current state
what is the current
situation?
.asOf state at point in past
how were things in the
past?
.since state since point in past
how have things
changed?
tx report
before / after / change view of a
tx
automated
event response
.with state with proposed additions
what would happen if
we did X?
.history timeless view of all history anything!

61. (d/q '[:find ?customer
:where [?customer :id]
[?customer :orders]]
(d/as-of db #inst "2013-01-01"))
Time travel

62. Query Engine is Local
• Local query engine, local cache
• If working set is in memory, everything is FAST
• Reads do not require any transactor interaction
• Use your own functions in the query

63. Extension with custom fns
[:find ?customer ?product
:where [?customer :shipAddress ?addr]
[?addr :zip ?zip]
[?product :product/weight ?weight]
[?product :product/price ?price]
[(Shipping/estimate ?zip ?weight) ?shipCost]
[(<= ?price ?shipCost)]]
“Find me the customer/product
combinations where the shipping cost
dominates the product cost.”
predicate

64. Extension with custom fns
[:find ?customer ?product
:where [?customer :shipAddress ?addr]
[?addr :zip ?zip]
[?product :product/weight ?weight]
[?product :product/price ?price]
[(Shipping/estimate ?zip ?weight) ?shipCost]
[(<= ?price ?shipCost)]]
“Find me the customer/product
combinations where the shipping cost
dominates the product cost.”
function

65. Rules

66. Rules
[(relatedProduct ?p1 ?p2)
[?p1 :category ?c]
[?p2 :category ?c]
[(!= ?p1 ?p2)]]
“Products are related if they
have a common category.”

67. Rules
[(relatedProduct ?p1 ?p2)
[?p1 :category ?c]
[?p2 :category ?c]
[(!= ?p1 ?p2)]]
“Products are related if they
have a common category.”
head is true ...

68. Rules
[(relatedProduct ?p1 ?p2)
[?p1 :category ?c]
[?p2 :category ?c]
[(!= ?p1 ?p2)]]
“Products are related if they
have a common category.”
if body is true

69. q("[:find ?p2
:in $ %
:where (expensiveChocolate p1)
(relatedProduct p1 p2)]",
db
rules)
“Find all products related to
expensive chocolate.”
Rule inputs
rules are a kind of input

70. q("[:find ?p2
:in $ %
:where (expensiveChocolate p1)
(relatedProduct p1 p2)",
db,
rules)
“Find all products related to
expensive chocolate.”
Naming rule inputs
rule names begin
with %

71. q("[:find ?p2
:in $ %
:where (expensiveChocolate p1)
(relatedProduct p1 p2)",
db,
rules)
“Find all products related to
expensive chocolate.”
Using rule patterns
rule patterns can
appear in :where clause

72. [[(relatedProduct ?p1 ?p2)
[?p1 :category ?c]
[?p2 :category ?c]
[(!= ?p1 ?p2)]]
[(relatedProduct ?p1 ?p2)
[?o :order/item ?item1]
[?item1 :order/product ?p1]
[?o :order/item ?item2]
[?item2 :order/product ?p2]
[(!= ?p1 ?p2)]]]
“Products are related if they have the same
category, or they have appeared in the
same order.”
Implicit or

73. ;; base case
[(story-comment ?story ?comment)
[?story :story/title]
[?story :new/comments ?comment]]
Recursive query
for graph navigation
it is a story comment if...

74. ;; base case
[(story-comment ?story ?comment)
[?story :story/title]
[?story :new/comments ?comment]]
Recursive query
for graph navigation
... there is a story ...

75. ;; base case
[(story-comment ?story ?comment)
[?story :story/title]
[?story :new/comments ?comment]]
Recursive query
for graph navigation
... with a comment

76. ;; recursion
[(story-comment ?story ?comment)
[?parent :news/comments ?comment)
(story-comment ?story ?parent)]
Recursive query
for graph navigation
or, it is a story comment if...

77. ;; recursion
[(story-comment ?story ?comment)
[?parent :news/comments ?comment]
(story-comment ?story ?parent)]
Recursive query
for graph navigation
... it has a parent comment ...

78. ;; recursion
[(story-comment ?story ?comment)
[?parent :news/comments ?comment)
(story-comment ?story ?parent)]
Recursive query
for graph navigation
which is itself a story comment

79. Indexes and Logs

80. Direct Index Access
• seek-datoms - walk the datoms in a
specified index between particular
transactions or dates
• entid-at - gets a fabricated entity id
based on a transaction id or date

81. Direct Log Access
• Walk the transaction log directly
• Starting from txid or point in time

82. Entities and Graph
Walking

83. Entity
• A collection of facts
• About one thing
• At a specific point in time

84. Entity
• A collection of datoms
• With the same entity ID
• At a specific point in time

85. Entity
• A collection of datoms
• With the same entity ID
• At a specific point in time
• Can be viewed as a map
• Traversable as a graph

86. Entities ⬌ Datoms
;; Entity
{:db/id 21005
:name "Stuart"
:likes #{{:db/id 1730
:food/name "coffee"}}
;; Datoms
[21005 :name "Stuart"]
[21005 :likes 1730]
[1730 :food/name "coffee"]

87. Entities
• Retrieve entity from the database as a map
• Follow references to lazily walk the graph
• Follow references in both directions
• Use touch to retrieve all entity attributes
(def ent (d/entity dbval 17592186045417))
=> {:db/id 17592186045417}
(d/touch ent)
=> {:db/doc "Hello world",
:db/id 17592186045417}

88. Sweet Spots
• Flexible data model
• Audit trail, provenance, time
• Transactional data of record
• Horizontal query scaling
• Cloud deployment

89. Leverage other
systems
• Blobs (images, sound, movies, giant text)
• Write churn (hit counter)
• Horizontal write scaling

90. Datomic Free
Datomic Pro
Starter
Datomic Pro
Cost
Redistributable?
Number of Peers
High-Availability
Transactor
Storage Services
Memcached
Support
$0 $0 per peer
Yes No No
2 2 by license
No No Yes
Local only All All
No No Yes
Community Community Enterprise