- What is Eva?
- Development
- Configuration
- About the Eva Data Model
- Components
- Running with Docker
- Additional Resources
Eva is a distributed database-system implementing an entity-attribute-value data-model that is time-aware, accumulative, and atomically consistent. Its API is by-and-large compatible with Datomic's. This software should be considered alpha for the purposes of quality and stability. Check out the FAQ for more info.
If you are brand new to Eva, we suggest reading through this entire readme to familiarize yourself with Eva as a whole. Afterwards, be sure to check out the Eva tutorial series, which break down and go over almost everything you will want to know.
First we kick off the repl with:
lein repl
Next we create a connection (conn) to an in-memory Eva database. We also need to define the fact (datom) we want to add to Eva. Finally we use the transact call to add the fact into the system.
(def conn (eva/connect {:local true}))
(def datom [:db/add (eva/tempid :db.part/user)
:db/doc "hello world"])
(deref (eva/transact conn [datom]))Note: deref can be used interchangeably with the @ symbol.
Now we can run a query to get this fact out of Eva. We don't use conn to make a query but rather we obtain an immutable database value like so:
(def db (eva/db conn))Next we execute a query that returns all entity ids in the system matching the doc string "hello world".
(eva/q '[:find ?e :where [?e :db/doc "hello world"]] db)If we want to return the full representation of these entities, we can do that by adding pull to our query.
(eva/q '[:find (pull ?e [*]) :where [?e :db/doc "hello world"]] db)project.cljcontains the project build configurationcore/*primary releasable codebase for Eva Transactor and Peer-librarycore/srcclojure source filescore/java-srcjava source filescore/testtest source filescore/resourcesnon-source files
dev/*codebase used during development, but not releaseddev/srcclojure source filesdev/java-srcjava source filesdev/testtest source filesdev/resourcesnon-source filesdev/test-resourcesnon-source files used to support integration testing
lein test
Eva exposes a number of configuration-properties that can be configured using java system-properties. Some specific configuration-properties can also be configured using environment-variables.
The eva.config namespace, linked here, contains
descriptions and default values for the config vars.
EAV data-entities consist of:
- a distinct entity-id
- 1-or-more attribute-value pairs associated with a single entity-id
EAV data can be represented in the following (equivalent) forms:
- as an object or map:
{:db/id 12345, :attribute1 "value1", :attribute2 "value2"} - as a list of EAV tuples:
[ [12345, :attribute1, "value1"], [12345, :attribute2, "value2"] ]
To make the EAV data-model time-aware, we extend the EAV-tuple into an EAVT-tuple containing the transaction-id (T) that introduced the tuple:
[
;; E A V T
[12345, :attribute1, "value1", 500],
[12345, :attribute2, "value2", 500]
]To make the EAVT data-model accumulative, we extend the EAVT-tuple with a final flag that indicates if the EAV information was added or removed at the transaction-id (T).
[
;; E A V T added?
[12345, :attribute1, "value1", 500, true],
[12345, :attribute2, "value2", 500, true]
]Under this model, common data operations (create, update, delete) are represented like this:
- Create: a single tuple with
added? == true
[[12345, :attribute1, "create entity 12345 with field :attribute1 at transaction 500", 500, true]]- Delete: a single tuple with
added? == false
[[12345, :attribute1, "create entity 12345 with field :attribute1 at transaction 500", 501, false]]- Update: a pair of deletion and creation tuples
[
;; At transaction 502
;; invalidate the old entry for :attribute2
[12345, :attribute2, "old-value", 502, false]
;; add a new entry for :attribute2
[12345, :attribute2, "new-value", 502, true]
]The complete history of the database is the cumulative list of these tuples.
Data-updates are submitted as transactions that are processed atomically. This means that when you submit a transaction, either all the changes in the transaction are applied, or none of the changes are applied.
Transactions are submitted as a list of data-modification commands.
The simplest data-modification commands (:db/add, :db/retract) correspond
to the accumulative tuples described above:
[
[:db/retract 12345 :attribute2 "old-value"]
[:db/add 12345 :attribute2 "new-value"]
]When this transaction is committed it will produce the following tuples in the database history
(where <next-tx> is the next transaction-number):
[
[12345, :attribute2, "old-value", <next-tx>, false]
[12345, :attribute2, "new-value", <next-tx>, true]
]In addition to the command-form, you can also create/update data using the object/map form of an entity:
[
{:db/id 12345
:attribute1 "value1"
:attribute2 "value2"}
]This form is equivalent to the command-form:
[
[:db/add 12345 :attribute1 "value1"]
[:db/add 12345 :attribute2 "value2"]
]Because all stored data reduces to EAVT tuples, schemas are defined per Attribute, rather than per Entity.
Schemas definitions are simply Entities that have special schema-attributes.
[
{:db/id #db/id[:db.part/db]
:db/ident :attribute1
:db/doc "Schema definition for attribute1"
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db.install/_attribute :db.part/db}
]Taking each key-value pair of the example in turn:
:db/id #db/id[:db.part/db]: declares a new entity-id in the:db.part/dbid-partition:db/ident :attribute1: declares that:attribute1is an alias for the entity-id:db/doc "Schema definition for attribute1": human-readable string documenting the purpose of:attribute1:db/valueType :db.type/string: declares that only string values are allowed for:attribute1:db/cardinality :db.cardinality/one: declares that an entity may no-more-than one:attribute1. This means that for an given entity-id, there will only ever be one current tuple of[<entit-id> :attribute1 <value>]. Adding a new tuple with this attribute will cause any existing tuple to be removed.:db.install/_attribute :db.part/db: declares that this:attribute1is registered with the database as an installed attribute
The included docker compose can be used to spin up a completely integrated Eva environment. This includes:
- transactor
- eva-catalog
- activemq
- maria-db image
To spin up said environment run the following commands:
make gen-docker-no-tests # to build Eva with the latest changes
make run-dockerTo shut down the the environment use the following command:
make stop-dockerIn order to open a repl container that can talk to the environment use:
make replAnd run the following to initially setup the repl environment:
(require '[eva.catalog.client.alpha.client :as catalog-client])
(def config (catalog-client/request-flat-config "http://eva-catalog:3000" "workiva" "eva-test-1" "test-db-1"))
(def conn (eva/connect config))Finally, test that everything is working with an empty transaction:
(deref (eva/transact conn []))A similar result to this should be expected:
{:tempids {}, :tx-data (#datom[4398046511105 15 #inst "2018-06-06T17:35:07.516-00:00" 4398046511105 true]), :db-before #DB[0], :db-after #DB[1]}- Additional Documentation.
- The reference documentation at http://docs.datomic.com/ is highly relevant for development within Eva.
- http://www.learndatalogtoday.org/ is a great resource for learning Datalog, the underlying query language in Eva.
- https://github.com/kristianmandrup/datascript-tutorial is another great resource for learning Datalog and how it applies to Datomic (or Eva).
No. Eva is its own project we built from the ground up. The API and high-level architecture are largely compatible with Datomic, but the database, up to some EPL code, was entirely built in-house. We have a list of the most notable API differences here.
If you are looking for an easy system to move to production quickly, almost certainly not. Eva is far less mature and has seen far less time in battle. Datomic Cloud is an amazing (and supported) product that is far easier to stand up and run with confidence. Eva is provided as-is.
There are a handful of small API differences in the Peer, whereas the Clients are quite distinct. For example, a Connection in Eva is constructed using a baroque configuration map, not a string. From an operational standpoint, Datomic is far more turn-key. There are also likely some low-level architectural differences between the systems that will cause them to exhibit different run-time characteristics. For example, our indexes are backed by a persistent B^๐-tree, whereas Datomic's indexes seem to exhibit properties more like a persistent log-structured merge-tree. For a more detailed list check here.
Workiva's business model requires fine-grained and scalable multi-tenancy with a degree of control and flexibility that fulfill our own evolving and unique compliance requirements. Additionally, development on Eva began before many powerful features of Datomic were released, including Datomic Cloud and Datomic Client.
The project by-and-large is nearly feature complete and we believe is generally technically sound. Workiva has decided to discontinue closed development on Eva, but sees a great deal of potential value in opening the code base to the OSS community. It is our hope that the community finds value in the project as a whole.
Eva will likely still continue to be maintained and matured in 10% time and by previous contributors on their personal time.
- Erik Petersen [email protected]
Listed, in transaction log style, in order of addition to the project:
- Ryan Heimbuch [email protected]
- Houston King [email protected]
- Timothy Dean [email protected]
- Ross Hendrickson [email protected]
- Erik Petersen [email protected]
- Brandon St. Jules [email protected]
- Tyler Wilding [email protected]
- Daniel Harasymiw [email protected]
- Martin Kreibe [email protected]
- Alex Alegre [email protected]
- Aleksandr Furmanov [email protected]
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