datoms-differ

Find the diff between two txes in datoms.

What the what?

Yeah, good question. Ahem. Basically, it's a way of keeping track of changes in data via a datomic/datascript-like API and data structure.

What's it for then?

This is a tool to use when you want to keep clients in sync with a backend. The client is initialized with a datascript db when connecting, and is fed updates in the form of tx-data when the backend changes.

So what does this do again?

You write transactions to assert information about entities from a blank slate. Then datoms-differ finds out what's changed since the last time. It's a differ that takes the old data plus a datascript-like transaction of entity maps, and outputs a set of tx-data additions and retractions.

What the what?

Yeah, still a good question.

Install

Add [datoms-differ "0.2.2"] to :dependencies in your project.clj.

API

Require [datoms-differ.core].

(create-conn schema)

Takes a datascript schema and creates a "connection" (really, an atom with an empty db). A datascript schema might look like this:

(def schema
{:route/number {:db/unique :db.unique/identity}
:route/vessels {:db/valueType :db.type/ref
:db/cardinality :db.cardinality/many}
:vessel/imo {:db/unique :db.unique/identity}})

Like datascript, datoms-differ only cares about :db.unique/identity, :db.type/ref, and :db.cardinality/many.

(transact! conn source entity-maps)

Takes a connection, a keyword source identifier and a list of entity maps, and transacts them into the connection.

• conn - the atom you created with create-conn
• source - you can put data into the connection from multiple sources, but since datoms-differ will retract missing values, it will only retract values previously asserted by the same source.
• entity-maps - a list of entity maps that are asserted by this source. Example:

[{:route/number "100"
:route/name "Stavanger-Tau"
:route/vessels [{:vessel/imo "123"
:vessel/mmsi "456}]}
{:vessel/imo "123"
:vessel/name "MF Hardanger"}]

Note that every entity map needs to contain one and only one attribute that is marked as :db.unique/identity in the schema.

Another interesting note about the above example is that since :route/vessels is marked as :db.type/ref, and :vessel/imo is :db.unique/identity, there will only be two entities as a result of this transaction. The route and one vessel with all three asserted attributes (imo, mmsi and name).

The returned value from transact! has :db-after, :db-before, but most interestingly it has a :tx-data list of datoms. This is the diff! Here it is!

Exporting to datascript

There's also some tools for exporting to datascript. This lets you create a datascript db in the client, and then keep it up to date with new txes.

Use [datoms-differ.export] with (export-db db) that gives you a string that can be read by clojurescript (when datascript is loaded) to create a datascript db.

If you want to reduce the amount of bytes sent over the wire, you can also use (prune-diffs schema tx-data) to remove retractions of values that are later asserted. This optimalisation is possible since datascript doesn't have a notion of history.