{{more citations needed, date=September 2015
An event store is a type of
database
In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases s ...
optimized for storage of events.
Conceptually, in an event store, only the ''events'' of a dossier or
policy
Policy is a deliberate system of guidelines to guide decisions and achieve rational outcomes. A policy is a statement of intent and is implemented as a procedure or protocol. Policies are generally adopted by a governance body within an orga ...
are stored. The idea behind it is that the dossier or policy can be derived from these events. The events (and their corresponding
data
In the pursuit of knowledge, data (; ) is a collection of discrete Value_(semiotics), values that convey information, describing quantity, qualitative property, quality, fact, statistics, other basic units of meaning, or simply sequences of sy ...
) are the only "real" facts that should be stored in the database. The instantiation of all other objects can be derived from these events. The code instantiates these objects in memory. In an event store database, this means that all objects that should be instantiated, are ''not'' stored in the database. Instead these objects are instantiated '
on the fly
On the fly is a phrase used to describe something that is being changed while the process that the change affects is ongoing. It is used in the automotive, computer, and culinary industries. In cars, on the fly can be used to describe the changing ...
' in memory by the code based on the events. After usage of these objects (e.g. shown in a
user interface
In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine f ...
), the instantiated objects are removed from memory.
For example, the event store concept of a database can be applied to insurance policies or pension dossiers. In these policies or dossiers the instantiation of each object that make up the dossier or policy (the
person
A person ( : people) is a being that has certain capacities or attributes such as reason, morality, consciousness or self-consciousness, and being a part of a culturally established form of social relations such as kinship, ownership of prope ...
,
partner
Partner, Partners, The Partner, or, The Partners may refer to:
Books
* ''The Partner'' (Grisham novel), by John Grisham, 1997
* ''The Partner'' (Jenaro Prieto novel), 1928
* ''The Partners'' (book), a 1983 book by James B. Stewart
* ''Partner'' (m ...
(s), employments, etc.) can be derived and can be instantiated in memory based on the real world events.
A crucial part of an event store database is that each event has a double
timeline
A timeline is a display of a list of events in chronological order. It is typically a graphic design showing a long bar labelled with dates paralleling it, and usually contemporaneous events.
Timelines can use any suitable scale represen ...
: This enables event stores to correct errors of events that have been entered into the event store database before.
* Valid date is the date at which the event has become valid.
* Transaction date is the date at which the event is entered into the database.
Another crucial part of an event store database is that events that are stored are not allowed to be changed. Once stored, also erroneous events are not changed anymore. The only way to change (or better: correct) these events is to instantiate a new event with the new values and using the double timeline. A correcting event would have the new values of the original event, with an event data of that corrected event, but a different transaction date. This mechanism ensures reproducibility at each moment in the time, even in the time period before the correction has taken place. It also allows to reproduce situations based on erroneous events (if required).
One advantage of the event store concept is that handling the effects of back dated events (events that take effect before previous events and that may even invalidate them) is much easier. In regular databases, handling backdated events to correct previous, erroneous events can be painful as it often results in rolling back all previous, erroneous transactions and objects and rolling up the new, correct transactions and objects. In an event store, only the new event (and its corresponding facts) are stored. The code will then redetermine the transactions and objects based on the new facts in memory.
An event store will simplify the code in that rolling back erroneous situations and rolling up the new, correct situations is not needed anymore.
Disadvantage may be that the code needs to re-instantiate all objects in memory based on the events each time a service call is received for a specific dossier of policy.
Temporal database
A temporal database stores data relating to time instances. It offers temporal data types and stores information relating to past, present and future time.
Temporal databases could be uni-temporal, bi-temporal or tri-temporal.
More specifically th ...