8/EMT

Entity Migration and Transformation

draft

Entity Migration and Transformation is a set of commands, events, protocols and other contracts to be used to enable migration and transformation of recorded entities.

License

Copyright (c) 2016 Yurii Rashkovskii

This Specification is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.

This Specification is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, see http://www.gnu.org/licenses.

Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

0. Goals

Once entity is recorded, it can't be modified. However, since entity (command, event) layouts evolve over time (more information becomes available, etc.) there needs to be a mechanism that will describe the evolution of the layouts.

The goals of this specification are:

  • To provide a shared, universal language for entities migration and transformation.
  • To keep this language as small as possible.
  • To facilitate the development of libraries that implement this shared language.
  • To enable standardized transformation of entities so that up-to-date queries can be issued against them.
  • To enable seamless integration with the language defined in 3/CEP.

1. Migration

1.1. Events

1.1.1. EntityLayoutIntroduced

  • Layout name: rfc.eventsourcing.com/spec:8/EMT/#EntityLayoutIntroduced
Type Property
ByteArray fingerprint
Optional[Layout] layout

This event describes an introduction of an entity layout. Fingerprint MUST be recorded in fingerprint.

The new layout itself MAY be recorded in layout.

1.1.2. EntityLayoutReplaced

  • Layout name: rfc.eventsourcing.com/spec:8/EMT/#EntityLayoutReplaced
Type Property
ByteArray fingerprint
UUID replacement

This event describes a replacement of an entity layout with a new one. Old layout's fingerprint MUST be recorded in fingerprint, and a corresponding EntityLayoutIntroduced's UUID MUST be recorded in replacement.

2. Transformation

2.1. Events

When event replacement happens, event->command causality is lost and for some types of applications it means information loss. In order to mitigate this, EventCausalityEstablished signals an establishment of a new causality relationship for an event.

2.2. Commands

In some cases, command transformation can be also beneficial (for example, when causal information contained in commands can be taken into an account). However, a command can't produce new commands.

It is RECOMMENDED to run a process that queries the journal for commands of a specific type and issues individual "matching" replacement commands for each command to be replaced with new events.

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