# Base assumption

Event handlers will always use **publicly available application API's**(services)

- already done in Sandbox API by overwriting **SELECT**, **UPDATE**, **READ** and **CREATE**

## Inbound data for validations

- req.target plus expand on related data
  - lazy loading on expand
- event facade could have an explicit publishing of specific services or documents (e.g. remote services)
- CQN Protocol adapter for subsequent reads --> req.data plus application service calls
- what is the CDS subset to put in?
  - req.data + target-rec (proxy, unloaded)
  - ORM type lazy loading (dereferenced)
  - application developer could actually provide custom proxies for specific functions
  - performance impact of multiple accesses to object graph and multiple DB roundtrips
  - can static code checking or developer annotations influence what is loaded into a graph?
  - alternative: Stripped-down SELECT limited to req.target and ID
    - application service only
    - access rights of user respected
- What about to-many relationships? For compositions essential, for associations to be questioned
- Application Service Reads
- outbound data for changes
- call remote services
  - register new remote services dynamically
  - CAP provides an API on remote services - connect doesn't need to be done by extension developer
  - alternative: declarative remote services plumbing with CDS service facade
    - model looks like static internal services, remote calls done transparently behind the scenes

-Emit Events

- choreography of extension points
  - deep inserts vs. fine grained operations
  - input validation may be suited for fine grained operations
  - today not in scope for performance reasons
  - two different use case: Insert new page to book vs. update order-header with items-constraints in place
- reject request, return errors and warnings - suitable for UI, too

*/

/*
Annotations available:

Entity level
  @expression.constraint : [{if: 'expression evaluates to bool'}, on: ['INSERT, UPDATE, DELETE'], error: 'Transaction Rollback and error message', warning: 'Transaction proceeds and warning message']
  @expression.computed : [{expression: 'ability to access request payload and modify it', on: ['INSERT, UPDATE']}]
  @event : [{if: 'expression evaluates to bool', on: ['INSERT, UPDATE, DELETE, READ'], when: 'before or after, default before', emit: 'Event Name', to: 'Messaging target, optional'}]
  @expresion.code :[{file: 'file name', on:['insert', 'update'], when: 'before or after, default before'},
                    {source: 'each => { if (each.stock > 111) {each.title += `-- 11% discount!`; each.price= each.price*0.9}', on:['insert', 'update'], when: 'before or after'}]  
Atribute Level
  @assert.constraint : {if: 'stock>=0 OR stock <1000', error: 'i18n/error102'};
  @event : {if: 'expression evaluates to bool', on: ['INSERT, UPDATE, DELETE, READ'], when: 'before or after', emit: 'Event Name', to: 'Messaging target, optional' }

Functions available:
  EXISTS(association target)
  COUNT,AVG,MIN,MAX,SUM: Composition items, arrays etc
  OLD:  before image
  EACH: loop over composition items

Events covered:
CRUD --> Longhand and Shorthand supported?
Upsert as one event?
Before and after:
  Before can change change request payload and stop transaction
  After should trigger only asynchronous messages
Specific Events for status changes? I think expression based event emitter suffices
*/

//Entity level annotations
@expression.constraint : [{if: 'stock>100 AND price>15)', on: ['INSERT', 'UPDATE'], error: 'No Book over price 15 should have more than 100 stock' }, // error, rollback transactions
{if: 'stock>90 AND price>15)', on: ['I', 'U'], warning: 'No Book over price 15 should have more than 100 stock' }] //warning, proceed with transaction but report warning back to UI
@expression.computed : {expression: 'if(stock>100) then price=price*0.9', on: ['INSERT']}  //ability to modify the payload of the request, but nothing beyond it
@expresion.code :[{file: 'sap.capire.bookshop-Books-beforeInsert', on:['insert', 'update'], when: 'before'},  //naming can be arbitrary?
{source: 'each => { if (each.stock > 111) {each.title += `-- 11% discount!`; each.price= each.price*0.9}', on:['insert', 'update'], when: 'before'}]  //alternative
@event : { if:'price>200', emit: 'Expensive Book', to: 'RulesEngine'}
entity Books : managed {
key ID : Integer;
title  : localized String(111); @event : {if: 'old.title="Hello"', emit: 'Hello changed' }  //old refers to before Image. No "to" clause means message is emitted to any subscriber interested
descr  : localized String(1111);
author : Association to Authors @assert.constraint: 'exists(author)';  //function calls need to evaluate to bool
genre  : Association to Genres;
stock  : Integer @assert.constraint : {if: 'stock>=0 OR stock <1000', error: 'Stock not within permitted parameters'}; //when operand is used, no auto-insert
price  : Decimal(9,2) @assert.constraint : '>0'; //insert operand on left side by default
currency : Currency;
image : LargeBinary @Core.MediaType : 'image/png';
stockWorth: Decimal(9,2) @expression.computed : 'stock*price'; //persisted on write. Overhead in runtime, but performance benefit on read. Payload ignored?
// stockWorth2 = stock*price;  -- long term goal from compiler team, not persisted on write, but calculated on read
stockWorth3 : Decimal @expression.computed: 'if (stock*price>1000) then stockWorth3=stock.price else stockworth3=1000';  //which altenative?
stockWorth4 : Decimal @expression.computed: {if: '(stock*price>1000)', then: 'stockWorth3=stock.price', else: 'stockworth3=1000'};
}
//@assert.expression: 'dateOfBirth<dateOfDeath'
entity Authors : managed {
key ID : Integer;
name   : String(111);
dateOfBirth  : Date ;
dateOfDeath  : Date @expression.constraint: '>dateOfBirth';
placeOfBirth : String;
placeOfDeath : String;
books  : Association to many Books on books.author = $self;
}

/** Hierarchically organized Code List for Genres */
entity Genres : sap.common.CodeList {
key ID   : Integer;
parent   : Association to Genres;
children : Composition of many Genres on children.parent = $self;
}

```

```