What is the difference between srv .on() .before() and .after() handlers in CAP CDS?

XAP Software · July 20, 2022

CAP CDS

srv.before() handler

This handler is called before the srv.on() handler. It is usually used for adding user’s input validation, for example:

this.before (['CREATE', 'UPDATE'],'Record', (req)=>{
  const record = req.data
  if (record.spentTime <= 0)  throw 'Spent time must be positive number'
})

There are another four things that you need to know about .before() handler:

  1. If you have more than one .before() handler, then they will be executed in order of their registration.
  2. If your handler executes asynchronous task, you need to return a promise.
  3. Execution of the .on() handlers starts, when all promises will be resolved.
  4. If your .before() handlers are running in asynchronous way, you can’t react on side-effects.

srv.on() handler

These handlers are run in a chain and every handler can terminate it. .on() handler helps you to create custom handlers replacing standard ones.

Please find below some examples for handling requests:

srv.on('READ','Records', async (req)=> req.reply(await cds.tx(req).run(...))
srv.on('READ','Records', (req)=> req.reply(...))
srv.on('READ','Records', (req)=> cds.tx(req).run(...))
srv.on('READ','Records', (req)=> [ ... ])
srv.on('READ','Records', ()=> SELECT.from(Records))
srv.on('READ','Records', (req,next)=>{
  if (...) return SELECT.from(Records) 
  else return next()  
})

srv.after() handler

The .after() handler is used to modify the response. It runs on the results, which are returned by the standard or custom .on() handler.

There are a few things you should know about this handler:

  1. Otherwise than the .on() handler, the .after() handler has two parameters - (result, req).
  2. You should use req parameter, if you need to reflect on an inbound request
  3. This method is used only to modify the result. You can’t replace the result, e.g. you can’t do the following: 
      this.after('READ','Records', (records)=>{
  return ...something...
})

That would be nice if you could make your code a little bit more expressive.

Vladislav Kobenko

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