wohali opened a new issue #1554: Additional Mango-based update handler / VDU 
URL: https://github.com/apache/couchdb/issues/1554
   This ticket is intended to gather ideas for what other sorts of update 
handler / VDU functionality we could add to Mango to provide a declarative 
replacement for JS-based update handlers and VDUs. This is intended as a 
"blue-sky" ticket to gather ideas that can be implemented simply in a 
declarative fashion.
   From #1534, I wrote:
   > There are other proposals to provide similar/useful functionality (#1532, 
#1533, #1498) if the operator decides to run without a JS engine (#1513), plus 
I suspect there are thoughts about doing Mango-based update functionality to 
filter fields, as well as apply database schema enforcement (type and format 
checking) in a declarative fashion for a NoJS mode. Could your embedded RasPi 
update function be written declaratively? I assume you're adding info such as 
incoming IP address, central timestamp, etc. which I could see being done 
automatically quite easily. This is a use case we can accelerate without the 
use of JS readily; we just need to agree on an API and how to write it down in 
a JSON blob.
   * Add datetime stamp (#1533)
   * Add IP address from incoming host, or other fields from the `req` object
   * Add additional content based on fields in the `req` object
       * Perhaps a lookup table that maps a `req` field value to a key:value 
pair (or JSON Patch) to insert?
   * Filter out (blacklist) or only allow (whitelist) certain fields (lists) - 
this is partial JSON schema support
   * Type-checking (field `type` is string, field `date` is ISO datetime|UNIX 
epoch, _etc._)
   * Field validation (format-checking, i.e. string must match a regex like 
`"$###.##"` or an integer must be within a certain range)
   * Consider using output of another Mango index/query for validating a field? 
For instance, a Mango query of documents that are of type `source` and 
extracting the field `ip_address`, then only allowing documents to come through 
that have a matching `req.ip_address` (for example).
       * Same as above, but to implement the lookup table. Example: look up 
source's IP address, auto-add name of sensor which is in the first document 
that matches in the Mango index

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