Add support for performing `attachToTangle` PoW locally (without relying on IRI node)

[todofixthis]

The PoW process can be done 100% offline, so the library should provide a mechanism for doing this locally, without having to send an attachToTangle request to a node.

Requirements not 100% nailed down yet, but here are some thoughts:

• Probably easiest for developers if we add e.g., a local_pow argument to Iota.__init__() (maybe StrictIota.__init__()?).
  • An alternative approach would be to use an adapter for this (a la https://pyota.readthedocs.io/en/develop/adapters.html#routingwrapper), but that would make it harder for developers to discover this feature.
  • Definitely open to other approaches, post a comment if you think there's a more intuitive/usable way to integrate this into the API (:
• Here are some existing implementations:
  • Java
  • C ("pearl_diver")
  • Javascript

Also, don't forget to update documentation and example scripts (:

[plenarius]

For reference here's how the js promoter creates a localAttachToTangle - might be helpful.

[scottbelden]

I don't really have experience with c extensions in python, but it seems like it would be more robust and faster if the c version could be wrapped and called via a c extension rather than porting to pure python.

[todofixthis]

Aye, I agree. Based on my experiences porting Curl-P to Python, I am not optimistic about how a pure-Python PearlDiver will perform.

We already have a C extension for Curl-P, and with a little bit of work, we could probably turn it into a wrapper for the ccurl library (which itself includes a PearlDiver implementation).

I think that would be a pretty sweet approach (:

[chrisoro]

I wouldn't port it to python but rather use the existing one. Could even use it directly by going the cython route..

For the overall feature itself I would like it to be very easy to use, like you said. I would recommend the RoutingWrapper though. Just add the possibility to 'route' that call locally to a different function.

Doing that, basically every existing function that calls (directly or indirectly) attachToTangle (like promote, replay, ..) can stay the same.

[olymk2]

The C extension would be nice to have, but a pure python implementation may be good for embedded devices running micro python it will be more portable as well this way.

Some libraries let you switch out between the pure python and the optimized version so that would be nice to have.

[mlouielu]

I have a Python version of pearldiver (PoW) here: https://github.com/mlouielu/iota-pearldiver

but CPython got poor performance, even with pypy still get bad performance...

[miili]

@mlouielu excellent! Do have the time to implement your Python solution into a RoutingWrapper for PyOTA?

[danielfaust]

What would you think of this: one place to specify where PoW is computed is globally in the Iota() constructor, and the other one is in each function which actually makes use of PoW.

from iota import Iota
from iota.adapter.pow import PoW

# Remote PoW for all functions that don't explicitly specify where it is computed
# api = Iota('https://service.iotasupport.com:14265')
# which equals to
# api = Iota('https://service.iotasupport.com:14265', pow=PoW('server'))

# Local PoW with C extension for all functions that don't explicitly specify where it is computed
api = Iota('https://service.iotasupport.com:14265', pow=PoW('ccurl'))

# Use a pure Python implementation of PoW only for the next function call
result = api.send_transfer(... , pow=PoW('python'))

# Use a remote server for PoW computation, where RoutingWrapper (if used)
# could help selecting the server.
# This would be helpful if the `Iota(...)` call specifies a local PoW but for this
# function call the PoW should be computed on the server
result = api.send_transfer(... , pow=PoW('server'))

# This would compute the PoW as selected in `Iota(...)` call, `ccurl` in this case
result = api.send_transfer(...)

Possibly the keyword argument could just take a string instead of a PoW() class.

I know that this approach would require a tedious modification of all the functions.

Else you would either end up with things like RoutingWrapper('https://service.iotasupport.com:14265', pow='server')
or

api =\
  Iota(
    PoWRoutingWrapper('https://service.iotasupport.com:14265', 'ccurl')
      .add_route('attachToTangle', 'http://localhost:14265', 'python')
      .add_route('interruptAttachingToTangle', 'http://localhost:14265', 'server')
  )

, since there might be a valid reason to use a attachToTangle routed to http://localhost:14265 without performing the PoW on the localhost server, maybe because it is a server in development which does not yet have the PoW computation implemented.

@todofixthis Any update about the local execution of the PoW ?

[imtypist]

I have a Python version of pearldiver (PoW) here: https://github.com/mlouielu/iota-pearldiver

but CPython got poor performance, even with pypy still get bad performance...

I tried to use this package, but I found that the running time changing trend is so strange when I increase mwm step by step...
comparing to the JavaScript implement, the running time is changing relatively more smoothly.
So I don't know if there is some implement difference?