Problem of making entities agree on some wavelength

I remember seeing this problem somewhere before, but don’t remember what it’s called and can’t find it again.

It was presented as two guys having $ n$ phones each that don’t ring when called. Each phone from one person is connected to one phone of the other person, in some random permutation. They can each test one phone every “round”, to check if they are using a connected pair of phones and thus able to communicate (this is in some way a consensus problem). They had to devise some strategy beforehand to be able to communicate (and minimize the amount of rounds necessary). Of course, the application behind this would be to make two entities connect on some same wavelength at the same time.

What is this problem called, also what are the best strategies?

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Questions while I’m making distributed key-value store

I’m CS student.

I’m currently trying to make distributed key-value store.

Since I’m new to distributed system concept.

I’m struggling with sort of concrete implementation and methodology to apply distributed system concept to key value store.

I’ve decided to use raft algorithm as my key-value store’s consensus algorithm.

As far as I know, raft-algorithm has leader,follower,candidate.

every specified interval, leader refreshes and synchronizes other nodes with up-to-date data.

My question is, If leader node is in charge of handling every write request(write requested data to key-value store and propagate the data to other nodes)or not. In case, leader takes overall responsible to write request, it looks really unreasonable to me.

I read many distributed RDBMS,K-V store’s architecture overview and user guide to know How they are doing this.

But none of them mentioned about my questions. I really struggled to answer to the questions myself!.. But I failed to make it.

As a last resort, I’m asking to stackoverflow users. my questions are

1. Does leader takes overall responsible for handling every write or read requests from clients?

If so, Should i redirect every requests to leader node?

2.raft-algorithm’s node(leader,candidate,follower) corresponds with node in node concept in key-value store?

Glossary of CockRoachDB says

Node is an individual machine running CockroachDB. Many nodes join together to create your cluster.

3.How often should distributed system synchronizes? Should it synchronize for every write transaction?

I know some of my questions(maybe all question) are dumb. Since I’m really eager to learn distributed system. It would be very glad to me if you let me know what i’m currently understanding distributed system incorrectly.

I really appreciate your answer and I want to apologize for my poor english in advance.

Raspberry Pi – Nextcloud – Snap – stuck on making ‘Read only Filesystem” writable

Installed nextcloud on Raspi with snap. Now I am stuck with editing the config.php file. Tried:

chomd 775 config.php 

also tried

sudo chmod 775 config.php 

It comes up:”changing permissions of ‘config.php’: Read-only file system”

Complete Path is: /snap/nextcloud/current/htdocs/config/config.php Has anybody a tip how to change it into write permission? Thank you!

Flowtype: How to properly use $ObjMap for making all functions chainable?

I’m working on some JS package and I want to provide separated flow type definitions as index.js.flow because internal behavior is slightly different.

I have this function for create component definition

function defineComponent(name, createFunc); 

createFunc is function which grabs elements and returns object containing particular user defined actions upon that component

so you can call defineComponent in this manner:

const loginForm = defineComponent("login form", () => {   ...   ...   return {     fillUsername: () => { ...doesn't matter what is return type... },     fillPassword: () => { ...doesn't matter what is return type... }   }; }); 

and those actions should be chainable, but I don’t want to burden user with always mentioning return type within each user defined action. So final chain should look like this:

loginForm   .fillUsername()   .fillPassword(). 

So internally defineComponent will wrap each user defined action like this:

function defineComponent(..., createFunc) {   const actions = createFunc(...);    return => {     return (...args) => {       action(...args);       return actions;     }   }) } 

I already tried this (my whole testing code):

type ComponentType<T> = $  ObjMap<T, <V>((V) => any) => V => ComponentType<T>>;  declare function defineComponent<T>(   name: string,   createFunc: () => T ): ComponentType<T>;  const Component = defineComponent("foo", () => {   return {     fooAction: () => {},     barAction: () => {}   }; });  Component.fooAction().barAction() 

I proceeds with flow’s No errors! but flow is showing no errors also when I do something like


And also VS Code provides no autocompletion above that :/


Making other phones ring [on hold]

Since I believe that there is no non-technical way to do this, I believe it falls under Information Security as it is related to phreaking.

Scenario: I live in a big house and I often have to check several rooms to find a family member. Each room has a landline telephone in, totalling about 5 phones throughout the house.

Question: Is there a way I can make every phone in my house ring from a phone, so I can then speak to someone without running around the house trying to find them? I am hoping for a sort of internal intercom system.

Using a mobile phone and ringing my house is not possible and out of scope.

Making Deals with Devils [closed]

I’m preparing for running my first campaign (Storm King’s Thunder) and I’ve got most of the basic information down in terms of running the campaign with the information provided but I wanted to add a little extra mechanics for the players to interact with if they so choose. I am especially fond of the idea of a Mephistopheles type demon presenting himself to the party and offering seemingly benign boons that have unintended consequences. I am worried, however, that giving them additional powers without sufficient drawbacks will unbalance the game. Simply signing over a soul seems to have no real in-game consequences that I can think of other than not being able to be resurrected after death.

I feel like a deal will need to have Boons, to tempt characters; Conditions, to make them feel like the deal had consequences; and Punishments, if the characters do not follow the conditions. Below are some examples I have come across of the three aspects of the deal laid out.


  • Life Insurance – After failing 3 death saving throws, the character gets 1 free instant Revivify. Can be renewed after Life Insurance is consumed, at a greater price
  • Get 3 luck dice (d20) to add to attack rolls, an ability checks, or saving throws. Must be used before outcome is determined. After luck dice is rolled, DM rolls d20 on 19-20 Luck dice value does not effect roll and die is consumed. Die can be regenerated by committing moderately evil acts (ex. destroy a merchant’s goods, desecrating temples, rob an orphanage, etc.)


  • One use of your voice at any time.
  • Collect on another contract made several decades ago. The terms of this contract remain unknown. The mortal who signed the first contract is advanced in years and either doesn’t remember the contract due to their memory failing, or starts begging that there is still time.
  • The fiend picks up a small rock off the ground and enchants it with a spell you cannot discern (on a successful Arcana check or casting of detect magic, you realize it’s from the Divination school). The fiend then demands that you always keep this rock on your person. You are magically bound to this: if the rock is left behind or discarded, it returns to your pack whenever you aren’t paying attention. The rock is always there and tends to get in the way, but imposes no mechanical penalties. Rock allows controlling entity to view the location of the holder at all times. Devil can sell or rent out scrying abilities as they see fit.


  • You pay with your mortal time, making you age rapidly into an old person, grey hair (and beard) with old skin and muscles within the next day. Reduce either Strength/Dexterity/Con by 2, or any two by 1.
  • From now any gamble you make results in an unwanted outcome
  • The devil send to collect on your debt he sends a champion (packs of lesser demons, another person who made a deal, etc.) to kill you
  • One of their eyes. Results in -2 penalty on Perception checks.

I have not fully fleshed out each detail and would like to hear criticisms and ideas to implement for each aspect.