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Add a write up of the state delta resolution algorithm #3122

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commented Apr 19, 2018

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\begin{algorithmic}
\STATE $to\_recalculate \leftarrow \text{empty set of state keys}$
\STATE $pending \leftarrow G_\delta$
\WHILE{$pending$ is empty}

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@krombel

krombel Apr 19, 2018

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\WHILE{$pending$ is not empty}

@richvdh

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commented Apr 20, 2018

\end{split}
\end{equation} which we call state maps, for $F, G \subset K$.

We can then compute the set of all ``unconflicted events":

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richvdh Apr 20, 2018

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better described as "unconflicted state keys" maybe?

\begin{split}
u_{f,g} : U_{f,g} \longrightarrow &\ E\\
x \longmapsto &\ \begin{cases}
f(x), & \text{if}\ f \in F \\

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richvdh Apr 20, 2018

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x \in F

\end{equation} which gets the unconflicted event for a given state key.


We can also define a function on $C_{f,g} = F \cup G \setminus U_{f,g}$:

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richvdh Apr 20, 2018

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parens around f \cup G ?

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@erikjohnston

erikjohnston Apr 20, 2018

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yup

We can also define a function on $C_{f,g} = F \cup G \setminus U_{f,g}$:
\begin{equation}
c_{f,g}: C_{f,g} \rightarrow E
\end{equation} which is used to resolve conflicts between $f$ and $g$. Note that $c_{f,g}$ is either $f(x)$ or $g(x)$.

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richvdh Apr 20, 2018

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ITYM $c_{f,g}(x)$ is either ...

I don't think it's true that c_{f,g} is either f or g

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erikjohnston Apr 20, 2018

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yup

\end{equation} which we call the resolved state of $f$ and $g$.

\begin{lemma}
$\forall x \in U_{f,g} \ s.t.\ g(x) = g'(x)$ then $r_{f,g}(x) = r_{f,g'}(x)$

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richvdh Apr 20, 2018

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this could do with clarifying. ITYM:

if $\forall x (g(x) = g'(x)), x \in U_{f_g}$, then ...

\end{split}
\end{equation} which we call the resolved state of $f$ and $g$.

\begin{lemma}

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richvdh Apr 20, 2018

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it looks like you're about to prove this, which is confusing. Suggest just stating it rather than making it a Lemma.

\alpha: E \rightarrow \mathbb{P}(K)
\end{equation} to be the mapping of an event to the type/state keys needed to auth the event, and
\begin{equation}
\alpha_{f,g}(x) = \alpha f(x) \cup \alpha g(x)

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richvdh Apr 20, 2018

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what's going on here? what does \alpha f(x) mean?

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@erikjohnston

erikjohnston Apr 20, 2018

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\alpha f(x) = \alpha(f(x)), i.e. for the event in the state f, \alpha f(x) are its auth state keys

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richvdh Apr 20, 2018

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yup makes sense. more parens ftw.


Further, we can define
\begin{equation}
a_{f,g}(x) = \bigcup_{n=0}^\infty (\alpha_{f,g})^n(x)

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richvdh Apr 20, 2018

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can't you use something other than a? (or something other than \alpha?) They look pretty similar.

Further, we can define
\begin{equation}
a_{f,g}(x) = \bigcup_{n=0}^\infty (\alpha_{f,g})^n(x)
\end{equation} to be the auth chain of $f(x)$ and $g(x)$. This is well defined as there are a finite number of elements in $F \cup G$ and $a_{f,g} \rightarrow F \cup G$.

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richvdh Apr 20, 2018

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I think more to the point, α → F ∩ G

a_{f,g}(x) = \bigcup_{n=0}^\infty (\alpha_{f,g})^n(x)
\end{equation} to be the auth chain of $f(x)$ and $g(x)$. This is well defined as there are a finite number of elements in $F \cup G$ and $a_{f,g} \rightarrow F \cup G$.

If we consider the implementation of $c_{f,g}$ in Synapse we can see that it depends not only on the values of $x$, but also on the resolved state of their auth events, i.e. $r_{f,g}(\alpha_{f,g}(x))$. By ``depends on" we mean that if those are the same for different values of $f$ and $g$, then the result of $c_{f,g}(x)$ is the same.

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richvdh Apr 20, 2018

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I'm failing to grok the By depends on sentence. "those are the same": which are the same?

the term "depends on" is used heavily in the later proofs, so I think this needs defining more precisely.

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erikjohnston Apr 20, 2018

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By depends on I mean, say: c_f(x) depends on r_f(x), then forall g where r_f(x) = r_g(x) then c_g(x) = c_f(x).

Its basically saying that c could be written as a pure function from f(x), g(x) and r_{f,g}(\alpha_{f,g}(x))

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richvdh Apr 20, 2018

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(i'd be tempted to abuse a proportionality symbol \propto to represent the 'depends on' relationship.

Or redefine it in terms of independence.

\end{lemma}

\begin{proof}
$c_{f,g}(x)$ depends on $x \in a_{f,g}(x)$, and $r_{f,g}(\alpha_{f,g}(x))$. Now:

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richvdh Apr 20, 2018

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why is x in a ?

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erikjohnston Apr 20, 2018

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a_{f,g}(x) = x \cup \alpha_{f,g}(x) \cup (\alpha_{f,g})^2(x) \cup ...

$c_{f,g}(x)$ depends on $x \in a_{f,g}(x)$, and $r_{f,g}(\alpha_{f,g}(x))$. Now:
\[
\begin{split}
r_{f,g}(\alpha_{f,g}(x))\ =\ & u_{f,g}(\alpha_{f,g}(x)) \cup c_{f,g}(\alpha_{f,g}(x))\\

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richvdh Apr 20, 2018

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u is only defined over the unconflicted part of the state key space. I think it's clear what you mean, but it might be good to be more precise here.

\begin{split}
r_{f,g}(\alpha_{f,g}(x))\ =\ & u_{f,g}(\alpha_{f,g}(x)) \cup c_{f,g}(\alpha_{f,g}(x))\\
\end{split}
\] but by definition $u_{f,g}(\alpha_{f,g}(x))$ depends only on $\alpha_{f,g}(x)$, so $r_{f,g}(\alpha_{f,g}(x))$ depends on $a_{f,g}(x)$ and $c_{f,g}\alpha_{f,g}(x)$.

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richvdh Apr 20, 2018

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could you be consistent about c_{f,g}\alpha_{f,g}(x) vs c_{f,g}(\alpha_{f,g}(x)) ? (with my preference heavily on the former). It's particularly weird to see r with extra parens in the same sentence as c without.

\begin{split}
r_{f,g}(\alpha_{f,g}(x))\ =\ & u_{f,g}(\alpha_{f,g}(x)) \cup c_{f,g}(\alpha_{f,g}(x))\\
\end{split}
\] but by definition $u_{f,g}(\alpha_{f,g}(x))$ depends only on $\alpha_{f,g}(x)$, so $r_{f,g}(\alpha_{f,g}(x))$ depends on $a_{f,g}(x)$ and $c_{f,g}\alpha_{f,g}(x)$.

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richvdh Apr 20, 2018

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I think s/depends on a/depends on α/

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richvdh Apr 20, 2018

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could you split the result of what r(...) depends on to a separate \begin{equation}, since you're about to use it for induction?

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@erikjohnston

erikjohnston Apr 20, 2018

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Err, yes, (though α is a subset of a, so its correct to say that it depends on a)

\end{split}
\] but by definition $u_{f,g}(\alpha_{f,g}(x))$ depends only on $\alpha_{f,g}(x)$, so $r_{f,g}(\alpha_{f,g}(x))$ depends on $a_{f,g}(x)$ and $c_{f,g}\alpha_{f,g}(x)$.

By induction, $c_{f,g}\alpha_{f,g}(x)$ depends on $a_{f,g}(x)$ and $c_{f,g}(\alpha_{f,g})^n(x), \forall n$. Since $(\alpha_{f,g})^n(x)$ repeats and we know $c_{f,g}$ is well defined, we can infer that $c_{f,g}(x)$ depends only on $\bigcup_{n=0}^\infty (\alpha_{f,g})^n(x) = a_{f,g}(x)$.

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richvdh Apr 20, 2018

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depends on α ?

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erikjohnston Apr 20, 2018

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Yes, if you mean the first "depends on". Though as above its correct to say it depends on a too.

@richvdh richvdh assigned erikjohnston and unassigned richvdh Apr 23, 2018

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commented Apr 26, 2018

@erikjohnston erikjohnston assigned richvdh and unassigned erikjohnston Apr 26, 2018

@richvdh richvdh removed their assignment Aug 16, 2018

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commented Mar 3, 2019

this would help with #1760 (i think?) as faster state res means that the number of extremities which need to be resolved is less of a consideration (especially if their intermediary resolution results have been cached and can be built on)

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