Vector clocks and distributed snapshots

vector clock

A vector clock is a vector composed of the logical clocks of all nodes. For a by NNA distributed system composed of$N$$N$ -dimensional vector, expressed as$[C_0,C_1,...,C_{n-1}]$。

Idea: Let each process know the logical clocks of all other processes in the system, so that the total order relationship can be achieved without additionally specifying the priority of the process.

Calculation method:
for process $i$ coming,$T_i\left[i\right]$ means process$i$ local logical time.
1. When the process$i$ When a new event occurs,$T_i[i]=T_i\left[i\right]+1$ .
2. When the process$When\; i$ sends a message, its vector clock$T_i$attached to the message.
3. When the process $After\; receiving\; the\; message,\; j$ will update the local vector time:$T_j[k]=max(T_j[k],T[k])$ ，其中 $k\in [1,N]$ (T is the vector time attached to the message).

$T_i<T_j\iff \forall x:T_i[x]⩽T_j[x]and\exists y：T_i[y]<T_j\left[y\right]$

For any two events $a$ and$b$，$a\to b\iff T_i<T_j$

distributed snapshot

How to "take a snapshot" of the system to obtain the global state of the system, which is used to restore the system to a normal state in the past when a failure occurs.

Master Lamport gave an example: a sky full of migratory birds, the scene is too big to be captured in one photo, how to take multiple photos and put them together to form a complete panorama?

Chandy-Lamport Algorithm
1. Initialization: Process $P_i$Record your own status and broadcast $marker\; to\; other\; processesand$ start recording incoming edge messages$.$

2. Spread the snapshot: process $P_j$Received $After\; themarker$ message, it will record its own status and start recording other incoming edge$messages$$.$Process$P_j$will pipe $C_{ij}$The state of the node is recorded as empty, and the snapshot is broadcast to other processes.

3. Terminate the snapshot: when all incoming edges have received $When\; themarker\; message\; issent$ , the snapshot is recorded.

(Once a process receives $marker$ , then receive$The\; channel\; ofmarker$ is set to empty, and the process no longer records the messages of this channel$)$