#SWIM

This is an attempt to implement SWIM in GO.

What is SWIM

I recommend you to read the paper in References sector first, i don't think i can explain better than the paper itself.

Here is the TL;DR version:

In distributed computing, a failure detector is an application or a subsystem that is responsible for detection of node failures or crashes in a distributed system.

A Traditional way to implement it is using heartbeat protocol: which either impose network loads that grow quadratically with group size, or compromise response times or false positive frequency w.r.t. detecting process crashes

The new system, called SWIM, provides a membership substrate that:

1. imposes a constant message load per group member;
2. detects a process failure in an (expected) constant time at some non-faulty process in the group;
3. provides a deterministic bound (as a function of group size) on the local time that a non-faulty process takes to detect failure of another process;
4. propagates membership updates, including information about failures, in infection-style (also gossip-style or epidemic-style); the dissemination latency in the group grows slowly (logarithmically) with the number of members;
5. provides a mechanism to reduce the rate of false positives by “suspecting” a process before “declaring” it as failed within the group

SWIM has two components: (1) a Failure Detector Component, that detects failures of members, and (2) a Dissemination Component, that disseminates information about members that have recently either joined or left the group, or failed.

Failure Detector

Given we have a cluster of n nodes. Each node has information about m nodes in the cluster (m <= n).

Every T' time unit (which called a protocol period), at node Mi

1. Increase the period pr

2. Select a random node in m nodes, called it Mj and ping it ping(Mi, Mj , pr). Wait for the worst-case message round-trip for an ack(Mi, Mj , pr).

   • If the ack message come back, that means Mj is still alive.
   • If not, move to step 3

3. Select k nodes in m nodes, ask them to ping Mj ping-req(Mi, Mj , pr)

   • If one of them receive the ack message ack(Mi, Mj , pr), the node is still alive
   • If no one receive the ack until the end of the period, declared Mj as failed.

4. To reduce the false positive in step 3, instead of mark Mj as failed immediately, we will mark it as suspected. After a prespecified time-out, it will be declared as failed. But if it response within the timeout, it will be declared as alive again.

As any given time, at node Mi

On receipt of ping-req(Mm, Mj , pr) message (Mj != Mi), send a ping(Mi, Mj , Mm, pr) message to Mj

On receipt of ack(Mi, Mj , Mm, pr) message from Mj, send an ack(Mm, Mj , pr) message to received to Mm

On receipt of ping(Mm, Mi, Ml, pr) message from Mm, reply with an ack(Mm, Mi, Ml, pr) message to Mm

On receipt of ping(Mm, Mi, pr) message from Mm, reply with an ack(Mm, Mi, pr) message to Mm

Dissemination Component (cont.)

References

1. On scalable and efficient distributed failure detectors.
2. SWIM: Scalable Weakly-consistent Infection-style Process Group Membership Protocol