August | 2015 | IT Technologies

[see only one docker nodes in docker swarm infos]:
The Docker nodes were all copies of the same virtual machine that had already started Docker before cloning, which ended up creating the same Docker ID in /etc/docker/key.json. I stopped Docker, removed the key file, and then restarted so each daemon would create a new key file.
Reference: https://forums.docker.com/t/docker-swarm-issues/2132

Filed under Cloud Computing, IT Management Tagged with docker, vm

CAP Theorem

2015/08/24

In a distributed system you can satisfy at most 2 out of the 3 guarantees:

Consistency: all nodes see same data at any time, or reads return lastest written value by any client.
- It means that even though there are multiple clients that are reading and writing the data. All the clients see the same data at any given point of time. The reads by any client return the latest written value by a particular client.
Availability: the system allows operations all the time, and operations return quickly.
- System allows read and write operations on all the keys all the time, and these operations return very quickly. Availability = Reads/writes complete reliably and quickly.
- Availability might be very essential for cloud computing providers, because measurements have shown that a 500ms increase in latency for reads/writes operations can cause a 20% drop in business revenue.
- Why low latency matters? More details can be found under the following links:
  http://www.activo.com/why-low-latency-does-matter-and-increases-online-sales/
  http://highscalability.com/latency-everywhere-and-it-costs-you-sales-how-crush-it
Partition-tolerance: the system continues to work in spite of network partitions.
- When the system is partitioned, some parts of the system fail, the system should still continue to work. It means that the remaining functioning partitions of system still need to guarantee both consistency and availability.
- Partitions can happen across data-centers when the internet gets disconnected through “Internet router outages”, “Under-sea cables cut” or “DNS not working”
- Partitions can also occur within a data-center, e.g. a rack switch outage.