Eventual Consistency Explained for Techies
Preamble: Have a look at my previous article titled “Eventually Consistency Explained for non-Techies”
Eventual and Weak Consistency
SimpleDB is eventually consistent. Eventual consistency is a version of weak consistency — you may not see the latest writes committed to the system.
Imagine that you have a system of N nodes. Of these, W nodes are involved in any write sent to the system and R nodes are contacted on any read from the system. Strong consistency can be achieved if R+W > N. In other words, if the read sets and write sets overlap, the read can discover the most recent write to the system.
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Eventual Consistency Explained for Non-techies
If you work in the Computer industry, especially the Internet industry, chances are good that you have encountered an eventually-consistent system.
For example, when managing an internet or IT business, you might have considered one of all of the following DB architectures:
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Use a single DB host
- e.g. MyHost
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Use a single DB host for your writes, but several for your reads
- e.g. MyWriteHost & MyReadHost1, MyReadHost2, MyReadHost 3, etc …
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Use multiple DB hosts
- e.g. MyHost1, MyHost2, etc….
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Use multiple DB hosts for your writes and your reads
- e.g. MyWriteHost1, MyWriteHost2, etc… & MyReadHost1, MyReadHost2, etc, ….
These 4 choices represent an increasing degree of data traffic partitioning, with 1 having no partitioning and 2 having the most partitioning.
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