There is a lot of discussion in the NoSQL community about consistency levels offered by NoSQL DBs and its relation to CAP/PACELC theorem. This article seeks to address this w.r.t MongoDB in-depth, thereby providing a broad overview of approaching CAP and PACLEC theorems from a NoSQL perspective. For a quick summary of consistency levels, please refer to Consistency Matrix towards the end of the article.
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MongoDB Notes for the Purpose of This Article
MongoDB always replicates data asynchronously from primary to secondaries. This cannot be changed.