Understanding Transaction Logs, Soft Commit and Commit in SolrCloud by Erick Erickson.
From the post:
As of Solr 4.0, there is a new “soft commit” capability, and a new parameter for hard commits – openSearcher. Currently, there’s quite a bit of confusion about the interplay between soft and hard commit actions, and especially what it all means for the transaction log. The stock solrconfig.xml file explains the options, but with the usual documentation-in-example limits, if there was a full explanation of everything, the example file would be about a 10M and nobody would ever read through the whole thing. This article outlines the consequences hard and soft commits and the new openSearcher option for hard commits. The release documentation can be found in the Solr Reference Guide, this post is a more leisurely overview of this topic. I persuaded a couple of the committers to give me some details. I’m sure I was told the accurate information, any transcription errors are mine!
The mantra
Repeat after me “Hard commits are about durability, soft commits are about visibility“. Hard and soft commits are related concepts, but serve different purposes. Concealed in this simple statement are many details; we’ll try to illuminate some of them.
Interested? 😉
No harm in knowing the details. Could come in very handy.