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That is a good challenge, as I am sure their reasons were extremely technical. Twitter also uses Lucene and also in different ways. And I am not at Amazon or Twitter, so I just go by their public presentations, quite technical ones. But basically I suspect it is like this. Vast Majority of actual search magic (vector analysis and heavy math and optimized index management) is in Lucene. But Lucene does not care much about getting data into the system (json, csv, xml, etc) or getting it out. It just has internal representation of documents and fields. Plus it does not do any sort of multi-index management, needed for scale. So, both Solr and Elasticsearch build on top of Lucene to do the user-friendly and scaling parts. Solr allows to send data in multiple formats, Elasticsearch sticks to JSON. Solr has multiple way to build search queries (url params, xml, json, changed over time), Elasticsearch sticks to JSON. Solr is a bit rigid about how schema (field collection and type definition) is done, Elastisearch is a bit more hands-off. Solr has composable pre-processor chains and explicit field analyzer chains, Elasticsearch focuses more on logs and has external pre-processors and custom scripting language. Solr uses Apache Zookeeper to coordinate distributed state, Elasticsearch rolled their own. Solr has one particular way to split data into replicas and shards and throw them from one server to another, Elasticsearch has another. Solr is perfectly happy to run a single node=server=collection=core. Elasticsearch starts with fully distributed cloud setup. All of these are trade-offs on top of actual search. That's why most of the consultants work with both Solr and Elasticsearch, the most difficult search optimization concepts are same for both. Bloomberg, I think, uses Solr directly and builds on top of that. They contributed Machine Learning ranking to Solr, for example. They also use Elasticsearch for log analysis, I believe. Amazon's needs are different again. Their data comes in through different routes (direct from databases?), their multi-tier replication and sharding strategy have different needs, they don't need user-facing user interface, etc. So, they take Lucene directly, and build the rest on top. And then contribute back to Lucene, which makes it available to both Solr and Elasticsearch. Everybody benefits in the end. |