GETting Results from ElasticSearch

ElasticSearch exposes most of its functionality via a RESTful API. When it comes to querying the data, you can either pass request parameters, e.g. the query string, or use the query DSL which structures the queries as JSON objects. For a talk I gave I used this example which executes a GET request with curl and passes the JSON query structure in the request body.

curl -XGET 'http://localhost:9200/jug/talk/_search' -d '{
    "query" : {
        "query_string" : {"query" : "suche"} 
    },
    "facets" : {
        "tags" : {
            "terms" : {"field" : "speaker"} 
        }
    }
}'

An alert listener(*) later told me that you can't use a request body with a GET request. While preparing the talk I also thought about this and only added it after testing it successfully. At least it is unusual and some software like proxy caches might not handle your request as intended. A lot of ElasticSearch examples I have seen are using POST requests instead but I think semantically requesting search results should be a GET.

The ElasticSearch docs explicitly allow the use of GET requests with a request body:

"Both HTTP GET and HTTP POST can be used to execute search with body. Since not all clients support GET with body, POST is allowed as well."

I didn't find a hint in the HTTP specification whether this should be allowed or not. This answer on Stackoverflow goes in the same direction as my initial concern, that you shouldn't do it because it might not be expected by users and also not supported by some stacks.

Finally, in this message Roy Fielding, one of the authors of the HTTP specification, discourages the use of a request body with GET.

"... any HTTP request message is allowed to contain a message body, and thus must parse messages with that in mind. Server semantics for GET, however, are restricted such that a body, if any, has no semantic meaning to the request."

As the query DSL influences the response it is clear that this is a semantic meaning, which by the words of Roy Fielding shouldn't be the done. So no consensus exists on this topic. By hindsight I am quite surprised that this works out that well with ElasticSearch and there aren't any problems I have heard about.

(*) I wish he didn't tell me, while talking about another talk, that he always looks for mistakes in slides when he's bored ;).