StreamQueries#streamResults does not clear scroll context when finished [DATAES-764]

[spring-projects-issues]

Sascha Woo opened DATAES-764 and commented

The frequent use of the stream implementation of Elasticsearch template e.g. in large iterations leads to the following warning since Elasticsearch 6.8.x and will be a problem in the next major release.

Trying to create more than 500 scroll contexts will not be allowed in the next major version by default. You can change the [search.max_open_scroll_context] setting to use a greater default value or lower the number of scrolls that you need to run in parallel.

The documentation of the Clear scroll API [1] point out the following:

Search context are automatically removed when the scroll timeout has been exceeded. However keeping scrolls open has a cost, as discussed in the previous section so scrolls should be explicitly cleared as soon as the scroll is not being used anymore using the clear-scroll API:

[1] https://www.elastic.co/guide/en/elasticsearch/reference/6.8/search-request-scroll.html#_clear_scroll_api

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Affects: 3.2.5 (Moore SR5)

Referenced from: pull request #406

Backported to: 3.2.6 (Moore SR6)