Question
Answer and Explanation
Retrieving all documents from Elasticsearch 5.6 can be achieved using the 'scan' and 'scroll' API. This method is designed for efficient retrieval of large datasets and is superior to simply fetching all results at once.
Here's a breakdown of how to do it:
1. Initiate the Scan:
- You start by sending a search request with the `search_type=scan` parameter. This will not return any results directly but will create a context for scrolling, along with a `scroll_id`. Set the size to how many documents you want to retrieve at a time. It's recommended to keep the size relatively large.
2. Perform the Scroll:
- After the scan, you use the `scroll` endpoint repeatedly with the obtained `scroll_id`. This allows you to retrieve batches of documents. You'll continue doing this until no more documents are returned, indicating that all have been retrieved.
3. Important parameters:
- `scroll`: this parameter specifies how long the scroll context should remain alive. After this time, the context will be deleted and you won't be able to retrieve next pages. For example '1m' (1 minute), '30s' (30 seconds) are valid.
- `size`: how many documents to be returned per scroll request. Setting a large size can potentially speed up the retrieval as it reduces the amount of requests.
Example using cURL:
First, initiate the scan with following request:
curl -XGET 'localhost:9200/your_index/_search?search_type=scan&scroll=1m&size=1000' -d '{"query": {"match_all": {}}}'
This command will output a response including a `_scroll_id`. Next step is to use the scroll API to get the actual results. For example, assuming `_scroll_id` is "dxNlY291dCBzY2FuOzU7MzI1MjE3MzpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NDpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NTpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NjpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NzpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MDs=":
curl -XGET 'localhost:9200/_search/scroll?scroll=1m' -d '{"scroll_id": "dxNlY291dCBzY2FuOzU7MzI1MjE3MzpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NDpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NTpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NjpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MzI1MjE3NzpwV05xN3hIalFzRjM1dFp5Ym9fVjg7MDs="}'
Continue using scroll endpoint until response does not contain any documents.
Important Notes:
- Replace `your_index` with the name of your Elasticsearch index.
- Ensure to handle the case where the scroll context expires and re-initiate the process if required.
- The `match_all` query in the example retrieves all documents. You can use any valid Elasticsearch query based on your needs.
This method ensures you efficiently retrieve all documents in Elasticsearch 5.6 without overwhelming the server or running into size limitations, making it a suitable choice for large datasets.