When we learned about bulk requests earlier in [bulk], you may have asked
yourself, `Why does the `bulk API require the funny format with the newline
characters, instead of just sending the requests wrapped in a JSON array, like
the mget API?''
To answer this, we need to explain a little background: Each document referenced in a bulk request may belong to a different primary
shard, each of which may be allocated to any of the nodes in the cluster. This
means that every action inside a bulk request needs to be forwarded to the
correct shard on the correct node.
If the individual requests were wrapped up in a JSON array, that would mean that we would need to do the following:
-
Parse the JSON into an array (including the document data, which can be very large)
-
Look at each request to determine which shard it should go to
-
Create an array of requests for each shard
-
Serialize these arrays into the internal transport format
-
Send the requests to each shard
It would work, but would need a lot of RAM to hold copies of essentially the same data, and would create many more data structures that the Java Virtual Machine (JVM) would have to spend time garbage collecting.
Instead, Elasticsearch reaches up into the networking buffer, where the raw request has been received, and reads the data directly. It uses the newline characters to identify and parse just the small action/metadata lines in order to decide which shard should handle each request.
These raw requests are forwarded directly to the correct shard. There is no redundant copying of data, no wasted data structures. The entire request process is handled in the smallest amount of memory possible.