Correct usage of _writev in node.js

Question

What is the correct usage of _writev() in node.js? The documentation says:

If a stream implementation is capable of processing multiple chunks of data at once, the writable._writev() method should be implemented.

It also says:

The primary intent of writable.cork() is to avoid a situation where writing many small chunks of data to a stream do not cause a backup in the internal buffer that would have an adverse impact on performance. In such situations, implementations that implement the writable._writev() method can perform buffered writes in a more optimized manner.

From a stream implementation perspective this is okay. But from a writable stream consumer perspective, the only way that write or writev gets invoked is through Writable.write() and writable.cork()

I would like to see a small example which would depict the practical use case of implementing _writev()

Answer 1

A writev method can be added to the instance, in addition to write, and if the streams contains several chunks that method will be picked instead of write. For example Elasticsearch allows you to bulk insert records; so if you are creating a Writable stream to wrap Elasticsearch, it makes sense to have a writev method doing a single bulk insert rather than several individual ones, it is far more efficient. The same holds true, for example, for MongoDB and so on.

This post (not mine) shows an Elasticsearch implementation https://medium.com/@mark.birbeck/using-writev-to-create-a-fast-writable-stream-for-elasticsearch-ac69bd010802

Answer 2

The _writev() will been invoked when using uncork(). There is a simple example in node document.

stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());

More See,

https://nodejs.org/api/stream.html#stream_writable_uncork https://github.com/nodejs/node/blob/master/lib/_stream_writable.js#L257