This page describes common patterns in pipelines with [custom I/O connectors]({{ site.baseurl }}/documentation/io/developing-io-overview/). Custom I/O connectors connect pipelines to databases that aren‘t supported by Beam’s [built-in I/O transforms]({{ site.baseurl }}/documentation/io/built-in/).
[Built-in I/O connectors]({{ site.baseurl }}/documentation/io/built-in/) are tested and hardened, so use them whenever possible. Only use custom I/O connectors when:
For instance, use a custom I/O connector to enrich pipeline elements with a small subset of source data. If you’re processing a sales order and adding information to each purchase, you can use a custom I/O connector to pull the small subset of data into your pipeline (instead of processing the entire source).
Beam distributes work across many threads, so custom I/O connectors can increase your data source’s load average. You can reduce the load with the startstart and finishfinish bundle annotations.