An EventEngine handles all cross-platform I/O, task execution, and DNS resolution for gRPC. A default, cross-platform implementation is provided with gRPC, but part of the intent here is to provide an interface for external integrators to bring their own functionality. This allows for integration with external event loops, siloing I/O and task execution between channels or servers, and other custom integrations that were previously unsupported.
WARNING: This is experimental code and is subject to change.
EventEngines are expected to internally create whatever threads are required to perform I/O and execute callbacks. For example, an EventEngine implementation may want to spawn separate thread pools for polling and callback execution.
At a high level, gRPC provides a ResourceQuota system that allows gRPC to reclaim memory and degrade gracefully when memory reaches application-defined thresholds. To enable this feature, the memory allocation of read/write buffers within an EventEngine must be acquired in the form of Slices from SliceAllocators. This is covered more fully in the gRFC and code.
Some callbacks may be expensive to run. EventEngines should decide on and document whether callback execution might block polling operations. This way, application developers can plan accordingly (e.g., run their expensive callbacks on a separate thread if necessary).
Assume that gRPC may use an EventEngine concurrently across multiple threads.