grn_ctx is the most important object. grn_ctx keeps the current infomation such as:
grn_ctx provides platform features such as:
Most APIs receive grn_ctx as the first argument.
You can't use the same grn_ctx from two or more threads. You need to create a grn_ctx for a thread. You can use two or more grn_ctx in a thread but it is not needed for usual use-case.
TODO...