More recent information FCM: sending FCM messages at scale

Quota Tokens, Token Buckets, and Refills: When sending messages against the FCM HTTP v1 API, each request consumes an allotted Quota Token in a given time window. This window, called a "Token Bucket", refills to full at the end of the time window. For example: the HTTP v1 API allots 600K Quota Tokens for each 1-minute Token Bucket, which refills to full at the end of each 1-minute window.