Best strategy for queueing when network/server is down

[utdrmac]

I have InfluxDB running on a server, and several raspberry pi's collecting data IoT style. When the network connection is lost between PIs and server, how should the client queue data for later retry/batch submission so as to not lose any data? Additionally, PIs have small amounts of RAM, so this queue would need to be disk-based. What is the best strategy here? Just keep appending wire-line strings to a file, or is there something better?