Orchestrating video transcoding in ruby

wroclove.rb 2019 talk by Michal Matyas (his 4th wroclove.rb, first time speaking). Originally titled 'Transcoding in Ruby: a story'; renamed because the actual transcoding was done by ffmpeg, not Ruby. Based on conversation logs rather than the actual code (speaker no longer has access). Narrates three-phase evolution of a Rails media platform's transcoding pipeline: (1) MVP with CarrierWave + carrierwave-video + carrierwave_backgrounder and ActiveRecord callbacks — ended up as spaghetti with a CarrierWave bug preventing reprocessing of a single version, and inability to transcode versions based on other versions; (2) one-DB-model-per-file rewrite that still used CarrierWave — suffered race conditions from multiple uploaders setting status in parallel and got stuck in permanent 'processing' state; (3) full rewrite with a single processing worker using streamio-ffmpeg with custom args, priority-based transcoding (lowest quality first, like YouTube), Sidekiq with separate queues for processing (CPU-bound) and uploads to S3 (network-bound). Explains why AWS Lambda (15-minute max execution), Zencoder, and Amazon Elastic Transcoder were rejected (cost). Covers the MP4 container format (ISO base media / QuickTime origin; ftyp, mdat, metadata atoms) and delivers 9 ffmpeg transcoding tips. Q&A asks about ffmpeg vulnerabilities.