Open source Java libraries that hold the microsecond path: no allocation, no blocking, no platform to operate. Jars you put in your own process, on your own threads.
Treats latency as a correctness property. FIX 4.2 to FIX Latest, type-safe encoders, store, logging and administration.
Asynchronous NIO. Selector-driven IO workers you can pin and balance sessions across, off the allocation path.
Lock-free MPMC and SPSC ring buffers whose slots are mutated in place, idle strategies across the whole trade-off, and a hashed wheel timer.
Raft elections inside your process over UDP: one instance leads, every other is told the moment that changes. A witness lets a two-node active/passive pair survive losing one node.