How Ephemeral RAM-Only Messaging Works: Inside the Ring Buffer Architecture

When you send a message on almost any mainstream chat platform, a complex chain of persistent storage operations is triggered. Your message is serialized, written to a database write-ahead log (WAL), replicated across multiple storage nodes, indexed for search, and backed up to cold storage. Even when a service offers "disappearing messages," deleting data from a B-tree or LSM-tree on an SSD often leaves uncompacted blocks and filesystem artifacts behind.

sling.chat takes a fundamentally different systems engineering approach: what is never written to disk can never be recovered from disk. Instead of a database, every channel on sling.chat uses a fixed-size, in-memory Ring Buffer (circular buffer) written in C++20.

What Is a Circular Ring Buffer?

A ring buffer is a contiguous block of pre-allocated memory—such as a std::vector<Message> of fixed capacity $N$—managed by two logical pointers or indices: a head index pointing to the oldest element, and a size counter tracking how many elements are currently stored.

• Phase 1 (Filling the Buffer): While size < capacity, each new message is written to index (head + size) % capacity, and size increments by 1.
• Phase 2 (Wrap-Around Overwrite): Once size == capacity (for example, 100 messages), the buffer is full. When message #101 arrives, it directly overwrites the memory slot at head, and head advances to (head + 1) % capacity.

Because the underlying memory slots are reused circularly, inserting a new message operates in strict $O(1)$ constant time with zero dynamic heap re-allocations once the channel is active.

Thread Synchronization with C++20 std::shared_mutex

In a multi-threaded WebSocket server, dozens of clients may join a channel at the same moment to read the recent message history while other participants are actively broadcasting new messages. Protecting the ring buffer with a naive exclusive mutex would serialize all readers behind one another.

To maximize throughput, sling.chat's RingBuffer<T> template uses a reader-writer lock (std::shared_mutex):

• Concurrent Readers (std::shared_lock): When a user joins #general or switches to #tech, the server takes a shared read lock to produce a chronological snapshot. Any number of threads can hold a shared lock simultaneously without blocking each other.
• Exclusive Writers (std::unique_lock): When a message passes SHA-256 Proof-of-Work verification, the worker thread acquires a brief exclusive lock for the few nanoseconds required to move the Message struct into the circular slot and advance the index.

Performance and Privacy Benefits

1. Sub-Millisecond Latency: Reading and writing to L3 cache and system RAM is orders of magnitude faster than waiting for disk I/O or network round-trips to an external database container.
2. Predictable Memory Footprint: Because both the number of messages per channel and maximum payload length are bounded, memory usage cannot grow unboundedly over time.
3. Guaranteed Ephemerality: Old messages are physically overwritten in RAM as the conversation progresses, and if the server container stops or restarts, the entire volatile memory space vanishes instantly.