This simulator lets you watch numbered writes enter a leader's append-only log and replay on two followers. You choose when the client is told a write succeeded, slow a follower down, or cut followers off the network, and read the consequences off the sequence counters.
• A 3D scene with the write client and acknowledgment stream, the leader's append-only log, the Follower 1 and Follower 2 replay logs, and an acknowledgment and quorum gate. • Controls for the acknowledgment policy (leader only asynchronous, or leader plus one follower), Follower 2 replay delay (1 to 6 seconds), and Disconnect follower 1 and Disconnect follower 2 switches. • Readouts for leader log sequence, each follower's sequence, the acknowledged prefix, unacknowledged writes and Follower 2's read lag. • Experiments for asynchronous lag, a blocked quorum and one follower isolated.
A follower's sequence is the number of leader writes older than its replay delay, while it is connected. With the leader-only policy, a write is acknowledged as soon as it is appended locally, so the client sees success while Follower 2 may still serve an older prefix. With the leader-plus-one-follower policy, the acknowledged prefix is the median of the leader and the two follower sequences, meaning at least two copies hold the write. If both followers are disconnected, the leader keeps appending but acknowledgments stop; if only one is isolated, the other can still satisfy the two-copy requirement.
The log is monotonic and ordered, with no leader elections, leader failure, conflicting writes, disk failures or bandwidth limits, and reconnecting followers catch up instantly. The fixed leader is deliberate so replication can be studied on its own. The acknowledgment policies are simplified illustrations and are not a complete specification of any database's consistency model.
Not necessarily. It depends on the policy: a two-copy quorum needs only the leader and one follower. The lab shows this when the acknowledged prefix is ahead of a slow or disconnected follower's sequence.
It is the highest write sequence that the current policy has confirmed to the client. Under leader-only it tracks the leader log; under leader-plus-one-follower it is the median of the three sequences, so two copies are guaranteed.
With the quorum policy, the leader continues to append writes, but no write can gain a second copy, so the acknowledged prefix stops advancing and unacknowledged writes pile up.
No. The leader stays fixed so you can isolate log replication, acknowledgment policy and partitions without the added complexity of failover.