Explain replication and read replicas.
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Understand the problem
replication
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Read the code
Read routing with read-your-writes
// Route reads to a follower for scale, but send a user to the leader
// briefly after they write so they never see their own stale data.
function pickReadDB(userId, lastWriteAt) {
const RECENT_MS = 3000; // longer than typical replication lag
if (Date.now() - (lastWriteAt[userId] ?? 0) < RECENT_MS) {
return leader; // read-your-writes: hit the primary
}
return followers[Math.floor(Math.random() * followers.length)];
}05
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