Translations:Manual:Database access/38/en

Lag primarily occurs when large write queries are sent to the master. Writes on the master are executed in parallel, but they are executed in serial when they are replicated to the slaves. The master writes the query to the binlog when the transaction is committed. The slaves poll the binlog and start executing the query as soon as it appears. They can service reads while they are performing a write query, but will not read anything more from the binlog and thus will perform no more writes. This means that if the write query runs for a long time, the slaves will lag behind the master for the time it takes for the write query to complete.