Question

What is the specific &#x27;read&#x27; guarantee that ensures a user always sees their own updates immediately after a write, even if the database is replicated?

Accepted Answer

The specific guarantee is called read-your-writes consistency, also known as read-after-write consistency. This guarantee ensures that once a user has submitted a data update, any subsequent read requests performed by that same user will return the updated value, even if the system uses database replication. Replication is the process of copying data across multiple servers to improve performance or availability. In a replicated system, there is often a time lag between writing data to a primary node and that data appearing on secondary nodes. Without this consistency guarantee, a user might write data to the primary node and then immediately read from a secondary node that has not yet received the update, resulting in the user seeing old or stale data. To implement read-your-writes consistency, the system must ensure that the user&#x27;s read requests are routed to the node containing their latest update, or that the system waits for the replication to complete before allowing the read. For example, if a user updates their profile picture, read-your-writes consistency ensures that the very next time they refresh their page, they see the new image rather than the old one, regardless of how many background database copies exist.

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What is the specific 'read' guarantee that ensures a user always sees their own updates immediately after a write, even if the database is replicated?