A read-only replica contains full logs, a MemTable, and an SSTable.
A read-only replica has the following characteristics:
Contains full logs, a MemTable, and an SSTable.
Cannot join a Paxos group or participate in log voting as a Paxos member, but acts as a listener to follow up logs of a Paxos member in real time and replays the logs locally. This type of replicas do not act as voting members and therefore cause no delay in transaction commits.
Provides read-only services if an application does not require strong consistency reads.
Can be converted into a full-featured replica.
The following table describes more features of such a replica.
| Feature | Description |
|---|---|
| Replica name and abbreviation | READONLY (R) |
| Whether logs are contained | It has asynchronous logs. It is only a listener instead of a member of the Paxos group (ASYNC_CLOG). |
| Whether a MemTable is contained | Yes (WITH_MEMSTORE) |
| Whether an SSTable is contained | Yes (WITH_SSSTORE) |
| Data security | Medium |
| Time to become the leader | Not supported |
| Resource cost | High |
| Service | Supports non-consistent read. |
| Limitations on replica type conversion | It can be converted only into a full-featured replica. |