The database is a core component for your Nuxeo cluster infrastructure, since it will store the document hierarchy, all document properties, and will be used as well for various queries. In addition to the document repository, the database is also used to persist directories and KeyValue service content.
Nuxeo supports many databases, both NoSQL and relational ones. Among them, MongoDB and PostgreSQL currently provide the best overall performances for Nuxeo usage.
A database system providing high availability. Each solution has its own options for this, therefore we can't go into further details here.
Best performances are obtained using MongoDB because it is a document based database. It is built to manage large volumes and is highly scalable.
If you have no choice but relying on a SQL based database, then go for PostgreSQL.
Each database has its own solutions for high availability, therefore we may not recommend a specific option here. You may however refer to our database configuration documentation for further details.