Read-only mounts

The implementation allows for a default mount, which is read-write, and for any number of additional mounts, which are read-only. This limitation is by design and is not expected to be removed in future Oak version.

There are two major aspects to this limitation

1. Having a commit run accross two or more multiple node stores is complicated in terms of implementation. Atomic commits will be very hard to ensure in a performant manner across multiple stores. Additionally, it will impose implementation burders to each NodeStore in order to support this special-case scenario.
2. There are multiple Oak subsystems that are not composite-aware and that would need to changed for that to happen, and this would again complicate the implementation for a special-case scenario.

Referenceable nodes

Referenceable nodes are not permitted in non-default mounts. The reason is cross-mount references can become invalid in scenarios where the set of mounts changes. Consider the following scenario:

Mounts:

• default mount D
• non-default mount N1, currently mounted under /tmp
• non-default mount N2, currently not mounted

In the repository, node /content/bar references referenceable node /tmp/foo (from N1). When the repository is shut down and reconfigureed to use N2 instead of N1, the reference can be broken unless we ensure that the reference stores used by N1 and N2 are the same. This does not happen today.

This constraint also means that:

• versionable nodes are not permitted in non-default mounts, as they are referenceable
• nt:resource nodes (usually found as children of nt:file nodes) are not permitted. It is recommended to replace them with oak:Resource ( see also OAK-4567 ).