Add library `lucene-backward-codecs` to migrate indices.
To be able to read and migrate Lucene indices from old (4.x)
formats to new (5.x) ones, add the `lucene-backward-codecs`
library to the project.
It is added to the `ext` directory and therefore to the classpath.
According to the Lucene documentation, having it in the classpath
can affect performance. But right now the `ext` directory is the
only one available and even for a separate tool for offline
migration the library would be needed.
3 files changed
