1. SQLite isn't heavy. There are probably even lighter-weight relational solutions, but I haven't needed one, so I haven't looked.
2. Even among non-relational document storage solutions, Mongo has a poor reputation. As someone else says elsewhere in the comments, it's hard to trust a team that let so many ridiculously dangerous bugs (and a basically unreliable design) out into production for so long.
2. Even among non-relational document storage solutions, Mongo has a poor reputation. As someone else says elsewhere in the comments, it's hard to trust a team that let so many ridiculously dangerous bugs (and a basically unreliable design) out into production for so long.