What is ToroDB Stampede?

ToroDB Stampede is a replication and mapping technology to maintain a live mirror of a MongoDB database (or sub-set) in a SQL database. ToroDB Stampede uses MongoDB's replica set oplog to keep track of the modifications in MongoDB.

ToroDB Stampede Structure

During replication ToroDB Stempede transforms MongoDB's JSON documents into a relational schema that allows certain queries (such as aggregates) to complete faster as running against JSON documents.

Mapping example

Current Limitations

SQL Target

Currently, ToroDB Stampede only supports the free open-source databases PostgreSQL and MySQL (and compatible databases) as targets.


ToroDB Stampede only supports MongoDB 3.2 and 3.4 at the moment.

The following MongoDB features are not yet supported:

  • Capped collections
  • The collMod command
  • The applyOps command (will stop the replication server)
  • The character \0 is escaped in strings because PostgreSQL doesn't support it.

The automatic creation of indexes in the target database is currently limited as follows:

  • Only simple one-key indexes (ascending and descending - those that ends in 1 and -1 when declared in MongoDB)
  • Index properties sparse and background are ignored
  • All keys path with the exception to the paths resolving in scalar value (e.g.: db.test.createIndex({"a": 1}) will not index value of key a for the document {"a": [1,2,3]})

Incompatible Document Designs

The main benefit of ToroDB Stampede is to flatten nested documents into tables. There are some patterns that cause the relational tables to end up with a very high number of columns—possibly up to the limitation of the SQL backend (in PostgreSQL about 1600 columns).

  • Pattern "key as values"
    Document key-names are turned into columns of tables. If you store values in the key-names, the table will have as many columns as you have distinct key-names.

  • Too many fields per document
    If several of them are optional and only some of them appear in each document, there might be thousands of columns.