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mongo-connector-postgresql is used to make the bridge between PostgreSQL and MongoDB. It acts as a river from MongoDB to PostgreSQL.

This connector is a plugin for Mongo Connector. Thanks a lot for their contribution !

DISCLAIMER

Please note: all tools and scripts in this repo are released for use "AS IS" without any warranties of any kind, including, but not limited to their installation, use, or performance. We disclaim any and all warranties, either express or implied, including but not limited to any warranty of noninfringement, merchantability, and/ or fitness for a particular purpose. We do not warrant that the technology will meet your requirements, that the operation thereof will be uninterrupted or error-free, or that any errors will be corrected. Any use of these scripts and tools is at your own risk. There is no guarantee that they have been through thorough testing in a comparable environment and we are not responsible for any damage or data loss incurred with their use. You are responsible for reviewing and testing any scripts you run thoroughly before use in any non-testing environment.

System Overview

mongo-connector-postgresql creates a pipeline from a MongoDB cluster to PostgreSQL. It synchronizes data in MongoDB to the target then tails the MongoDB oplog, keeping up with operations in MongoDB in real-time. It has been tested with Python 2.7 and PostgreSQL 9.5.

All the works of tailing the oplog, performing the initial sync is made by the mongo-connector. So the first documentation you should read is on their website.

Getting Started

For some context (in french) : See our blog article

Installation

The easiest way to install mongo-connector and our extension is with:

pip install mongo-connector mongo-connector-postgresql

Using this connector

For the configuration of the connector, please check out the mongo-connector documentation beforehand since only the pg-mongo-connector specifics are written here.

Just call the mongo-connector and specify this plugin with the option -d:

mongo-connector  -t postgresql://localhost -d postgresql_manager -c config.json

The url could follow any of the syntax defined by libpq connection string.

This connector use its own mapping file to determine the fields that should be written in PostgreSQL and their types. This file should be named mappings.json. Here is a sample :

{
    "my_mongo_database":{
        "my_mongo_collection":{
            "pk":"id",
            "indices":[
                "UNIQUE INDEX subproperty_idx ON my_postgre_table(subproperty)"
            ],
            "_id":{
                "dest":"id",
                "type":"TEXT"
            },
            "description":{
                "type":"TEXT",
                "index": true,
            },
            "enable":{
                "dest":"enabled",
                "type":"BOOLEAN"
            },
            "myobject.subproperty":{
                "dest":"subproperty",
                "type":"TEXT"
            }
        }
    }
}

Please notice the following :

• The pk field is mandatory and should point to the destination's primary key
• By default, the dest field is set to the original Mongo field name.
• If the original document in mongodb has a embedded document, everything is flattened to be inserted in PostgreSQL
• One can define indices in two different ways : Using the array indices and a SQL definition or autogenerate index

by setting the index field to true

The connector also supports arrays of documents. Let say your Mongo database stores the following documents :

{
    "posts":{
        "name":"Check out the mongo -> postgres connector",
        "content":"Inspiring blog post",
        "comments":[
            {
                "user":"Elon Musk",
                "comment":"What a revolution !"
            },
            {
                "user":"Kevin P. Ryan",
                "comment":"Nice !"
            }
        ]
    }
}

To allow the connector to map the post objects AND its comments, you should use the following mapping :

{
    "my_mongo_database":{
        "posts":{
            "pk":"id",
            "_id":{
                "dest":"id",
                "type":"TEXT"
            },
            "content":{
                "type":"TEXT"
            },
            "comments":{
                "type":"_ARRAY",
                "fk":"post_id"
            }
        },
        "comments":{
            "pk":"id",
            "post_id":{
                "dest":"post_id",
                "type":"TEXT"
            },
            "user":{
                "dest":"user",
                "type":"TEXT"
            },
            "comment":{
                "dest":"comment",
                "type":"TEXT"
            }
        }
    }
}

Please notice the following :

• The type _ARRAY is used to indicate to the connector that the field is an array
• The additional field fk is provided to indicate to the connector where to store the root document id. This field is mandatory for an array
• The comments' mapping declares only the primary key but no mapping exists. The connector will generate the identifier automatically
• The foreign key must be declared in the comments table so it's created with the schema initialization

Finally, the connector supports arrays of scalar. Let say your Mongo database stores the following documents in the timeline collection :

{
    "author": "507f1f77bcf86cd799439011",
    "posts": [{
            "name": "Check out the mongo -> postgres connector",
            "tags": [
                    "Awesome",
                    "Article",
                    "Postgres"
            ]
    }]
}

One can use the following mapping :

{
    "my_mongo_database": {
            "timeline": {
                    "pk": "id",
                    "_id": {
                            "dest": "id",
                            "type": "TEXT"
                    },
                    "author": {
                            "type": "TEXT"
                    },
                    "posts": {
                            "type": "_ARRAY",
                            "dest": "timeline_posts",
                            "fk": "timeline_id"
                    }
            },
            "timeline_posts": {
                    "pk": "id",
                    "_id": {
                            "dest": "id",
                            "type": "TEXT"
                    },
                    "name": {
                            "type": "TEXT"
                    },
                    "tags": {
                            "dest": "timeline_posts_tags",
                            "type": "_ARRAY_OF_SCALARS",
                            "fk": "timeline_post_id",
                            "valueField": "tag"
                    }
            },
            "timeline_posts_tags": {
                    "pk": "id",
                    "_id": {
                            "dest": "id",
                            "type": "TEXT"
                    },
                    "tag": {
                            "type": "TEXT"
                    }
            }
    }
}

Contribution / Limitations

We searched for a software like this for quiet a time and we are confident that other people would need it. However, some features/improvements are currently lacking :

• A better documentation ?
• Their is no way to map a mongo collection to a differently named postgres table
• There is virtually no error handling, especially if the mapping is wrong (e.g. missing pk field)
• Rollbacks are not supported
• System commands are not supported (e.g. create collection)
• Only operations on the 'public' schema are allowed
• Currently, because of our use of the ON CONFLICT directive, only PostgreSQL >= 9.5 can be used

Even if this code is battle tested (we use it in production and fix for bugs every time we see one), it is far from perfect. Indeed, it was written quickly, by Python aware (but still Java specialized) developers in a "proof of concept" perspective. Therefore, one can easily improve the code quality, the test coverage (which is currently 0%), the architecture or write more 'Pythonic' code.