Mongo, Restheart, and Dokku

Mongo is a popular NoSQL database nowadays. It has several advantages compared to relational databases (schema-less, key-value store, scalable, distributed, propagated consistency) but the main advantage, IMHO, is that JSON is the lingua franca for the data structures it stores and the queries you use. JSON is a nice, simple data structure representation. It can be easily passed between the database and the application; and it can be converted to/from a string and an object in the native language you use. Mongo has many native-language APIs available for interfacing your application with the database, so you should be able to connect your application with Mongo.

Unfortunately, Xamarin Forms applications, written in C#, are Portable Class Library (PCL) applications, and the C# API for Mongo can’t work. Unfortunately, PCL apps and libraries can only link with other PCL libraries that have the same or additional target platforms. So, a PCL app that is targeting Windows, Android, and iOS cannot link with a library that only targets Windows, or only Android, or only iOS.

However, all is not lost. Restheart is an open source REST API server for Mongo databases. You can easily write a layer to encapsulate the calls that you need to access your Mongo database. There aren’t many functions you have to write beyond the usual CRUD, and the signatures for the method are just strings.

Setup

The installations instructions are on the Mongo.org website. Get the latest production version of Mongo. Then, execute the installation file. After it completes, create a directory for the database.

md data/db

To run Mongo, type

mongod --dbpath data/db

To set up Restheart, either copy the latest from Restheart.org, or clone the git repository for Restheart. If you are going to build it from scratch, you will need Maven, Java, and various other tools. Note, I highly recommend you build Restheart from git because the documentation is quite poor, and the only really good way to understand the server is to debug it.

To run Restheart, type java -jar ./restheart/target/restheart.jar, or the appropriate jar file. You can specify a configuration file for Restheart as a parameter when executing the jar.

CRUD

In a SQL database, data are stored in tables. In Mongo, data are stored in a collection. As with SQL, Mongo has CRUD statements for access to the database. The equivalent in Mongo of a row in a SQL table is a document, and is simply a JSON string. A projection of a JSON structure is equivalent to a projection in SQL. If you convert the JSON into a Newtonsoft JObject, you can make a projection of one key/value pair using the [] operator. As in SQL, an index can be created to improve search time.

Examples

SQL Op	Operation	Example	Example in Mongo shell	Example in Restheart
 Create database	 Create a database named db1.	 Create a database named db1.	use db1 NOTE: you need to insert a collection to actually create the database while using the Mongo shell.	 PUT http://localhost:8080/db1/ NOTE: Do not set data or you will create a record with that data in the database. It can’t be deleted!
Create table	Create collection	 Create a collection named “col”.	db.createCollection(“col”)	PUT http://localhost:8080/db1/col
Insert a row in a table	Insert a document in a collecton	Insert {“a1″:”b1”} in collection col.	 db.col.insert({“a1″:”b1”})	POST http://localhost:8080/db1/col payload is {“a1″:”b1”} header is Content-Type: application/json NOTE: The record type/value doesn’t have to be unique. So, you can insert duplicates if you like.
Read a row	Read a document	Find {} — find all records in collection col.	db.col.find({}) or just db.col.find()	GET http://localhost:8080/db1/col header is Content-Type: application/json
Read a row	Read a document	Find { “a1” : <value> } — find all records with key a1 in collection col.	db.col.find({“a1”:{$regex : “.*” }})	GET http://localhost:8080/db1/col?filter={“a1″:{$regex:”.*”}} NOTE: Make sure to note the correct syntax. The Restheart arg parser is very, very poor. E.g., using parentheses instead of equal sign: http://localhost:8080/db1/col?filter={“a1″:{$regex:”.*”}} This will return all records with a1, which is not what you intended! To limit the find to the “first” record (whatever that means), add ‘&pagelimit=1’ to the args of the GET url.
Read a row	Read a document with projection	Find { “a1” : <value> } — find all records with key a1 in collection col, but only show keys of “cc”. For example, suppose there is one record, {{“aa”:”bb”}, {“cc”:”dd”}}. Return {“cc”:”dd”}.	db.col.find({“aa”:{$regex : “.*” }}).projection({“cc”:1})	http://localhost:8080/db1/col?filter={“aa”:{$regex:”.*”}}&keys={“cc”:1}
 Update a row	 Update a document	Update {“a1″:”*.”} to {“a1″:”asdf”}	db.col.update({“a1″:{$regex:”.*”}},{“a1″:”xx”})	PATCH http://localhost:8080/tb3/col/*?filter={“a1″:”.*”} header is Content-Type: application/json payload is {“a1″:”asdf”}
 Delete a row	 Delete a document	 Delete a record with a particular key/value, { “a1” : <value> }.	 db.col.remove({“a1″:{$regex:”.*”}})	 DELETE http://localhost:8080/db1/col/*?filter={“aa”:{$regex:”.*”}}
 Delete a table	 Delete a collection	 Nuke an entire collection col.	 db.col.drop()	 DELETE http://localhost:8080/db1/col
 Delete database	 Delete database	 Nuke the entire database db1.	 db.dropDatabase()	 DELETE header contains If-Match:<the id of the database>, e.g., 570e3dc71d9563251070fab3 http://localhost:8080/tb1/

 

NOTE: Restheart normally requires an “ETag” header entry, i.e., “If-Match: 570e3dc71d9563251070fab3”, where that number is an ID of the record to delete. You can turn off ETag checking in a configuration file, which you can specify on the java command line.

To get a value, you will have to parse the json for the field of interest. You can use projection to help you with that.

Running Mongo and Restheart on Dokku

To run a Mongo database and a Restheart server on Dokku, you can get a copy of my Github example, then modify it for your needs. The example is here.

References

https://softinstigate.atlassian.net/wiki/display/RH/Installation+and+Setup#InstallationandSetup-7.3.MongoDBauthenticationwithjustenoughpermissionsauth-with-jep

https://softinstigate.atlassian.net/wiki/display/RH/Query+Documents

http://www.tutorialspoint.com/mongodb/mongodb_query_document.htm

https://docs.mongodb.org/manual/reference/method/db.collection.find/

https://www.mongodb.org/downloads#production

http://www.codeproject.com/Articles/1087008/Mongo-DB-Tutorial-and-Mapping-of-SQL-and-Mongo-DB

http://stackoverflow.com/questions/tagged/restheart

https://en.wikipedia.org/wiki/MongoDB