Transactions

Support for transactions is available from MongoDB v4.0.

Setting up a Replica Set

As described in the MongoDB Manual, "multi-document transactions are available for replica sets only. Transactions for sharded clusters are scheduled for MongoDB 4.2".

Follow these steps to start a replica set. The following script will create a replica set with 3 nodes:

mkdir db1
mkdir db2
mkdir db3
mongod --dbpath ./db1 --port 27021 --replSet "rs0" --bind_ip 127.0.0.1
mongod --dbpath ./db2 --port 27022 --replSet "rs0" --bind_ip 127.0.0.1
mongod --dbpath ./db3 --port 27023 --replSet "rs0" --bind_ip 127.0.0.1
mongo --port 27021 replica_set_initiate.js

The contents of replica_set_initiate.js are:

rs.initiate( {
   _id : "rs0",
   members: [
      { _id: 0, host: "127.0.0.1:27021" },
      { _id: 1, host: "127.0.0.1:27022" },
      { _id: 2, host: "127.0.0.1:27023" }
   ]
})

Executing Transactions

In MongoDB, transactions are bound to Sessions.

In Mongoc.jl, use the function Mongoc.transaction with do-syntax to execute a transaction, and use the argument session to get database and collection references bound to the session that will execute the transaction.

Just use the session object the same way you would use a Client.

Example

import Mongoc

# connect to a Replica Set
client = Mongoc.Client("mongodb://127.0.0.1:27021,127.0.0.1:27022,127.0.0.1:27023/?replicaSet=rs0")

# this collection reference is not bounded to the transaction
collection_unbounded = client["my_database"]["my_collection"]

# insert a dummy document, just to make sure the collection exists
push!(collection_unbounded, Mongoc.BSON("""{ "test" : 1 }"""))
empty!(collection_unbounded)

Mongoc.transaction(client) do session
    database = session["my_database"]
    collection = database["my_collection"]
    new_item = Mongoc.BSON()
    new_item["inserted"] = true
    push!(collection, new_item)
    println("collection_bounded is empty? ", isempty(collection_unbounded))
    println("collection is empty? ", isempty(collection))
end

println(collect(collection_unbounded))

The script output is:

collection_bounded is empty? true
collection is empty? false
Mongoc.BSON[BSON("{ "_id" : { "$oid" : "5ba4251f3192e3298b62c5a3" }, "inserted" : true }")]