GIANT Stories at MongoDB

Introduction to MongoDB Transactions in Python

Multi-document transactions arrived in MongoDB 4.0 in June 2018. MongoDB has always been transactional around updates to a single document. Now, with multi-document transactions we can wrap a set of database operations inside a start and commit transaction call. This ensures that even with inserts and/or updates happening across multiple collections and/or databases, the external view of the data meets ACID constraints.

To demonstrate transactions in the wild we use a trivial example app that emulates a flight booking for an online airline application. In this simplified booking we need to undertake three operations:

  1. Allocate a seat (seat_collection)
  2. Pay for the seat (payment_collection)
  3. Update the count of allocated seats and sales (audit_collection)

For this application we will use three separate collections for these documents as detailed in bold above. The code in updates these collections in serial unless the --usetxns argument is used. We then wrap the complete set of operations inside an ACID transaction. The code in is built directly using the MongoDB Python driver (Pymongo 3.7.1). See the section on client sessions for an overview of the new transactions API in 3.7.1.

The goal of this code is to demonstrate to the Python developer just how easy it is to covert existing code to transactions if required or to port older SQL based systems.

Setting up your environment

The following files can be found in the associated github repo, pymongo-transactions.

  • gitignore : Standard Github .gitignore for Python
  • LICENSE : Apache's 2.0 (standard Github) license
  • Makefile : Makefile with targets for default operations
  • : Run a set of writes with and without transactions. Run python -h for help.
  • : The file containing the transactions retry functions.
  • : Use a MongoDB change stream to watch collections as they change when is running
  • : Starts a MongoDB replica set (on port 7100) and kills the primary on a regular basis. This is used to emulate an election happening in the middle of a transaction.
  • : check and/or set feature compatibility for the database (it needs to be set to "4.0" for transactions)
You can clone this repo and work alongside us during this blog post (please file any problems on the Issues tab for the repo).

We assume for all that follows that you have Python 3.6 or greater correctly installed and on your path.

The Makefile outlines the operations that are required to setup the test environment.

All the programs in this example use a port range starting at 27100 to ensure that this example does not clash with an existing MongoDB installation.


To setup the environment you can run through the following steps manually. People that have make can speed up installation by using the make install command.

Set a python virtualenv

$ cd pymongo-transactions
$ virtualenv -p python3 venv
$ source venv/bin/activate

Install Python MongoDB Driver, pymongo

Install the latest version of the PyMongo MongoDB Driver (3.7.1 at the time of writing).

pip install --upgrade pymongo

Install Mtools

MTools is a collection of helper scripts to parse, filter, and visualize MongoDB log files (mongod, mongos). mtools also includes mlaunch, a utility to quickly set up complex MongoDB test environments on a local machine. For this demo we are only going to use the mlaunch program.

pip install mtools

the mlaunch program also requires the psutil package.

pip install psutil

The mlaunch program gives us a simple command to start a MongoDB replica set as transactions are only supported on a replica set

Start a replica set whose name is txntest. (see the make init_server make target) for details:

mlaunch init --port 27100 --replicaset --name "txntest"

Using the Makefile for configuration

There is a Makefile with targets for all these operations. For those of you on platforms without access to Make it should be easy enough to cut and paste the commands out of the targets and run them on the command line.

Running the Makefile

cd pymongo-transactions

You will need to have MongoDB 4.0 on your path. There are other convenience targets for starting the demo programs:

  • make notxns : start the transactions client without using transactions
  • make usetxns : start the transactions client with transactions enabled
  • make watch_seats : watch the seats collection changing
  • make watch_payments : watch the payment collection changing

Running the transactions example

The transactions example consists of two python programs. and


$ python -h
usage: [-h] [--host HOST] [--usetxns] [--delay DELAY]
                           [--iterations ITERATIONS]
                           [--randdelay RANDDELAY RANDDELAY]

optional arguments:
  -h, --help            show this help message and exit
  --host HOST           MongoDB URI [default: mongodb://localhost:27100,localh
  --usetxns             Use transactions [default: False]
  --delay DELAY         Delay between two insertion events [default: 1.0]
  --iterations ITERATIONS
                        Run N iterations. O means run forever
                        Create a delay set randomly between the two bounds
                        [default: None]

You can choose to use --delay or --randdelay. if you use both --delay takes precedence. The--randdelay parameter creates a random delay between a lower and an upper bound that will be added between each insertion event.

The program knows to use the txntest replica set and the right default port range.

To run the program without transactions you can run it with no arguments:

$ python
using collection: SEATSDB.seats
using collection: PAYMENTSDB.payments
using collection: AUDITDB.audit
Using a fixed delay of 1.0

1. Booking seat: '1A'
1. Sleeping: 1.000
1. Paying 330 for seat '1A'
2. Booking seat: '2A'
2. Sleeping: 1.000
2. Paying 450 for seat '2A'
3. Booking seat: '3A'
3. Sleeping: 1.000
3. Paying 490 for seat '3A'
4. Booking seat: '4A'
4. Sleeping: 1.000

The program runs a function called book_seat() which books a seat on a plane by adding documents to three collections. First it adds the seat allocation to the seats_collection, then it adds a payment to the payments_collection`, finally it updates an audit count in the audit_collection. (This is a much simplified booking process used purely for illustration).

The default is to run the program without using transactions. To use transactions we have to add the command line flag --usetxns. Run this to test that you are running MongoDB 4.0 and that the correct featureCompatibility is configured (it must be set to 4.0). If you install MongoDB 4.0 over an existing /data directory containing 3.6 databases then featureCompatibility will be set to 3.6 by default and transactions will not be available.

Note: If you get the following error running python --usetxns that means you are picking up an older version of pymongo (older than 3.7.x) for which there is no multi-document transactions support.

Traceback (most recent call last):
  File "", line 175, in 
    total_delay = total_delay + run_transaction_with_retry( booking_functor, session)
  File "/Users/jdrumgoole/GIT/pymongo-transactions/", line 52, in run_transaction_with_retry
    with session.start_transaction():
AttributeError: 'ClientSession' object has no attribute 'start_transaction'

Watching Transactions

To actually see the effect of transactions we need to watch what is happening inside the collections SEATSDB.seats and PAYMENTSDB.payments.

We can do this with This script uses MongoDB Change Streams to see what's happening inside a collection in real-time. We need to run two of these in parallel so it's best to line them up side by side.

Here is the program:

$ python -h
usage: [-h] [--host HOST] [--collection COLLECTION]

optional arguments:
  -h, --help            show this help message and exit
  --host HOST           mongodb URI for connecting to server [default:
  --collection COLLECTION
                        Watch  [default:

We need to watch each collection so in two separate terminal windows start the watcher.

Window 1:

$ python --watch seats
Watching: seats

Window 2:

$ python --watch payments
Watching: payments

What Happens when you run without transactions?

Lets run the code without transactions first. If you examine the code you will see a function book_seats.

def book_seat(seats, payments, audit, seat_no, delay_range, session=None):
    Run two inserts in sequence.
    If session is not None we are in a transaction

    :param seats: seats collection
    :param payments: payments collection
    :param seat_no: the number of the seat to be booked (defaults to row A)
    :param delay_range: A tuple indicating a random delay between two ranges or a single float fixed delay
    :param session: Session object required by a MongoDB transaction
    :return: the delay_period for this transaction
    price = random.randrange(200, 500, 10)
    if type(delay_range) == tuple:
        delay_period = random.uniform(delay_range[0], delay_range[1])
        delay_period = delay_range

    # Book Seat
    seat_str = "{}A".format(seat_no)
    print(count( i, "Booking seat: '{}'".format(seat_str)))
    seats.insert_one({"flight_no" : "EI178",
                      "seat"      : seat_str,
                      "date"      : datetime.datetime.utcnow()},
    print(count( seat_no, "Sleeping: {:02.3f}".format(delay_period)))
    #pay for seat
    payments.insert_one({"flight_no" : "EI178",
                         "seat"      : seat_str,
                         "date"      : datetime.datetime.utcnow(),
                         "price"     : price},
    audit.update_one({ "audit" : "seats"}, { "$inc" : { "count" : 1}}, upsert=True)
    print(count(seat_no, "Paying {} for seat '{}'".format(price, seat_str)))

    return delay_period

This program emulates a very simplified airline booking with a seat being allocated and then paid for. These are often separated by a reasonable time frame (e.f. seat allocation vs external credit card validation and anti-fraud check) and we emulate this by inserting a delay. The default is 1 second.

Now with the two scripts running for seats_collection and payments_collection we can run as follows:

$ python

The first run is with no transactions enabled.

The bottom window shows running. On the top left we are watching the inserts to the seats collection. On the top right we are watching inserts to the payments collection.

watching without transactions

We can see that the payments window lags the seats window as the watchers only update when the insert is complete. Thus seats sold cannot be easily reconciled with corresponding payments. If after the third seat has been booked we CTRL-C the program we can see that the program exits before writing the payment. This is reflected in the Change Stream for the payments collection which only shows payments for seat 1A and 2A versus seat allocations for 1A, 2A and 3A.

If we want payments and seats to be instantly reconcilable and consistent we must execute the inserts inside a transaction.

What happens when you run with Transactions?

Now lets run the same system with --usetxns enabled.

$ python --usetxns

We run with the exact same setup but now set --usetxns.

watching with transactions

Note now how the change streams are interlocked and are updated in parallel. This is because all the updates only become visible when the transaction is committed. Note how we aborted the third transaction by hitting CTRL-C. Now neither the seat nor the payment appear in the change streams unlike the first example where the seat went through.

This is where transactions shine in world where all or nothing is the watchword. We never want to keeps seats allocated unless they are paid for.

What happens during failure?

In a MongoDB replica set all writes are directed to the Primary node. If the primary node fails or becomes inaccessible (e.g. due to a network partition) writes in flight may fail. In a non-transactional scenario the driver will recover from a single failure and retry the write. In a multi-document transaction we must recover and retry in the event of these kinds of transient failures. This code is encapsulated in We both retry the transaction and retry the commit to handle scenarios where the primary fails within the transaction and/or the commit operation.

def commit_with_retry(session):
    while True:
            # Commit uses write concern set at transaction start.
            print("Transaction committed.")
        except (pymongo.errors.ConnectionFailure, pymongo.errors.OperationFailure) as exc:
            # Can retry commit
            if exc.has_error_label("UnknownTransactionCommitResult"):
                print("UnknownTransactionCommitResult, retrying "
                      "commit operation ...")
                print("Error during commit ...")

def run_transaction_with_retry(functor, session):
    assert (isinstance(functor, Transaction_Functor))
    while True:
            with session.start_transaction():
                result=functor(session)  # performs transaction
        except (pymongo.errors.ConnectionFailure, pymongo.errors.OperationFailure) as exc:
            # If transient error, retry the whole transaction
            if exc.has_error_label("TransientTransactionError"):
                print("TransientTransactionError, retrying "
                      "transaction ...")

    return result

In order to observe what happens during elections we can use the script This script will start a replica-set and continuously kill the primary.

$ make kill_primary
. venv/bin/activate && python
no nodes started.
Current electionTimeoutMillis: 500
1. (Re)starting replica-set
no nodes started.
1. Getting list of mongod processes
Process list written to mlaunch.procs
1. Getting replica set status
1. Killing primary node: 31029
1. Sleeping: 1.0
2. (Re)starting replica-set
launching: "/usr/local/mongodb/bin/mongod" on port 27101
2. Getting list of mongod processes
Process list written to mlaunch.procs
2. Getting replica set status
2. Killing primary node: 31045
2. Sleeping: 1.0
3. (Re)starting replica-set
launching: "/usr/local/mongodb/bin/mongod" on port 27102
3. Getting list of mongod processes
Process list written to mlaunch.procs
3. Getting replica set status
3. Killing primary node: 31137
3. Sleeping: 1.0 resets electionTimeOutMillis to 500ms from its default of 10000ms (10 seconds). This allows elections to resolve more quickly for the purposes of this test as we are running everything locally.

Once is running we can start up again using the --usetxns argument.

$ make usetxns
. venv/bin/activate && python --usetxns
Forcing collection creation (you can't create collections inside a txn)
Collections created
using collection: PYTHON_TXNS_EXAMPLE.seats
using collection: PYTHON_TXNS_EXAMPLE.payments
using collection: PYTHON_TXNS_EXAMPLE.audit
Using a fixed delay of 1.0
Using transactions

1. Booking seat: '1A'
1. Sleeping: 1.000
1. Paying 440 for seat '1A'
Transaction committed.
2. Booking seat: '2A'
2. Sleeping: 1.000
2. Paying 330 for seat '2A'
Transaction committed.
3. Booking seat: '3A'
3. Sleeping: 1.000
TransientTransactionError, retrying transaction ...
3. Booking seat: '3A'
3. Sleeping: 1.000
3. Paying 240 for seat '3A'
Transaction committed.
4. Booking seat: '4A'
4. Sleeping: 1.000
4. Paying 410 for seat '4A'
Transaction committed.
5. Booking seat: '5A'
5. Sleeping: 1.000
5. Paying 260 for seat '5A'
Transaction committed.
6. Booking seat: '6A'
6. Sleeping: 1.000
TransientTransactionError, retrying transaction ...
6. Booking seat: '6A'
6. Sleeping: 1.000
6. Paying 380 for seat '6A'
Transaction committed.

As you can see during elections the transaction will be aborted and must be retried. If you look at the code you will see how this happens. If a write operation encounters an error it will throw one of the following exceptions:

Within these exceptions there will be a label called TransientTransactionError. This label can be detected using the has_error_label(label) function which is available in pymongo 3.7.x. Transient errors can be recovered from and the retry code in has code that retries for both writes and commits (see above).


Multi-document transactions are the final piece of the jigsaw for SQL developers who have been shying away from trying MongoDB. ACID transactions make the programmer's job easier and give teams that are migrating from an existing SQL schema a much more consistent and convenient transition path.

As most migrations involving a move from highly normalised data structures to more natural and flexible nested JSON documents one would expect that the number of required multi-document transactions will be less in a properly constructed MongoDB application. But where multi-document transactions are required programmers can now include them using very similar syntax to SQL.

With ACID transactions in MongoDB 4.0 it can now be the first choice for an even broader range of application use cases.

Why not try our transactions today by setting up your first cluster on MongoDB Atlas our Database as a Service offering.

To try it locally download MongoDB 4.0.

Join us at MongoDB Europe 2018 for deep-dive technical sessions and hands-on tutorials.

Introducing the Best Database for Modern Applications

The announcements we made today at MongoDB World 2018 represent a significant milestone in the evolution of MongoDB, making it the database of choice for all modern applications. Broadly speaking, there are three reasons for this:

  1. The document data model – presenting you the best way to work with data.
  2. It’s distributed by design – allowing you to intelligently put data where you want it.
  3. A unified experience that gives you the freedom to run anywhere – allowing you to future-proof your work and eliminate vendor lock-in.

There is a ton of new stuff, and so I wanted to give you a summary of what I covered during my keynote, with links to key resources so you can learn more.

Best Way to Work with Data

Today we released MongoDB Server 4.0 for General Availability. The highlight of the release is multi-document ACID transactions, which we previewed back in February with a beta program that attracted thousands of members of the community, putting transactions through their paces and providing invaluable feedback to the engineering team. We’ve implemented transactions so they feel just like the transactions you are familiar with from relational databases. They enforce snapshot isolation to provide a consistent view of data, and all-or-nothing execution to maintain data integrity. And while the document model means multi-document transactions aren’t necessary for most operations, with them it’s even easier for you to address a complete range of use cases with MongoDB.

It’s no secret how much I love MongoDB’s aggregation framework. Building queries stage-by-stage, checking your output as you go is by far a better way to write your most complicated queries than dealing with a monolithic snarl of SQL. To make that workflow even better, we’ve enhanced MongoDB Compass with the aggregation pipeline builder, which provides stage-by-stage, real-time feedback on the documents flowing through your pipelines. It’s easier than ever to deploy sophisticated processing pipelines that transform, aggregate, and analyze your data, all from the simple and intuitive MongoDB Compass GUI. You can then export the pipelines, and any other queries you create in Compass, to the native code of your preferred programming language. Server 4.0 also adds type conversions to the aggregation pipeline. With the new $convert operator you can transform mixed data types into standardized, cleansed formats natively within the database, preparing it for BI and machine learning, while eliminating costly, slow, and fragile ETL processes.

Extending the tools you can use to work with data managed by the server, we announced the public beta of MongoDB Charts, which provides the fastest and easiest way to get insights into your operational data, in real time. With Charts, you can create and share visualisations of your MongoDB data, using a document-native interface, without needing to move it into other systems or leverage third-party tools.

Documents and MongoDB’s query language are the best way to work with data, and to bring that power out of the datacenter and into the hands of app developers, MongoDB Stitch, which is GA as of today, provides two of its four services: QueryAnywhere and Functions. Using the authentication and declarative access control rules of Stitch QueryAnywhere, we can end the horrid practice of implementing shadow query languages in REST on top of application servers that just turn those REST calls into real query languages. With a native SDK, developers can make use of the full power of MongoDB from mobile and JavaScript applications, while Stitch makes sure that the right permissions are observed. Stitch Functions, JavaScript functions that execute with full access to application context, let developers compose their business logic with access to Atlas and calls to external services. With these two services, it’s easy to build complete applications without standing up a single application server.

Intelligently Put Data Where you Want It

As a distributed system, MongoDB enables you to spread data out across a cluster of nodes for resilience, scalability, and workload isolation. Unlike other distributed databases that randomly spray data around a cluster, MongoDB allows you to define controls that place data on specific nodes, for example in a specific region for low latency reads and writes, and for compliance with new privacy regulations.

The new Global Clusters introduced to MongoDB Atlas allow you to deploy a geographically distributed, fully managed database that provides low latency writes and reads to users anywhere, with data placement controls for regulatory compliance. We also announced Atlas Enterprise, offering new security controls including LDAP integration, the encrypted storage engine with bring-your-own key management, and database-level auditing. Organizations can now also use databases managed by MongoDB Atlas to build HIPAA-compliant applications under an executed Business Associate Agreement (BAA) with MongoDB, Inc. With these now announcements, MongoDB Atlas is the most secure cloud database service available anywhere.

Coding in a distributed world also means that the traditional means of responding to events in a database are no longer viable. So Stitch Triggers, also GA today, makes it possible by building on the Change Streams introduced in MongoDB 3.6. When you create a trigger, Stitch manages a change stream on your behalf, providing real-time notifications to Stitch Functions, which can react in all the ways functions can, from updating analytics rollup collections, to sending email or text messages, or kicking off other external services like Kafka or Kinesis.

Freedom to Run Anywhere

Whether you want to consume your database as a service in MongoDB Atlas, or manage it yourself on your own infrastructure, the announcements today make that even easier. We deliver a data platform that runs the same everywhere, that leverages the benefits of multi-cloud strategy with no lock-in, and is available in 50+ regions across the major cloud providers.

If you want to run MongoDB yourself, then we have released our new free MongoDB monitoring cloud service. The service is available to all MongoDB users, without needing to install an agent, navigate a paywall, or complete a registration form. You will be able to see the metrics and topology about your environment from the moment free monitoring is enabled. You can enable free monitoring easily using the MongoDB shell, MongoDB Compass, or by starting the mongod process with the new db.enableFreeMonitoring() command line option, and you can opt out at any time.

We’re seeing more DevOps teams leveraging the power of containerization and technologies like Kubernetes and Red Hat OpenShift to manage containerized clusters. Today we announced beta of the new MongoDB Enterprise Operator for Kubernetes, enabling you to deploy and manage MongoDB clusters from within the Kubernetes API, without having to connect separately to Ops Manager. You can learn more by reading our Red Hat OpenShift and MongoDB blog, and checking out the repository on GitHub.

Announced today, MongoDB Mobile takes MongoDB to a new frontier. Available in beta, MongoDB Mobile extends your ability to put data where you need it, all the way out to the edge of the network on IoT assets and iOS and Android mobile devices. MongoDB Mobile provides a single database, query language, and the intuitive Stitch SDK that runs consistently for data held on mobile clients, through to the backend server.

MongoDB Mobile provides the power and flexibility of MongoDB in a compact form that is power- and performance-aware with a low disk and memory footprint. It supports 64 bit iOS and Android operating systems and is easily embedded into mobile and IoT devices for fast and reliable local storage of JSON documents. With secondary indexing, access to the full MongoDB query language and aggregations, users can query data any way they want. With local reads and writes, MongoDB Mobile lets you build the fastest, most reactive apps. And Stitch Mobile Sync, which is in private beta now, will automatically synchronize data changes between data held locally and your backend database, helping resolve any conflicts – even after the device has been offline. The beta program is open now, and you can sign up for access on the MongoDB Mobile product page.

What’s Next?

So as you can see, that’s a ton of stuff. The announcements today represent our biggest set of releases yet, and we’re incredibly excited to get it into your hands and see what amazing things you do with them. Head over to our MongoDB World 2018 announcements page for more resources on each of these new products and services.

Scaling Your Replica Set: Non-Blocking Secondary Reads in MongoDB 4.0

MongoDB 4.0 adds the ability to read from secondaries while replication is simultaneously processing writes. To see why this is new and important let's look at secondary read behavior in versions prior to 4.0.


From the outset MongoDB has been designed so that when you have sequences of writes on the primary, each of the secondary nodes must show the writes in the same order. If you change field "A" in a document and then change field "B", it is not possible to see that document with changed field "B" and not changed field "A". Eventually consistent systems allow you to see it, but MongoDB does not, and never has.

On secondary nodes, we apply writes in batches, because applying them sequentially would likely cause secondaries to fall behind the primary. When writes are applied in batches, we must block reads so that applications cannot see data applied in the "wrong" order. This is why when reading from secondaries, periodically the readers have to wait for replication batches to be applied. The heavier the write load, the more likely that your secondary reads will have these occasional "pauses", impacting your latency metrics. Given that applications frequently use secondary reads to reduce the latency of queries (for example when they use "nearest" readPreference) having to wait for replication batches to be applied defeats the goal of getting lowest latency on your reads.

In addition to readers having to wait for replication batch writes to finish, the writing of batches needs a lock that requires all reads to complete before it can be taken. That means that in the presence of high number of reads, the replication writes can start lagging – an issue that is compounded when chain replication is enabled.

What was our goal in MongoDB 4.0?

Our goal was to allow reads during oplog application to decrease read latency and secondary lag, and increase maximum throughput of the replica set. For replica sets with a high write load, not having to wait for readers between applying oplog batches allows for lower lag and quicker confirmation of majority writes, resulting in less cache pressure on the primary and better performance overall.

How did we do it?

Starting with MongoDB 4.0 we took advantage of the fact that we implemented support for timestamps in the storage engine, which allows transactions to get a consistent view of data at a specific "cluster time". For more details about this see the video: WiredTiger timestamps.

Secondary reads can now also take advantage of the snapshots, by reading from the latest consistent snapshot prior to the current replication batch that's being applied. Reading from that snapshot guarantees a consistent view of the data, and since applying current replication batch doesn't change these earlier records, we can now relax the replication lock and allow all these secondary reads at the same time the writes are happening.

How much difference does this make?

A lot! The range of performance improvements for throughput could range from none (if you were not impacted by the replication lock - that is your write load is relatively low) to 2X.

Most importantly, this improves latency for secondary reads – for those who use readPreference "nearest" because they want to reduce latency from the application to the database – this feature means their latency in the database will also be as low as possible. We saw significant improvement in 95 and 99th percentile latency in these tests.

Thread levels 8 16 32 64
Feature off 1 2 3 5
Feature on 0 1 1 0

95th percentile read latency (ms)

Best part of this new feature? You don't need to do anything to enable it or opt-into it. All secondary reads in 4.0 will read from snapshot without waiting for replication writes.

This is just one of a number of great new features coming in MongoDB 4.0. Take a look at our blog on the 4.0 release candidate to learn more. And don’t forget, you’ve still got time to register for MongoDB World where you can meet with the engineers who are building all of these great new features.

MongoDB 4.0 Release Candidate 0 Has Landed

MongoDB enables you to meet the demands of modern apps with a technology foundation that enables you through:

  1. The document data model – presenting you the best way to work with data.
  2. A distributed systems design – allowing you to intelligently put data where you want it.
  3. A unified experience that gives you the freedom to run anywhere – future-proofing your work and eliminating vendor lock-in.

Building on the foundations above, MongoDB 4.0 is a significant milestone in the evolution of MongoDB, and we’ve just shipped the first Release Candidate (RC), ready for you to test.

Why is it so significant? Let’s take a quick tour of the key new features. And remember, you can learn about all of this and much more at MongoDB World'18 (June 26-27).

Multi-Document ACID Transactions

Previewed back in February, multi-document ACID transactions are part of the 4.0 RC. With snapshot isolation and all-or-nothing execution, transactions extend MongoDB ACID data integrity guarantees to multiple statements and multiple documents across one or many collections. They feel just like the transactions you are familiar with from relational databases, are easy to add to any application that needs them, and and don't change the way non-transactional operations are performed. With multi-document transactions it’s easier than ever for all developers to address a complete range of use cases with MongoDB, while for many of them, simply knowing that they are available will provide critical peace of mind that they can meet any requirement in the future. In MongoDB 4.0 transactions work within a replica set, and MongoDB 4.2 will support transactions across a sharded cluster*.

To give you a flavor of what multi-document transactions look like, here is a Python code snippet of the transactions API.

with client.start_session() as s:
        collection.insert_one(doc1, session=s)
        collection.insert_one(doc2, session=s)

And now, the transactions API for Java.

try (ClientSession clientSession = client.startSession()) {
           try {
                   collection.insertOne(clientSession, docOne);
                   collection.insertOne(clientSession, docTwo);
          } catch (Exception e) {

Our path to transactions represents a multi-year engineering effort, beginning over 3 years ago with the integration of the WiredTiger storage engine. We’ve laid the groundwork in practically every part of the platform – from the storage layer itself to the replication consensus protocol, to the sharding architecture. We’ve built out fine-grained consistency and durability guarantees, introduced a global logical clock, refactored cluster metadata management, and more. And we’ve exposed all of these enhancements through APIs that are fully consumable by our drivers. We are feature complete in bringing multi-document transactions to replica sets, and 90% done on implementing the remaining features needed to deliver transactions across a sharded cluster.

Take a look at our multi-document ACID transactions web page where you can hear directly from the MongoDB engineers who have built transactions, review code snippets, and access key resources to get started.

Aggregation Pipeline Type Conversions

One of the major advantages of MongoDB over rigid tabular databases is its flexible data model. Data can be written to the database without first having to predefine its structure. This helps you to build apps faster and respond easily to rapidly evolving application changes. It is also essential in supporting initiatives such as single customer view or operational data lakes to support real-time analytics where data is ingested from multiple sources. Of course, with MongoDB’s schema validation, this flexibility is fully tunable, enabling you to enforce strict controls on data structure, type, and content when you need more control.

So while MongoDB makes it easy to ingest data without complex cleansing of individual fields, it means working with this data can be more difficult when a consuming application expects uniform data types for specific fields across all documents. Handling different data types pushes more complexity to the application, and available ETL tools have provided only limited support for transformations. With MongoDB 4.0, you can maintain all of the advantages of a flexible data model, while prepping data within the database itself for downstream processes.

The new $convert operator enables the aggregation pipeline to transform mixed data types into standardized formats natively within the database. Ingested data can be cast into a standardized, cleansed format and exposed to multiple consuming applications – such as the MongoDB BI and Spark connectors for high-performance visualizations, advanced analytics and machine learning algorithms, or directly to a UI. Casting data into cleansed types makes it easier for your apps to to process, sort, and compare data. For example, financial data inserted as a long can be converted into a decimal, enabling lossless and high precision processing. Similarly, dates inserted as strings can be transformed into the native date type.

When $convert is combined with over 100 different operators available as part of the MongoDB aggregation pipeline, you can reshape, transform, and cleanse your documents without having to incur the complexity, fragility, and latency of running data through external ETL processes.

Non-Blocking Secondary Reads

To ensure that reads can never return data that is not in the same causal order as the primary replica, MongoDB blocks readers while oplog entries are applied in batches to the secondary. This can cause secondary reads to have variable latency, which becomes more pronounced when the cluster is serving write-intensive workloads. Why does MongoDB need to block secondary reads? When you apply a sequence of writes to a document, then MongoDB is designed so that each of the nodes must show the writes in the same causal order. So if you change field "A" in a document and then change field "B", it is not possible to see that document with changed field "B" and not changed field "A". Eventually consistent systems suffer from this behavior, but MongoDB does not, and never has.

By taking advantage of storage engine timestamps and snapshots implemented for multi-document ACID transactions, secondary reads in MongoDB 4.0 become non-blocking. With non-blocking secondary reads, you now get predictable, low read latencies and increased throughput from the replica set, while maintaining a consistent view of data. Workloads that see the greatest benefits are those where data is batch loaded to the database, and those where distributed clients are accessing low latency local replicas that are geographically remote from the primary replica.

40% Faster Data Migrations

Very few of today’s workloads are static. For example, the launch of a new product or game, or seasonal reporting cycles can drive sudden spikes in load that can bring a database to its knees unless additional capacity can be quickly provisioned. If and when demand subsides, you should be able to scale your cluster back in, rightsizing for capacity and cost.

To respond to these fluctuations in demand, MongoDB enables you to elastically add and remove nodes from a sharded cluster in real time, automatically rebalancing the data across nodes in response. The sharded cluster balancer, responsible for evenly distributing data across the cluster, has been significantly improved in MongoDB 4.0. By concurrently fetching and applying documents, shards can complete chunk migrations up to 40% faster, allowing you to more quickly bring new nodes into service at just the moment they are needed, and scale back down when load returns to normal levels.

Extensions to Change Streams

Change streams, released with MongoDB 3.6, enable developers to build reactive, real-time, web, mobile, and IoT apps that can view, filter, and act on data changes as they occur in the database. Change streams enable seamless data movement across distributed database and application estates, making it simple to stream data changes and trigger actions wherever they are needed, using a fully reactive programming style.

With MongoDB 4.0, Change Streams can now be configured to track changes across an entire database or whole cluster. Additionally, change streams will now return a cluster time associated with an event, which can be used by the application to provide an associated wall clock time for the event.

Getting Started with MongoDB 4.0

Hopefully this gives you a taste of what’s coming in 4.0. There’s a stack of other stuff we haven’t covered today, but you can learn about it all in the resources below.

To get started with the RC now:

  1. Head over to the MongoDB download center to pick up the latest development build.
  2. Review the 4.0 release notes.
  3. Sign up for the forthcoming MongoDB University training on 4.0.

And you can meet our engineering team and other MongoDB users at MongoDB World'18 (June 26-27).


* Safe Harbor Statement

This blog post contains “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended. Such forward-looking statements are subject to a number of risks, uncertainties, assumptions and other factors that could cause actual results and the timing of certain events to differ materially from future results expressed or implied by the forward-looking statements. Factors that could cause or contribute to such differences include, but are not limited to, those identified our filings with the Securities and Exchange Commission. You should not rely upon forward-looking statements as predictions of future events. Furthermore, such forward-looking statements speak only as of the date of this presentation.

In particular, the development, release, and timing of any features or functionality described for MongoDB products remains at MongoDB’s sole discretion. This information is merely intended to outline our general product direction and it should not be relied on in making a purchasing decision nor is this a commitment, promise or legal obligation to deliver any material, code, or functionality. Except as required by law, we undertake no obligation to update any forward-looking statements to reflect events or circumstances after the date of such statements.