When the Message Broker Becomes the Bottleneck: Scaling Push and SMS With MongoDB Polling

A back-end team extracted notification delivery from a large Ruby monolith into a centralized Go service, only to discover that their shared RabbitMQ cluster could not reliably support the new workload. Here’s how they solved the problem using MongoDB as a polling queue and the trade-offs that came with it.

TL;DR - Key Takeaways

• Wheely extracted push, SMS and status notifications from its Ruby monolith into a centralized, stateless Go service after fragmented delivery logic became difficult to scale and manage.
• When its shared RabbitMQ cluster struggled with large campaign spikes, the team used MongoDB as a persistent polling queue with atomic task claiming, retries, exponential backoff, idempotency and dead-letter handling.
• The new architecture handled several times the previous peak volume without broker-related delivery failures, but introduced polling latency, database overhead and additional functionality for the team to maintain.

The Scaling Problem Inside the Monolith

Back in 2019, the engineering team at Wheely, a popular ride-hailing platform, had a coordination problem that emerged with the product. User-facing notifications such as push alerts, SMS confirmations and status updates were scattered around the large Ruby monolith. Various parts of the codebase made direct attempts to send messages without a shared logic, unified retry strategy and consistent way to track delivery outcomes.

As the system continued to grow, new back-end microservices were introduced, with each one requiring its own path to the user’s device. Push campaigns made things worse. Sending thousands of notifications at once exposed limits that routine traffic had never revealed. The monolith wasn’t designed as a notification delivery system, and scaling it that way implied additional risks to the core business logic already implemented there. The team needed a dedicated, centralized service that could serve as a single point of entry for all outbound communications, both for the monolith and future microservices alike.

Why the Existing Message Broker Wasn’t Enough

The natural architecture for this kind of service involves a message broker. The monolith and microservices publish notification events, workers consume them and delivery happens asynchronously. The team’s infrastructure already had RabbitMQ in place, so the initial design assumed it would handle this workload. In practice, the shared RabbitMQ cluster proved unreliable under the notification service’s workload. Unstable behavior of the cluster during large push campaigns caused several delivery failures, which were difficult to diagnose, hard to retry correctly and disruptive to other services that used the same broker.

RabbitMQ Internals

To understand why RabbitMQ turned out to be a bottleneck, it’s worth taking a quick look at its internal architecture. RabbitMQ is built on top of the Erlang virtual machine: BEAM, where almost every broker component runs as a separate Erlang process. Queues, connections, channels and consumers are independent lightweight processes, communicating with each other via messaging. This model provides strong fault tolerance and scalability in various scenarios. However, under a very heavy load, the overhead of managing these processes can become noticeable.

Moreover, RabbitMQ is also sensitive to the type of load it receives.

• The first factor is the number of queues. Each queue is a separate Erlang process with its own internal data structures. As the number of queues increases, more memory is used, the Erlang VM scheduler starts working harder and there is an additional operational overhead from operating the broker.
• Second, the length of the queue matters. RabbitMQ was designed as a system to process continuous message flow, not to store very large backlogs for long periods. If producers write faster than consumers can read, the queue starts growing quickly. It results in higher memory usage, puts additional pressure on the disk for persistent messages and significantly increases delivery delay.
• Finally, RabbitMQ clusters are sensitive to network conditions. Replication between nodes, synchronization of queues after recovery and network latency can all significantly impact the system’s performance and cluster recovery time after failures.

These characteristics are not flaws in RabbitMQ; they simply define the type of loads it is best suited to handle. When designing a high-load system, it is important to consider not only the broker’s capabilities but also the workload profile it will need to support.

Why RabbitMQ Was No Longer the Right Fit

RabbitMQ had handled its role well for a long period. The problem was not with the broker itself, but the change in workload that took place over time.

• First, the system was increasingly dealing with large-scale push and SMS campaigns. A single campaign generated tens of thousands of individual messages, all arriving in RabbitMQ practically at the same time. This created sharp load spikes, made queues grow and increased delivery delays.
• Second, some consumers were still running inside the Ruby monolith. Compared to the newer Go services, they were slower at processing messages, allowing queues to accumulate much faster than they could be drained. Simply increasing the number of consumers wasn’t a practical option. Performance of the monolith had already become a limiting factor, and scaling it would have been complicated due to the current system’s architecture.
• Finally, RabbitMQ served as a shared infrastructure component for various services. Every new communication-related feature added load to the same cluster, making it an increasingly critical point of failure.

Extracting a Centralized Go Notification Service

The new service was implemented as a single, stateless API responsible for accepting send requests and handling everything else: Routing to the target provider, retrying failed deliveries and recording the outcome. The monolith and other microservices sent requests to a single endpoint without having to manage the delivery process themselves. Go was chosen for the implementation of the new service. It was containerized and deployed using Docker on the existing AWS infrastructure, consistent with the rest of the microservices on the platform.

On the mobile side, the service integrates with the Apple Push Notification service (APNs) and Firebase Cloud Messaging (FCM). SMS delivery was handled via integration with one or more third-party providers.

Using MongoDB as a Polling Queue

Since it was not possible to rely on any broker, it was necessary to find some alternative way of buffering delivery tasks and processing them. The alternative chosen by the team was the usage of MongoDB as a polling queue. MongoDB was one of the tools used in the current stack.

When a notification request arrives, the service writes a document to a dedicated MongoDB collection. Each document represented a single delivery task and contained all the information needed to be processed independently: The recipient, the message payload, the channel, the current status and metadata to retry tracking.

A simplified document life cycle looked like this:

pending → processing → delivered
         ↘ retry → (back to pending, up to N attempts)
         ↘ failed (dead-letter)

Workers polled the collection at short intervals, querying for documents in the pending state. To process two workers trying to process the same task, MongoDB’s atomic findOneAndUpdate operation was used to transition a document from pending to processing in a single step. Only one worker succeeded in claiming any given record.

Retry policy was embedded in the document itself. Each failed attempt incremented a counter, as well as updated a retry_after timestamp using an exponential backoff strategy. Workers excluded documents whose retry_after time had not yet passed. Documents that reached the maximum retry count were moved to the failed state rather than being tried to be processed again.

Idempotency was maintained by creating a stable identifier for each delivery task when it was created. In case the same request arrived twice due to the retry on the caller’s side, the service could detect the duplicate before writing a second document.

Processed and failed records were either archived into another collection or dropped after the retention window.

Index design was critical. The collection required compound indexes covering status, channel and scheduling fields for efficient polling queries. Without the right indexes, polling at scale would have caused unacceptable read load on MongoDB.

Optimizing MongoDB for a Write-Heavy Workload

Using MongoDB as a persistent queue required careful data model design.

• First, we separated the queue into a write-heavy collection, fully isolated from the rest of the application data. This way, it was possible to optimize it independently by choosing its own indexes, configuring a dedicated retention policy and ensuring that intensive writes did not affect the rest of the system.
• At the same time, we attempted to implement an append-heavy data model. Messages were created once and then updated just a few times while moving through the delivery pipeline before being either archived or deleted. This life cycle aligned well with MongoDB’s strengths and helped maintain stable write performance.
• Queue documents were intentionally kept compact. They included only information needed for routing, delivery and status tracking. This reduced the amount of data transferred, lowered disk and replication load and allowed more of the working set to fit in memory.
• Indexes required particular attention. We deliberately limited them to those needed for efficient polling and status-based lookups. Any additional index increases the write cost, as MongoDB must update it on every insert or update. For write-heavy workloads, keeping the number of indexes minimal becomes more important than maximizing read performance.
• Messages were polled in batches instead of one at a time. During each poll cycle, the worker would claim only a certain number of tasks, which significantly reduces the number of database operations, thereby making the load more predictable in case of many incoming messages.
• Finally, to prevent the collection from growing indefinitely, we either periodically archived processed messages or automatically removed them using TTL indexes. This kept both the collection and its indexes relatively small, improving write speed and the efficiency of polling queries.

It was the combination of these decisions that made MongoDB an efficient and predictable persistent queue for this workload.

Observability relied on tooling already in use at Wheely: The ELK stack for log aggregation and Datadog for metrics and alerting.

The Trade-Offs We Accepted

MongoDB was not built to be a message broker, and using it as one comes at a cost. During each poll cycle, database read operations were generated regardless of whether the queue was empty. At scale, it added measurable load to MongoDB. This load was then associated directly with notifications throughput instead of being isolated in a dedicated broker. Despite a careful index design and a sensible polling interval, it was still possible to reduce but not eliminate this overhead.

Polling introduces inherent latency. A message written to the queue would not be processed until the next poll cycle. It is a reasonable decision for most notification use cases, but it was a deliberate compromise compared to the near-immediate delivery that one can expect from a properly functioning broker.

The team also had to implement, test and maintain functionality that mature message brokers typically provide out of the box: Atomic claiming, retry scheduling, dead-letter handling, duplicate suppression and observability. Each of these capabilities added complexity to the service itself.

There was also an organizational risk. Solutions described as temporary often become permanent. Teams adopting this pattern should clearly define under what conditions that would justify migrating to a dedicated broker, and they should build sufficient observability to detect when those conditions are met.

A dedicated message broker continues to be the right choice in the case of a stable infrastructure, when the team has the operational expertise to operate it and the delivery latency requirements are strict. This architectural approach was chosen based on specific sets of constraints, not as a general recommendation.

Operational Results and Lessons

After the migration, the service handled notification volumes several times higher than the previous peak without delivery failures caused by broker instability. The service achieved a much better delivery success rate and now serves as the shared communication layer for multiple back-end services across the platform.

Three lessons worth carrying forward:

• First, architecture must reflect the maturity of the infrastructure it runs on. A theoretically correct design built on an operationally unreliable dependency can perform worse than a pragmatic design built on infrastructure the team already operates well.
• Second, database polling can be a viable delivery mechanism, but it requires a thorough implementation. Idempotence, atomic claiming, backoff, dead-letter handling and observability are not optional additions; but are the key to making the pattern safe. Skipping even one of them leads to failure modes that are difficult to diagnose under load.
• Third, extracting a shared capability from a monolith is an operational change, not just a code refactoring exercise.