March 3, 2022

Startup Foundations: Exploring AWS Options for Serverless APIs

Mark Perkin, Vice President and Architect, GS Accelerate

In a firm of nearly 45,000 employees from across the globe, new ideas are generated daily. But in the buzz of everyday business, complex transactions, and never-ending activity, what support exists to take these ideas and really execute on them? This was the premise that sparked the beginnings of Goldman Sachs' internal incubator GS Accelerate. Founded in 2018, GS Accelerate is a firmwide platform that aims to support the firm’s long-standing culture of innovation and experimentation and seeds commercial, innovative, and growth businesses for Goldman Sachs. Accelerate provides our people with the capital, resources, and support to transform ideas into viable new businesses and products for our clients, with a focus on the firm’s growth. Funded businesses focus on building client solutions, leveraging Goldman Sachs’ unique strengths and offering employees the opportunity to work in a fast-paced, entrepreneurial environment.

GS Accelerate was where the X25 project began. Funded through Accelerate in 2020, X25 seeks to provide a digital-first, cloud-based solution to transform the financial product landscape. X25's engineers leveraged a 150-year-old firm with its resources and deep bench of experience to pioneer cutting-edge digital transformation in the alternative investments industry. In this blog post, we will share how the X25 team laid the foundation of our startup through exploring an AWS serverless solution for our client-facing APIs.

X25 Operating Environment

X25 is a multi-tenant platform for servicing a specific class of complex financial instruments. The majority of computations are batch operations triggered by each instrument's schedule, which defines one or more events that need(s) to be processed daily. The timing of these events leads to multiple computations across instruments triggering simultaneously, with higher peaks of activity at specific days in the business calendar. This operating domain exerts a highly variant compute demand characterized by long periods of relatively low activity interspersed with brief periods of very high demand. Compute demand can easily vary 1000-fold from hour-to-hour. We wanted our cost for the infrastructure to align with demand, so exploring AWS serverless options was an obvious decision. Serverless architecture allows customers to scale up and scale down on demand. 

In addition to these scheduled operations, X25 exposes an API to its tenants, which is used by the X25 web portal and any tenant-authored integration services. The API does not require the same scale as the back-end scheduled events. X25 is built for only a relatively small number of tenants (less than 100) who only need access to the API intermittently. The advantage of serverless here is not limited to elastic scaling; serverless also means that there are no servers to manage, which reduces operational overhead. Unlike back-end scheduled event handling, latency was an important consideration for the X25 API, as any noticeable latency would impair the user experience. 

Cloud Vendor Selection

In order to best deliver the firm's services, X25's first decision was to decide which cloud vendor(s) to select. Teams that are new to cloud computing may wonder whether they should be developing their software in a cloud-neutral fashion and be able to switch vendors if required, or if should they select a target vendor and lean in to that vendor's services and APIs. The former option is quite challenging as the services and APIs offered by each vendor vary. Aiming to create a cloud-neutral solution means that only baseline fundamental infrastructure-as-a-service offerings (which are most easily switchable) should be used. However, such an approach leads to more code for these teams to develop, and therefore results in a longer time to market. On the other hand, leaning in to a specific vendor's platform and software-as-a-service solutions locks teams into that vendor, which has its own drawbacks.

X25 follows a microservice architecture where service-to-service communication is conducted strictly via HTTP URLs and each service is developed, tested, and deployed independently. Each microservice can then, in theory, select a different cloud vendor, but each would lean in to that vendor's offerings. In this way, the complexity of a vendor neutral solution and single vendor lock-in are avoided. Migrating a single microservice is still a re-engineering effort, but is quite feasible if microservices are sufficiently granular. Migration from one vendor to another can then be done service by service to mitigate the risk of a big bang switchover to another cloud vendor.

That said, in X25's initial minimum viable product (MVP), all services were elected to run on AWS’ cloud infrastructure, primarily driven by the fact that Goldman Sachs has already successfully migrated/built multiple businesses onto AWS.

AWS Serverless API

In building the X25 API on AWS, we experimented with AWS's two serverless compute options: Lambda and ECS Fargate.

Lambda is event-driven and provisions compute on-demand. Customers only pay for what they use, in terms of memory and computation time. ECS is AWS's managed container orchestration service. ECS can run containers on EC2 instances provisioned by the customer or, with ECS Fargate, on instances provisioned and managed by AWS. As a container orchestrator, ECS can only work with containers. When initially launched, Lambda only worked with ZIP files containing functions run on an AWS provided container. More recently, AWS has added support for customer containers on Lambda. 

Lambda provisions compute on-demand and will re-use a previously provisioned compute for subsequent events. However, after a period of inactivity, AWS will reclaim the compute. A subsequent request then requires a cold-start, which incurs some latency. Also, during higher periods of demand, AWS will spin-up additional computes, each of which incurs a cold start. Customers can configure a minimum provisioned concurrency to ensure that a defined number of instances are always available. Having this baseline of compute availability incurs a cost even when there is zero activity. 

With ECS Fargate, customers specify a minimum and maximum number of containers to continuously run (similar to Lambda's provisioned concurrency), plus an auto-scaling policy. The policy can be driven by different metrics, such as request rate, CPU, or memory usage. By specifying suitable thresholds, additional instances can be provisioned before the current number reaches capacity/saturates. These can then be scaled down as the demand subsides.

The advantage of Lambda is the ability to scale on-demand without any policy to manage, and the ability to scale down to zero when there is no demand. On the downside, however, latency is not predictable and may spike from cold starts. ECS Fargate provides a minimal, always-available compute and is therefore more predictable with lower latency, but does incur some costs when there is no demand. 

ECS Fargate is tailored for containerized microservices, while Lambda is more geared towards function-as-a-service (FaaS) architectures. However, with AWS adding support for containers and provisioned concurrency to Lambda, the distinction versus ECS Fargate starts to blur. The X25 team initially explored the Lambda .ZIP file approach.

X25 services are built using the Python Flask framework. In deploying to Lambda, the team simply packaged the whole application in a single .ZIP file fronted by an application load balancer. AWS has documented that this is a Lambda anti-pattern and recommends that when using Lambda, customers should embrace FaaS and deploy multiple small Lambda functions, potentially one per API route, fronted with an AWS API Gateway. Essentially, the API gateway becomes the REST framework. While this may be the best way to use Lambda, it does make testing more challenging, as an AWS environment is required. By building a standalone flask application, it's easier for developers to spin up and test on their local machines.

Building and deploying multiple separate functions introduces additional CI/CD challenges. Since each function can, in theory, be deployed separately, if one is careless, it is possible that a production environment may contain a combination of separately-deployed function versions that have not been tested together in any pre-production environment. Deploying the functions together as single container does not suffer from this problem, as the container is a single, atomic deployable unit.

When Flask-based servers were first tested, they exhibited a noticeable start-up delay when creating the Flask application and connecting to the database. This latency is an unacceptable delay in the portal user experience. This led us to the following choices; the team could either:

  1. Stick with the Lambda monolith but find a way to optimize the application startup.
  2. Migrate to Lambda FaaS. 
  3. Migrate to ECS Fargate with the existing Flask app.

With option 1, there are a few optimizations that could be explored. Flask could be replaced with a framework that offers a faster time to make the first API call (e.g. FastAPI), then, explore leveraging an AWS database connection pooling for faster database connectivity.

Option 2 would arguably be the most natural fit for using Lambda as an API. However, this would require investment into our CI/CD infrastructure (e.g. ensuring a simpler way for developers to test locally in a non-AWS environment) to ensure that combinations of functions cannot be deployed until proven in the pre-production environment. 

The X25 team ultimately chose option 3, as it reduces latency, requires the least additional investment, and is a very common, traditional solution that is well-known and proven by other teams within GS.

X25 developers now enjoy a streamlined, low friction CI/CD as a consequence of this decision:

  • Developers can continue to spin up their services on their local machine and test against localhost
  • Multiple developers can work separately, and independently, on multiple feature branches within the same service. Each feature branch maps to a distinct service container in Fargate. Each container has a feature branch specific URL for client testing.
  • Latency spikes accessing the service APIs have been eliminated.


  • Either Lambda or ECS Fargate can be used to build serverless applications. When using Lambda, it is essential that teams optimize any initialization logic. Otherwise, cold-start times could become unacceptable. Provisioned concurrency does not offer a complete solution, as cold-starts will still occur during periods of high demand. 
  • It's not obvious that the Lambda monolith is an anti-pattern. Both the Lambda monolith and the granular Lambda FaaS solutions have their pros and cons, and both could suffer from high latency on cold-starts without sufficient care.
  • Building a standalone REST service as a containerized image has its benefits as all the request routing, request validation, authentication and authorization can all be provided by the REST framework (Flask and most other frameworks provide all the hooks needed). Developers can then easily test locally. 
  • For operating environments at lower scaling demand, the ECS Fargate container-based solution offers the simplest option and lowest friction for developers. It also means that developers do not need to worry about optimizing their startup times.

What's Next for the X25 Team?

Ultimately, the X25 team aims to deploy its APIs across multiple regions and provide an always-available solution, even if an entire AWS region fails. These globally-deployed services need to be able to handle any event in any region at any time. This introduces a new challenge in interacting with a database. The team needs to ensure that multiple concurrent events cannot create an inconsistent state in the databases. In a future blog, the team will explore how an always-available, globally-consistent DB architecture meets this challenge.

It's been a truly energizing journey building X25 through GS Accelerate, and assessing the serverless options for building our APIs is just one of the myriad of critical decisions that have been made to move our cloud-native business forward. 

About GS Accelerate

Since launching in 2018, Goldman Sachs employees have submitted more than 1,800 ideas, many of which, including X25, have been funded, resulting in seven new products available for Goldman Sachs clients and people. GS Accelerate businesses are hiring across teams and functions in offices across the globe including New York, Dallas, and Birmingham. To view open roles, visit here.

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