Banks can modernize payments without replacing their core by deploying a parallel ledger, often called a sidecar, that runs cloud-native alongside the existing system. This architecture moves card issuance and ledger logic off the core progressively, without a rip-and-replace mandate. The core continues to function as it always has. New products and capabilities run on modern infrastructure from day one. The bank retains full operational stability while building toward the architecture it actually needs.
Legacy cores weren't built for the pace of change that banks now face. They were built for stability, and they deliver it. But that same stability creates a cost: adding new products is slow, compliance updates require vendor coordination, and the gap between what your customers expect and what your platform can deliver grows wider each quarter.
The traditional answer to this problem has been core replacement. Full migration. Rip and replace. It's an answer that sounds logical until you examine the track record. Core replacement projects routinely stretch beyond their planned timelines. Costs exceed initial estimates by significant margins. And when something goes wrong during cutover, the consequences can be severe for both the bank and it’s customers.
Banks aren't wrong to be cautious. But caution without a path forward isn't a strategy. Regulators expect resilience. Customers expect products that match what neobanks offer. Competitors are moving. The question isn't whether to modernize. The question is how to do it without putting the institution at risk.
Sidecar payment modernization is the answer that resolves that tension.
A sidecar, or parallel ledger, is a separate, cloud-native ledger and card infrastructure layer that runs alongside your existing core. It connects via APIs. It doesn't replace the core. Banks launch new products and serve customers on modern infrastructure without touching other critical systems that continue to run on the legacy core.
The core continues to do what it does. The sidecar does what the core cannot.
This isn't a workaround or a stopgap. It's an architectural pattern used by some of the largest financial institutions in the world. HSBC's PayMe, for example, runs on Episode Six infrastructure alongside existing core banking systems. It's a production deployment at Tier 1 scale, not a pilot or a proof of concept.
The parallel ledger approach changes the calculus of modernization. Instead of a single, high-stakes project with a fixed go-live date, you have a controlled, incremental path. Products move off the core when the bank is ready. Not when a vendor contract expires or a regulator demands it.
Sidecar payment modernization follows a staged approach. Understanding the phases helps set realistic expectations for your board, your risk committee, and your operational teams.
Phase one: define the beachhead product. Not every product is the right starting point. The beachhead should be a product where the business case is clear, where the legacy limitation is most painful, and where the risk of moving is bounded. The right choice varies by institution. What matters is that the first product gives your teams a contained, high-value environment to validate the integration, build operational familiarity with the new platform, and demonstrate measurable outcomes to leadership before the program expands.
Phase two: connect the sidecar. The parallel ledger connects to your existing infrastructure via APIs. This isn't a big-bang integration. It's a targeted connection that allows the sidecar to read relevant data from the core and write new transaction records to its own ledger. Your existing risk, fraud, and compliance systems remain in place. They feed into the new environment, not around it.
Phase three: launch and validate. The beachhead product goes live on the sidecar. Your operational teams learn the new configuration environment. Your compliance and audit functions verify that the governance requirements they need are met. This is a production deployment with real customers and real transactions, but it's contained to the scope you defined.
Phase four: expand deliberately. With the beachhead live, the bank has evidence. Risk and compliance have seen the platform perform. Product teams have configured parameters without waiting on vendor sprints. Leadership has a working model for what progressive modernization looks like. Additional products move to the sidecar on a timeline the bank controls, based on readiness and strategic priority rather than external pressure.
Each phase is designed to preserve what banks value most: operational stability and the ability to meet regulator expectations without operational disruption.
The banks that get the most from sidecar payment modernization do the same thing before the first integration call: they get aligned internally on a handful of decisions that determine how well the program runs once it's live. Answering these questions early removes ambiguity from governance, integration design, vendor selection, and scope, so the program builds momentum rather than stalling on decisions that should have been made before kickoff.
Who owns configuration authority? A parallel ledger gives your teams the ability to create and modify product parameters without vendor development cycles. That's the point. But it requires the bank to decide in advance who is authorized to make those changes, what the approval workflow looks like before a change reaches production, and what audit trail the platform needs to produce for regulators. Getting this right before go-live means your governance model is in place from day one, not retrofitted after your first regulatory review.
How will data flows between the core and the sidecar be mapped? The parallel ledger reads from your existing core and writes to its own ledger. The integration design needs to specify exactly how those data flows work, including input and output validation, field mapping from existing message formats, and how the two environments reconcile at end of day. Banks that invest in this mapping upfront move through integration testing significantly faster than those who leave it to be resolved during go-live.
What does the vendor's configuration model actually look like? The goal of sidecar modernization is to give your teams direct control over product parameters. It's worth verifying that the platform you're evaluating genuinely supports that. Ask specifically whether your teams can manage configuration changes within your own governance workflows, or whether changes still require vendor development sprints. The answer determines whether you're gaining operational independence or simply relocating the bottleneck.
What's the scope of the first phase, and what's explicitly out of scope? The staged approach works because it keeps risk bounded and learning concentrated. Defining clearly which products move in phase one — and committing to what stays on the core until a later phase — gives your operational teams a manageable go-live target and gives risk and compliance a contained environment to review. The discipline of a well-defined first phase is what makes each subsequent phase easier to approve and execute.
The architecture is only as good as the platform running it. Banks evaluating a parallel ledger partner should assess across several dimensions.
Enterprise-grade uptime. A sidecar that introduces new availability risk defeats its own purpose. Look for documented uptime at 99.95% or better, with disaster recovery built into the architecture rather than bolted on. AWS regional deployments across multiple geographies provide the redundancy that banking operations require.
Proven deployment at Tier 1 scale. Proofs of concept and small-scale pilots don't demonstrate that a platform can handle millions of cards and billions in payment volume. Ask for evidence of production deployments at Tier 1 banks, across multiple markets, at real transaction scale.
Governed configurability. This is the most important capability to evaluate, and the hardest to assess from a features list. Ask specifically: how does the platform handle authorized configuration updates? What's the approval workflow? What does the audit trail look like for a regulator? Configuration-based product management with role-based access is the standard you should require.
Integration with existing infrastructure. A parallel ledger that demands the bank replace its fraud or dispute management systems isn't a sidecar. It's a full replacement with extra steps. The right partner brings integration experience with existing bank infrastructure and a deployment model that supports coexistence until full decoupling is warranted.
Multi-product capability across customer segments. The bank's needs will expand. A sidecar that supports consumer credit, commercial cards, virtual accounts, and installment products on a single platform prevents the proliferation of point solutions and the integration debt that follows.
Banks that successfully deploy a parallel ledger architecture consistently report benefits that go beyond the cost savings most modernization programs cite.
Product teams gain the ability to configure and validate new product parameters within your governance framework, without waiting on vendor development cycles. When a regulator changes a requirement, you respond with precision. When a commercial customer asks for a different control structure on their card program, your team configures it within your existing approval workflows.
Compliance and audit teams gain better visibility, not less. A purpose-built modern ledger provides audit-trailed configuration updates and reporting structures that are often clearer than what legacy systems produce. Regulators that have reviewed Episode Six deployments at institutions like HSBC have seen what bank-grade governance looks like on modern infrastructure.
Executive leadership gains optionality. The parallel ledger doesn't commit the bank to a future migration date. It creates the conditions under which a migration becomes possible when the institution is ready, rather than under duress from a contract deadline or a legacy system failure.
Episode Six is a global provider of enterprise-grade ledger and card infrastructure, operating across 50+ countries. The Episode Six platform is designed specifically for institutions that require bank-grade governance alongside modern product capability, including 99.95% uptime, deployment across multiple AWS regions, and configuration-based product management built for audit and compliance requirements.
If your institution is evaluating sidecar payment modernization or facing pressure to move off a legacy processor, the conversation starts with your architecture. Speak with the Episode Six team about how the Parallel Ledger deploys into your specific environment.
Explore Episode Six for Tier 1 banks | Learn about the Episode Six ledger platform
Episode Six is The World's Local Processor™. As a global provider of enterprise-grade card issuing and ledger infrastructure for financial technology companies, banks, and brands, Episode Six delivers the innovative capabilities needed to compete with disruptors and lead the market. Flexibility, adaptability, and resilience are built into the core of Episode Six's platform, ensuring clients maintain operational resilience and competitive advantage. Episode Six operates in over 50 countries, powering millions of accounts and billions in payments globally, with an expanding team located in the US, Canada, UK, Europe, Japan, Singapore, Hong Kong, Australia, and India. Investors include HSBC, Mastercard, SBI Investment Co Ltd, Anthos Capital, Avenir, and Japan Airlines.