Uncovering the Seams in Mainframes for Incremental Modernisation

A Case Study of Legacy Displacement

Mainframe systems continue to run much of the world's computing workload, but it's often difficult to add new features to support growing business needs. Furthermore the architectural challenges that make them slow to enhance also make them hard to replace. To reduce the risk involved, we should use an incremental approach to legacy displacement, gradually replacing legacy capabilities with implementations in modern technology. This strategy requires us to introduce seams into the mainframe system: points in which we could divert logic flow into newer services. In a recent project we investigated several approaches to introduce these seams into a long-lived mainframe system.

10 April 2024

Photo of Alessio Ferri

Alessio Ferri is a Technical Lead at Thoughtworks. He is a developer and a passionate technologist who applies his hands-on experience to help clients and delivery teams meet their objectives.

Photo of Tom Coggrave

Tom Coggrave is a Principal Technologist at Thoughtworks where he helps clients with their modernisation challenges, lately with a particular focus on mainframe modernisation.

In a recent project, we were tasked with designing how we would replace a Mainframe system with a cloud native application, building a roadmap and a business case to secure funding for the multi-year modernisation effort required. We were wary of the risks and potential pitfalls of a Big Design Up Front, so we advised our client to work on a ‘just enough, and just in time’ upfront design, with engineering during the first phase. Our client liked our approach and selected us as their partner.

The system was built for a UK-based client’s Data Platform and customer-facing products. This was a very complex and challenging task given the size of the Mainframe, which had been built over 40 years, with a variety of technologies that have significantly changed since they were first released.

Our approach is based on incrementally moving capabilities from the mainframe to the cloud, allowing a gradual legacy displacement rather than a “Big Bang” cutover. In order to do this we needed to identify places in the mainframe design where we could create seams: places where we can insert new behavior with the smallest possible changes to the mainframe’s code. We can then use these seams to create duplicate capabilities on the cloud, dual run them with the mainframe to verify their behavior, and then retire the mainframe capability.

Thoughtworks were involved for the first year of the programme, after which we handed over our work to our client to take it forward. In that timeframe, we did not put our work into production, nevertheless, we trialled multiple approaches that can help you get started more quickly and ease your own Mainframe modernisation journeys. This article provides an overview of the context in which we worked, and outlines the approach we followed for incrementally moving capabilities off the Mainframe.

Contextual Background

The Mainframe hosted a diverse range of services crucial to the client's business operations. Our programme specifically focused on the data platform designed for insights on Consumers in UK&I (United Kingdom & Ireland). This particular subsystem on the Mainframe comprised approximately 7 million lines of code, developed over a span of 40 years. It provided roughly ~50% of the capabilities of the UK&I estate, but accounted for ~80% of MIPS (Million instructions per second) from a runtime perspective. The system was significantly complex, the complexity was further exacerbated by domain responsibilities and concerns spread across multiple layers of the legacy environment.

Several reasons drove the client's decision to transition away from the Mainframe environment, these are the following:

  1. Changes to the system were slow and expensive. The business therefore had challenges keeping pace with the rapidly evolving market, preventing innovation.
  2. Operational costs associated with running the Mainframe system were high; the client faced a commercial risk with an imminent price increase from a core software vendor.
  3. Whilst our client had the necessary skill sets for running the Mainframe, it had proven to be hard to find new professionals with expertise in this tech stack, as the pool of skilled engineers in this domain is limited. Furthermore, the job market does not offer as many opportunities for Mainframes, thus people are not incentivised to learn how to develop and operate them.

High-level view of Consumer Subsystem

The following diagram shows, from a high-level perspective, the various components and actors in the Consumer subsystem.

The Mainframe supported two distinct types of workloads: batch processing and, for the product API layers, online transactions. The batch workloads resembled what is commonly referred to as a data pipeline. They involved the ingestion of semi-structured data from external providers/sources, or other internal Mainframe systems, followed by data cleansing and modelling to align with the requirements of the Consumer Subsystem. These pipelines incorporated various complexities, including the implementation of the Identity searching logic: in the United Kingdom, unlike the United States with its social security number, there is no universally unique identifier for residents. Consequently, companies operating in the UK&I must employ customised algorithms to accurately determine the individual identities associated with that data.

The online workload also presented significant complexities. The orchestration of API requests was managed by multiple internally developed frameworks, which determined the program execution flow by lookups in datastores, alongside handling conditional branches by analysing the output of the code. We should not overlook the level of customisation this framework applied for each customer. For example, some flows were orchestrated with ad-hoc configuration, catering for implementation details or specific needs of the systems interacting with our client’s online products. These configurations were exceptional at first, but they likely became the norm over time, as our client augmented their online offerings.
This was implemented through an Entitlements engine which operated across layers to ensure that customers accessing products and underlying data were authenticated and authorised to retrieve either raw or aggregated data, which would then be exposed to them through an API response.

Incremental Legacy Displacement: Principles, Benefits, and Considerations

Considering the scope, risks, and complexity of the Consumer Subsystem, we believed the following principles would be tightly linked with us succeeding with the programme:

  • Early Risk Reduction: With engineering starting from the beginning, the implementation of a “Fail-Fast” approach would help us identify potential pitfalls and uncertainties early, thus preventing delays from a programme delivery standpoint. These were:
    • Outcome Parity: The client emphasised the importance of upholding outcome parity between the existing legacy system and the new system (It is important to note that this concept differs from Feature Parity). In the client’s Legacy system, various attributes were generated for each consumer, and given the strict industry regulations, maintaining continuity was essential to ensure contractual compliance. We needed to proactively identify discrepancies in data early on, promptly address or explain them, and establish trust and confidence with both our client and their respective customers at an early stage.
    • Cross-functional requirements: The Mainframe is a highly performant machine, and there were uncertainties that a solution on the Cloud would satisfy the Cross-functional requirements.
  • Deliver Value Early: Collaboration with the client would ensure we could identify a subset of the most critical Business Capabilities we could deliver early, ensuring we could break the system apart into smaller increments. These represented thin-slices of the overall system. Our goal was to build upon these slices iteratively and frequently, helping us accelerate our overall learning in the domain. Furthermore, working through a thin-slice helps reduce the cognitive load required from the team, thus preventing analysis paralysis and ensuring value would be consistently delivered. To achieve this, a platform built around the Mainframe that provides better control over clients' migration strategies plays a vital role. Using patterns such as Dark Launching and Canary Release would place us in the driver's seat for a smooth transition to the Cloud. Our goal was to achieve a silent migration process, where customers would seamlessly transition between systems without any noticeable impact. This could only be possible through comprehensive comparison testing and continuous monitoring of outputs from both systems.

With the above principles and requirements in mind, we opted for an Incremental Legacy Displacement approach in conjunction with Dual Run. Effectively, for each slice of the system we were rebuilding on the Cloud, we were planning to feed both the new and as-is system with the same inputs and run them in parallel. This allows us to extract both systems’ outputs and check if they are the same, or at least within an acceptable tolerance. In this context, we defined Incremental Dual Run as: using a Transitional Architecture to support slice-by-slice displacement of capability away from a legacy environment, thereby enabling target and as-is systems to run temporarily in parallel and deliver value.

We decided to adopt this architectural pattern to strike a balance between delivering value, discovering and managing risks early on, ensuring outcome parity, and maintaining a smooth transition for our client throughout the duration of the programme.

Incremental Legacy Displacement approach

To accomplish the offloading of capabilities to our target architecture, the team worked closely with Mainframe SMEs (Subject Matter Experts) and our client’s engineers. This collaboration facilitated a just enough understanding of the current as-is landscape, in terms of both technical and business capabilities; it helped us design a Transitional Architecture to connect the existing Mainframe to the Cloud-based system, the latter being developed by other delivery workstreams in the programme.

Our approach began with the decomposition of the Consumer subsystem into specific business and technical domains, including data load, data retrieval & aggregation, and the product layer accessible through external-facing APIs.
Because of our client’s business purpose, we recognised early that we could exploit a major technical boundary to organise our programme. The client’s workload was largely analytical, processing mostly external data to produce insight which was sold on to clients. We therefore saw an opportunity to split our transformation programme in two parts, one around data curation, the other around data serving and product use cases using data interactions as a seam. This was the first high level seam identified.
Following that, we then needed to further break down the programme into smaller increments.
On the data curation side, we identified that the data sets were managed largely independently of each other; that is, while there were upstream and downstream dependencies, there was no entanglement of the datasets during curation, i.e. ingested data sets had a one to one mapping to their input files. .
We then collaborated closely with SMEs to identify the seams within the technical implementation (laid out below) to plan how we could deliver a cloud migration for any given data set, eventually to the level where they could be delivered in any order (Database Writers Processing Pipeline Seam, Coarse Seam: Batch Pipeline Step Handoff as Seam, and Most Granular: Data Characteristic Seam). As long as up- and downstream dependencies could exchange data from the new cloud system, these workloads could be modernised independently of each other.
On the serving and product side, we found that any given product used 80% of the capabilities and data sets that our client had created. We needed to find a different approach. After investigation of the way access was sold to customers, we found that we could take a “customer segment” approach to deliver the work incrementally. This entailed finding an initial subset of customers who had purchased a smaller percentage of the capabilities and data, reducing the scope and time needed to deliver the first increment. Subsequent increments would build on top of prior work, enabling further customer segments to be cut over from the as-is to the target architecture. This required using a different set of seams and transitional architecture, which we discuss in Database Readers and Downstream processing as a Seam.
Effectively, we ran a thorough analysis of the components that, from a business perspective, functioned as a cohesive whole but were built as distinct elements that could be migrated independently to the Cloud and laid this out as a programme of sequenced increments.


Our transitional architecture was mostly influenced by the Legacy seams we could uncover within the Mainframe. You can think of them as the junction points where code, programs, or modules meet. In a legacy system, they may have been intentionally designed at strategic places for better modularity, extensibility, and maintainability. If this is the case, they will likely stand out throughout the code, although when a system has been under development for a number of decades, these seams tend to hide themselves amongst the complexity of the code. Seams are particularly valuable because they can be employed strategically to alter the behaviour of applications, for example to intercept data flows within the Mainframe allowing for capabilities to be offloaded to a new system.

Determining technical seams and valuable delivery increments was a symbiotic process; possibilities in the technical area fed the options that we could use to plan increments, which in turn drove the transitional architecture needed to support the programme. Here, we step a level lower in technical detail to discuss solutions we planned and designed to enable Incremental Legacy Displacement for our client. It is important to note that these were continuously refined throughout our engagement as we acquired more knowledge; some went as far as being deployed to test environments, whilst others were spikes. As we adopt this approach on other large-scale Mainframe modernisation programmes, these approaches will be further refined with our most up to date hands-on experience.

External interfaces

We examined the external interfaces exposed by the Mainframe to data Providers and our client’s Customers. We could apply Event Interception on these integration points to allow the transition of external-facing workload to the cloud, so the migration would be silent from their perspective. There were two types of interfaces into the Mainframe: a file-based transfer for Providers to supply data to our client, and a web-based set of APIs for Customers to interact with the product layer.

Batch input as seam

The first external seam that we found was the file-transfer service.
Providers could transfer files containing data in a semi-structured format via two routes: a web-based GUI (Graphical User Interface) for file uploads interacting with the underlying file transfer service, or an FTP-based file transfer to the service directly for programmatic access.

The file transfer service determined, on a per provider and file basis, what datasets on the Mainframe should be updated. These would in turn execute the relevant pipelines through dataset triggers, which were configured on the batch job scheduler.

Assuming we could rebuild each pipeline as a whole on the Cloud (note that later we will dive deeper into breaking down larger pipelines into workable chunks), our approach was to build an individual pipeline on the cloud, and dual run it with the mainframe to verify they were producing the same outputs. In our case, this was possible through applying additional configurations on the File transfer service, which forked uploads to both Mainframe and Cloud. We were able to test this approach using a production-like File transfer service, but with dummy data, running on test environments.

This would allow us to Dual Run each pipeline both on Cloud and Mainframe, for as long as required, to gain confidence that there were no discrepancies. Eventually, our approach would have been to apply an additional configuration to the File transfer service, preventing further updates to the Mainframe datasets, therefore leaving as-is pipelines deprecated. We did not get to test this last step ourselves as we did not complete the rebuild of a pipeline end to end, but our technical SMEs were familiar with the configurations required on the File transfer service to effectively deprecate a Mainframe pipeline.

API Access as Seam

Furthermore, we adopted a similar strategy for the external facing APIs, identifying a seam around the pre-existing API Gateway exposed to Customers, representing their entrypoint to the Consumer Subsystem.

Drawing from Dual Run, the approach we designed would be to put a proxy high up the chain of HTTPS calls, as close to users as possible. We were looking for something that could parallel run both streams of calls (the As-Is mainframe and newly built APIs on Cloud), and report back on their outcomes.

Effectively, we were planning to use Dark Launching for the new Product layer, to gain early confidence in the artefact through extensive and continuous monitoring of their outputs. We did not prioritise building this proxy in the first year; to exploit its value, we needed to have the majority of functionality rebuilt at the product level. However, our intentions were to build it as soon as any meaningful comparison tests could be run at the API layer, as this component would play a key role for orchestrating dark launch comparison tests. Additionally, our analysis highlighted we needed to watch out for any side-effects generated by the Products layer. In our case, the Mainframe produced side effects, such as billing events. As a result, we would have needed to make intrusive Mainframe code changes to prevent duplication and ensure that customers would not get billed twice.

Similarly to the Batch input seam, we could run these requests in parallel for as long as it was required. Ultimately though, we would use Canary Release at the proxy layer to cut over customer-by-customer to the Cloud, hence reducing, incrementally, the workload executed on the Mainframe.

Internal interfaces

Following that, we conducted an analysis of the internal components within the Mainframe to pinpoint the specific seams we could leverage to migrate more granular capabilities to the Cloud.

Coarse Seam: Data interactions as a Seam

One of the primary areas of focus was the pervasive database accesses across programs. Here, we started our analysis by identifying the programs that were either writing, reading, or doing both with the database. Treating the database itself as a seam allowed us to break apart flows that relied on it being the connection between programs.

Database Readers

Regarding Database readers, to enable new Data API development in the Cloud environment, both the Mainframe and the Cloud system needed access to the same data. We analysed the database tables accessed by the product we picked as a first candidate for migrating the first customer segment, and worked with client teams to deliver a data replication solution. This replicated the required tables from the test database to the Cloud using Change Data Capture (CDC) techniques to synchronise sources to targets. By leveraging a CDC tool, we were able to replicate the required subset of data in a near-real time fashion across target stores on Cloud. Also, replicating data gave us opportunities to redesign its model, as our client would now have access to stores that were not only relational (e.g. Document stores, Events, Key-Value and Graphs were considered). Criterias such as access patterns, query complexity, and schema flexibility helped determine, for each subset of data, what tech stack to replicate into. During the first year, we built replication streams from DB2 to both Kafka and Postgres.

At this point, capabilities implemented through programs reading from the database could be rebuilt and later migrated to the Cloud, incrementally.

Database Writers

In regards to database writers, which were mostly made up of batch workloads running on the Mainframe, after careful analysis of the data flowing through and out of them, we were able to apply Extract Product Lines to identify separate domains that could execute independently of each other (running as part of the same flow was just an implementation detail we could change).

Working with such atomic units, and around their respective seams, allowed other workstreams to start rebuilding some of these pipelines on the cloud and comparing the outputs with the Mainframe.

In addition to building the transitional architecture, our team was responsible for providing a range of services that were used by other workstreams to engineer their data pipelines and products. In this specific case, we built batch jobs on Mainframe, executed programmatically by dropping a file in the file transfer service, that would extract and format the journals that those pipelines were producing on the Mainframe, thus allowing our colleagues to have tight feedback loops on their work through automated comparison testing. After ensuring that outcomes remained the same, our approach for the future would have been to enable other teams to cutover each sub-pipeline one by one.

The artefacts produced by a sub-pipeline may be required on the Mainframe for further processing (e.g. Online transactions). Thus, the approach we opted for, when these pipelines would later be complete and on the Cloud, was to use Legacy Mimic and replicate data back to the Mainframe, for as long as the capability dependant on this data would be moved to Cloud too. To achieve this, we were considering employing the same CDC tool for replication to the Cloud. In this scenario, records processed on Cloud would be stored as events on a stream. Having the Mainframe consume this stream directly seemed complex, both to build and to test the system for regressions, and it demanded a more invasive approach on the legacy code. In order to mitigate this risk, we designed an adaption layer that would transform the data back into the format the Mainframe could work with, as if that data had been produced by the Mainframe itself. These transformation functions, if straightforward, may be supported by your chosen replication tool, but in our case we assumed we needed custom software to be built alongside the replication tool to cater for additional requirements from the Cloud. This is a common scenario we see in which businesses take the opportunity, coming from rebuilding existing processing from scratch, to improve them (e.g. by making them more efficient).

In summary, working closely with SMEs from the client-side helped us challenge the existing implementation of Batch workloads on the Mainframe, and work out alternative discrete pipelines with clearer data boundaries. Note that the pipelines we were dealing with did not overlap on the same records, due to the boundaries we had defined with the SMEs. In a later section, we will examine more complex cases that we have had to deal with.

Coarse Seam: Batch Pipeline Step Handoff

Likely, the database won’t be the only seam you can work with. In our case, we had data pipelines that, in addition to persisting their outputs on the database, were serving curated data to downstream pipelines for further processing.

For these scenarios, we first identified the handshakes between pipelines. These consist usually of state persisted in flat / VSAM (Virtual Storage Access Method) files, or potentially TSQs (Temporary Storage Queues). The following shows these hand-offs between pipeline steps.

As an example, we were looking at designs for migrating a downstream pipeline reading a curated flat file stored upstream. This downstream pipeline on the Mainframe produced a VSAM file that would be queried by online transactions. As we were planning to build this event-driven pipeline on the Cloud, we chose to leverage the CDC tool to get this data off the mainframe, which in turn would get converted into a stream of events for the Cloud data pipelines to consume. Similarly to what we have reported before, our Transitional Architecture needed to use an Adaptation layer (e.g. Schema translation) and the CDC tool to copy the artefacts produced on Cloud back to the Mainframe.

Through employing these handshakes that we had previously identified, we were able to build and test this interception for one exemplary pipeline, and design further migrations of upstream/downstream pipelines on the Cloud with the same approach, using Legacy Mimic to feed back the Mainframe with the necessary data to proceed with downstream processing. Adjacent to these handshakes, we were making non-trivial changes to the Mainframe to allow data to be extracted and fed back. However, we were still minimising risks by reusing the same batch workloads at the core with different job triggers at the edges.

Granular Seam: Data Characteristic

In some cases the above approaches for internal seam findings and transition strategies do not suffice, as it happened with our project due to the size of the workload that we were looking to cutover, thus translating into higher risks for the business. In one of our scenarios, we were working with a discrete module feeding off the data load pipelines: Identity curation.

Consumer Identity curation was a complex space, and in our case it was a differentiator for our client; thus, they could not afford to have an outcome from the new system less accurate than the Mainframe for the UK&I population. To successfully migrate the entire module to the Cloud, we would need to build tens of identity search rules and their required database operations. Therefore, we needed to break this down further to keep changes small, and enable delivering frequently to keep risks low.

We worked closely with the SMEs and Engineering teams with the aim to identify characteristics in the data and rules, and use them as seams, that would allow us to incrementally cutover this module to the Cloud. Upon analysis, we categorised these rules into two distinct groups: Simple and Complex.
Simple rules could run on both systems, provided they fed on different data segments (i.e. separate pipelines upstream), thus they represented an opportunity to further break apart the identity module space. They represented the majority (circa 70%) triggered during the ingestion of a file. These rules were responsible for establishing an association between an already existing identity, and a new data record.
On the other hand, the Complex rules were triggered by cases where a data record indicated the need for an identity change, such as creation, deletion, or updation. These rules required careful handling and could not be migrated incrementally. This is because an update to an identity can be triggered by multiple data segments, and operating these rules in both systems in parallel could lead to identity drift and data quality loss. They required a single system minting identities at one point in time, thus we designed for a big bang migration approach.

In our original understanding of the Identity module on the Mainframe, pipelines ingesting data triggered changes on DB2 resulting in an up to date view of the identities, data records, and their associations.

Additionally, we identified a discrete Identity module and refined this model to reflect a deeper understanding of the system that we had discovered with the SMEs. This module fed data from multiple data pipelines, and applied Simple and Complex rules to DB2.

Now, we could apply the same techniques we wrote about earlier for data pipelines, but we required a more granular and incremental approach for the Identity one.
We planned to tackle the Simple rules that could run on both systems, with a caveat that they operated on different data segments, as we were constrained to having only one system maintaining identity data. We worked on a design that used Batch Pipeline Step Handoff and applied Event Interception to capture and fork the data (temporarily until we can confirm that no data is lost between system handoffs) feeding the Identity pipeline on the Mainframe. This would allow us to take a divide and conquer approach with the files ingested, running a parallel workload on the Cloud which would execute the Simple rules and apply changes to identities on the Mainframe, and build it incrementally. There were many rules that fell under the Simple bucket, therefore we needed a capability on the target Identity module to fall back to the Mainframe in case a rule which was not yet implemented needed to be triggered. This looked like the following:

As new builds of the Cloud Identity module get released, we would see less rules belonging to the Simple bucket being applied through the fallback mechanism. Eventually only the Complex ones will be observable through that leg. As we previously mentioned, these needed to be migrated all in one go to minimise the impact of identity drift. Our plan was to build Complex rules incrementally against a Cloud database replica and validate their outcomes through extensive comparison testing.

Once all rules were built, we would release this code and disable the fallback strategy to the Mainframe. Bear in mind that upon releasing this, the Mainframe Identities and Associations data becomes effectively a replica of the new Primary store managed by the Cloud Identity module. Therefore, replication is needed to keep the mainframe functioning as is.

As previously mentioned in other sections, our design employed Legacy Mimic and an Anti-Corruption Layer that would translate data from the Mainframe to the Cloud model and vice versa. This layer consisted of a series of Adapters across the systems, ensuring data would flow out as a stream from the Mainframe for the Cloud to consume using event-driven data pipelines, and as flat files back to the Mainframe to allow existing Batch jobs to process them. For simplicity, the diagrams above don’t show these adapters, but they would be implemented each time data flowed across systems, regardless of how granular the seam was. Unfortunately, our work here was mostly analysis and design and we were not able to take it to the next step and validate our assumptions end to end, apart from running Spikes to ensure that a CDC tool and the File transfer service could be employed to send data in and out of the Mainframe, in the required format. The time required to build the required scaffolding around the Mainframe, and reverse engineer the as-is pipelines to gather the requirements was considerable and beyond the timeframe of the first phase of the programme.

Granular Seam: Downstream processing handoff

Similar to the approach employed for upstream pipelines to feed downstream batch workloads, Legacy Mimic Adapters were employed for the migration of the Online flow. In the existing system, a customer API call triggers a series of programs producing side-effects, such as billing and audit trails, which get persisted in appropriate datastores (mostly Journals) on the Mainframe.

To successfully transition incrementally the online flow to the Cloud, we needed to ensure these side-effects would either be handled by the new system directly, thus increasing scope on the Cloud, or provide adapters back to the Mainframe to execute and orchestrate the underlying program flows responsible for them. In our case, we opted for the latter using CICS web services. The solution we built was tested for functional requirements; cross-functional ones (such as Latency and Performance) could not be validated as it proved challenging to get production-like Mainframe test environments in the first phase. The following diagram shows, according to the implementation of our Adapter, what the flow for a migrated customer would look like.

It is worth noting that Adapters were planned to be temporary scaffolding. They would not have served a valid purpose when the Cloud was able to handle these side-effects by itself after which point we planned to replicate the data back to the Mainframe for as long as required for continuity.

Data Replication to enable new product development

Building on the incremental approach above, organisations may have product ideas that are based primarily on analytical or aggregated data from the core data held on the Mainframe. These are typically where there is less of a need for up-to-date information, such as reporting use cases or summarising data over trailing periods. In these situations, it’s possible to unlock business benefits earlier through the judicious use of data replication.
When done well, this can enable new product development through a relatively smaller investment earlier which in turn brings momentum to the modernisation effort.
In our recent project, our client had already departed on this journey, using a CDC tool to replicate core tables from DB2 to the Cloud.

While this was great in terms of enabling new products to be launched, it wasn’t without its downsides.

Unless you take steps to abstract the schema when replicating a database, then your new cloud products will be coupled to the legacy schema as soon as they’re built. This will likely hamper any subsequent innovation that you may wish to do in your target environment as you’ve now got an additional drag factor on changing the core of the application; but this time it’s worse as you won’t want to invest again in changing the new product you’ve just funded. Therefore, our proposed design consisted of further projections from the replica database into optimised stores and schemas, upon which new products would be built.

This would give us the opportunity to refactor the Schema, and at times move parts of the data model into non-relational stores, which would better handle the query patterns observed with the SMEs.

Upon migration of batch workloads, in order to keep all stores in sync, you may want to consider either a write back strategy to the new Primary directly (what was previously known as the Replica), which in turn feeds back DB2 on the Mainframe (though there will be higher coupling from the batches to the old schema), or revert the CDC & Adaptation layer direction from the Optimised store as a source and the new Primary as a target (you will likely need to manage replication separately for each data segment i.e. one data segment replicates from Replica to Optimised store, another segment the other way around).


There are multiple things to consider when offloading from the mainframe. Depending on the size of the system that you wish to migrate off the mainframe, this work can take a considerable amount of time, and Incremental Dual Run costs are non-negligible. How much this will cost depends on various factors, but you cannot expect to save on costs via dual running two systems in parallel. Thus, the business should look at generating value early to get buy-in from stakeholders, and fund a multi-year modernisation programme. We see Incremental Dual Run as an enabler for teams to respond fast to the demand of the business, going hand in hand with Agile and Continuous Delivery practices.

Firstly, you have to understand the overall system landscape and what the entry points to your system are. These interfaces play an essential role, allowing for the migration of external users/applications to the new system you are building. You are free to redesign your external contracts throughout this migration, but it will require an adaptation layer between the Mainframe and Cloud.

SeamPatterns of Legacy DisplacementSummary
Batch inputEvent interception, Dual runCapture and redirect external inputs to batch systems
API accessEvent interception, Dark launching, Dual run, Canary releaseCapture and redirect calls to APIs

Secondly, you have to identify the business capabilities the Mainframe system offers, and identify the seams between the underlying programs implementing them. Being capability-driven helps ensure that you are not building another tangled system, and keeps responsibilities and concerns separate at their appropriate layers. You will find yourself building a series of Adapters that will either expose APIs, consume events, or replicate data back to the Mainframe. This ensures that other systems running on the Mainframe can keep functioning as is. It is best practice to build these adapters as reusable components, as you can employ them in multiple areas of the system, according to the specific requirements you have.

SeamPatterns of Legacy DisplacementSummary
Data interactionsExtract product lines, Dual run, Legacy mimicIdentify readers and writers, and unhook them, backfilling where necessary
Batch pipeline step handoffLegacy mimic, Transitional architectureInterpose new steps within existing batch flows
Data characteristicEvent Interception, Transitional architectureModernise workloads incrementally by segmenting data
Downstream processing handoffLegacy mimic, Transitional architectureCall back to legacy to retain necessary side effects

Thirdly, assuming the capability you are trying to migrate is stateful, you will likely require a replica of the data that the Mainframe has access to. A CDC tool to replicate data can be employed here. It is important to understand the CFRs (Cross Functional Requirements) for data replication, some data may need a fast replication lane to the Cloud and your chosen tool should provide this, ideally. There are now a lot of tools and frameworks to consider and investigate for your specific scenario. There are a plethora of CDC tools that can be assessed, for instance we looked at Qlik Replicate for DB2 tables and Precisely Connect more specifically for VSAM stores.

Cloud Service Providers are also launching new offerings in this area; for instance, Dual Run by Google Cloud recently launched its own proprietary data replication approach.

For a more holistic view on mobilising a team of teams to deliver a programme of work of this scale, please refer to the article “Eating the Elephant” by our colleague, Sophie Holden.

Ultimately, there are other things to consider which were briefly mentioned as part of this article. Amongst these, the testing strategy will play a role of paramount importance to ensure you are building the new system right. Automated testing shortens the feedback loop for delivery teams building the target system. Comparison testing ensures both systems exhibit the same behaviour from a technical perspective. These strategies, used in conjunction with Synthetic data generation and Production data obfuscation techniques, give finer control over the scenarios you intend to trigger and validate their outcomes. Last but not least, production comparison testing ensures the system running in Dual Run, over time, produces the same outcome as the legacy one on its own. When needed, outcomes are compared from an external observer’s point of view as a minimum, such as a customer interacting with the system. Additionally, we can compare intermediary system outcomes.

Hopefully, this article brings to life what you would need to consider when embarking on a Mainframe offloading journey. Our involvement was at the very first few months of a multi-year programme and some of the solutions we have discussed were at a very early stage of inception. Nevertheless, we learnt a great deal from this work and we find these ideas worth sharing. Breaking down your journey into viable valuable steps will always require context, but we hope our learnings and approaches can help you getting started so you can take this the extra mile, into production, and enable your own roadmap.

Significant Revisions

10 April 2024: Published final installment on using data replication

04 April 2024: Published internal seams on batch pipelines

02 April 2024: Published internal seams around database

27 March 2024: Published seams on external interfaces

26 March 2024: Published first installment