Lead Java Developer/Solution Architect (6-month contract)

About the Position

Lead Java Developer/Solution Architect (6-month contract)

• Location: London, England, United Kingdom (Remote)
• Employment Type: FULL_TIME

gravity9 is recruiting Java engineers and architects for a portfolio of engagements across UK central government. These are large-scale, data-intensive modernization programs: replacing legacy systems that carry statutory weight, decomposing monoliths that serve millions of users, and building secure data platforms where access control and auditability are first-class architectural concerns rather than late additions.

We are hiring across two overlapping profiles - Senior Java Developer and Application / Migration Architect - and expect most people to move between programs as engagements start and complete. Several of these are hands-on architect roles: you will own design decisions and also write production code.

Responsibilities

Architecture and Design

• Produce high- and low-level designs, security and data architectures, and architecture decision records - and take them through client design authorities and formal governance gates.
• Make and defend architectural trade-offs in front of both engineering and non-technical stakeholders, including client technical authorities and third-party suppliers.
• Analyze real access patterns and validate target MongoDB document models against genuine throughput, query, and sharding requirements before they are baselined.
• Design integration boundaries in multi-vendor programs where you own one component and must collaborate across the seams of others.

Hands-on Build

• Build production Java services and RESTful APIs (Spring Boot or similar), designed for integration by multiple consuming systems and documented to OpenAPI.
• Design and implement MongoDB document models, aggregation pipelines, indexing, and sharding strategies, and integrate them cleanly into Java service layers rather than through a leaky abstraction.
• Build data pipelines: CDC and streaming ingest, transformation frameworks, checkpointed batch extract workers, and MongoDB sink configuration and tuning at terabyte scale.
• Implement access control, encryption, and audit within the application and data layer, including enforcement inside the MongoDB query path, rather than bolted on around it.
• Write the tests: unit, integration, golden-record fixtures, parallel-run reconciliation, and load and stress harnesses that prove performance under realistic volumes.
• Work across both MongoDB Atlas and Enterprise Advanced, including Kubernetes-hosted deployments, and with the migration and sync tooling around them.

Data Modernization and Migration

• Model relational, XML, or other legacy schemas onto MongoDB document models, handling schema and semantic drift through versioned, configuration-driven mappings rather than forked code.
• Design and prove cutover mechanisms - dual-write, backfill, reconcile, read-switch, rollback - for systems where a failed migration is not an acceptable outcome.
• Build validation and reconciliation tooling that satisfies audit: completeness, field-level fidelity, and query-level equivalence between old and new.

Security, Access Control, and Assurance

• Design and implement attribute- or role-based access control models, including enforcement at the data layer and policy administration.
• Work with application-layer and field-level encryption - MongoDB client-side field-level and queryable encryption, and externally managed key material.
• Produce tamper-evident audit trails and observability that answer not just what happened but why.

Client-Facing Delivery and Enablement

• Work as an embedded member of a small delivery team, frequently alongside client engineers and other suppliers.
• Run workshops, playbacks, and sprint demonstrations; produce design documentation, integration guides, and operational runbooks.
• Support knowledge transfer and upskilling of client teams - on these programs, enablement is a deliverable, not an afterthought.
• Use AI-assisted engineering tooling as part of normal delivery, within the constraints client security policy sets.

Requirements

Essential

• Strong hands-on Java - building and running production services and RESTful APIs with Spring Boot or an equivalent framework.
• MongoDB - solid working experience covering document modeling, querying, aggregation pipelines, and indexing, and integrating MongoDB into Java application layers. Atlas and/or Enterprise Advanced.
• Data-intensive systems - demonstrable experience of pipelines, migrations, or integrations at meaningful scale, and sound reasoning about throughput, ordering, idempotency, and failure recovery.
• API design - clean, well-documented REST interfaces suitable for integration by multiple consuming systems.
• Testing - genuine automated testing discipline, plus practical experience of load and stress testing against realistic volumes.
• Infrastructure and deployment - Terraform or equivalent infrastructure as code, containerized deployment on Kubernetes, and AWS or a comparable cloud.
• Communication - able to write clear design documentation and hold your own in a client design authority or architecture review.
• UK resident, and either holding current UK SC clearance or clearly eligible to obtain it.

Desirable

• Proven design authority on a delivery of comparable size and sensitivity, while remaining hands-on enough to prototype, pair, and review code.
• Experience operating inside multi-vendor or multi-supplier programs, including holding a technical position when other suppliers hold a different one.
• UK public sector, or another regulated and security-conscious environment.
• Kafka and change-data-capture tooling such as Debezium; stream processing.
• Legacy modernization patterns - strangler fig, operational data layers, dual running, and phased cutover.
• Domain-driven design and bounded-context analysis; monolith decomposition.
• Attribute-based access control and policy-as-code frameworks (OPA, Casbin, XACML-based systems).
• Field-level or client-side encryption, key management, HSM/KMS, and KMIP integration.
• Search - Lucene-based or Atlas Search, vector search, hybrid retrieval, and RAG patterns.
• Metadata standards such as DCAT or Dublin Core; data cataloging, data mesh, and federated data architecture concepts.
• Event-driven architecture and consistency patterns in distributed systems (saga, eventual consistency).
• Oracle, COBOL, or other mainframe and legacy platform familiarity.
• Time-series or geospatial data; deployment to constrained or edge hardware.
• AI-assisted development workflows and agentic coding tools.

Working Context

• Engagement: Contract. Typically three to twelve months per program, with a strong likelihood of rolling onto the next. Some roles begin part-time through a design phase before going full-time for build.
• Clearance: SC clearance is the standard requirement for these roles, and current SC will always be the fastest route onto a program. We will consider especially strong candidates who hold BPSS only and can sponsor SC in those cases, though clearance takes roughly three months and start dates will reflect that.
• Residency: UK resident. All work on these programs is delivered from the UK by UK-onshore staff.
• Working Model: Predominantly remote with periodic onsite attendance. Locations vary by program - expect occasional travel to client sites including London, Manchester, and Leeds.
• Environment: Work is typically delivered through client-supplied devices or VDI, with VPN access to client environments. You should be comfortable operating within defined information-handling and access constraints, and with the pace that government security and onboarding processes set.
• Team Shape: Small, focused, multi-disciplinary teams. On several programs, gravity9 delivers alongside partner organizations and other suppliers, and you will need to work effectively across those boundaries.
• Timing: These roles support a growing pipeline of central government work. Some engagements are contracted and mobilizing; others are in contract negotiation with expected start dates in the coming months. We are building the bench now so that we can mobilize quickly, and clearance lead times mean early conversations are worthwhile even where a start date is not yet fixed.