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

gravity9

12d•Hybrid

About The Position

gravity9 is recruiting Java engineers and architects for a portfolio of engagements across UK central government. These are large-scale, data-intensive modernisation programmes: 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 programmes as engagements start and complete. Several of these are hands-on architect roles: you will own design decisions and also write production code.

Requirements

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 modelling, 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, containerised 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
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 programmes, including holding a technical position when other suppliers hold a different one

Nice To Haves

UK public sector, or another regulated and security-conscious environment
Kafka and change-data-capture tooling such as Debezium; stream processing
Legacy modernisation 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 cataloguing, 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

Responsibilities

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
Analyse 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 programmes where you own one component and must collaborate across the seams of others
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
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
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
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 programmes enablement is a deliverable, not an afterthought
Use AI-assisted engineering tooling as part of normal delivery, within the constraints client security policy sets

Benefits

SC clearance sponsorship
Contract roles typically three to twelve months per programme, with a strong likelihood of rolling onto the next
Some roles begin part-time through a design phase before going full-time for build
Predominantly remote with periodic onsite attendance
Occasional travel to client sites including London, Manchester and Leeds
Work typically delivered through client-supplied devices or VDI, with VPN access to client environments
Comfortable operating within defined information-handling and access constraints
Comfortable with the pace that government security and onboarding processes set
Small, focused, multi-disciplinary teams
Work effectively across boundaries with partner organisations and other suppliers