Senior Software Engineer - Elasticsearch in London

About 9fin: 9fin is the AI platform powering global debt markets - the world's largest asset class at over $145 trillion. Debt markets are vast, global, and mission-critical, yet still run on fragmented data, PDFs, and manual workflows. 9fin replaces this broken infrastructure with a single platform that centralises proprietary credit data, deep analysis, and high-value workflows across global markets. Today, 9fin powers teams at 300+ blue-chip institutions worldwide, including global banks, asset managers, private equity firms, law firms, and advisors. The business is scaling at exceptional speed, with rapid expansion in the US and best-in-class retention driven by deep workflow adoption.

What you’ll work on: The team maintains a strong focus on impact, using technology to deliver value for the business. While we have a high-level vision from the business, we have autonomy to set our team goals and define our own success. We take responsibility for our outcomes and emphasize proactive behaviours like ownership and accountability. As part of the retrieval team, you will be one of the core maintainers of the Elasticsearch cluster and its configuration. You’ll work closely with a senior team of both backend, AI and Data engineers to own and scale the search infrastructure at 9fin.

Responsibilities:

• Exposure to the development and engineering on the backend services and web application hosted tools and systems of the 9fin platform.
• Play an impactful role as a senior IC, running code reviews, mentoring other developers, leadership & involvement in early research and experimentation with new technologies providing input for new features and products.
• Continually improve the 9fin codebase taking the chance to automate, refactor and document when it’s most impactful.
• Very diverse web platforms, from complex financial data modelling, complex access control business logic, capturing environmental, emission, energy data, incorporation of ML/AI prediction services, data visualisation serving and automation systems.
• Collaboration and teamwork: Cross functional teams are formed for all projects including the appropriate domain expertise from 9fin financial, legal and editorial colleagues.
• Your ability to explain and break down complex projects to non-technical audiences will enable productive and efficient team progress.
• Developer Experience: Through managed automated preview environments, feature flags and well maintained non-production environments.

Our Backend Stack:

• Python is the dominant backend language used.
• Flask & FastAPI.
• Typing is used within our Python stack to power automated documentation and API definitions.
• Relational Databases (PostgreSQL) at the heart of majority of services.
• We have our own internal developer platform with standardised paved road approaches to infrastructure to bring your code into production as quickly as possible.
• We’re AWS for Cloud hosted services.
• Heavy use of AWS lambda Serverless.
• Docker (AWS ECS) for uniform development/deployment from dev to prod.
• Elastic Search deployment hosted in Elastic Cloud.

Requirements: This role will be a great fit if you:

• Have experience operating as a Senior IC Engineer, leading systems design and infrastructure discussions, mentoring more junior team members, and scoping work within cross-functional product teams.
• Have a deep understanding of Elasticsearch internals (inverted index, shards/replicas, segment merging, refresh/flush cycles) and how they impact performance and consistency.
• Have strong experience designing and tuning mappings, analyzers, and tokenizers (custom analyzers, multi-fields, synonyms, stemming strategies) for precise search relevance.
• Have proven ability to optimize query performance (DSL queries, filters vs queries, aggregations, caching, profiling, avoiding expensive operations like script queries when possible).
• Have hands-on experience operating clusters in production (index lifecycle management, shard allocation, scaling, monitoring, failure recovery, snapshot/restore).
• Have a solid knowledge of distributed systems concepts (consistency, partitioning, replication, eventual consistency) and how Elasticsearch behaves under load and failure scenarios.
• Plan and reason about software effectively, clearly and compellingly communicating its purpose within systems and its business impact on clients.
• Have hands-on experience designing data flow through software, from persistent storage to API endpoints, while applying industry best practices for validation.
• Experience with Python and its web frameworks (we currently use FastAPI with strong typing & Pydantic).
• Strong experience delivering APIs, Service Oriented Architecture.
• Strong working knowledge of SQL statements for retrieving data.
• A good collaborator and communicator with cross functional engineering teammates and others within 9fin.
• Code Quality: Through writing testable, maintainable code, and by engaging in code review for your own work and for reviewing others.

Benefits: We’re a scaling start up and we enjoy sharing our success, when the company succeeds, we always reinvest that in our people. We also offer huge amounts of responsibility, an abundance of opportunity for growth and a platform to truly excel.

Financial & Insurance:

• Competitive Salary (our salary bands are benchmarked at the top end of the market).
• Equity options.
• Pension (your minimum contributions are 4% with 9fin matching up to 7%).
• Private Medical Insurance.
• Paid sick leave with Income Protection for long periods of illness.
• Group Life Assurance.
• Season Ticket Loan & Cycle to Work schemes.

Time off:

• 25 holiday days per year.
• Local public holidays (with the ability to exchange them for alternative days).
• Hybrid working model, to allow you the flexibility to decide how, where and when you do your best work.
• Work abroad for up to 3 months a year.
• 1 month paid sabbatical after 5 years of service.
• Enhanced parental leave & flexible working arrangements available.

Training & Culture:

• Professional learning and development budget.
• Quarterly team socials.
• Summer and Winter company social events.

9fin is an equal opportunities employer. At 9fin we are dedicated to building and promoting a fair and inclusive workplace where everyone can reach their full potential and truly belong. We recognize that building diverse teams enables a more creative and productive environment. If you’re excited about this role but your experience doesn’t perfectly align with the job description, we encourage you to apply anyway. You might just be who we’re looking for - either for this role, or perhaps another.