Esper
Esper is a language, compiler, and runtime for complex event processing and streaming analytics in Java and .NET.
What Esper is
Esper is a language, compiler, and runtime for complex event processing and streaming analytics in Java and .NET. Its Event Processing Language, or EPL, extends SQL with expressions that operate across events and time. The compiler converts EPL into bytecode that can be packaged in JAR files for distribution and execution. Esper and NEsper are components embedded in Java and .NET processes, not standalone servers. The in-memory runtime is described as memory-efficient and low-latency, and does not require external storage or services. Its adapters cover CSV, JMS, HTTP, Kafka, databases, and sockets, with JMX metrics exposure. Applications can join event data with external information from web services and relational databases. In Esper Enterprise Edition, scale-out features use Apache Kafka and Apache Zookeeper for elastic scaling, load balancing, fault tolerance, and multi-datacenter support. Listed use cases include business process monitoring, financial applications such as fraud detection, network and application monitoring, and sensor networks. Esper 9.0.0 is listed under GNU GPL (GPL v2) at no cost. EsperTech also offers commercial redistribution licenses and production support; Developer Seat support answers how-to questions but does not fix bugs or provide training.
Good
- EPL extends SQL for expressions over events and time.
- Adapters include Kafka, HTTP, databases, and sockets.
- External data can be joined with event streams.
- Esper 9.0.0 is listed under GNU GPL (GPL v2).
Watch out
- Esper and NEsper are components, not standalone servers.
- Developer Seat support excludes bug fixes and training.
- Enterprise scale-out features are described for Esper Enterprise Edition.
Who it suits
Esper may suit Java or .NET developers building event-processing applications that can embed a runtime in their processes. Its listed use cases include monitoring, financial applications, and sensor networks.
Verdict
Esper offers an embeddable streaming engine with an SQL-like event language and adapters for a range of inputs. Teams should account for its component-based deployment model and distinguish the Enterprise Edition scale-out features from the GPL-listed release.
Plans and prices
From Esper's own pricing page, checked 3 Oct 2026.
| Plan | Price | What it gives you |
|---|---|---|
| Esper 9.0.0 | Free | GNU GPL (GPL v2) |
| Esper Open Source | Free | GPLv2 license · Java/JVM and .NET/NEsper · software supplied as-is |
| Esper Enterprise Edition | Priced on request | closed-source commercial product · horizontal scaling · REST services · GUI · JDBC/JMX · commercial support |
| EsperHA | Priced on request | high-availability add-on · state resilience · fast recovery · enterprise support |
| Esper support | Priced on request | Production support: Bronze, Silver, Gold · Development support: Developer Seat · per year and per instance or per seat |
The facts
Each one read from Esper's own pages, with the date it was checked.
What it does
- API documentation
- Official documentation provides separate JavaDoc for the API, compiler, runtime, common modules, Avro and EsperIO.checked 15 Oct 2026
- Commercial licensing
- EsperTech offers EULA and OEM licenses that remove GPL copyleft obligations, with redistribution rights for OEMs.checked 15 Oct 2026
- Compilation
- The Esper compiler compiles EPL into bytecode that can be packaged in JAR files for distribution and execution.checked 3 Oct 2026
- Continuous processing
- The engine stores queries and evaluates incoming events continuously instead of polling stored records.checked 15 Oct 2026
- Current release
- The documentation page identifies Esper and EsperIO 9.0.0 as the current release.checked 3 Oct 2026
- Deployment model
- Esper and NEsper are embeddable components for Java and .NET processes rather than standalone servers.checked 3 Oct 2026
- Developer tooling
- EsperTech provides a free Visual Studio Code extension and EPL language server.checked 15 Oct 2026
- Documentation depth
- EsperTech publishes reference manuals, API JavaDocs, examples, training material and a knowledge base across many releases.checked 15 Oct 2026
- Downloads
- Core downloads are public, while Enterprise Edition and EsperHA downloads require registration or login.checked 15 Oct 2026
- Embedding
- Esper is designed to run inside Java or .NET processes and does not require external services or storage.checked 15 Oct 2026
- Enterprise security
- Esper Enterprise Edition says EsperHQ and EsperJMX can access CEP engines remotely over secured connections with user credentials.checked 3 Oct 2026
- Event formats
- Supported event representations include Java objects, maps, JSON, XML, object arrays and Avro.checked 15 Oct 2026
- Event language
- Its Event Processing Language (EPL) implements and extends SQL for expressions over events and time.checked 3 Oct 2026
- Event Processing Language
- Its EPL language extends SQL-style querying with event-time and stream-processing operations.checked 15 Oct 2026
- Extension APIs
- Extension points include plug-in loaders, data-flow operators, virtual data windows, method invocation and external service calls.checked 15 Oct 2026
- External data
- Esper supports joining external data, including web services, relational databases through SQL-query joins and method invocation joins.checked 3 Oct 2026
- Independent assessment
- Bloor Research describes Esper as an early open-source CEP engine and says commercial licenses are needed for OEM use.checked 15 Oct 2026
- Independent deployment finding
- Bloor reports that Esper can run as an embedded component, self-contained stack or Docker container across edge, on-premises and cloud environments.checked 15 Oct 2026
- Independent Kafka finding
- Bloor reports that Enterprise Edition can add instances dynamically and use Kafka-based failover when an instance goes down.checked 15 Oct 2026
- Independent technical evidence
- A peer-reviewed study selected Esper for reported performance, operator versatility and technological maturity when integrating CEP with message brokers.checked 15 Oct 2026
- Integrations
- The feature list names CSV, JMS, HTTP, Kafka, database and socket input/output adapters, and JMX metrics exposure.checked 3 Oct 2026
The company
- Company legal name
- The maker identifies itself as EsperTech Inc.checked 15 Oct 2026
- Founded
- Esper is documented as created in 2006, with its first public release in early 2006.checked 15 Oct 2026
- Headquarters
- EsperTech lists a mailing address in Wayne, New Jersey, USA.checked 15 Oct 2026
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