Common Expression Language
The Common Expression Language (CEL) implements common semantics for expression evaluation, enabling different applications to more easily interoperate.
Key Applications
- Security policy: organization have complex infrastructure and need common tooling to reason about the system as a whole
- Protocols: expressions are a useful data type and require interoperability across programming languages and platforms.
Guiding philosophy:
- Keep it small & fast.
- CEL evaluates in linear time, is mutation free, and not Turing-complete. This limitation is a feature of the language design, which allows the implementation to evaluate orders of magnitude faster than equivalently sandboxed JavaScript.
- Make it extensible.
- CEL is designed to be embedded in applications, and allows for extensibility via its context which allows for functions and data to be provided by the software that embeds it.
- Developer-friendly
- The language is approachable to developers. The initial spec was based on the experience of developing Firebase Rules and usability testing many prior iterations.
- The library itself and accompanying toolings should be easy to adopt by teams that seek to integrate CEL into their platforms.
The required components of a system that supports CEL are:
- The textual representation of an expression as written by a developer. It is of similar syntax of expressions in C/C++/Java/JavaScript
- A binary representation of an expression. It is an abstract syntax tree (AST).
- A compiler library that converts the textual representation to the binary representation. This can be done ahead of time (in the control plane) or just before evaluation (in the data plane).
- A context containing one or more typed variables, often protobuf messages. Most use-case will use attribute_context.proto
- An evaluator library that takes the binary format in the context and produces a result, usually a Boolean.
Example of boolean conditions and object construction:
// Condition
account.balance >= transaction.withdrawal
|| (account.overdraftProtection
&& account.overdraftLimit >= transaction.withdrawal - account.balance)
// Object construction
common.GeoPoint{ latitude: 10.0, longitude: -5.5 }For more detail, see:
Released under the Apache License.
Disclaimer: This is not an official Google product.