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The Common Expression Language (CEL) implements common semantics for expressionevaluation, enabling different applications to more easily interoperate.

Key Applications

• Security policy: organizations have complex infrastructure and need commontooling to reason about the system as a whole
• Protocols: expressions are a useful data type and require interoperabilityacross programming languages and platforms.

Guiding philosophy:

1. 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 theimplementation to evaluate orders of magnitude faster than equivalentlysandboxed JavaScript.
2. Make it extensible.
   • CEL is designed to be embedded in applications, and allows forextensibility via its context which allows for functions and data to beprovided by the software that embeds it.
3. Developer-friendly.
   • The language is approachable to developers. The initial spec was basedon the experience of developing Firebase Rules and usability testingmany prior iterations.
   • The library itself and accompanying toolings should be easy to adopt byteams 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 isof similar syntax to expressions in C/C++/Java/JavaScript
• A representation of the program's abstract syntax tree (AST).
• A compiler library that converts the textual representation to the binaryrepresentation. This can be done ahead of time (in the control plane) orjust before evaluation (in the data plane).
• A context containing one or more typed variables, often protobuf messages.Most use-cases will useattribute_context.proto
• An evaluator library that takes the binary format in the context andproduces a result, usually a Boolean.

For use cases which require persistence or cross-process communcation, it ishighly recommended to serialize the type-checked expression as a protocolbuffer. The CEL team will maintains canonical protocol buffers for ASTs andwill keep these versions identical and wire-compatible in perpetuity:

• CEL canonical
• CEL v1alpha1

Example of boolean conditions and object construction:

// Conditionaccount.balance >=transaction.withdrawal|| (account.overdraftProtection&&account.overdraftLimit >=transaction.withdrawal-account.balance)// Object constructioncommon.GeoPoint{latitude:10.0,longitude:-5.5 }

For more detail, see:

• Introduction
• Language Definition

Released under theApache License.