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Syntax for Models

  • A model CONF should have at least four sections: [request_definition], [policy_definition], [policy_effect], [matchers].

  • If a model uses RBAC, it should also add the [role_definition] section.

  • A model CONF can contain comments. The comments start with #, and # will comment the rest of the line.

Request definition​

[request_definition] is the definition for the access request. It defines the arguments in e.Enforce(...) function.

[request_definition]
r = sub, obj, act

sub, obj, act represents the classic triple: accessing entity (Subject), accessed resource (Object) and the access method (Action). However, you can customize your own request form, like sub, act if you don't need to specify an particular resource, or sub, sub2, obj, act if you somehow have two accessing entities.

Policy definition​

[policy_definition] is the definition for the policy. It defines the meaning of the policy. For example, we have the following model:

[policy_definition]
p = sub, obj, act
p2 = sub, act

And we have the following policy (if in a policy file)

p, alice, data1, read
p2, bob, write-all-objects

Each line in a policy is called a policy rule. Each policy rule starts with a policy type, e.g., p, p2. It is used to match the policy definition if there are multiple definitions. The above policy shows the following binding. The binding can be used in the matcher.

(alice, data1, read) -> (p.sub, p.obj, p.act)
(bob, write-all-objects) -> (p2.sub, p2.act)
tip

The elements in a policy rule are always regarded asstring. If you have any question about this, please see the discussion at: https://github.com/casbin/casbin/issues/113

Policy effect​

[policy_effect] is the definition for the policy effect. It defines whether the access request should be approved if multiple policy rules match the request. For example, one rule permits and the other denies.

[policy_effect]
e = some(where (p.eft == allow))

The above policy effect means if there's any matched policy rule of allow, the final effect is allow (aka allow-override). p.eft is the effect for a policy, it can be allow or deny. It's optional and the default value is allow. So as we didn't specify it above, it uses the default value.

Another example for policy effect is:

[policy_effect]
e = !some(where (p.eft == deny))

It means if there's no matched policy rules ofdeny, the final effect is allow (aka deny-override). some means: if there exists one matched policy rule. any means: all matched policy rules (not used here). The policy effect can even be connected with logic expressions:

[policy_effect]
e = some(where (p.eft == allow)) && !some(where (p.eft == deny))

It means at least one matched policy rule ofallow, and there is no matched policy rule ofdeny. So in this way, both the allow and deny authorizations are supported, and the deny overrides.

note

Although we designed the syntax of policy effect as above, the current implementations only use hard-coded policy effect, as we found there's no much need for that sort of flexibility. So for now, you must use one of the built-in policy effects instead of customizing your own one.

The supported built-in policy effects are:

Policy effectMeaningExample
some(where (p.eft == allow))allow-overrideACL, RBAC, etc.
!some(where (p.eft == deny))deny-overrideDeny-override
some(where (p.eft == allow)) && !some(where (p.eft == deny))allow-and-denyAllow-and-deny
priority(p.eft) || denypriorityPriority
subjectPriority(p.eft)priority base on roleSubject-Priority

Matchers​

[matchers] is the definition for policy matchers. The matchers are expressions. It defines how the policy rules are evaluated against the request.

[matchers]
m = r.sub == p.sub && r.obj == p.obj && r.act == p.act

The above matcher is the simplest, it means that the subject, object and action in a request should match the ones in a policy rule.

You can use arithmetic like +, -, *, / and logical operators like &&, ||, ! in matchers.

Multiple sections type​

If you need multiple policy definitions or multiple matcher, you can use like p2, m2. In fact, all of the above four sections can use multiple types and the syntax is r+number, such as r2, e2. By default these four sections should correspond one to one. Such as your r2 will only use matcher m2 to match policies p2.

You can pass in EnforceContext as the first parameter of enforce method to specify the types, the EnforceContext is like this

EnforceContext{"r2","p2","e2","m2"}
type EnforceContext struct {
    RType string
    PType string
    EType string
    MType string
}

Example usage, see model and policy, the request is as follows

// Pass in a suffix as parameter to NewEnforceContext,such as 2 or 3 and it will create r2,p2,etc..
enforceContext := NewEnforceContext("2")
// You can also specify a certain type individually
enforceContext.EType = "e"
// Don't pass in EnforceContext,the default is r,p,e,m
e.Enforce("alice", "data2", "read")     // true
// pass in EnforceContext
e.Enforce(enforceContext, struct{ Age int }{Age: 70}, "/data1", "read")     //false
e.Enforce(enforceContext, struct{ Age int }{Age: 30}, "/data1", "read")     //true

Special Grammer​

You could also use in, the only operator with a text name. This operator checks the right-hand side array to see if it contains a value that is equal to the left-side value. Equality is determined by the use of the == operator, and this library doesn't check types between the values. Any two values, when cast to interface{}, and can still be checked for equality with == will act as expected. Note that you can use a parameter for the array, but it must be an []interface{}.

Also refer to rbac_model_matcher_using_in_op, keyget2_model and keyget_model

Example:

[request_definition]
r = sub, obj
...
[matchers]
m = r.sub.Name in (r.obj.Admins)
e.Enforce(Sub{Name: "alice"}, Obj{Name: "a book", Admins: []interface{}{"alice", "bob"}})

Expression evaluator​

The matcher evaluation in Casbin is implemented by expression evaluators in each language. Casbin integrates their powers to provide the unified PERM language. Besides all the model syntax provided here, those expression evaluators may provide extra functionality, which may be not supported by another language or implementation. Use it at your own risk.

The expression evaluators used by each Casbin implementation are:

ImplementationLanguageExpression evaluator
CasbinGolanghttps://github.com/Knetic/govaluate
jCasbinJavahttps://github.com/killme2008/aviator
Node-CasbinNode.jshttps://github.com/donmccurdy/expression-eval
PHP-CasbinPHPhttps://github.com/symfony/expression-language
PyCasbinPythonhttps://github.com/danthedeckie/simpleeval
Casbin.NETC#https://github.com/davideicardi/DynamicExpresso
Casbin4DDelphihttps://github.com/casbin4d/Casbin4D/tree/master/SourceCode/Common/Third%20Party/TExpressionParser
casbin-rsRusthttps://github.com/jonathandturner/rhai
casbin-cppC++https://github.com/ArashPartow/exprtk
note

If you encounter performance issue about Casbin, it's probably caused by the low efficiency of the expression evaluator. You can both send issue to Casbin or the expression evaluator directly for advice to speed up. See Benchmarks section for details.