WebAssembly Edit

What is WebAssembly (Wasm)?

As described on https://webassembly.org/

WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable target for compilation of high-level languages like C/C++/Rust, enabling deployment on the web for client and server applications.

Overview

OPA is able to compile Rego policies into executable Wasm modules that can be evaluated with different inputs and external data. This is not running the OPA server in Wasm, nor is this just cross-compiled Golang code. The compiled Wasm module is a planned evaluation path for the source policy and query.

Current Status

The core language is supported fully but there are a number of built-in functions that are not, and probably won’t be natively supported in Wasm (e.g., http.send). Built-in functions that are not natively supported can be implemented in the host environment (e.g., JavaScript).

Compiling Policies

You can compile Rego policies into Wasm modules using the opa build subcommand.

For example, the opa build command below compiles the example.rego file into a Wasm module and packages it into an OPA bundle. The wasm target requires at least one entrypoint rule (specified by -e, or a metadata entrypoint annotation).

opa build -t wasm -e example/allow example.rego

The output of a Wasm module built this way contain the result of evaluating the entrypoint rule. For example:

[
  {
    "result": <value of data.example.allow>
  }
]

The output of policy evaluation is a set of variable assignments. The variable assignments specify values that satisfy the expressions in the policy query (i.e., if the variables in the query are replaced with the values from the assignments, all of the expressions in the query would be defined and not false.)

When policies are compiled into Wasm, the user provides the path of the policy decision that should be exposed by the Wasm module. The policy decision is assigned to a variable named result. The policy decision can be ANY JSON value (boolean, string, object, etc.) but there will be at-most-one assignment. This means that callers should first check if the set of variable assignments is empty (indicating an undefined policy decision) otherwise they should select the "result" key out of the variable assignment set.

For more information on opa build run opa build --help.

Advanced Compiling Options

You can also compile Rego policies into Wasm modules from Go using the lower-level rego API that produces raw Wasm executables and the higher-level compile API that produces OPA bundle files. The compile API is recommended.

Using Compiled Policies

JavaScript SDK

There is a JavaScript SDK available that simplifies the process of loading and evaluating compiled policies. If you want to evaluate Rego policies inside JavaScript we recommend you use the Javascript SDK. There is also an example NodeJS application provided for reference.

Other Languages

A number of other languages have OPA Wasm support too via various community SDKs. Community Wasm SDKs and integrations (6 projects) are listed on the Wasm integrations page in the OPA Ecosystem section.

From Scratch

If you want to integrate Wasm compiled policies into a language or runtime that does not have SDK support, read this section.

Instantiating the Wasm Module

Before you can evaluate Wasm compiled policies you need to instantiate the Wasm module produced by the compilation process described earlier on this page.

To load the compiled Wasm module refer the documentation for the Wasm runtime that you are using. At a high-level you must provide a memory buffer and a set of import functions. The memory buffer is a contiguous, mutable byte-array that allows you to pass data to the policy and receive output from the policy. The import functions are dependencies of the compiled policies.

ABI Versions

Wasm modules built using OPA 0.27.0 onwards contain a global variable named opa_wasm_abi_version that has a constant i32 value indicating the ABI version this module requires. Described below you find ABI versions 1.x.

There’s another i32 constant exported, opa_wasm_abi_minor_version, used to track backwards-compatible changes.

Using tools like wasm-objdump (wasm-objdump -x policy.wasm), the ABI version can be found here:

Global[3]:
 - global[0] i32 mutable=1 - init i32=121904
 - global[1] i32 mutable=0 <opa_wasm_abi_version> - init i32=1
 - global[2] i32 mutable=0 <opa_wasm_abi_minor_version> - init i32=0
Export[19]:
[...]
 - global[1] -> "opa_wasm_abi_version"
 - global[2] -> "opa_wasm_abi_minor_version"

Note the i32=1 of global[1], exported by the name of opa_wasm_abi_version.

Version notes

ABI	Notes
1.0	Start of ABI versioning.
1.1	Adds export `memory`.
1.2	Adds exported function `opa_eval`.
1.3	Adds exported functions `opa_value_free`, `opa_heap_blocks_stash`, `opa_heap_blocks_restore`, `opa_heap_stash_clear`.

Exports

The primary exported functions for interacting with policy modules are listed below. In the ABI column, you can find the ABI version with which the export was introduced.

Function	Description	ABI
`eval`	`int32 eval(ctx_addr)` Evaluates the loaded policy with the provided evaluation context. The return value is reserved for future use.	1.0
`builtins`	`value_addr builtins(void)` Returns the address of a mapping of built-in function names to numeric identifiers that are required by the policy.	1.0
`entrypoints`	`value_addr entrypoints(void)` Returns the address of a mapping of entrypoints to numeric identifiers that can be selected when evaluating the policy.	1.0
`opa_eval_ctx_new`	`ctx_addr opa_eval_ctx_new(void)` Returns the address of a newly allocated evaluation context.	1.0
`opa_eval_ctx_set_input`	`void opa_eval_ctx_set_input(ctx_addr, value_addr)` Set the input value to use during evaluation. This must be called before each `eval()` call. If the input value is not set before evaluation, references to the `input` document result produce no results (i.e., they are undefined.)	1.0
`opa_eval_ctx_set_data`	`void opa_eval_ctx_set_data(ctx_addr, value_addr)` Set the data value to use during evaluation. This should be called before each `eval()` call. If the data value is not set before evaluation, references to base `data` documents produce no results (i.e., they are undefined.)	1.0
`opa_eval_ctx_set_entrypoint`	`void opa_eval_ctx_set_entrypoint(ctx_addr, entrypoint_id)` Set the entrypoint to evaluate. By default, entrypoint with id `0` is evaluated.	1.0
`opa_eval_ctx_get_result`	`value_addr opa_eval_ctx_get_result(ctx_addr)` Get the result set produced by the evaluation process.	1.0
`opa_malloc`	`addr opa_malloc(int32 size)` Allocates size bytes in the shared memory and returns the starting address.	1.0
`opa_free`	`void opa_free(addr)` Free a pointer. Calls `opa_abort` on error.	1.0
`opa_json_parse`	`value_addr opa_json_parse(str_addr, size)` Parses the JSON serialized value starting at str_addr of size bytes and returns the address of the parsed value. The parsed value may refer to a null, boolean, number, string, array, or object value.	1.0
`opa_value_parse`	`value_addr opa_value_parse(str_addr, size)` The same as `opa_json_parse` except Rego set literals are supported.	1.0
`opa_json_dump`	`str_addr opa_json_dump(value_addr)` Dumps the value referred to by `value_addr` to a null-terminated JSON serialized string and returns the address of the start of the string. Rego sets are serialized as JSON arrays. Non-string Rego object keys are serialized as strings.	1.0
`opa_value_dump`	`str_addr opa_value_dump(value_addr)` The same as `opa_json_dump` except Rego sets are serialized using the literal syntax and non-string Rego object keys are not serialized as strings.	1.0
`opa_heap_ptr_set`	`void opa_heap_ptr_set(addr)` Set the heap pointer for the next evaluation.	1.0
`opa_heap_ptr_get`	`addr opa_heap_ptr_get(void)` Get the current heap pointer.	1.0
`opa_value_add_path`	`int32 opa_value_add_path(base_value_addr, path_value_addr, value_addr)` Add the value at the `value_addr` into the object referenced by `base_value_addr` at the given path. The `path_value_addr` must point to an array value with string keys (eg: `["a", "b", "c"]`). Existing values will be updated. On success the value at `value_addr` is no longer owned by the caller, it will be freed with the base value. The path value must be freed by the caller after use by calling `opa_value_free`. (The original path string passed to `opa_json_parse` or `opa_value_parse` to create the value must be freed by calling `opa_free`.) If an error occurs the base value will remain unchanged. Example: base object `{"a": {"b": 123}}`, path `["a", "x", "y"]`, and value `{"foo": "bar"}` will yield `{"a": {"b": 123, "x": {"y": {"foo": "bar"}}}}`. Returns an error code (see below).	1.0
`opa_value_remove_path`	`int32 opa_value_remove_path(base_value_addr, path_value_addr)` Remove the value from the object referenced by `base_value_addr` at the given path. Values removed will be freed. The path value must be freed by the caller after use by calling `opa_value_free`. (The original path string parsed by `opa_json_parse` or `opa_value_parse` must be released using `opa_free`.) The `path_value_addr` must point to an array value with string keys (eg: `["a", "b", "c"]`). Returns an error code (see below).	1.0
`opa_value_free`	`void opa_value_free(value_addr)` Free a value such as one generated by `opa_value_parse` or `opa_json_parse` reference at `value_addr`	1.3
`opa_heap_blocks_stash`	`void opa_heap_blocks_stash(void)` Stash free heap blocks in a shadow heap to enable `eval` or `opa_eval` to allocate only blocks that it can subsequently free with a call to `opa_heap_ptr_set`. The caller should subsequently call `opa_heap_ptr_get` and store the value to save before calling `opa_heap_bloks_restore`	1.3
`opa_heap_blocks_restore`	`void opa_heap_blocks_restore(void)` Restore heap blocks stored by `opa_heap_blocks_stash` to the heap. This should only be called after a `opa_heap_ptr_set` to the a heap pointer recorded by `opa_heap_ptr_get` after the previous call to `opa_heap_blocks_stash`.	1.3
`opa_heap_stash_clear`	`void opa_heap_stash_clear(void)` Drop all heap blocks saved by `opa_heap_blocks_stash`. This leaks memory in the VM unless the caller subsequently invokes `opa_heap_ptr_set` to a value taken prior to calling `opa_heap_blocks_stash`. (see below)	1.3
`opa_eval`	`str_addr opa_eval(_ addr, entrypoint_id int32, data value_addr, input str_addr, input_len int32, heap_ptr addr, format int32)` One-off policy evaluation method. Its arguments are everything needed to evaluate: entrypoint, address of data in memory, address and length of input JSON string in memory, heap address to use, and the output format (`0` is JSON, `1` is “value”, i.e. serialized Rego values). The first argument is reserved for future use and must be `0`. Returns the address to the serialised result value.	1.2

The addresses passed and returned by the policy modules are 32-bit integer offsets into the shared memory region. The value_addr parameters and return values refer to OPA value data structures: null, boolean, number, string, array, object, and set.

Error codes:

OPA Wasm Error codes are int32 values defined as:

Value	Name	Description
0	OPA_ERR_OK	No error.
1	OPA_ERR_INTERNAL	Unrecoverable internal error.
2	OPA_ERR_INVALID_TYPE	Invalid value type was encountered.
3	OPA_ERR_INVALID_PATH	Invalid object path reference.

Imports

Policy modules require the following function imports at instantiation-time:

Namespace	Name	Params	Result	Description
`env`	`opa_abort`	`(addr)`	`void`	Called if an internal error occurs. The `addr` refers to a null-terminated string in the shared memory buffer.
`env`	`opa_println`	`(addr)`	`void`	Called to emit a message from the policy evaluation. The `addr` refers to a null-terminated string in the shared memory buffer.
`env`	`opa_builtin0`	`(builtin_id, ctx)`	`addr`	Called to dispatch the built-in function identified by the `builtin_id`. The `ctx` parameter reserved for future use. The result `addr` must refer to a value in the shared-memory buffer. The function accepts 0 arguments.
`env`	`opa_builtin1`	`(builtin_id, ctx, _1)`	`addr`	Same as previous except the function accepts 1 argument.
`env`	`opa_builtin2`	`(builtin_id, ctx, _1, _2)`	`addr`	Same as previous except the function accepts 2 arguments.
`env`	`opa_builtin3`	`(builtin_id, ctx, _1, _2, _3)`	`addr`	Same as previous except the function accepts 3 arguments.
`env`	`opa_builtin4`	`(builtin_id, ctx, _1, _2, _3, _4)`	`addr`	Same as previous except the function accepts 4 arguments.

The policy module also requires a shared memory buffer named env.memory.

Memory Buffer

A shared memory buffer must be provided as an import for the policy module with the name env.memory. The buffer must be large enough to accommodate the input, provided data, and result of evaluation.

Built-in Functions

After instantiating the policy module, call the exported builtins function to receive a mapping of built-in functions required during evaluation. The result maps required built-in function names to the identifiers supplied to the built-in function callbacks (e.g., opa_builtin0, opa_builtin1, etc.)

For example:

const memory = new WebAssembly.Memory({ initial: 5 });
const policy_module = await WebAssembly.instantiate(byte_buffer, /* import object */);
const addr = policy_module.instance.exports.builtins();
const str_addr =  policy_module.instance.exports.opa_json_dump(addr);
const builtin_map = deserialize_null_terminated_JSON_string(memory, str_addr);

The built-in function mapping will contain all of the built-in functions that may be required during evaluation. For example, the following query refers to the http.send built-in function which is not included in the policy module:

result := http.send({"method": "get", "url": "https://example.com/api/lookup/12345"})

If this query was compiled to Wasm the built-in map would contain a single element:

{
    "http.send": 0
}

When the evaluation runs, the opa_builtin1 callback would invoked with builtin_id set to 0.

Evaluation

Once instantiated, the policy module is ready to be evaluated. Use the opa_eval_ctx_new exported function to create an evaluation context. Use the opa_eval_ctx_set_input and opa_eval_ctx_set_data exported functions to specify the values of the input and base data documents to use during evaluation.

To evaluate, call to the exported eval function with the eval context address as the only parameter.

Input

The (optional) input document for a policy can be provided by loading a JSON string into the shared memory buffer. Use the opa_malloc exported function to allocate a buffer the size of the JSON string and copy the contents in at the returned address. After the raw string is loaded into memory you will need to call the opa_json_parse exported method to get an address to the parsed input document for use in evaluations. Set the address via the opa_eval_ctx_set_input exported function supplying the evaluation context address and parsed input document address.

External Data

External data can be loaded for use in evaluation. Similar to the input this is done by loading a JSON string into the shared memory buffer. Use opa_malloc and opa_json_parse followed by opa_eval_ctx_set_data to set the address on the evaluation context.

Data can be updated by using the opa_value_add_path and opa_value_remove_path and providing the same value address as the base. Similarly, use opa_malloc and opa_json_parse for the updated value and creating the path.

After loading the external data use the opa_heap_ptr_get exported method to save the current point in the heap before evaluation. After evaluation this should be reset by calling opa_heap_ptr_set to ensure that evaluation restarts back at the saved data and re-uses heap space. This is particularly important if re-evaluating many times with the same data.

If you want to continue to update data in between query evaluations then the calling convention is a little more sophisticated due to the way that eval and opa_eval release temporary memory between queries. The functions opa_heap_blocks_stash and opa_heap_blocks_restore provide a safe way to stash free heap memory during queries and then restore it for use when adding or removing further external data. Without using these, the eval and opa_eval calls will leak all heap blocks below the heap pointer. The calling convention is as follows:

It’s always prudent at VM initialization to call opa_malloc with a size of 0 and then opa_heap_ptr_get to obtain the initial value of the heap pointer for the VM. Call this the “initial heap pointer”.
If you never load external data, you can use the “initial heap pointer” as your “data heap pointer” for calls to eval or opa_eval.
On the first load of external data, after calling opa_heap_ptr_get also call opa_heap_blocks_stash. This will save free heap memory for reuse later and prevent calls to eval or opa_eval from leaking that memory. Call the saved heap pointer after the initial data document load the “data heap pointer”. This “data heap pointer” is the value that should be used in eval or opa_eval calls.
On subsequent calls to modify the data document using opa_value_add_path or opa_value_remove_path, do the following:
1. Call opa_heap_ptr_set passing the “data heap pointer” to reset the heap and clear any memory left from eval or opa_eval calls.
2. Call opa_heap_blocks_restore to reinstate the heap stashed heap memory.
3. Call opa_malloc/opa_json_parse/opa_free to create the “path” and “value” arguments (in WASM value form) as usual.
4. Call opa_value_add_path or opa_value_remove_path as usual.
5. Call opa_value_free on the “path” argument to release it as usual.
6. Call opa_value_blocks_stash to stash any free heap blocks to protect them during eval or opa_eval calls.
7. Call opa_heap_ptr_get to get a new “data heap pointer”. It may be larger or smaller than the previous value depending upon which internal memory the calls allocated or released.
The calling convention for eval and opa_eval don’t change at all.
If, at any point, you wish to reset the VM to an initial state with regard to the policy data then do the following:
1. Call opa_heap_stash_clear to drop all stashed heap blocks (if any).
2. Call opa_heap_ptr_set with the “initial heap pointer” to reset the heap to its initial state.
3. Use the “initial heap pointer” as your new “data heap pointer” until the next time you add external data in the VM.

It might seem counter-intuitive to hide available heap memory from calls to eval or opa_eval. But that memory was never truly available for queries in the first place. The very first call to opa_heap_ptr_set (either before eval or which opa_eval calls internally) resets the heap and leaks any free blocks on the heap. In versions of the ABI prior to 1.3 this memory was simply lost. Note, however, that multiple queries would not continue to leak memory since they would always reset the heap pointer to the same value. The WASM engine would only leak further memory if there were subsequent calls to opa_value_add_path or opa_value_remove_path followed by more queries. ABI 1.3 introduced the calling convention using opa_heap_blocks_stash and opa_heap_blocks_restore to allow for interleaving query evaluations with incremental data document modifications.

Entrypoints

The compiled policy may have one or more entrypoints. If no entrypoint is set on the evaluation context the default entrypoint (0) will be evaluated. SDKs can call entrypoints() after instantiating the module to retrieve the entrypoint name to entrypoint identifier mapping. SDKs can set the entrypoint to evaluate by calling opa_eval_ctx_set_entrypoint on the evaluation context. If an invalid entrypoint identifier is passed, the eval function will invoke opa_abort.

Results

After evaluation results can be retrieved via the exported opa_eval_ctx_get_result function. Pass in the evaluation context address. The return value is an address in the shared memory buffer to the structured result. To access the JSON result use the opa_json_dump exported function to retrieve a pointer in shared memory to a null terminated JSON string.

The result of evaluation is the set variable bindings that satisfy the expressions in the query. For example, the query x = 1; y = 2; y > x would produce the following result set:

[
    {
        "x": 1,
        "y": 2
    }
]

Sets are represented as JSON arrays.

Ecosystem Projects

Wasm is a great way to integrate OPA into applications where the Go SDK is unavailable.

Below are ecosystem projects related to OPA Wasm. Integrations are ordered by the amount of linked content.

OPA Wasm Javascript Module

Styra

This project implements the OPA Wasm module interface in JavaScript.

View OPA Wasm Javascript Module Details

Wasm .NET Package (me-viper)

Andrii Kurochka

This project implements Open Policy Agent (OPA) WebAssembly .NET core SDK.

View Wasm .NET Package (me-viper) Details

Wasm .NET Package (christophwille)

Christoph Wille

This project implements the OPA Wasm module interface in C#.

View Details

OPA Wasm Rust Crate

matrix.org

This project implements the OPA Wasm module interface in Rust.

View OPA Wasm Rust Crate Details

Cloudflare Worker Enforcement of OPA Policies Using Wasm

This example project in OPA contrib uses the NodeJS OPA Wasm Module to enforce policy at the edge of Cloudflare’s network.

View Details

Wasm Java Gradle SDK (sangkeon)

Sangkeon Nam

This project implements the OPA Wasm module interface in Java.

View Details

View these projects in the OPA Ecosystem.

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