The Enso Protocol

Enso is a sophisticated language, but in order to provide a great user experience to our users we also need the ability to provide great tooling. This tooling means a language server, but it also means a set of extra peripheral components that ensure we can run Enso in a way that the product requires.

These services are responsible for providing the whole-host of language- and project-level tooling to the IDE components, whether they’re hosted in the cloud or locally on a user’s machine.

This document contains the architectural and functional specification of the Enso protocol.

For a detailed specification of all of the messages that make up the protocol, please see the protocol message specifications.


The divisions of responsibility between the backend engine services are dictated purely by necessity. As multi-client editing necessitates careful synchronisation and conflict resolution, between the actions of multiple clients. This section deals with the intended architecture for the Engine Services.

The engine services are divided into two main components:

  1. The Project Manager: This component is responsible for listing and managing user projects, as well as spawning the language server for a given project when it is opened.
  2. The Language Server: This component is responsible for dealing with incoming connections and resolving conflicts between multiple clients. It is also responsible for servicing all of the requests from the clients.

Both components will be implemented as akka actors such that we can defer the decision as to run them in different processes until the requirements become more clear.

The Project Manager

The project manager service is responsible for both allowing users to work with their projects but also the setup and teardown of the language server itself. Its responsibilities can be summarised as follows:

  • Allowing users to manage their projects.
  • Starting up the language server for a given project upon user selection.
  • Notifying the language server of a pending shutdown on project exit to allow it to persist any state that it needs to disk.

Language Server

The language server is responsible for managing incoming connections and communicating with the clients, as well as resolving any potential conflicts between the clients. It is responsible for the following:

  • Negotiating and accepting connections from multiple clients.
  • Resolving conflicts between messages from multiple clients.
  • Optimising incoming requests wherever possible.

It is also responsible for actually servicing all of the incoming requests, which includes but isn’t limited to:

  • Completion Information: It should be able to provide a set of candidate completions for a given location in the code.
  • Introspection Information: It should be able to provide introspection information from the running interpreter, which consists primarily of types and values.
  • Textual Diff Management: It needs to be able to accept and publish diffs of the program source code. As part of this, it needs to keep the node metadata up to date.
  • Analysis Operations: It should be able to service various IDE-style analysis requests (e.g. jump-to-definition or find usages)
  • Arbitrary Code Execution: It should be able to execute arbitrary Enso code on values in scope.
  • Refactoring: Common refactoring operations for Enso programs, including renaming, code formatting, and so on.
  • IO Management: Though this is arguably a feature of the runtime rather than the language server itself, this refers to the ability to watch files and monitor IO in order to recompute minimal subsets of the program.

It should be noted that the language server explicitly does not talk using LSP. The component is solely responsible for servicing requests, instead of dealing with the minutiae of connection negotiation and request handling.

Additionally, it is very important to note that the language server must not depend directly on the runtime (as a compile-time dependency). This would introduce significant coupling between the runtime implementation and the language server. Instead, the LS should only depend on org.graalvm.polyglot to interface with the runtime.

Textual Protocol

This section is partially out of date.

The protocol refers to the communication format that all of the above services speak between each other and to the GUI. This protocol is not specialised only to language server operations, as instead it needs to work for all of the various services in this set.

The protocol we are using intends to be fully compatible with the Microsoft LSP specification (version 3.15). In essence, we will operate as follows:

  • Where our use case matches with a function provided by LSP, we will use the specified LSP message (e.g. completions).
  • Where our use-case does not match a message provided by LSP, we will use the following process:
    1. If we can implement this on top of one of LSP’s extensible mechanisms (e.g. commands) we will do so.
    2. If this is not possible, we will specify an extension to the protocol. This extension will be well-specified within this document, and should be in the spirit of the existing protocol. If relevant, we may propose it as a future extension to the specification.

Aside from the language server protocol-based operations, we will definitely need a protocol extension to support Enso’s custom language functionality.

Textual Protocol Communication Patterns

Whatever protocol we decide on will need to have support for a couple of main communication patterns:

  • Pub/Sub: A standard publisher/subscriber model, the server will need to be able to support this kind of connection to deal with events that do not occur strictly in response to client actions (e.g. updates to observed values).
  • Req/Res: A standard request/response model, the server will need to be able to support this kind of connection to deal with one-off requests from the client, and potentially to make requests to the client (e.g. list modules in the current project, please refresh your file state).

There are also certain messages that follow the request/response model but where the responses are trivial acknowledgements. For simplicity’s sake these are currently subsumed by the generic request-response model.

As we have decided to remain compatible with LSP, we can use any communication pattern that we desire, either by employing existing LSP messages, or writing our own protocol extensions. Both of the above-listed patterns are supported by LSP.

We can support additional patterns through LSP’s mechanisms:

  • Asynchronous responses can be sent as notifications.
  • Protocol-level acknowledgements is supported directly in LSP.

Textual Protocol Transport

The transport of the protocol refers to the underlying layer over which its messages (discussed in the protocol format below) are sent. As we are maintaining compatibility with LSP, the protocol transport format is already defined for us.

  • Textual messages are sent using JSON-RPC over a WebSocket connection (as defined in the LSP spec).
  • As a protocol extension we also negotiate a secondary binary WebSocket connection for sending visualisation data. This transport is independent of the LSP spec, and hence is defined entirely by us.

The actionables for this section are:

  • Determine the details for the binary WebSocket, including how we want to encode messages and pointers into the stream, as well as how we set it up and tear it down.

The Protocol Format

Protocol messages are defined by LSP. Any extensions to the messages defined in the standard should use similar patterns such that they are not incongruous with LSP messages. The following notes apply:

  • Textual messages should be sent as LSP messages or extensions to them.
  • We have a hybrid extension to the protocol to allow us to send binary data (for visualisations) over a second WebSocket connection.

This means that we have two pipes: one is the textual WebSocket defined by LSP, and the other is a binary WebSocket.

Textual Protocol Functionality

This entire section deals with the functional requirements placed upon the protocol used by the engine services. These requirements are overwhelmingly imposed by the IDE, but also include additional functionality for the future evolution of the language.

All of the following pieces of functionality that are explained in detail are those expected for the 2.0 release. Any additional functionality beyond this milestone is described in a dedicated section.

Textual Diff Management

The engine services need to support robust handling of textual diffs. This is simply because it is the primary form of communication for synchronising source code between the IDE and the engine. It will need to support the following operations:

  • Synchronisation requests to ensure that the engine and IDE have the same view of the files in the project.
  • Diff update requests, that send a textual diff between client and server (or vice versa).

Both of these are supported natively within the LSP, and we will be using those messages to implement this.

It should be noted that we explicitly do not intend to handle updates to node metadata within the language server.

  • These updates should be sent as part of each diff the client provides (as a separate segment in the didChange message).
  • We may support this in the future, but we do not for now.

We place the following requirements upon the implementation of this:

  • We must be able to handle diffs of any size, even though we prefer that the client sends us minimal diffs. We do not know what all clients will do, and hence to remain compatible we must handle all diffs.
  • Diffs may require some AST-based or semantic minimisation in order to assist in the compiler’s incremental pipeline.
  • The gateway must handle diffs from multiple clients properly.

The implementation is as follows:

  • Support for the LSP messages didOpen, didChange, willSaveWaitUntil, didSave, didClose, and support for informing the runtime on each of these.
  • It must track which files are currently open in the editor.
  • The language server and runtime should watch the project folder in order to track updates as necessary.

Handling Multiple Clients

Multiple-client support will be implemented while remaining compatible with the LSP specification.

  • In the initial implementation, we will work on the principle of ‘write lock’, where only one of the multiple connected clients has the ability to write to the file.
  • In the future we will work on true conflict resolution of edits, mediated by the language server.

It will work as follows:

  • We will use as much of the LSP initialisation and connection flow as possible to connect additional clients. The extensions should be minimal and should not break compatibility with LSP.
  • We will extend the protocol (or use extension mechanisms) to implement write-lock negotiation. The first client to connect is granted write-lock, but this may be changed later via negotiation.
  • If any client sends a didChange message without holding the write lock, it should receive an applyEdit message that reverts the change, as well as a notification of the error.
  • The language server / gateway is responsible for synchronising state with new clients as they connect. As part of initialisation it should receive client state, and then applyEdit to synchronise views of the code.

Project State Management

One of the most important functionalities for this service set is the ability to manage the state of a project in general. The project state refers to the whole set of the project files and metadata and needs to support the following functionalities:

  • Get project metadata (name, maintainer, version, dependencies, and so on)
  • Change requests for the above

All file-based operations in the project can be handled by the editor directly, or the language server (when doing refactoring operations), and as such need no support in this section of the protocol.

At the current time, the language server has a 1:1 correspondence with a project. In the future, however, we may want to add LSP support for multiple projects in a single engine, as this would allow users to work with multiple related projects in a single instance of the IDE.

File Management and Storage

The nature of LSP means that file management and storage is not handled by the language server, and is instead handled by the editor. The protocol makes a distinction between:

  • Open Files: These are the ones currently open in the editor, and are ‘owned’ by the editor. The language server has to work with an in-memory representation for these files.
  • Closed Files: These are the ones not open in the editor, and can be accessed and modified directly by the language server.

The language server must have direct access to the project directory on the machine where it is running (either the local machine or in the Enso cloud), and file operations between the IDE and that machine are handled indepdendently of the language server.

Execution Management

The language server process will need to be able to respond to requests for various kinds of execution of Enso code. Furthermore, it needs to be able to respond to requests to ‘listen’ to the execution of various portions of code. This implies that the following functionalities are needed:

  • Execution of a function with provided arguments.
  • Execution of a function from a given call site (stack position and code position).
  • Attach an execution listener to an arbitrary code span.
  • Detach an execution listener by ID.
  • Implement heartbeat messages for execution listeners. If a heartbeat response isn’t received before some time-out, the language server should detach the listener.
  • Force cache invalidation for arbitrary code spans.
  • Attach an automatic execution request.
  • Detach an automatic execution request.
  • Redirect stdout/stdin/stderr to and from the IDE.

All of these functionalities will need to take the form of custom extensions to the LSP, as they do not fit well into any of the available extension points. To that end, these extensions should fit well with the LSP.

A subscription (execution listener) is applied to an arbitrary span of code at a given position in the call stack.

  • A subscription may encompass multiple nodes or a single node. Information is received for all nodes covered by the provided span.
  • A subscription will ensure that the client receives information on changes in:
    • Execution state (whether the node is being computed or is cached)
    • Profiling information
    • Values
    • Types
    • Where we are in the call stack (useful for recursive execution)
  • Such subscriptions must be accompanied by heartbeat messages in order to allow the language server to cull unused subscriptions.
  • Additionally, it will be important for each subscription to be able to configure a rate limit, such that the update messages do not overwhelm the client. If unspecified this should be set to a sensible default.


One of the most important elements of execution management for the language server is the ability to control and interact with the execution cache state in the runtime.

  • This cache stores intermediate values, and every value can be in one of three states: invalid, valid but evicted, and valid but present.
  • The cache works based on dependencies between data, such that if foo is used by bar, then changing foo must recompute bar.

The cache eviction strategy is one that will need to evolve. This comes down to the simple fact that we do not yet have the tools to implement sophisticated strategies, but we need to be correct.

  • In the initial version we will invalidate all call sites for a given method name when a name is changed. Internally this is implemented as the invalidation of all occurrences of a dynamic symbol by name, while ignoring the type it was defined on.
  • We also need to account for dependencies between data such that if there is a dependency b => a, then a change to a must invalidate the cache result of b.
  • In future, the typechecker will be able to help constrain the set of evicted methods by exploiting dependencies between values and types with more information.
  • There may also be non-obvious data dependencies that can be exploited to make better cache-eviction decisions.

Progress Reporting

In the future it will be desirable for long running computations to provide real-time progress information (e.g. for training a neural network it would be great to know which epoch is running).

  • This could be achieved by a special kind of Monadic context (similar to writer, but mutable buffer based).
  • This would allow the function to log values without needing to return.
  • These would be sent as visualisations for use in the IDE.

LSP provides an inbuilt mechanism for reporting progress, but that will not work with visualisations. As a result that should be reserved for reporting progress of long-running operations within the language server rather than in user code.


The IDE needs the ability to request completions for some target point (cursor position) in the source code. In essence, this boils down to some kind of smart completion. The completion should provide the following:

  • Sensible suggestions at the cursor position, ranked by relevance.
  • Local variables, where relevant.
  • Suggestions for symbols that would make sense (e.g. by type) but are not imported. To support this, selection of such a symbol will trigger the automatic addition of the relevant import on the language server.
  • Searching in tags and documentation. This metadata-based search functionality should be used to refine suggestions and help suggest functionality relevant to user tasks.
  • Browsing the symbol hierarchy. The user should be able to click through modules to browse the various symbols contained within.
  • Import Completion for when a user has typed import and hits <tab>. This feature should suggest libraries that are available, along with provide their top-level documentation to give users an idea of what they can be used for.

Hints should be gathered by the runtime in an un-ranked fashion based upon the above criteria. This will involve combining knowledge from both the compiler and the interpreter to deliver a sensible set of hints.

  • Hints should be scored on a type match. For example, if we have a type 5, foo : 5 -> String scores higher than bar : Nat -> Dynamic, scores higher than baz : Any -> Any. This should be done by heuristics initially, and later by querying the typechecker for subsumption relationships (the notion of specificity discussed in the types design document).
  • Information contained in the tags section of the documentation should also be used to rank candidates.
  • Please note that this ranking algorithm will be required to get more complex in the future, so please design it for extensibility and high performance.
  • Local variables should rank higher than global symbols.

From an implementation perspective, the following notes apply:

  • This will be implemented on top of the completion and completionResolve messages provided by the LSP spec.
  • We will extend these messages with an optional field that specifies the type being queried upon. This is a stop-gap solution until inference can determine the type.
  • The request does not contain the query string, as text matching is handled by the IDE. The language server only handles candidate completions.
  • We should determine if there are any sensible ways in which this process can be optimised, as we have the potential to return very large completion sets. It is probably worth waiting to see if this is necessary before implementing any optimisations here.

Analysis Operations

We also want to be able to support a useful set of semantic analysis operations to help users navigate their code. As these rely on knowledge of the language semantics, they must be explicitly supported by the language server:

  • List Symbols in Scope: The scope should be specified by a code span.
  • Insert Import for Symbol: This should use an applyEdit message to ask the IDE to insert an import for the symbol specified in the request. If the file is closed, then the edit should be made directly, as the LSP specifies.

Functionality Post 2.0

In addition to the functionality discussed in detail above, there are further augmentations that could sensibly be made to the Engine services to support a much better editing and user-experience for Enso. These are listed briefly below and will be expanded upon as necessary in the future.

  • Refactoring Operations: As all of these operations rely on a semantic analysis of the source program, they must be performed by the language server. These should include (but may not be limited to) the renaming, moving, extraction and inlining of entities. In future this could be expanded to include refactoring hints a la IntelliJ.
  • Arbitrary Visualisation Code: Visualisations should be able to be defined using Enso code and will require additional support.
  • IO Manager: The ability to do sophisticated IO monitoring, such as watching for file changes, in order to support minimal re-execution of analysis pipelines.
  • Enhanced Type-Manipulation: Get fits for holes, case splitting, insert type, refine type, solve type, and so on. Inspiration for these operations can be taken from programs that provide for interactive type-driven development.
  • REPL: Protocol messages to support a REPL-style of interactive development. This should include, at a minimum, the ability to execute arbitrary code statements in a REPL, but could be enhanced by the ability to execute code from the editor in the REPL, and send changes back from the REPL to the file.
  • Debugging:: The user should be able to place break-points, and easily inspect values during execution of a program. This debugging functionality should allow for hot-reloading of code, changing of values within a live program, and various other debugger functionality (step over, step in, step out, continue, etc). Future debugger functionality should be based on the standard debug adapter protocol.
  • Profiling Information: Profiling information for the executing code, able to be displayed visually in Enso Studio.
  • Code Formatting: Automatic formatting of Enso code using the One True Style ™.
  • Server-Side Metadata Management: The lack of node metadata management in the language server currently means that any language client other than Enso Studio is guaranteed to corrupt the node metadata when editing Enso code. This will reset the node layout and can be quite annoying.
  • True Multi-Client Support: The initial release will only support multiple connected clients through the use of a write lock. This is not a great user experience, and in future we should instead use proper conflict resolution for true collaborative editing. This will use a combination of didChange and applyEdit messages to reconcile all clients’ views of the files. This is also why willSaveWaitUntil is important, as it can ensure that no client editor saves until it has the authority to do so (all changes are reconciled).
  • Enhanced Semantic Analysis: Enhanced semantic analysis operations that rely on compiler analysis and typechecking. This includes things like “find usages”, “jump to definition” and “find symbol”, as these can greatly enhance a user’s development experience.
  • LSP Spec Completeness: We should also support all LSP messages that are relevant to our language. Currently we only support a small subset thereof.

Binary Protocol

The binary protocol refers to the auxiliary protocol used to transport raw binary data between the engine and the client. This functionality is entirely extraneous to the operation of the textual protocol, and is used for transferring large amounts of data between Enso components.

As the protocol is a binary transport, it is mediated and controlled by messages that exist as part of the textual protocol.

In order to deserialize a family of messages and correlate responses with requests, each request/response/notification is wrapped in an envelope structure. There is a separate envelope for incoming and outgoing messages:

namespace org.enso.languageserver.protocol.binary;

//A mapping between payload enum and inbound payload types.
union InboundPayload {
  INIT_SESSION_CMD: InitSessionCommand,
  WRITE_FILE_CMD: WriteFileCommand,
  READ_FILE_CMD: ReadFileCommand

//An envelope for inbound requests and commands.
table InboundMessage {

  //A unique id of the message sent to the server.
  messageId: EnsoUUID (required);

  //An optional correlation id used to correlate a response with a request.
  correlationId: EnsoUUID;

  //A message payload that carries requests sent by a client.
  payload: InboundPayload (required);

namespace org.enso.languageserver.protocol.binary;

//A mapping between payload enum and outbound payload types.
union OutboundPayload {
  ERROR: Error,
  SUCCESS: Success,
  VISUALISATION_UPDATE: VisualisationUpdate,
  FILE_CONTENTS_REPLY: FileContentsReply

//An envelope for outbound responses.
table OutboundMessage {

  //A unique id of the message sent from the server.
  messageId: EnsoUUID (required);

  //An optional correlation id used to correlate a response with a request.
  correlationId: EnsoUUID;

  //A message payload that carries responses and notifications sent by a server
  payload: OutboundPayload (required);

namespace org.enso.languageserver.protocol.binary;

//This message type is used to indicate failure of some operation performed.
table Error {

  //A unique error code identifying error type.
  code: int;

  //An error message.
  message: string;


//Indicates an operation has succeeded.
table Success {}

Binary Protocol Communication Patterns

The binary protocol currently only supports a single type of communication pattern:

  • Push: Messages containing data are pushed in response to operations performed using the textual protocol.

Binary Protocol Transport

The binary protocol uses flatbuffers for the protocol transport format. This choice has been made for a few reasons:

  • Robust multi-language support, including Rust and Java on the JVM.
  • High performance, including support for zero-copy data handling and streaming data.
  • Robust, schema-based messages.

Binary Protocol Functionality

The binary protocol exists in order to serve the high-bandwidth data transfer requirements of the engine and the GUI.

Displaying Visualisations

A major part of Enso Studio’s functionality is the rich embedded visualisations that it supports. This means that the following functionality is necessary:

  • Execution of an arbitrary Enso expression on a cached value designated by a source location.
  • The ability to create and destroy visualisation subscriptions with an arbitrary piece of Enso code as the preprocessing function.
  • The ability to update existing subscriptions with a new preprocessing function.

Visualisations in Enso are able to output arbitrary data for display in the GUI, which requires a mechanism for transferring arbitrary data between the engine and the GUI. These visualisations can output data in common formats, which will be serialised by the transport (e.g. text), but they can also write arbitrary binary data that can then be interpreted by the visualisation component itself in any language that can be used from within the IDE.

From the implementation perspective:

  • This will need to be an entirely separate set of protocol messages that should be specified in detail in this document.
  • Visualisations should work on a pub/sub model, where an update is sent every time the underlying data is recomputed.
  • Protocol responses must contain a pointer into the binary pipe carrying the visualisation data to identify an update.

Service Connection Setup

As these services need to support multiple clients in future, there is some rigmarole around setting up the various connections needed by each client. The process for spawning and connecting to an engine instance is as follows:

  1. Spawn the Server: The project manager spawns the language server, passing the socket information as part of the initialisation flow.
  2. Client ID Generation: The client generates and stores a UUID that will be used to identify the client while it is connected.
  3. Protocol Connection Initialisation: The client performs the init for the textual protocol connection, passing its client identifier as it does so. See session/initProtocolConnection for more information.
  4. Data Connection Initialisation: The client performs the init for the data connection, passing its client identifier as it does so. See session/initDataConnection below more information.

Service Connection Teardown

As the engine performs sophisticated caching and persisting of data where possible, it is very important that the client informs the engine of the end of its session. In contrast to the initialisation flow above, this is not an involved process.

  1. Notify the Engine: Prior to disconnecting from the sockets, the client must send session/end to the server.
  2. Disconnect: Once that message has been sent, the client may disconnect at any time.