The Language Server Protocol developed by Microsoft for Visual Studio Code is a language and IDE agnostic protocol which clearly separates language semantics from UI presentation. Language developers can implement the protocol and benefit from immediate support in all IDEs, while IDE developers, who implement the protocol get automatic support for all these languages without having to write any language-specific code. This session will let you learn more about the innards of the LSP. We will also have an overview of the current implementations in Eclipse, and outside Eclipse as well. 

1. The Language Server Protocol:
Why the Hype?
Mikael Barbero
Eclipse Foundation
Eclipse Summit India 2017

2. Dear IDE, please
support this new trendy
language!

3. Dear IDE, please
support this new trendy
language!
Dear language, please
provide integration with
my favorite IDE!

4. Dear IDE, please
support this new trendy
language!
Dear language, please
provide integration with
my favorite IDE!

5. Dear IDE, please
support this new trendy
language!
Dear language, please
provide integration with
my favorite IDE!
Language Server Protocol helps building language support!

6. Implementing language support for an IDE is hard
Expert of the tool Expert of the language
Parser
Semantic analysis
Type system
UI Workflows
APIs
Integration

7. Java Javascript C/C++ Go Json Rust CSS ...
Eclipse
Eclipse Che
Eclipse Orion
Atom
VisualStudio
Code
...

8. What is the so-called
Language Server Protocol (LSP)?

9. –Phil Karlton
“There are only two hard things in Computer Science:
cache invalidation and naming things.”

10. –Phil Karlton
“There are only two hard things in Computer Science:
cache invalidation and naming things.”

11. What is the so-called
Language Server Protocol (LSP)?

12. “A bad name for a Remote Procedure Call (RPC) API that is
used between by a tool and a language smartness provider to
integrate features like auto complete, goto definition, find all
references and alike into the tool”

13. “A bad name for a Remote Procedure Call (RPC) API that is
used between by a tool and a language smartness provider to
integrate features like auto complete, goto definition, find all
references and alike into the tool”

14. https://github.com/Microsoft/language-server-protocol

15. LSP
(RPC API)

16. Program A
LSP
(RPC API)

17. Program A Program B
LSP
(RPC API)

18. Tool
Program A Program B
LSP
(RPC API)

19. Language
Smartness
Provider
Tool
Program A Program B
LSP
(RPC API)

20. Language
Smartness
Provider
Tool
Program A Program B
LSP
(RPC API)

21. Language
Smartness
Provider
Tool
Program A Program B
LSP
(Client)
(RPC API)

22. Language
Smartness
Provider
Tool
Program A Program B
LSP
(Client) (Server)
(RPC API)

23. Language
Smartness
Provider
Tool
Program A Program B
LSP

24. Language
Smartness
Provider
Tool
Program A Program B
LSP

25. Tool
Tool
Tool
Language
Smartness Provider

26. Tool
Tool
Tool
Language
Smartness Provider
N
o
m
ultitenancy

27. Implementing language support for an IDE is easier
Expert of the tool Expert of the language
Parser
Semantic analysis
Type system
UI Workflows
APIs
Integration
LSP

28. Eclipse
Eclipse Che
Eclipse Orion
Atom
VisualStudio
Code
...
Java
Javascript
C/C++
Go
Json
Rust
CSS
...
LSP
DRY
(don't repeat yourself)

29. Current Status - Clients Support
Eclipse CheEclipse (LSP4e) NeoVim
Visual Studio
Code
Sublime TextEmacs VimAtom
Work in Progress
Eclipse Orion

30. Current Status - Languages Support

31. Current Status - LSP SDKs
node.js MS vscode-languageserver-node
C# MS work in progress by David Wilson
Java Eclipse, TypeFox Eclipse LSP4J
Haxe @nadako language-server-protocol-haxe
PHP Felix Becker php-language-server
Rust Bruno Medeiros RustLSP
Haskell Alan Zimmerman Haskell-LSP
C# OmniSharp C#-LSP
C# Inomata Kentaro LanguageServerProtocol
SDK/libraries support implementing the protocol in a particular language.

32. LSP in depth

33. JSON-RPC based API
)]}'

34. JSON-RPC
Stateless RPC
protocol
(use "id" used by client to
track responses)
JSON as data
format of request
and responses
Agnostic of
transport layer
(can be Sockets, Shared
Memory, PIPE, HTTP, ...)

35. JSON-RPC
Program A Program B

36. JSON-RPC
{
"jsonrpc": "2.0",
"method": "subtract",
"params": [42, 23],
"id": 1
}
Program A Program B

37. JSON-RPC
{
"jsonrpc": "2.0",
"result": 19,
"id": 1
}
{
"jsonrpc": "2.0",
"method": "subtract",
"params": [42, 23],
"id": 1
}
Program A Program B

38. Agnostic of
transport layer
(can be Sockets, Shared
Memory, PIPE, HTTP, ...)
Clients can only communicate with servers if they
talk on the same transport channel.
e.g., if a server only supports pipes (stdin / stdout),
a client that uses sockets won’t be able to use it.

39. LSP in a nutshell
A set of JSON-RPC requests,
responses and notifications to integrate
tools (clients) and language smartness
providers (servers)

40. LSP: Client Requests
Completion
Requests a list of completion
items from a document
position. Resolution can be
done in two steps
Hover
Requests hover information at a
given text document position.
Result is a markdown string or
a pair of a language and a code
block value
Signature Help
Requests signature
information at a given cursor
position (e.g. used to display a
tooltip with signature when
typing a method call)
References
Requests to resolve project-
wide references for the
symbol denoted by the given
text document position
Workspace Symbols
Requests to list project-wide
symbols matching the query
string (usually the name /
name prefix of the symbol)
Document Symbols
Requests to list all symbols
found in a given text
document

41. LSP: Client Requests (cont'd)
Formatting
Requests to format a whole
document
Range Formatting
Requests to format a given
range in a document
On Type Formatting
Requests to format parts of
the document during typing.
Trigger characters are
configured at registration time
Go to Definition
Requests to resolve the
definition location of a symbol
at a given text document
position
Code Action
Requests to compute
commands for a given text
document and range. Can be
used for instance to get the
list of quick fixes for a given
problem
Code Lens
Request to compute code
lenses for a given text
document. It returns a list of
commands to be executed for
some text ranges (e.g.
number of references)

42. LSP: Client Requests (cont'd again)
Rename
Requests to perform a
workspace-wide rename of a
symbol
Document Links
Requests the location of links
in a document. Resolution can
be done in two steps
Document Highlight
Requests to resolve a
document highlights for a
given text document position
(e.g., to mark all occurrences
of the variable for a given
position)

43. Typical lifecycle (startup)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
Client Tool
(e.g., Eclipse, VSCode)

44. Typical lifecycle (startup)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
Start
Client Tool
(e.g., Eclipse, VSCode)
(not defined by LSP)
Can be child process, daemon, run image in
container, HEAD http://server/status, …

45. Typical lifecycle (startup)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
Start
Initialize
Params: rootUri, the root URI of the workspace + client
capabilities
The protocol does not define how the rootUri parameter should be used by the servers. Most servers expect file: URIs. JDT-LS uses
this URI to provision an Eclipse Workspace (i.e. imports all Eclipse, Gradle and Maven projects it can find in the rootUri hierarchy).
Client Tool
(e.g., Eclipse, VSCode)
(not defined by LSP)
Can be child process, daemon, run image in
container, HEAD http://server/status, …

46. Typical lifecycle (startup)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
Start
Initialize
Params: rootUri, the root URI of the workspace + client
capabilities
Returns server capabilities (like how text documents are
synced)
The protocol does not define how the rootUri parameter should be used by the servers. Most servers expect file: URIs. JDT-LS uses
this URI to provision an Eclipse Workspace (i.e. imports all Eclipse, Gradle and Maven projects it can find in the rootUri hierarchy).
Client Tool
(e.g., Eclipse, VSCode)
(not defined by LSP)
Can be child process, daemon, run image in
container, HEAD http://server/status, …

47. Typical lifecycle (editing)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
Client Tool
(e.g., Eclipse, VSCode)

48. Typical lifecycle (editing)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
didOpen
Client Tool
(e.g., Eclipse, VSCode)
Notifies the server that the document's truth is now
managed by the client and the server must not try to
read the document's truth using the document's uri

49. Typical lifecycle (editing)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
didOpen
didChange
Signals changes to a text document (the granularity of the
reported changes are defined by server capabilities)
Client Tool
(e.g., Eclipse, VSCode)
Notifies the server that the document's truth is now
managed by the client and the server must not try to
read the document's truth using the document's uri

50. Typical lifecycle (editing)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
didOpen
didChange
Signals changes to a text document (the granularity of the
reported changes are defined by server capabilities)
Usually will notify about new diagnostics, some time later
Client Tool
(e.g., Eclipse, VSCode)
Notifies the server that the document's truth is now
managed by the client and the server must not try to
read the document's truth using the document's uri

51. Typical lifecycle (editing)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
didOpen
didChange
Signals changes to a text document (the granularity of the
reported changes are defined by server capabilities)
Usually will notify about new diagnostics, some time later
The didOpen, didChange (and willSave, save, didClose) are not mandatory. Some servers expose via their capabilities that they do not
need to be notified about these events.
Client Tool
(e.g., Eclipse, VSCode)
Notifies the server that the document's truth is now
managed by the client and the server must not try to
read the document's truth using the document's uri

52. Typical lifecycle (saving)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
Client Tool
(e.g., Eclipse, VSCode)
The didOpen, didChange (and willSave, save, didClose) are not mandatory. Some servers expose via their capabilities that they do not
need to be notified about these events.

53. Typical lifecycle (saving)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
willSave | willSaveWaitUntil
Client Tool
(e.g., Eclipse, VSCode)
+ Reason of the save (manual, after delay, focus out). "Wait until" is a way to
get all the edits that should be apply before saving (e.g. if a refactoring has
been requested before and it still being computed on the server side)
The didOpen, didChange (and willSave, save, didClose) are not mandatory. Some servers expose via their capabilities that they do not
need to be notified about these events.

54. Typical lifecycle (saving)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
willSave | willSaveWaitUntil
didSave
Can include the whole text document if server capabilities
require it
Client Tool
(e.g., Eclipse, VSCode)
+ Reason of the save (manual, after delay, focus out). "Wait until" is a way to
get all the edits that should be apply before saving (e.g. if a refactoring has
been requested before and it still being computed on the server side)
The didOpen, didChange (and willSave, save, didClose) are not mandatory. Some servers expose via their capabilities that they do not
need to be notified about these events.

55. Typical lifecycle (saving)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
willSave | willSaveWaitUntil
didSave
Can include the whole text document if server capabilities
require it
The document's truth now exists where the document's uri
points to.
Client Tool
(e.g., Eclipse, VSCode)
+ Reason of the save (manual, after delay, focus out). "Wait until" is a way to
get all the edits that should be apply before saving (e.g. if a refactoring has
been requested before and it still being computed on the server side)
didClose
The didOpen, didChange (and willSave, save, didClose) are not mandatory. Some servers expose via their capabilities that they do not
need to be notified about these events.

56. Typical lifecycle (shutdown)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
Client Tool
(e.g., Eclipse, VSCode)

57. Typical lifecycle (shutdown)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
shutdown
Client Tool
(e.g., Eclipse, VSCode)
Asks the server to shut down, but to not exit (otherwise the
response might not be delivered correctly to the client)

58. Typical lifecycle (shutdown)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
shutdown
May return errors
Client Tool
(e.g., Eclipse, VSCode)
Asks the server to shut down, but to not exit (otherwise the
response might not be delivered correctly to the client)

59. Typical lifecycle (shutdown)
Client
(e.g., VSCode, Eclipse)
Language Server
(e.g., JDT-LS, PHP-LS)
shutdown
exit
May return errors
Asks the server to exit its process. The server should exit
with success code 0 if the shutdown request has been
received before; otherwise with error code 1.
The specification explicitly talk about exiting a process on the ‘exit’ notification. Of course, some clients that will interact with daemons,
containers or HTTP servers will use this notification differently.
Client Tool
(e.g., Eclipse, VSCode)
Asks the server to shut down, but to not exit (otherwise the
response might not be delivered correctly to the client)

60. Server does no editing
All editing is done on the client side
• The server can read files, but all file modifications
are sent to the clients in the form of a
WorkspaceEdit.
• The client applies the edits and notifies the server
about the changes so he can refresh changed
files.
• To execute a command (e.g. quick fix or a
rename), the client send a request that the server
acknowledge or not. If it does, then the server
compute the changes and send a request to the
client for him to apply the modifications

61. LSP Limitations

62. Text Editing Only
No support for running, testing or debugging
Ongoing work for debug
https://github.com/Microsoft/vscode-debugadapter-node

63. No Advanced Tooling
Types hierarchy Advanced refactoring

64. Syntax Highlighting, Bracket Matchings and Code Folding
Deferred to the client's editor
Requires tokenizer and / or grammar in addition to the language server

65. No Communication between Language Servers
No cross-languages go to
definition or refactoring

66. No Packaging Standard
• Currently: specific to each editor
• Language servers can have a lot
of native dependencies
• Possible solution: use docker to
package and run LS

67. No standard transport or transport negotiation
Potential solutions:
• Force transport negotiation via a
specific transport during startup
phase
• Store metadata about the language
servers in a registry / marketplace

68. ??
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69. Key Takeaways: why the hype?
Mikaël Barbero
mikael.barbero@eclipse-foundation.org
@mikbarbero
https://eclipse.org/community/eclipse_newsletter/2017/may/

70. Key Takeaways: why the hype?
Promising and
powerful
abstraction for
language tooling
development
Mikaël Barbero
mikael.barbero@eclipse-foundation.org
@mikbarbero
https://eclipse.org/community/eclipse_newsletter/2017/may/

71. Key Takeaways: why the hype?
Promising and
powerful
abstraction for
language tooling
development
Still young and has
shortcomings
Mikaël Barbero
mikael.barbero@eclipse-foundation.org
@mikbarbero
https://eclipse.org/community/eclipse_newsletter/2017/may/

72. Key Takeaways: why the hype?
Promising and
powerful
abstraction for
language tooling
development
Still young and has
shortcomings
Paradigm shift in
tooling
development:
microservices
Mikaël Barbero
mikael.barbero@eclipse-foundation.org
@mikbarbero
https://eclipse.org/community/eclipse_newsletter/2017/may/