Migration of LINE’s Android App from SPDY to HTTP/2

Masakuni Oishi
LINE / LINE Communication Platform Engineering / Senior Software Engineer

https://linedevday.linecorp.com/2021/ja/sessions/130
https://linedevday.linecorp.com/2021/en/sessions/130
https://linedevday.linecorp.com/2021/ko/sessions/130

Transcript

1. View Slide

2. Agenda - Problems of LINE app’s network stack
   - What we did to revamp the network stack
   - What we achieved with this project
   

   View Slide

3. େੴ ক๜
   Masakuni Oishi
   LINE Communication Platform
   Engineering
   

   View Slide

4. Background
   - SPDY protocol + our own push mechanism
   - It was introduced in the early days of LINE app for efficient API calls.
   - But, the successor protocols, HTTP/2 and HTTP/3, have been standardized.
   SPDY should be replaced by them.
   - The code had become difficult to maintain.
   - Because, we had made our own customizations to Netty library.
   The network stack of LINE Android app had several problems
   

   View Slide

5. Project Goals
   - Migrate SPDY to HTTP/2
   - In addition, consider upgrading to HTTP/3 in the future.
   - Comply with standard specifications
   - Reduce our own customizations as much as possible.
   - Safely complete the migration of the network stack
   - A failure in the network stack may affect a huge number of LINE users.
   

   View Slide

6. Difficulties
   - Migrate all API calls in the app “at once”.
   - Implement bidirectional communication with the server over HTTP/2.
   - Release incrementally in a way that allows for rollback.
   

   View Slide

7. Why we need to migrate AT ONCE
   SPDY protocol stack has a connection pool.
   Server
   Features
   SPDY stack
   Connection
   pool
   API X API X over SPDY
   Client app
   

   View Slide

8. Why we need to migrate AT ONCE
   HTTP/2 protocol stack also has its own connection pool.
   Server
   Features
   HTTP/2 stack
   Connection
   pool
   API Y
   Client app
   API Y over HTTP/2
   

   View Slide

9. Why we need to migrate AT ONCE
   If both SPDY and HTTP/2 are used at the same time,
   the number of TCP connections to the server will be doubled.
   Server
   Features
   SPDY stack
   Connection
   pool
   API X API X over SPDY
   HTTP/2 stack
   Connection
   pool
   API Y API Y over HTTP/2
   Client app
   

   View Slide

10. Solution
    Introduced a common network layer so that the protocol to be used can be switched
    by a feature flag.
    Features
    SPDY stack
    Connection
    pool
    API X,Y
    HTTP/2 stack
    Connection
    pool
    Common network layer
    Switch by
    a feature flag
    

    View Slide

11. Bidirectional communication with server
    - For real-time chatting and other purposes, LINE client app uses bidirectional
    communication with the server.
    - Previously, LINE app used a mechanism called “SPDY Push”, which utilizes
    server-initiated streams of the SPDY protocol.
    

    View Slide

12. SPDY Push
    When the server pushes a message to the client, it is sent as a server-initiated
    stream (SYN_STREAM + DATA frames) on the SPDY connection.
    Server
    Client SPDY stack
    SPDY connection
    SYN_STREAM + DATA
    

    View Slide

13. Problems of SPDY Push
    - Most network libraries do not support server-initiated streams.
    - For LINE Android app, we forked Netty library and made our own
    customizations.
    - These customizations were the main cause of making the code difficult to
    maintain.
    - Therefore, for HTTP/2, we have developed a new mechanism called
    “Streaming Push”.
    

    View Slide

14. Streaming Push
    Client makes a call of HTTP POST method, and uses its request body and
    response body as streams, respectively.
    Server
    Client HTTP/2 stack
    HTTP POST method
    Request body →
    ← Response body
    

    View Slide

15. Implementation of Streaming Push
    - The mechanism of Streaming Push is a proven one that is also used in
    gRPC.
    - It fully follows the semantics of HTTP/2.
    - In LINE Android app, Streaming Push is implemented on top of the bare,
    un-customized OkHttp library.
    

    View Slide

16. Prepare for rollout
    - SPDY and HTTP/2 protocol stack can be switched by a feature flag.
    - SPDY Push and Streaming Push are also switched by this flag.
    - We added a mechanism that allows this flag to be changed for each user by
    server-side settings.
    

    View Slide

17. Test and Rollout
    We spent more than a month conducting in-house tests of HTTP/2 protocol,
    gradually increasing the number of test targets.
    Finally, we rolled out the HTTP/2 protocol to all users and completed it
    successfully without any trouble.
    LINE client
    developers
    All LINE
    employees
    5% of users 100% of users
    

    View Slide

18. What we achieved with this project
    - LINE Android app has completely migrated from SPDY to HTTP/2.
    - A new push mechanism that complies with HTTP/2 semantics has been
    introduced.
    - Successfully completed the rollout without any trouble.
    - Client-side code is now in a modern architecture using OkHttp and Kotlin
    Coroutines.
    

    View Slide

19. QUIC and HTTP/3…
    - The semantics of HTTP/3 have not changed from HTTP/2.
    - Streaming Push mechanism should also work on HTTP/3.
    - Currently, OkHttp does not support QUIC and HTTP/3. However, when
    OkHttp supports them, LINE Android app should only need to upgrade the
    version of the OkHttp library.
    

    View Slide

20. OSS library created for this project
    - We released an OSS library created for this project.
    - Lich OkHttp & Lich Thrift : https://github.com/line/lich
    - Coroutine-aware extensions for OkHttp & Apache Thrift
    - e.g. Perform a resumable download using Range requests.
    

    View Slide

21. Thank you
    

    View Slide