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MOBILE2025.12.28

Message Ordering in Chat Applications

Message ordering is a fundamental aspect of any chat application. Learn about the challenges and approaches to implementing reliable message ordering.

Message ordering is a crucial part of any chat application. It ensures that conversations feel natural and are easy to follow. When messages appear in the wrong order, users can quickly become confused and frustrated. What seems like a simple requirement (showing messages in chronological order) actually involves several challenging edge cases, especially on mobile devices.

The Challenges of Message Ordering

Some factors that disrupt the natural flow of messages.

  • Network delays can cause messages to arrive out of sequence.
  • Offline messages need to be integrated into the conversation once the user reconnects.
  • Different devices may have unsynchronized system clocks, leading to inconsistencies.
  • Both past messages and new updates must maintain a consistent and accurate order.

    • How to Implement Message Ordering ?

    To implement reliable message ordering, a key design decision is choosing which field to use for sorting messages. The data model typically provides two main options which are a server-generated messageId and a client-generated createdAt timestamp.

    Order by messageId

    In this approach, the backend assigns a unique, sequential ID to each message as it is processed. This ensures a consistent and authoritative order across all devices.

    Advantages

  • The backend remains the single source of truth, guaranteeing that all clients display the same message order.

    • Disadvantages

  • The client does not know the messageId until the backend generates it, which can introduce latency.
  • This approach also does not effectively support offline messaging.

    • Order by createdAt Timestamp

    Here, the client sets a timestamp when the message is created. This allows for immediate display of the message in the UI, even before it has been sent to the server.

    Advantages

  • This method provides low latency and supports offline messaging, as the message can be displayed instantly.

    • Disadvantages

  • It is vulnerable to inconsistencies due to clock differences between devices and can be susceptible to client-side manipulation.

    • Synchronizing Clocks

    A third option is to use a createdAt timestamp but with backend clock synchronization. This combines the benefits of timestamps with improved reliability (read Simple Precision Time Protocol (SPTP)).

    Advantages

  • This reduces ordering issues caused by clock drift.

    • Disadvantages

  • Implementing accurate clock synchronization across diverse networks is complex and can be costly.

    • Trade-offs at a Glance

    Order byAdvantagesDisadvantages
    messageIdA server-generated ID ensures consistent ordering across clients.The client doesn't have the messageId until the backend generates it.
    createdAt timestampLow latency. Immediate message display without waiting for the backend.Vulnerable to inconsistencies from device clock differences.
    createdAt with syncCombines timestamp benefits with improved reliability.Clock drift between devices can still cause problems.

    A Hybrid Approach for a Seamless Experience

    To mitigate the downsides of each approach, many modern chat applications use a hybrid strategy. This method provides a smooth user experience by combining the speed of client-side timestamps with the reliability of server-side IDs.

  • Successfully sent messages are those that have been acknowledged by the server and are ordered using the server-generated messageId.
  • Pending messages are those that are still waiting to be sent or are in the process of being sent. They are ordered based on the client-generated createdAt timestamp and always appear at the end of the message list.

    • This ensures that newly sent messages appear instantly, while the final, authoritative order is maintained by the server.

    The Message Lifecycle

    To make a better decision on screen message ordering, it is important to understand how messages flow through the system. The following diagram illustrates the journey of a message from the sender to the receiver in a typical chat system.

    Message Lifecycle Diagram

    Simply follow this flow

    1 Immediate Storage & Display - The Messages repository immediately creates and stores new messages, and the UI displays them on screen right away to provide instant feedback.

    2 Remote Forwarding - Behind the scenes, the repository forwards this message to the Messages remote data source, which packages it as a NewMessageRequest and sends it to the backend via HTTP.

    3 Server Processing - The backend processes this request, generating a unique messageId for the message.

    4 Dual Notification - The backend then takes two parallel actions.

    (4.1) It sends a 201 response back to the sender with a NewMessageResponse object.

    (4.2) It notifies the recipient, either through a WebSocket if they are active, or a push notification if they are inactive.

    5 Local Update - Once the sender's client receives the HTTP response, it updates the message in local storage with the new messageId. Lastly, the UI refreshes to show successful delivery.

    Message ordering in chat applications is a complex problem with no single perfect solution. By understanding the trade-offs between different approaches, developers can choose the best strategy for their specific needs. The hybrid approach, combining client-side timestamps for pending messages and server-side IDs for confirmed messages, offers a robust and user-friendly solution that is widely adopted in the industry.

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