4.9. Time synchronization persistence support

4.9.1. Overview

This document complements NTP-based time synchronization and explains how to persist the state of the NTP Time Synchronization Object.

The NTP module is designed to integrate with the generic persistence mechanism used in Anjay Lite. This allows the application to easily save the NTP object state across device restarts.

Note

Code related to this tutorial can be found under examples/tutorial/AT-TimeSynchronizationPersistence in the Anjay Lite source directory.

Note

A detailed description of the persistence mechanism, including object storage and restoration patterns, is provided in the Persistence support tutorial.

4.9.2. Built-in object persistence

The Time Synchronization Object instance managed by the NTP module can be saved to and restored from non-volatile storage using the helper functions:

• anj_ntp_obj_restore() - restores the NTP Object instance from previously stored data during startup,

• anj_ntp_obj_store() - serializes the current NTP Object instance, so it can be written to persistent storage.

To use persistence API set the CMake option ANJ_WITH_PERSISTENCE.

4.9.3. Initialization with persistence restore

When the application starts, it attempts to restore the NTP Object state from persistent storage using anj_ntp_obj_restore(). If the restoration succeeds, the NTP module continues using the previously saved configuration. Please note that anj_ntp_obj_restore() is called after anj_ntp_init() and this is slightly different from the usual pattern described in the Persistence support tutorial.

if (anj_ntp_init(&anj, &ntp, &ntp_config)) {
    log(L_ERROR, "Failed to initialize NTP module");
    return -1;
}

FILE *file = fopen(PERSISTENCE_NTP_FILE, "r");
if (file) {
    anj_persistence_context_t ctx =
            anj_persistence_restore_context_create(persistence_read, file);
    if (anj_ntp_obj_restore(&ntp, &ctx)) {
        fclose(file);
        remove(PERSISTENCE_NTP_FILE);
    } else {
        fclose(file);
    }
}

4.9.4. Persisting state after object update

When the LwM2M Server updates Time Synchronization Object resources (for example, changes the NTP server address or synchronization period), the NTP module reports this via the ANJ_NTP_STATUS_OBJECT_UPDATED status in the event callback. The application can treat this status as a trigger to call anj_ntp_obj_store() and persist the new configuration together with other client state.

case ANJ_NTP_STATUS_OBJECT_UPDATED: {
    FILE *file = fopen(PERSISTENCE_NTP_FILE, "w+");
    if (!file) {
        log(L_ERROR, "Could not open persistence file for writing");
        return;
    }
    anj_persistence_context_t ctx =
            anj_persistence_store_context_create(persistence_write, file);
    if (anj_ntp_obj_store(ntp, &ctx)) {
        log(L_ERROR, "Failed to persist NTP object state");
        fclose(file);
        remove(PERSISTENCE_NTP_FILE);
        return;
    }
    fclose(file);
    log(L_INFO, "NTP object state persisted successfully");
    break;
}

4.9.5. Summary

This tutorial explained how to use Anjay Lite’s persistence mechanism to store and restore the state of the NTP Time Synchronization Object. By integrating persistence into the application, the NTP module can maintain its configuration across device restarts, ensuring consistent time synchronization behavior.